F. Herbert Bormann Gene E. Likens Pattern and Process in a Forested Ecosystemwebpage.pace.edu/dnabirahni/rahnidocs/law802/Chapter 5... · 2006-06-28 · 140 Pattern and Process in

F. Herbert Bormann

Gene E. Likens

Pattern and Process in aForested EcosystemDisturbance, Development and the Steady StateBased on the Hubbard Brook Ecosystem Study

Springer-Verlag

CHAPTER 5

Reorganization: Recoveryof Biotic Regulation

The aggrading northern hardwood ecosystem has a considerable capacityto exercise control over hydrology and biogeochemistry and to regulatethe flow and use of solar energy (Chapter 2). When this control is at itsmaximum, the ecosystem is most stable, with highly predictable and low

net losses of nutrients and a fairly constant annual evapotranspirationrate. Regulation is rooted in internal ecosystem processes such as,-__.- --transpiration, nutrient uptake, d e c o m p o s i t i o n , m i n e r a l i z a t i o n - d -

‘~&%%I-Clear-cutting.-results in marked changes in internal ecosystem-7;~.<distinct 1-f. biotic regulation -over -energ@ow,processes

biogeochemistry, and hvdrology-(Chapter 3).a-_~m---tie i%.+- examine the recovery of biotic regulation as the variousspecies respond, through growth and reproductive strategies, to the newconditions of resource availability created by clear-cutting.

QsLoj>gulation is reflected in markedly increased-6?e_xpBw-rand nutrients from the ecosystem and some increase.-& soil erosion..___ _~ ~..depending on the degree of disturbance. On the other hand, recoven of

the ecosystem may-be -stimulated by temp_orary_enrich_ment of t&&c__--. .-_.

,?A 1 with available soil moisture during the summer, increased radiant energy

$ypi-I .?A’--& the ground level, and a marked in&ease in the concentrations of_.- ~~ ~~

21 c&&ed nute$nts i n t h eedy --

soil solution_ (Figure 5-l). In a sense, the

y ecosystem has an abundance of solar energy, is fertilized. with nutrientsdrawn out of the nutrient capital of the system and is imgated.

In the remainder of this chapter, we shall consider how the autotrophlcbiota respond to these conditions of site enrichment and how the

ecosystem recovers biotic regulation over export patterns. In the course,.7:-.-

Reorganization: Recovery of Biotic Regulation 139

Clear Cutting/ 1-1

AlltiapathicNutrientUptakeMarkedly

VegetationDiminished

Relaxed

Decomposition,

Increased

A

Biotic Regulationof ErosionRelativelyUnchanged

j*

4. I

+Increased Modest IncreaseStreamf low In Erosion Accelerated Dissolved-

ExportNutrient Loss

Figure 5-l. Hydrologic and biogeochemical responses of the northern hardwoodecosystem during the first two seasons after cutting. Erosion response assumeslittle damage to the soil during the harvesting process. Items in double-lined boxesrepresent marked increases in resource availability within the ecosystem.

of this discussion, we shall attempt to illustrate the array of mechanismsby which the short- and long-term stability of the ecosystem is main-

,, tained,the cost of such maintenance, and to present a generalized view of\ ecosystem-liomeostasis.ic- ---~ I__-.-. .J

PRIMARY PRODUCTIVITY

Initial Response

Conditions created by clear-cutting activate the entire array of reproduc-tive and growth strategies discussed in Chapter 4, and the clear-cut site issoon covered with vegetation. However, these general observations leaveseveral basic questions unanswered. Howdapi_d!y does prima_ryproduc-

-tionrscsyer? How does production compare to that prior to disturbance?--e.--- -- .How closely is production coupled to the~~~tie&rotrophic&o&ses dis-cussed in Chapter 3? How quickly does the ecosystem reestablish maxi-mum biotic regulation over energy flow, biogeochemistry, and hydrology?

Studies in the vicinity of Hubbard Brook (Figure 5-2) indicate thatafter a lag of about 1 year, rew of net timarymductivity @JPP) od.-- - - - - -.-~.-~~~ -__.-b&ret sites is rapid. Within-&ears after &&.NPPaveragesabou~38% of a-%yr-old fores?Tceaf area index CLAI) is about 3 Q&-M ’wominated by raspbergyz;y, i!%@~~~compared to a value of

140 Pattern and Process in a Forested Ecosystem

about 6.0 for the older forest (Whittaker et al.. 1974). At 4wP_and* LA1 are-about_equaL.to values for a 55-yr-old forest (Figure 5-2; Marks.

‘lq$. Between Years 4 and 6, NPP may rise substantially above that ofthe intact forest, but whether it does so appears to be influenced by therelative importance of exploitive species in the revegetation process. Allof the points above the regression line in Figure 5-2 represent data fromplots almost wholly dominated by exploitive species (raspberry or pin

cherry).The rapid average rate of NPP increase, 232 g/m’-yr,’ (Figure 5-2),

/- . .

.1400-

1200-

LZI' IOOO-

“E\CT

Ez 800-

-0c

$ 600-

0,22 400-

200-

I1

OO I 2 3 4 5 6

Years After Clear-Cutting

Figure 5-2. The relationship of aboveground net primary productivity to time afterclear-cutting. Data are from the following sources: (0) Marks (1974); (0) Whitney

(1978); (+) S. Bicknell, F. H. Bormann, and P. Marks (unpublished data); (A) P.

Murphy (unpublished); (x) D. Ryan (unpublished). Murphy’s data are for above-ground current growth and hence are a low estimate of aboveground NPP. All dataare from plