PRIMARY SUCCESSION BEACH PLAIN:

Speciesand nuTrient

relationaccumulation.

biomass

isman

dynamics

OIS" "' I) Lf')J

PRIMARY SUCCESSION ON A BEACH PLAIN:

SPECIES DYNAMICS IN RELATION TO BIOMASS AND NUTRIENT ACCUMULATION

Jef Huisman

Begeleiding: Han 01ff

RijksUniversiteit GroningenDoctoraal—onderwerp Plantenoecologie

Vakgroep Biologie van Planten1990

Rijksunlversitelt Groflingefl

BbllOtheek BjOIOOISCh Centrum

Kerklaafl 30 — postbus 149750 AA HAREN

De resultaten vermeld in dit versiag worden niomenteel bewerkt voor publicatie.Overname of gebruik van gegevens is dan ook niet toegestaan.

CONTENTS

Summary 3

Introduction 4

Materials and methodsThe study area 6

The permanent transects 7

Analyses on the data of species dynamics 7

Reconstruction of changes in biomass, light and soil 9

ResultsAnalyses on the data of species dynamics 12Reconstruction of changes in biomass, light and soil 14

Discussion 17

References 22

Tables & figures 24

SUMMARY

The present study reports on a primary succession series which started on the

bare beach of the Dutch island Schiermonnikoog after the building of a sand

dike in 1959. Vegetational changes were studied by means of permanent

transects. Soil development and vegetation structure during this successional

series could be reconstructed, since several stages of this succession were

present in the same part of the island. Salinity, moisture content and

flooding are considered to be major determinants of the spatial variation,

ranging from saline, wet plains to dry, fresh dunes, and are likely to be

responsible for the year to year fluctuations of short—living species.

However, these factors did not show a trend in time. From soil analyses it is

argued that nitrogen limits the total biomass. In about 16 years the total

amount of nitrogen in the organic layer of the soil increased from 7 to 50 g

N m in the plains and from 1 to 15 g N nf2 on the dunes. The accumulation of

nitrogen during succession is accompanied by an increased biomass, a decreased

light penetration to the soil—surface, a decreased proportion root, an

increased proportion stem, an increased plant height at maturity, and a

decreasing abundance of short—living species. Only during the first stage on

the dry dunes sand—blowing is considered to prevent small species fromestablishing. With the exception of this early dune stage, our observations

are in agreement with the resource ratio hypothesis of Tilman (1985). Causes

of the high rates of nitrogen accumulation in the soil are discussed. The

observed nitrogen accumulation, which is considered to be an importantdeterminant of the successional dynamics, is thought to be facilitated by the

vegetation.

3

INTRODUCTION

Primary succession starts on a bare soil, without plant propagules and organicmatter. This can occur after for instance sand dune formation, the retreat ofglaciers, heavy erosion or volcanic eruptions. Except for nitrogen, all of themineral elements required by plants occur in the parent material in which mostsoils form (Jenny, 1980). In general, the bare substrates on which primarysuccessions start are therefore very low in nitrogen (Gerlach 1988, Crockerand Major, 1955). Therefore, nitrogen will often be the limiting soil resourceduring the early stages of primary successions.

The rate of accuu1ation of nitrogen in the system will be determined by forexample the abundance of nitrogen—fixing organisms and the rate of atmosphericinput. The total amount of the limiting nutrient in an ecosystem imposes anabsolute limit on the maximal possible biomass of the system (DeAngelis et al,1989)Robertson and Vitousek (1981) and Robertson (1982) found that the

mineralization potential of the sand dunes at Lake Michigan increased withtheir total nitrogen content. The same was found by Gerlach (1988) on the sanddunes of Spiekeroog. Willis (1963) observed that N—addition at the dunes ofBraunton Burrows, England, was followed by an increased above—ground standingcrop and a change in species composition. In his experiments P—addition didnot increase the standing crop, but N+P—addition was followed by a largerstanding crop compared to the N—addition, with other species reachingdominance. The addition of only P, trace—elements and K, Ca and Mg had noeffects.

An increased above—ground standing crop in a more productive habitat willlead to a decrease in the amount of light penetrating to the soil—surface.This observation led Tilman (1985) to the formulation of the resource ratiohypothesis of succession (Tilman, 1985). This hypothesis is based on the factthat plants face a trade—off while foraging for light and below—groundresources (nutrients, water). This implicates that plants which are superiorcompetitors for light will be less efficient in competing for nutrients andvice versa. Succession starting on a poor soil driven by the accumulation ofnitrogen in the system will therefore implicate competitive replacement ofnutrient competitors by light competitors, since the relative availabilitiesof nutrients and light changes through time. This hypothesis states thatcompetition will be a major determinant of species dynamics during primarysuccession.

An increase of the total amount of a limiting nutrient, accompanied with anincrease of the availability of this nutrient, and a constant loss rate, willlead to an increased total plant bioniass and a decreased light penetration tothe soil—surface. In such a time series the resource ratio hypothesis predictsa succession characterized by a decrease in the allocation to root, anincrease in the allocation to stem, an increase in the plant height atmaturity and a decreasing in abundance of short—living species.

The predictions of the resource ration hypothesis were not in correspondencewith the data on two different secondary succession series. Tilman had toreject his hypothesis as an explanation for the first 60 years of secondaryold field succession on a nitrogen poor Minnesota sand plain (Gleeson andTilnian 1989). During this series the allocation to roots increased withincreasing nutrient availability. Berendse and Elberse (1989) and 01ff et al.(1990), studying Dutch grassland—successions under hay—making, found thatspecies from relatively poor successional stages had a higher shoot/root ratiothan species from richer stages.

4

Other hypothesis have been proposed to explain successional dynamicsstarting on a bare soil. Early species might allocate more to many small seedswhich makes them good colonists. Later species might be poorer colonists butgood nutrient competitors. This trade—off is likely to explain the old fieldsuccession at the Minnesota sand plain (Gleeson and Tilman, 1989) and comesclose to a shift from 'Ruderals' to 'Competitors' sensu Grime (1979). Atrade—off between maximal relative growth rate and competitive abilities isconsidered to be important to explain succession patterns starting onnutrient—rich soils (Grime, 1979; Chapin, 1980; Tilman, 1988).Other trade—offs might be of importance when dealing with other physical

constraints on plant growth. A change in these constraints might thereforealso lead to a change in vegetation composition. Halofyts, for instance,invest a lot in physiological and morphological properties which enable themto withstand high salt concentrations. However, most halofyts seem to havetheir physiological growth optimum in freshwater conditions (Rozema, 1978).A critical revision of the literature on halophytes led Barbour (1970) toconclude an absence of any obligate halophyts among the angiosperms. Yet a lotof reports have described successions where desalination was considered to bethe main cause of species replacements (see for instance Feekes (1936) andJoenje (1974)). Apparently, under fresh conditions, the investments ofhalofyts only lead to growth reductions in competitive situations. Trade—offsbeteen investments in avoidance and tolerance mechanisms against harmfuleffects of high salinity and investments in competitive ability for nutrientsand light can be expected.Although Tilman (1988) gives examples of primary successions at Glacier Bay,

Alaska (Crocker and Major, 1955; Lawrence, 1967), and at the sand dunes ofLake Nichigan (Olson, 1957), which seem to agree with his resource—ratiohypothesis, no detailed comparison between the pattern of a primary successionand the predictions of the resource—ratio hypothesis has yet been published.This paper will deal with succession on a former beach, which got more and

more vegetated after the building of a sand dike.The development of this vegetation is considered to be a good example of apriniary succession starting on a poor soil.

This successional series will be described in terms of species patterns,changes in biornass, soil development and allocation patterns, whereby trendswill be separated from fluctuations.The outcome of these analyses is compared with the predictions of conceptualmodels of plant succession.

5

MATERIALS AND METHODS

The study area

The study area, the 'Beach Plain', extends from benchmark 7.4 to 10.4 on the

eastern side of the Waddensea—island Schiermonnikoog (The Netherlands,

53°29'NL, 6°12'EL, Fig 1).Several authors did earlier research in this area. The ecophysiology of some

halophytes was related to the vegetation zonation of the Beach Plain (Rozema,

1978). A reconstruction of the vegetational development by aerial photographes

and a few vegetation records was made by Van der Laan (1980). Wapenaar (1980)

performed a vegetation mapping in 1979. Annual fluctuations of some

theophytes were related to annual and seasonal fluctuations in the

precipitation balance (Van Tooren, Schat and Ter Borg, 1982). The ecophysio—

logy and population ecology of some therofytes and short—living perennials on

the Beach Plain was studied by Schat (1982).Originally the Beach Plain was a nearly bare sand flat, with scattered

embryo—dunes up to 2 meters in height. These dunes were sparsely vegetated

with Elymus farctus and Ammophila arenaria. The lower parts were bare, or

sparsely covered with the annuals Salicornia stricta, Spergularia marina and

Suaeda maritima (Van der Laan,l980). In 1959, with the construction of a sand

ridge, the area was screaried off from the direct influence of the North Sea.

However, during heavy storms in 1972, a large opening was formed betweenbenchmark 10 and 10.2. In most winters, when the water table is higher than

at least 1.8 meter above NAP (Dutch Ordnance Level), the area is flooded by

North Sea water coming through this opening. At higher tides, when the water,

table is at least 2.1 meter above NAP, also water from the Wadden Sea can

enter the area.The seawater may stay in the Beach Plain for months. This occurs because the

relatively low position of the flat parts of the Beach Plain prevents the

inundation water from flowing out. In April or May, the water table in the

lower parts usually falls below the soil surface, normally reaching a depth

of about 40 — 70 cm. below soil surface in summer. In extremely dry summers

a depth of 1 meter may be reached.Fluctuations in the lenght of the inundation period and in the precipitation/—

evaporation balance result in a strongly fluctuating salinity of the upper

soil layers of the lower parts (Van Tooren, Schat and Ter Borg 1983). Rozema

(1978) found, during the summer, a 3 to 15 cm. higher groundwater table with

a much lower salinity (50 mM NaC1) in the central parts of the small embryo—

dunes when compared to the groundwater of the flat area (295 mM), which

indicates the existence of a fresh water lens in these small dunes. The

capillary rise of the soil moisture was estimated at about 110 cm. (Rozema,

1978). This means that the higher parts of the dunes cannot be supplied with

(fresh) capillary groundwater in contrast to the lower zones.

At present, the small dunes are densely covered with Hippophae rhamnoides, a

thorny shrub able to fix nitrogen, and the lower parts are dominated by Juncus

gerardii, Scirpus maritimus and Phragmites australis.

6

The permanent transects

The wegetation development was recorded from 1972 onwards by Ter Borg, VanToor en, Zonneveld, Keizer and a lot of undergraduate students, using a seriesof 1 Ine transects lying near benchmark 8.2. This location comprises a flat lowpart and a dune slope with a northern exposure (Fig. 2), with the lowest pointbeing 1.3 meter above NAP. Twelve permanent transects of 20 meters length eachwere laid out perpendicular on the heightlines of the site, at a distance of1 me -cer from each other. Every year in August, the presence of every specieswas recorded in 1 meter long and 40 cm. wide strips along these transects,resu1ting in data from 12 X 20 240 rectangles. In some years, mainly before1979 , the recording was not complete: parts of the plot and/or some specieswere not recorded. But in all cases it was known which parts and which specieswere not recorded.

Analysis of the data on species dynamics

T- nd

Becat15e the main scope of our study was to analyze vegetation dynamics underdifferent sets of physical constraints, the transect—area was divided intofive sub—areas, with the spatial variation within each sub—area reduced as faras possible (Table 1, Fig. 2). Criteria used for this division were meanheight and soil—salinity (measured in summer by the electrical conductivityof the upper 10 cm. (unpublished data) of the rectangular strips.

Successional trend derived from presence/absence data can be seen as achange in time (t) of the chance (p) to find a certain species in a givensurface, in this case the surface of a strip (0.4 m2', in a given area. Trendswere calculated for each combination of species and subarea, given that aspecies occurred in a subarea for at least four years. Five different modelswere used for determining the shape of p(t) as variations on the general model

p(t) f(t)/(1+f(t)) * g(t)/(1+g(t)) (Eqn. 1)

where the functions f(t) and g(t) were defined as

f(t) exp(a), g(t) (model I, Eqn. 2)

f(t) exp(a+b*t), g(t) (model II, Eqn. 3)

f(t) exp(a+b*t), g(t) exp(c) (model III, Eqn. 4)

f(t) exp(a+b*t+c*t2), g(t) (model IV, Eqn. 5)

f(t) exp(a+b*t), g(t) exp(c+d*t) (model V, Eqn. 7)

where a, b, c and d are the coefficients to be estimated. p(t) will be calledthe expected frequency of a species. These models can be ranked according toan increasing number of parameters determining their shape, and are thereforeof increasing complexity. Model I implies no trend (constant p(t), model IIan increasing or decreasing trend, model III an increasing or decreasing trendwith an upper limit, model IV implies an increase and a decrease at the samerate ("symmetrical around a maximum") and model V implies an increase anddecrease with different rates ("skewed with a maximum"). Working with models

7

of increasing complexity allows to choose for the simplest possible model

which sufficiently explains the observed pattern. The coefficients of model

1,11 and IV were estimated by logistic regression. The coefficients of model

III and V were estimated using non—linear regression in SPSSPC v.3.1 (1989).

The Levenberg—Marquardt algorithm was used to solve the unconstrained models.

Logistic regression minimizes the statistic —2LL by iteration, defined as

—2LL SUM(_2*log(L0/Ll))(Eqn 8)

where LO is the predicted probability of membership in the correct group in

this model (for example, suppose the chance of finding a certain species in

a strip is predicted to be 0.8 then, when this species is found LO — 0.8, when

this species is not found LO 0.2) and Li is the predicted probability in

a perfect model, which is always one, because in a perfect model the

prediction is always correct. This simplifies to

—2LL SUM(—2*log(L0))(Eqn. 9)

Nonlinear regression minimizes the residual sum of squares by iteration, so

for model III and V, —2LL was calculated after fitting the regression model.

Because —2LL has a Chi—Square distribution, with degrees of freedom equal

to the number of cases minus the number of coefficients used in the model

(SPSS v.3.1, 1989), the decrease of —2LL when putting in more coefficients has

a Chi—Square distribution with degrees of freedom equal to the number of extra

coefficients. Thus it can be tested whether this decrease is significant.

Starting with model I, only more complex models were accepted when the new

model lead to a significant decrease in —2LL (p<.O5). Model III and IV have

the same number of coefficients, therefore a choice between model III and

model IV cannot be based on a significant improvement. Whenever a choice

between model III and IV was necessary the one with the lowest —2LL was

chosen.Missing data were accounted for by weighting each year by the number of

strips recorded. The criteria used to stop the iterations where the default

criteria of SPSS v.3.1.The trends were also used for calculations on species—richness, on the rate

of succession and on the occurrence of species with different life—forms and

maximal heights. By using the trends in stead of the observed values for each

year, the predicted patterns were not obfuscated by annual fluctuations.

The expected species—richness of a strip is just the summation over all

occurring species of the expected chance to find them in a strip. Because the

chance to find a species on a surface of a strip was estimated, also the

chance to find a species on a surface of two strips could be estimated. So the

expected species—richness could also be calculated for larger surfaces.

The rate of succession was computed as the percentage dissimilarity (PD) for

each year with the previous year with 1973 and with 1989 (01ff & Bakker, in

press), with

PD(y) — 100 * (1 — (2c/(a+b)) (Eqn. 10)

where a is the sum over all species of their expected chances in year y, b is

the sum over all species of their expected chances in the year with which year

y is compared, c is the sum over all species of the minimum of these two

expected chances. The classification of Raunkiaer (1934) of life forms was

used and maximal heights of the species were taken from the Dutch botanical

database (CBS, 1987). Life—form spectra per year per subarea were calculated

by weighting each species by it's expected frequency. The 6 species which

8

couTJd behave both as hemicryptofyt and geofyt were assigned to a separategro'i The weights of the 3 other species able to exhibit more than one life—form were equally divided among their life—form groups. Mean maximal plantheLlt for each subarea and each year was calculated as the mean of the lOlogtrasformed maximal heights weighted by the expected chance of each species.

F],uc tuations

The standardized residual (SR) is defined as the residual (observed relativefrequency minus expected chance) divided by an estimate of its standarddeviatiOfl, in this case:

residual(t)SR(t) — (Eqn. 11)

SQRT(p(t)*(l—p(t))

Mu1tJ-le regressions were computed using SR(t) as the dependent variable andprecIPitation deficit over the period april—june and maximal height of theflooding seawater in the period september—april as independent variables.MetOr0l0giCal data, collected 2 km west of the permanent plot, were suppliedby the Geological Institute of the Free University of Amsterdam. Precipitationdeficit is defined as actual precipitation minus evaporation, according toPenma1. Data on the waterlevel of the WaddenSea, collected 3 km to the south,were derived from the annual reports of Rijkswaterstaat, The Hague.

CorilatiOflS between species

It might be expected that decreasing species are negatively correlated withincreasing species. But whenever the disappearance of a species is directlycaused by competitive replacement due to the appearance of another species,it might be expected that the maximum chance of finding the decreasing speciesin a very small, homogeneous area is 1 minus the chance to find the increasingspecies. It is the maximum chance, because other factors (e.g. dispersal,competition with other species) might cause the chance to be lower. SinceHjppphae rhamnoides is considered to be a major determinant of the structureof the community, the decrease of other species was related to the increaseof this species.The age of the shrub Hippophae rhamnoides in a certain year in a certain stripwas estimated by counting the number of years Hippophae had already beenpresent in this strip. The first year the age was taken zero (seedlings);whenever in a certain year in a certain strip Hippophae was absent, thecounting started anew. The ages derived in this way were in agreement with thepresent ages of the shrubs of the permanent transects.

Reconstruction of changes in biomass, light and soil

The data of the line—transects only yielded information on the frequency ofoccurrence, not on the abundance of the different species. Also changes inbiomaS5 or in the distribution of this biomass over the diffent functionalunits of a plant are unknown and no data on soil—development, amount ofnutrients or light—profile were gathered. Therefore, to get some insight inthe mechanisms of succession in the Beach Plain a reconstruction of thechanges in the above—mentioned factors had to be made.

9

The vegetation of Plain, Slope and Dune during the intervals 1972/73, 1980/81

and 1988/89 were characterized by means of the trends. In and around the Beach

Plain, we searched for sites were the vegetation showed close resemblance to

these 9 characteristics. In these vegetations 3 plots per characteristic weresituated. These plots were considered to be representative for the situation

which occurred at the line—transects at the place and time of their

characteristics. The subsites will be called Plain'72, Plain'80, and so on.The location of the plots is shown in fig.1. The Plain'72, Slope'72 and

Dune'72 plots were placed in front of the gap at benchmark 10.2. The plots of

'80 were located east of this gap in a dune area which was bare sand,

according to aerial photographs, until about ten years ago. The '88 plots were

laid down in the neighborhood of the permanent line transects.Line—transects were made in the beginning of September in the same way asdescribed earlier but with 3 cords of 5 meters length resulting in 15 strips

per plot. This yielded to the possibility to investigate if the plots were a

good representation for their successional stage in their spatial context. All

other samplings at the plots were done during 1—15 july 1990.

Biomass and light

The above—ground standing crop, root biomass, light profile and percentagecover were measured for each plot.Above—ground standing crop, without the shrub Hippophae, was sampled by

clipping an area of 0.4 x 0.4 m. per plot. These samples were sorted to living

species and dead organic material, dried to constant mass at 70°C and

weighted. The above—ground biomass of Hippophae was estimated by a non—destructive procedure, sampling over a larger area.For this, we measured the above—ground fresh weight (B, in grams) and 70°C

dried biomass, height (h, in cm), diameter at the bottom of the stem (d, in

cm) and age of several Hiptovhae shrubs in the Beach Plain, ranging in height

from 30 to 250 cm. The age of each shrub was determined as the number of

yearrings counted at the bottom of the stem. The above—ground biomass of each

individual shrub could be estimated by B 2.8l5*(d2*h)°9094 (r 0.99, N =

13). Dry weight of the shrub was calculated by multiplying the fresh weight

by 0.478 (r 0.98, N 7). Avarage total above—ground biomass (dry—weight)of 1-Iippophae rhamnoides was estimated by taking an area around each plot of

at least 15 m2 in which we estimated the dry—weight of each shrub by measuring

h and d.Roots were collected by taking two 20 cm. deep, 7 cm. diameter soil cores per

plot. Nearly all roots were within these 20 cm. The cores were rinsed of soil

and litter under a fine water spray to obtain the roots, which were dried and

weighed. The roots could not be separated into living and dead. At each plot

the vertical light profile in the vegetation was measured using 5 centimeter

intervals, using a PAR collector (400—700 rim) with a measuring surface of 100

x 1 cm. The light extinction at each height was expressed as a fraction of the

ambiant light intensity above the vegetation.

Allocation patterns

Proportion root was calculated as root biomass divided by total biomass. For

a lot of Cramineae, Juncaceae and Cyperaceae it was quit arbitrary todiscriminate between leaves and stems. Therefore, as a measure for the"proportion stem", we divided the vegetation height (height at which 5% of the

light was intercepted) by total biomass. This measure was called height per

biomass (HB). Similarly, the cover per biomass (CB, percentage cover divided

by total biomass) was used as measure for "proportion leaves".

10

:L measurements

The thickness of the organic layer was measured at ten randomly chosen sitesin .ach plot. The dark brown organic layer showed a sharp boundary with thejrLying yellow to greyish sand, which will be referred to as the minerallayr Samples were taken from the organic layer and from the mineral layerat depth between 10 and 15 cm, at least at from five samples taken near each

These cores were pooled and mixed until at least 700 gram per sample wasgat1eth This resulted in one organic—layer—sample and one mineral—layer—sanipJe per plot. Soil samples taken near each plot were analyzed for moisturecontt, specific weight of the soil, NaC1, CaCO3, pl1(l-120), pH(KC1), organicmatt content, total carbon, total nitrogen and total phosphorus, usingstaridard procedures. The moisture content and specific dry—weight weremea1ed by taking 2 samples per plot from the upper 5 centimeters of the soiland 2 samples from the mineral layer at a depth between 10 and 15 cm. using100 l. pF—rings. The samples of the upper—soil were cut of at the depth ofthe rganic layer and for these samples only this organic layer was used. Thesamples were freshly weighted, dried at 70°C and weighed again.At .1ain'72, Dune'72 and Dune'80, the organic layer was too poorly developedfor .sampling it separately; in these cases the organic—layer—samples weresampl of the upper 2 cm. of the soil.

The total amounts of organic matter, C, N and F in the organic layer (A) wereestirmat for all plots, except the Plain'72, Dune'72 and Dune'80 plots, byth[AJ x sw[AJ x ct{A], where ththickness, sw=specific weight and ctcontent.The anmounts in the organic layer at Plain'72, Dune'72 and Dune'80 wereestixit by 2 x sw[AJ x ct[AJ — (2 — th[AJ) x sw[BJ x ct[B) where Bminerallayei. The total amount of organic matter in the upper 20 cm. of the soil wascalcL1'at by th[A] x sw[AJ x ct[AJ + (20 — th[A)) x sw[BJ x ct[BJ withth[AJ=2 for Plain'72, Dune'72 and Dune'80.

analyses

The effects of time interval ('72,'80 and '88) and place (Plain, Slope andDune) on biomass and on soil characteristics of the organic and of the minerallayer were determined using two—way analyses of variance with Student—Newman—KeulS contrasts among treatment means.

11

RESULTS

Analysis of the data on species dynamics

Trends

The PlainDuring the first years, hemicryptofyts like Glaux inaritima and Agrostis

stolonifera are very abundant in the Plain (Fig. 3 and Fig. 6A). Liinonium

vulgare and Plantago inaritima, both halofyts, can be found too. Also a lot of

therofytic species like the halofyts Spergularia spec, and Salicornia spec.

and the glycofyts Odontites verna ssp. serotina, Centauriuln pulchelluin and

Centauriurn littorale (a biannual) occur. The rhizomatous species Juncus

gerardii, Scirpus maritimus, Juncus maritimus and Phragmites australis and

the hemicryptofyt Potentilla anserina gradually increase, whereas Glaux

maritima, Limoniuni vulgare, Plantago maritima and the afore mentioned

therofyts decrease.At the end of the research period therofyts can still be found, mainly

Atriplex prostrata and Spergularia spec.. Juncus gerardii is over it's peak

abundance, while Agrostis stolonifera is still very common and the tall

Scirpus maritimus and Phragmites australis are increasing.

The mean maximal plant height slightly increases in time (Fig. 7A). The

expected species richness is rather constant until about 1980 and then

gradually declines (Fig. 8).

The SlopeThe Slope can initially be characterized by a very high frequency of the

grasses Agrostis stolonifera and Festuca rubra and by the occurence of the

therofyts Centaurium littorale, Linuni catharticuni and Odontites verna ssp.

serotina (Fig. 4 and Fig. 6B). Also Ammophila arenaria and Sonchus arvensis

(geo/hemicryptofyts) are common. Festuca rubra increases in frequency until

it is present in every strip, whereas Agrostis stolonifera decreases. Small

hemicryptofyts like Leontodon nudicaulis, Sagina nodosa (which can behave as

a chainaefyt as well as a hemicryptofyt), Armeria maritima, Trifolium repi

and Trifoliusn fragiferuni even as the monocots Juncus alpino—articulalatUS,

Carex distans and Carex flacca increase after they have colonized the area.

Furthermore, flippophae rhamnoides and Potentilla anserina, both present from

the beginning, increase.From 1981 onwards all the afore mentioned therofyts and monocots disappear.

Arnmophila arenaria, Sonchus arvensis and Festuca rubra decrease. In contrast,

Hippohae rhamnoides, Potentilla anserina, Cirsiuni arvense, Chamaenerion

angustifoliuni and the tall grasses Calamagrostis epigejos, Poa pratensis and

Holcus lanatus occur more frequently.

Mean maximal plant height increases (Fig. 7B). Species richness increases

until about 1981, when on average 12 species were counted per strip. In total,

on an area of about 15 in2 40 species were found. But after 1981 this very high

species richness decreases very rapidly and is about halved in 1989 (Fig. 8).

The DuneLike on the Slope in the early succession stages Agrostis stolonifera, Festuc

rubra, Amniophila arenaria and Sonchus arvensis are the abundant species,

although the last two occur in higher frequency compared to the slope. But in

contrast, therofyts are almost absent in this stage (Fig. 5 and Fig. 6C).

12

Sonchus arvensis and Agrostis stolonifera decrease from the beginning. Thesumirer annuals Lir.um catharticum, Euphrasia stricta, and the winter annualsCerastium semidecadruin and Aira praecox enter the succession area andincrease in frequency. The small charnaefyts Cerastium fontanum, Sedum acre andSagina nodosa reach their maximal frequency around 1980. From 1981 onwards,all therofyts and small chamaefyts disappear and Festuca rubra and Ammophilaarenaria decrease, while Flippophae rhamnoides, Chamaenerion angustifolium andthe Large grasses Poa pratensis, Calamagrostis epigejos, Elymus pycnanthus andHolcus lanatus increase. Mean maximal plant height is rather high in 1972,reaches a minimum in 1980 and then increases (Fig. 7C). The trend in speciesrichness is the same as on the Slope, although the number of species on theDune is always lower (Fig. 8).

Rate of successionThe rate of succession measured by changes in percentage dissimilarity (PD),is iore or less constant in time in each area (fig. 9A). In the Plain, therate of succession seems somewh lower than on the Slope and the Dune, Incourse of time the vegetation co position of Plain, Slope and Dune differedmore and more from 1973 (Fig. 9B). The vegetation of the Plain correspondedmore and more to the latest year 1989, whereas Slope and Dune showed anincreasing correspondence with 1989 from only 1981 onwards (Fig. 9C).

Fluctuations

Although the highest rainfall—deficit over the months april, may, june and thehighest flooding occurred in the same year (Fig. 10), there was no significantcorrelation between rainfall deficit and height of highest flooding (r —

0.19, N 18, p > 0.1). Also no trends in time of these two variables couldbe found (rainfall—deficit: r 0.12, N 18, p > 0.1; height of highestflooding: r 003, N = 18, p > 0.1). All bi-J1est floodings of the periodsseptember—april occurred between november ai march, with the exception of1973 (3 april) and 1979 (12 september 1978). In almost all winters the areawas flooded by seawater from both the Waddensea and the North Sea.

The standardized residuals of the trends in 40 % of the therofytic specieswere significantly explained by rainfall—deficit in 'spring' and/or height ofthe highest flooding, whereas 13% of the perennial species—trends showed asignificant effect (table 2).The standardized residual of the trend had a positive correlation with theheight of the highest flooding for five halofytic surnnier—annuals, whichspecies mainly occurred in the lower subareas. It had a negative correlationwith the rainfall—deficit over the period april—june for five non—halofytictherofyts and for the perennials Plantago coronopus and Cerastium fontanum,all mainly occurring on Slope and Dune.In the Plain the difference between observed species richness/strip and

expected species richness/strip increased with height of if g -st flooding butnot significantly (r 0.57, N 11, P 0.066).

Correlations between species

The increase in height (h, in cm,) with increasing age (x, in years) ofHippophae rhamnoides could be described by the equation h(x) 278*x/(7.83+x)

(r 0.90, p<.OOl, N — 25). The decrease of several species was very closelycorrelated with the increase of Hippophae rhamnoides. The relative frequenciesof these species were at maximum 1 minus the relative frequencies of Hippophaerhamnoides of a given minimal age (Fig. 11); whereby this mininal age ofHippophae, thus it's minimal height, had to be taken higher to reach this

13

maximum, whenever the maximal heights of these species were higher.

Reconstruction of changes in biomass, light and soil

The plots

The percentage dissimilarity of the vegetations of the plots with thecharacterized vegetations of the successional stages these plots representedwas always lower than with the other characterized vegetations, with theexception of the somewhat higher PD of the Slope '72 plots (table 3). Thisimplicated that the plots were a good representation of the chronosequence.

Biomass and light

In all three subareas, the living above—ground standing crop significantlyincreased, with Dune finally reaching the highest standing crop and Plain thelowest (Fig. 12).Plain'72 is dominated by Glaux maritija, Agrostis stolonifera and Scirpusmaritimus (Fig. 13A). Plain'80 and '88 are dominated by Juncus gerardii. InPlain'88 Glaux maritima is absent and Agrostis stolonifera takes only a smallpart in the total standing crop, although Agrostis stolonifera occurred inalmost all strips of the Plain'88 plots. The proportion of Scirpus maritimusincreases from Plain'80 to Plain'88.Slope'72 is dominated by Agrostis stolonifera, Festuca rubra and Centaurium

littorale (Fig. l3B). In Slope'EO Agrostis stolonifera has a much smallerportion than in Slope'72, the portion of Centaurium littorale is about thesame, the portion of Festuca ru1 has doubled. The standing crop of Slope'88consisted for 70% of Hippcphae rhaninoides, Centaurium littorale has

disappeared.Dune'72 is dominated by Ammophila arenaria; also Sonchus arvensis and

Festuca rubra are important contributors to the total standing crop (Fig.13C). The portion of Festuca rubra has doubled in Dune'BO, Ammophila arenariaand Sonchus arvensis contribute less. In '88 Hippophae rhamnoides shares againabout 70% of the standing crop. Chamaenerion angustifolium takes about halfof the 30% left. In all the plots the therofyts only take a very small part(18.5% at maximum in Slope'72, most of a biannual species) in the totalstanding crop. In all plots only a few species (5 species at maximum inSlope'80) account for more than 90% of the standing crop.The amount of above—ground dead organic material is low in Plain'72 and

Slope'72 in contrast with Dune'72, where a lot of dead standing leaves ofAmmophila arenaria were found (Fig. 14).Above—ground dead organic material is less than 10% of the total above—

ground organic matter in Plain'72, Slope'88 and Dune'88; but even more than50% in Slope'80, Dune'72 and Dune'80. In Plain and Dune root biomass increasessignificantly in time, on the Slope root biomass is constant (Fig. 15).

Highest root biomass is achieved by Plain'80 and Plain'88; in these plotsrhizomes are very abundant and account for about 25% of the root biomass.Total biomass increases significantly with time (Fig. 16). In '88 the totalbiomass of all three subareas is the same, but with a very differentdistribution over roots and shoots. In '88 the above—ground standing crop offorbs and grasses is also the same in the three subareas.

The '72 plots were only scarcely vegetated, with a lot of bare sand; between40 and 90% of the PAR reached the soil—surface. In the '88 plots this is atmaximum 10% (Fig. 17). The vegetation is much higher in the '88 plots than in

14

the '72 plots. The vegetation—height of Plain'72 and Slope'72, about 10 cm.,is in sharp contrast with the height of Dune'72, being about 50 cm. Standingcrops of more than 300 g/rn2 resulted in low light penetrations at the soil—surface (Fig. 18).

Allocation iatterns

In the Plain, the proportion roots decreased from 95% in '72 to 85% in '88(FLg. 19). From '72 to '80 there is an increase in cover per unit biomass(GB), while hight per unit bioinass (HB) is constant. From '80 to '88 it is theother way round: GB is constant and FIB increases. On the Slope, the proportionroot declines from 95% to 50%. CR is maximal in '80 and FIB increases. From '72to '80 on the Dune, FIB falls down very rapidly, GB also declines, butproportion root increases from 65 to 85%. From '80 to '88 the allocationpattern on the Dune is the same as on the Slope.

SoL1 measurements

The specific weight of the organic layer was strongly decreasing with time inall three subareas (Fig. 20). In the mineral layer there was only a slightlydecrease of the specific weight on the Dune. Both in the organic and in themineral layer the moisture content of the Dune was much lower than themoisture content of Plain and Slope (Fig. 21). In almost all cases the organiclayer had a higher moisture content than the mineral layer. No trends in timewere observed, except a slight increase on the Dune.The NaCl content is much higher in the Plain than on Slope and Dune (Fig.

22) . Whereas on the Slope some NaC1 can be found, the Dune was alreadycompletely. desalinated during the early stages of succession. In the Plain andon the Slope the organic layer had a higher NaC1 content than the minerallayer. No major trends in time were observed.Although in all three subareas a decrease in the CaCO3 content of the

mineral layer was found, this decrease was only significant on the Dune (Fig.

23) . The GaCO3 content of the organic layer showed a strong decline from the'80 plots to the '88 plots. The CaCO3 content of the organic layers of the '88plots was less than 0.3%.The pH(H20) and pH(KC1) of the mineral layers were between 7.8 and 8.2 (Fig.24 and Fig. 25). On the Slope, a significant increase in pH(H20) was found,while on the Dune the pI-1(KG1) significantly decreased. The organic layer ofboth Plain and Dune showed a significant decrease of pH(H20) and pH(KC1) intime. The organic layer of the Slope has no significant change in pH(H20) orpH(KC1). In all three subareas there is a significant increase of the

thickness of the organic layer in time (Fig. 26). In the Dune'72 plots and intwo of the three Plain'72 plots no well defined organic layer could be found.Slope has a much thicker organic layer than Plain and Dune both in '72 and in'88. In all three subareas there is a significant increase of the organicmatter content of the organic layer in time (Fig. 27). The content is loweston the Dune for every year. In the mineral layer almost no organic mattercould be found, although there is a significant increase in time on the Dune.In a lot of plots the total organic matter content of the upper 20 cm. of thesoil consisted almost completely of roots (Fig. 28). One Plain'88 plot had athicker organic layer with a higher organic matter content than the other twoPlain'88 plots, whereas root biornass was about the same. In the Slope'88 plotsthere is also considerable accumulation of organic matter other than roots.

The C—content (C), N—content (N) and organic matter content of the samples

15

were all closely correlated, according to C — —0.048 + l0.8*N (fig. 29, r

0.98, N 54), C — —0.046 + 0.332*OM (Fig. 30A, r — 0.98, N — 54) and N

0.0026 + 0.0295*OM (Fig. 30B, r — 0.96, N — 54). No effects of time, placeand/or layer on these correlations were found and the intercepts were notsignificantly different from zero. Stated otherwise: in all plots the C—content was 33.2% of the organic matter content, the N—content was 2.95% of

the organic matter content and the C/N—ratio was 10.8. Thus C— and N—contentshow the same pattern as the organic matter content: in all three subareas theC— and N—content of the organic layer increase significantly in time, withlowest contents on the Dune; and the C— and N—content in the mineral layer islow, although the increase in time is significant (Fig. 31 and Fig. 32).P—content (P) was not that closely correlated with organic matter content asC— and N—content; also the intercept is not that low compared to the slopethat it could be neglected: P 0.0108 + 0.0021*OM (Fig. 30C, r — 0.86, N

54). The P—content of the mineral layer is not that low when compared to theorganic layer (Fig. 33). Plain had a significantly higher content than Slopeand Dune. The P—content significantly increased with time in the organic layer

of Plain and Slope from '72 to '80, but not from '80 to '88. In Dune nosignificant change was found, although the direction of change is comparable.The P content of Dune'80 and '88 is significantly lower than the P content ofPlain'80, Plain'88 and Slope'80. The C/P—ratio in the organic layer

significantly increased with time in all subareas, reaching a ratio of about

100 in '88 (Fig. 34). The total amount of organic matter, N and P in theorganic layer increased significantly in time, with Dune having the lowestaccuiu1ation (Fig. 35 and Fig. 36). The average rates of N accumulation in thesoil were 3 g N m2 year' for Plain and Slope and 1 g N nf2 year' for Dune.

16

DISCUSSION

The vegetation composition at the plots, on which the reconstruction is based,were very similar to the vegetation at the permanent transects area, bothaccording to their percentage dissimilarity (table 3) and to their structureas could be seen from photographs of the permanent transects area. The PD ofSlope'72 with it's own characteristic is somewhat too high, this is a

consequence of the absence of Linum catharticum and a high frequency of Glauxniarit.ma. Maybe these plots were laid down at a place slightly wetter or moresaline than the Slope of the permanent transects. Apart from these two speciesthe vegetation of the Slope'72 plots closely resembled to the successionalstage it represented. The succession series at the permanent transects isprobably a good representation for what happened in most aprts of the BeachPlain. Rozema (1978), Van der Laan (1980), Wapenaar (1980) and Schat (1982)describe vegetation zonations from different years and from different placesof the Beach Plain. These descriptions are in agreement with the observedpatterns at the permanent transects area. Rozema (1978) determined the biomassper species along a transect from plain to dune. These biomasses weresomewhere intermediate between the biomasses found at the '72 plots and thoseat the '80 plots. At present all the dunes from the most Eastern point tobenchmark 9 are densely covered with Hippophae. All the plain areas frombenchmark 7.8 to 9 are covered with vegetations of Juncus gerardii, Scirpusmaritimus, Phragmites australis, etc.The succession is still going on. Phragmites australis, the tallest speciesof the Plain, is increasing. In older dune slacks on the island which are alsoflooded by seawater, Phragmites australis is the dominant species. On somedunes in the Beach Plain young Sambucus nigra, shrubs taller at maturity than1-lippophae rhamnoides, are growing up. On older dunes near the Beach Plain,Sambucus nigra has replaced Hippophae rhamnoides. The tree Acer pseudoplatanusis a possible candidate for following up Sambucus nigra.Species growing on the Plain have to cope with high salinity and with

flooding in the winterperiod. Waterlogging of the soil will probably also insummer impose anaerobic root conditions for long periods. A lot of species inthe Plain are known as halofyts. Most of the species occurring in this areahave no above—ground biomass in winter. The therofyts are almost all summerannuals. The shoots of Glaux maritima, Juncus gerardii, Scirpus maritimus andPhragmites australis die off in autunin. Glaux maritima has a special kind ofhibernating buds (Rozenia, 1978); the geofyts can survive the winterperiod inunderground rhizomes. Also the leaves of Potentilla anserina die off inautumn. In spring and summer Agrostis stolonifera and Potentilla anserina canrapidly spread by their stolons. Fluctuations of annuals were positivelycorrelated with the height of flooding, probably because high floodings createlarge gaps in the vegetation, where annuals can establish.The Slope is much less saline than the Plain, but still has a rather high

moisture content. The species are mainly glycofyts. Fluctuations of therofytsand some perennials were negatively correlated with rainfall—deficit inspring. A high rainfall— deficit will cause a low soil moisture content.Lowering of the soil moisture content will increase the salinity (Van Tooren,Schat and Ter Borg, 1982). Both a low soil moisture content and a highsalinity will have a negative effect on the germination and establishment ofthe glycofytic therofyts, Odontites verna ssp. serotina, Centauriumpulchellum, Centaurium littorale and Linum catharticuni all germinate in

spring; Plantago coronopus mainly germinates in spring, but can also germinatein the autumn (Schat, 1982).Salinity and soil moisture content are both low on the Dune. Winterannuals

like Cerastium semidecandruin, Arenaria serpyllifolia and Aira praecox flower

17

in early spring, die after seed—setting in may/june and survive during the drysummer as seeds (Rozijn, 1984). Odontites verna sst. serotina, a summerannual, appears on the Dune only after wet springs. Sedum acre can reduceevaporation, by closing the stomata at daytime (CAM—metabolism). Hippophaerhamnoides and Ammophila arenaria can root rather deep. Arrunophila arenaria andElyrnus pycnanthus fold their leaves when it gets dry, burrying their stomatadeeply between the ribs. Thus most species of the Dune are adapted to longperiods of drought.

It can be concluded that differences in salinity, water availability andflooding are the physical constraints determining the differences in speciescomposition of Plain, Slope and Dune. Year to year fluctuations of the samefactors seem to be responsible for fluctuations in the occurrence of a lot ofshort—living species.However, non of these factors can be held responsible for the succession

patterns observed. NaCl—content did not change with time, except a slightdecrease on the Slope, which can also be an effect of the site where theSlope'72 plots were laid. Van Tooren, Schat and Ter Borg (1982), measured theconductivity of the soil moisture of the Plain in august from 1973 to 1980 anddid not observe any desalination. Moisture content only increased on the Dune,accompanied by an increased organic matter content (larger water holdingcapacity) and an increased above—ground standing crop (moist microclimate).So this might be rather a consequence than a cause of the observed succession.No trends in the height of flooding or in the rainfall—deficit in spring didoccur. Also no changes in the access of flooding seawater to the area wereobserved.

Loss—rates imposed by herbivores (rabbits, hares and geese) are thought tobe not high enough to be of great influence on the observed successionalpatterns at the Beach Plain. As discussed above, loss—rates in the Plain arehigher than on Slope and Dune, because of loss of above—ground biomass inautumn and of losses due to high floodings.

The rate at which P is made available in soils with high pH and high CaCO3—content is low, But this low rate does not necessarily mean that in principlenot all P can be made available. The total amount of P in the upper soil isenough to sustain a much higher biomass than observed in '88. Assuming thatplants have a P—content of 0.2%, P—content in the mineral soil is 0.1 g/kg andspecific weight of the soil is 1.4 kg/dm3, then the total amount of P in a 10

cm. thick layer can sustain a plant biomass of 7 kg/rn2. Plants will usenutrients from layers thicker than 10 cm. The K—content of the mineral layerson the Beach Plain ranges from 0.2 to 1 g/kg dry soil, with a considerablewater—soluble part (Rozema, 1978). This K—content is also enough to sustaina much higher biomass than observed, Assuming that plants have a K—content of1% and K—content in the mineral soil is 0.2 g/kg, then the total amount of Kin a 10 cm. thick layer can sustain a plant biornass of 2.8 kg/rn2. The N—content is strictly correlated with the organic matter content, suggestingthat all N is organic N. A C/N—ratio of about 11, as on the Beach Plain, wasalso found at the first stages of succession at both the dunes of LakeMichigan (Olson, 1957) and the recessing glaciers in Alaska (Crocker andDickson, 1957). This C/N—ratio is in the same order as the C/N—ratio of mostliving organisms. Almost all organic matter on the Beach Plain consisted ofroots, although the fraction dead roots is not known. The total amount of Nat the '72 plots seems too less to sustain the biomasses observed in the '88plots. Assuming that plants have a N—content of 1% (which is three times lowerthan the regression—coefficient of N—content with organic matter content) andN—content in the mineral soil is 0.06 g/kg, then the total amount of N in a10 cm. thick mineral layer can sustain a plant biomass of only 0.8 kg/rn2. Theamount of N in the organic layer increases rapidly, but is in '72 still very

18

low. This leads to the conclusion that the total amount of N limits the totalbionass at the Beach Plain. This conclusion is in agreement with the findingsof Villis (1963) at the dunes of Braunton Burrows. The soils at BrauntonBurrows are comparable with the Beach Plain: pH is between 8 and 9, P—contentis about 0.1 g/kg and N—content ranges from 0.11 to 2.8 g/kg. Addition of onlyP, only K or P+K didn't lead to an increased plant biomass, but after N—addition biomass increased (Willis, 1963). In his thesis Rozema (1978)

mentions an unpublished N—addition experiment at the Beach Plain, wherebioniass increased 10—20 fold compared to control plots.

On Slope and Dune the increasing amount of N is accompanied by an increasein frequency and height of Hippophae rhamnoides. The decrease of a lot ofspecies directly follows the increase of 1-Iippophae, with taller speciesdecreasing when Hippophae gets taller. This suggests that Hippophae takes awaythe light, thereby outcompeting these species. Indeed a lot of light can betaken away in a Hippophae stand; at Dune'88 only 10% of the incident PARreaches a height of 50 cm. above the soil—surface. Some other species on Slopeand Dune seem to disappear for other reasons; they decrease before they canbe shaded by Hivophae.On the Plain an increased amount of N is also accompanied by an increasingheight of the vegetation, an increased above—ground standing crop and adecreased light penetration to the soil. The decrease and/or disappearance ofsmall species, for instance Glaux rnaritima, Spergularia spec, and Salicorniaspec., might also be caused, at least partly, by shading by taller species.On Slope and Dune species richness reaches an optimum around 1980. Species

richness on the Plain is lower and fluctuates more. Species richness willpartly depend on the number of species which can tolerate extreme values ofphysical factors as salinity, flooding and drought. The number of species fromthe Dutch flora which can survive moist and fresh conditions, as on the Slope,is niuch larger compared to the number of species which survive saline,anaerobic and often flooded conditions, as on the Plain. Species richness willalso partly depend on competitive interactions and the heterogeneity ofresources and constraints in space and time. This heterogeneity can beexpected to be high on the Slope, since it is the small transition zonebetween a wet, saline subarea and a dry, fresh subarea, yielding possibly acheckerboard of conditions of the two subareas, Furthermore, the heterogeneitya species experiences will depend on the ratio between the scale ofenvironmental heterogeneity and the size of the species. The decrease ofspecies richness on Slope and Dune might be a consequence of the increase oftaller species. Disturbances can cause a loss of competitively gaineddominance. In gaps caused by floodings a lot of species can enter, therebyincreasing species richness.The competitive abilities of plant species will be greatly influenced by

their allocation patterns. The height which a plant, given it's biomass, canreach, will depend on it's allocation to stern. Suppose an increase inheight(h) is a simple function of stem biomass(B):

h .=c*Bs' 1=> h

B5As*B J

with B total biomass, A — allocation to stem, c and x are constants. Whenstem—biomass is not only used for an increase in height, but also for increasein diameter, then x < 1. An increasing diameter is necessary for bearing ahigher weight. What does happen with HB; height divided by total biornass:

FIB h/B = c*AsX*Bl

19

When the allocation pattern is fixed (As is constant) and x < 1, 1-lB will

decrease with increasing biomass. Thus, when x < 1, HB can only increase withincreasing biomass, when A8 increases. Therefore HB is a good measure for the"proportion stem".On Plain and Slope not only mean plant height increases, but also HB,suggesting an increased allocation to stem. In contrast, there is a decreasein the mean plant height and HR until about 1980 at the Dune. In 1972 smallspecies were almost absent at the Dune, which was dominated by Ainmophilaarenaria and Sonchus arvensis, both being able to reach a height of more than1 meter. These species are characteristic for places with much sand—blowing.Sand—blowing explains the absence of small species, they will drown in thesand. The biomass of roots deeper than 20 cm. were not estimated. On Plain andSlope no roots going deeper than 20 cm. were observed. On the Dune some rootsof Hipophae rhamnoides and Anmiophila arenaria went deeper. Rut because therewere not much roots going deeper, the vast portion of root biomass is thoughtto be sampled. Proportion root decreases at Plain and Slope and from '80 to'88 also on the Dune.

Although on the early Dune stages sand—blowing will have been an importantconstraint, leading to somewhat other successional patterns than predictedfrom a hypothesis about resource interactions, all other observations, alsoat the later Dune stages, are in agreement with the predictions of theresource—ratio—hypothesis (Tilman, 1985): An increase of the total amount ofN is accompanied by an increased total plant biomass, a decreased lightpenetration to the soil—surface, a decreased proportion root, an increasedproportion stem, an increased plant height at maturity and a decreasingabundance of short—living species.This does not imply that differences in colonization abilities or RGRmax didnot play a role in this successional dynamics. These traits will for examplebe of importance for species entering the gaps created by high floodings. Rutin general soil—driven dynamics (sensu Tilman, 1988) seem to be of overrulingimportance for this succession series.

On the Reach Plain the total amount of N will have increased by an atmosfericinput, nitrogen—fixing organisms and the input of N by flooding seawater. Onthe East Frisian islands, Germany, the atmosferic input from dry and wetdeposition is estimated at about 1.5 g N m2 yr' (Gerlach, 1989). On the Dutchmain land this input is estimated at 4 g N m2 yr'. Hippophae shrubs can fixbetween 1.5 g N m2 yr' (Akkermans, 1971) and 17.9 g N rn2 yr' (Becking,1970). Stewart (1965) estimates the nitrogen fixation by free—living bacteriain dune—slacks at 6 g N m2 yr'. In the period 1973—1989 North Sea water, 10km. West from the Reach Plain, had a total N—content of 1.33 0.64 mg/l (N

95; Rijkswaterstaat, personal communication); when 20 cm. of flooded NorthSea water damps in, it looses about 0.26 g N m2 to the soil. Leaching willdepend on the nitrogen uptake by the vegetation. Other N—losses might besuffered from denitrification, mainly on the wet Plain and Slope, and probablyfrom allocation of biomass by floodings. The relative importance of theselosses has not yet been estimated for sites comparable with the Beach Plain.We estimated an increase of 45 g N m2 in 16 years for the organic layer ofPlain and Slope. This is comparable with the rate of N—accunulation in thesoil at the first 20 years of succession after recession of glaciers inAlaska, which was about 4 g N m2 year' (Crocker and Major, 1955; Crocker andDickson, 1957). On the Dune, about 15 g N m2 accumulated in the soil in 16years, this is comparable with the accumulation at the early successionalstages of the dunes of both Lake Michigan (Olson, 1957) and Spiekeroog(Gerlach, 1989). The total increase of N on the Reach Plain and at the other

20

successions must have been higher, because the increasing above—ground biomasswill also have increased above—ground N. At the glaciers and the dunes of LakeMichigan the rate of N—accumulation decreased in time.The increase of the P—content of the organic layer might be due to inputs fromoutside, but also from reallocation of P in the soil by the vegetation.By 'influencing fixation, denitrification and leaching and by uptake ofnitrogen, the vegetation is an important determinant of the rate of nitrogenaccumulation. It is not likely that without a vegetation, the total amountsof nitrogen at the Beach Plain and in other primary successions had everbecome as much as they are now, because without storage, mainly in plants,nitrogen easily leaches out from these sandy soils. When discussing theimportance of nitrogen—fixation at Glacier Bay, Lawrence et al.(l967, p.812)argue: " it is only the latter [the nitrogen—fixing Alnus] that raises thenitrogen supply to a level enabling the spruce forest to achieve dominance".Thus, although a lot of primary successions might be mainly "soil—driven" withthe outcome of competition determined by the interactions between organismsand resources, soil—development in terms of N—accumulation might be mainly"plant—driven", which makes the usefulness of the first term doubtfull.Examples of theoretical and emperical results about positive feedbacks betweenthe growth of autotrophs and the total amount of limiting nutrients in opensystems (i.e. systems with inputs and outputs of nutrients) are reviewed byDeAngelis et al (1989), Such positive feedbacks come close to the facilitationhypothesis of Clements (1916), although it is facilitation in an indirect way,distrusted by so many plant ecologists (Drury and Nisbet, 1973; Colinvaux,1973; Connell and Slatyer, 1977; Peet and Christensen, 1980; Miles, 1979). Onthe Beach Plain nitrogen accumulation, probably facilitated by plants, seemsto be the main cause of the observed successional patterns.

21

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Robertson, GP Vitousek, PM (1981) Nitrification potentials in primary andsecondary succession. Ecology 62: 376—386.

Rozema, J (1978) On the ecology of some halophytes from a beach plain in theNetherlands. PhD Thesis, Free University of Amsterdam,

Rozijn, NAMG (1984) Adaptive strategies of some dune annuals. PhD Thesis,Free University of Amsterdam.

Schat, H (1982) On the ecology of some Dutch dune slack plants. PhD Thesis,Free University of Amsterdam.

Stewart, WDP (1965) Nitrogen turnover in marine and brackish habitats. Ann.Bot: 229—239.

Tilman, D (1985) The resource—ratio hypothesis of plant succession. AmericanNaturalist 125: 827—852.

Tilman, D (1988) Dynamics and Structure of Plant Communities. PrincetonUniversity Press, Princeton.

Van Tooren, BF Schat, H Ter Borg, SJ (1983) Succession and fluctuation in thevegetation of a Dutch Beach Plain. Vegetatio 53: 139—151.

Van der Laan, F (1980) Synthese van het vegetatiekundig onderzoek in destrandvlakte op Schiermonnikoog van 1952 t/m 1979. MSc Thesis, FreeUniversity of Amsterdam.

Wapenaar, P (1980) Vegetatiebeschrijving en —kartering van de Strandvlaktevan Schiermonnikoog in 1979. MSc Thesis, Free University of Amsterdam.

Willis, AJ Folkes, BF Hope—Simpson, JF Yemm, EW (1959) Braunton Burrows: Thedune system and its vegetation. Journal of Ecology 47: 1—24.

Willis, AJ (1963) Braunton Burrows: The effects on the vegetation of theaddition ofmineral nutrients to the dune soils, Journal of Ecology 51:353—374.

23

TABLES & FIGURES

Fig.l: (A) Schiermonnikoog, an island in the Wadden Sea. The study area, theBeach Plain, is shown within the rectangle. Arrow indicates permanent tran-sects area. (B) The Beach Plain, with the locations of the '72, '80 and '88

plots. The box, close to the '88 plots, indicates the permanent transectsarea.

Fig.2: The permanent transects area, (A) a three—dimensional view and (B)mean height of each strip above the lowest point and the division in fivesubareas.

Fig.3: Trends in the changes in frequency (based on 132 strips per year) ofthe most frequent species of the Plain.(A) AS = Agrostis stolonifera, CP Centaurium puichellum, GM = Glauxmaritima, JG = Juncus gerardii, OV Odontites verna ssp. serotina, PA =Potentilla anserina, PH = Phragmites australis, SM = Scirpus maritimus.

(B) AP = Atriplex prostrata, CL = Centaurium littorale, JB = Juncus bufonius,

JM = Juncus maritimus, PM Plantago maritima, SC = Salicornia spec. , SS =

Spergularia spec..

(C) CD = Carex distans, CR = Chenopodium rubrum, ES = Eleocharis spec, LV =

Limonium vulgare, PS Puccinellia spec., SU = Suaeda maritima.

Fig.4: Trends in the changes in frequency (based on 38 strips per year) ofthe most frequent species of the Slope.(A) AA Ammophila arenaria, AS = Agrostis stolonifera, CL = Centauriumlittorale, FR = Festuca rubra, HR Hippophae rhamnoides, JA = Juncus alpino—

articulatus, PA Potentilla anserina, PP = Poa pratensis.

(B) CA = Cirsium arvense, CE = Calamarostis epigeios, CF Cerastium fon—

tanum, LC = Linum catharticum, LN = Leontodon nudicaulis, OV = Odontitesverna ssp. serotina, SA = Sonchus arvensis, SN Sagina nodosa.

(C) AM = Armeria maritima, CD Carex distans, CH = Chamaenerion angusti—

folium, CX Carex flacca, GM = Glaux maritima, HL = Holcus lanatus, TF =

Trifolium fragiferum, TR Trifolium repens.

Fig.5: Trends in the changes in frequency (based on 32 strips per year) ofthe most frequent species of the Dune.(A) AA = Amrnophila arenaria, AS Agrostis stolonifera, CA = Chamaenerionangustifolium, FR = Festuca rubra, HR Hippophae rhamnoides, PP =pratensis, SA = Sonchus arvensis, SM = Sedum acre.

(B) AP = Aira praecox, CE = Calamagrostis epigejos, CF = Cerastium fontanum,

CS = Cerastium semidecandrum, EP = Elymus pycnanthus, LN Leontodon nudi—

caulis, SN = Sagina nodosa, SJ = Senecio iacobaea.

(C) CI = Cirsium arvense, ES = Euphrasia stricta, HL = Holcus lanatus, LC =

Linum catharticum, PC = Plantago coronopus.

Fig.6: Trends in changes in the life—form spectra. Each species was weightedfor its frequency as estimated by the trends. (A) Plain, (B) Slope, (C) Dune.

Fig.7: Maximal plant height plotted for each species for the years it

occurred. Symbols are given in five different classes, according to thefrequency of the species as estimated by the trends. Trend of the mean of thelOlog transformed maximal heights weighted by the frequencies as estimatedby the trends is indicated by the line. (A) Plain, (B) Slope, (C) Dune.

24

Fig.8: Changes in species richness with time. Symbols indicate observedspecies richness while the lines show species richness as estimated by thetrends. In years where symbols are missing, some species were not recorded.(A) Number of species per strip, (B) total number of species in the subarea.

Fig.9: Rate of succession expressed as percentage dissimilarity (PD) as

estimated by the trends. (A) PD with preceding year, (B) PD with 1973, (C)

PD with 1989.

Fig.l0: Rainfall—deficits over the months april, may, june and height of thehighest flooding preceding the growing season,

Fig.ll: Observed frequencies of some species plotted against the observedfrequencies of Hippophae rhamnoides of an estimated given minimal height. Thelines show the maximal frequencies species can reach, when Hippophae of agiven minimal height directly and immediately causes these species to dis-appear. Maximal heights of the species are given behind each species. Minimalheight of Hippophae (A) 30 cm, (B) 56 cm, (C) 110 cm.

Fig.12: Reconstruction of the living above—ground standing crop of Plain,Slope and Dune at three different stages of the succession, given in theyears these stages occurred as deduced from permanent transects.

Fig.13: Relative contribution of the dominant species to the total livingabove—ground standing crop. (A) Plain, (B) Slope, (C) Dune.

Fig.l4: Reconstruction of the above—ground dead organic matter of Plain,Slope and Dune at three different stages of the succession, given in theyears these stages occurred as deduced from permanent transects.

Fig.l5: Reconstruction of the root biomass in the upper 20 cm. of the soilof Plain, Slope and Dune at three different stages of the succession, givenin the years these stages occurred as deduced from permanent transects.

Fig.16: Reconstruction of the total biomass of Plain, Slope and Dune at threedifferent stages of the succession, given in the years these stages occurredas deduced from permanent transects.Root biomass is determined in the upper 20 cm. of the soil.

Fig.17: Reconstruction of the light profiles of Plain, Slope and Dune atthree different stages of the succession, given in the years these stagesoccurred as deduced from permanent transects.

Fig.18: Relation between light penetration to the soil—surface and above—ground standing crop for all the plots of Plain, Slope and Dune.

Fig.19: Reconstruction of the allocation patterns of Plain, Slope and Duneat three different stages of the succession, given in the years these stagesoccurred as deduced from permanent transects. As measure for the allocationto. root the proportion of the roots in the total biomass is used. As measurefor the investment in stem—function the height divided by total biomass isused. As measure for the investment in leave—function the percentage coverdivided by total biomass is used.

Fig.20: Reconstruction of the specific weight of the soils of Plain, Slopeand Dune at three different stages of the succession, given in the yearsthese stages occurred as deduced from permanent transects. (A) Organic layer(B) mineral layer.

25

Fi,2l: Reconstruction of soil moisture content of Plain, Slope and Dune atthree different stages of the succession, given in the years these stagesoccurred as deduced from permanent transects. (A) Organic layer (B) minerallayer.

Fig.22: Reconstruction of the NaC1—content of the soils of Plain, Slope andDune at three different stages of the succession, given in the years thesestages occurred as deduced from permanent transects. (A) Organic layer (B)mineral layer.

Fig.23: Reconstruction of the CaCO3—content of the soils of Plain, Slope andDune at three different stages of the succession, given in the years thesestages occurred as deduced from permanent transects. (A) Organic layer (B)mineral layer.

Fig.24: Reconstruction of pH(H2O) of the soils of Plain, Slope and Dune atthree different stages of the succession, given in the years these stagesoccurred as deduced from permanent transects. (A) Organic layer (B) minerallayer.

Fig.25: Reconstruction of pH(KC1) of the soils of Plain, Slope and Dune atthree different stages of the succession, given in the years these stagesoccurred as deduced from permanent transects. (A) Organic layer (B) minerallayer.

Fig.26: Reconstruction of the thickness of the organic layers of Plain, Slopeand Dune at three different stages of the succession, given in the yearsthese stages occurred as deduced from permanent transects.

Fig.27: Reconstruction of the organic matter content of the soils of Plain,Slope and Dune at three different stages of the succession, given in theyears these stages occurred as deduced from permanent transects. (A) Organiclayer (B) mineral layer.

Fig.28: Root biomass in the upper 20 cm. of the soil against the total amountof organic matter in the upper 20 cm. of the soil for all the plots of Plain,Slope and Dune. The line indicates all cases in which the total amount oforganic matter is equal to the root biomass.

Fig.29: Correlation between C—content and N—content for all the plots ofPlain, Slope and Dune. Only the contents of the organic layer are given, butthe correlation is based on the contents of both organic and mineral layer.For all plots the N—content of the mineral layer is less than 0.25 g/kg.

Fig.30: Correlation of C—content (A), N—content (B) and P— content (C) withthe organic matter content for all the plots of Plain, Slope and Dune. Onlythe contents of the organic layer are given, but the correlation is based onthe contents of both organic and mineral layer. For all plots the organicmatter content of the mineral layer is less than 10 g/kg.

Fig.3l: Reconstruction of soil C—content of Plain, Slope and Dune at threedifferent stages of the succession, given in the years these stages occurredas deduced from permanent transects. (A) Organic layer (B) mineral layer.

Fig.32: Reconstruction of soil N—content of Plain, Slope and Dune at threedifferent stages of the succession, given in the years these stages occurredas deduced from permanent transects. (A) Organic layer (B) mineral layer.

26

Fig.33: Reconstruction of soil P—content of Plain, Slope and Dune at threedifferent stages of the succession, given in the years these stages occurredas deduced from permanent transects. (A) Organic layer (B) mineral layer.

Fig.34: Reconstruction of the C/P—ratio in the organic layers of Plain, Slopeand Dune at three different stages of the succession, given in the yearsthese stages occurred as deduced from permanent transects.

Fig.35: Reconstruction of the total amounts of N and organic matter in theorganic layers of Plain, Slope and Dune at three different stages of thesuccession, given in the years these stages occurred as deduced from per—manent transects. They can be shown in one picture, because they were soclosely correlated.

Fig.36: Reconstruction of the total amounts of P in the organic layers ofPlain, Slope and Dune at three different stages of the succession, given int1e years these stages occurred as deduced from permanent transects.

27

Table 1: Used criteria leading to the division of the permanent transectsarea in five subareas.

electrical con-ductivity(microS/cm)

number ofstrips

height abovelowest point

(cm.)

"Creek" < 25 2200 — 17000 17

"Plain" 25 — 55 750 — 6300 132

"Foot" 25 — 55 180 — 1200 21

"Slope" 55 — 100 50 — 200 38

"Dune" > 100 12 — 200 32

TabLe 2: PartiaL regression—coefficients of the standardized residuals of the caLcuLated trends againstheight of the highest fLooding (F) and rainfalL—deficit over the period apriL—june CR) for each of the fivesubareas. ALL species which showed a significant effect are shown. — no trend caLculated, because speciesoccurred in Less than four years, n.s. = multiple regression not significant (p>.05).

CREEK PLAIN FOOT SLOPE DUNE

F R F R F R F R F R

The rofyts

Salicornia spc. 0.56 n.s. 0.61 n.s. — —

Chenopodium rub. 0.61 n.s. n.s. n.s. n.s. n.s.

Spergularia spc. n.s. n,s. 0.68 n.s. 0.88 n.s.

Suaeda maritima 0.57 n.s. 0.63 n.s. 0.62 n.s. — —

Atriplex prostr. n.s. n.s. 0.54 n.s. 0.61 n.s. n.s. n.s.

Ocioritites verna n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n,s. —0.60

Centaurium put. — — n.s. —0.57 n.s. —0.63 n.s. n.s. — —

Centaurium titt, — — n.s. —0.63 n.s. n.s. n.s. n.s. n.s. n.s.

Linum cath. — — n.s. n.s. n.s. n.s. n.s. —0,55 —0.72 n.s,

Arenaria serp. — — — — — — n.s. n.s. n.s. —0.77

Charnaefyts

Cerastiurn font. — — — — n.s. n.s. n.s. —0.67 n.s. n.s.

Sedum acre — — — — — — n.s. n.s. —0.56 n.s.

Hemi cryptofyts

Festuca rubra — — n.s. n.s. n.s. n.s. 0.60 n.s. n.s. n.s.

Calamagrostis e. — — — — n.s. n.s. n.s. 0.63 n.s. n.s.

Trifolium repens — — — — n.s. n.s. n.s. 0.67 n.s. n.s.

Plantago cor. — — — — — — —0.64 —0.65 n.s. n.s.

Cirsiurn vulgare — — — — — — —0.65 n.s. n.s. n.s.

Geofyts

Juncus maritimus n,s. n.s. n,s. n.s. 0.62 n.s,

Scirpus mar. n.s. n.s. —0.61 0.53 n.s. n.s. n.s. n.s.

Phragmites aus, — — 0.77 n.s. — — — —

Cirsium arvense — — n,s. n.s. n.s. n,s. n.s. 0.55 n,s. n.s.

Sonchus arvensis — — n.s. n.s. n.s. 0.56 n.s. n.s. —0.60 n.s.

Arrinophila are. — — — — 0.58 n.s. n.s. n,s. —0.55 n.s.

F = FLOWINGR = RAINFALL DEFICIT

Table 3: Percentage dissimilarity between the characterizedvegetations at the permanent transects and the vegetations at theplots used for the reconstruction.

PERMANENT TRANSECTS

PLAIN SLOPE DUNE

'72 '80 '88 '72 '80 '88 '72 '80 '88

Plain'72 36 46 65 77 86 94 83 95 99

Plain'80 41 30 47 69 79 88 85 94 99

Plain'BB 66 43 30 74 81 83 86 94 99

L0TS

Slope'72

Slope'BO

Slope'88

Dune'72

44

60

78

92

52

66

73

95

80

80

68

96

43

35

61

56

51

31

57

64

79

75

26

72

70

56

61

33

73

58

68

53

84

74

38

66

Dune'80 74 81 85 42 39 75 42 36 69

Dune'88 88 90 88 68 63 33 48 54 20

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0.072 75 78 81 84 87 90

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o TRIFOLIUM F.SLOPE; 25 CM

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1800

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0 25 50 75 100 125

HEIGHT FROM SOIL SURFACE (CM)

100

80

60

40

20

0

100

80

60

40

20

0

150

DUNE

—0— 72 80 —+— 88

0 25 50 75 100 125

HEIGHT FROM SOIL SURFACE (CM)

150

• PLAIN SLOPE 0 DUNE

0 400 800 1200 1600

STANDING CROP (g/m2)

120

80

40

70 74

— HEIGHT(cm/kg)

120

80

40

70 74

— HEIGHT(cm/kg)

DUNE

120

80

40

070 74

— HEIGHT(cm/kg)

78

YEAR

— COVER(%/kg)

82 86

— ROOTS(%)

90

PLAIN

09078 82 86

YEAR

COVER ROOTS(%/kg) (%)

SLOPE

0---_ —0____— — —

S

— —

S

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YEAR

COVER — ROOTS(%/kg) (%)

SP

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72 80 88 72 80 88 72 80 88

70

60

50

40

30

20

10

0

70

60

50

40

30

20

10

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DUNE

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72 80 88 72 80 88 72 80 88SLOPE DUNE

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3000

2000

1000

00 1000 2000 3000 4000

ORGANIC MATTER (g/m2)

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ORGANIC MATTER CONTENT (g/kg)

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ORGANIC MATTER CONTENT (g/kg)

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a a a ab

ab

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72 80 88 72 80 88PLAIN SLOPE DUNE

72 80 88

N—

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T(g

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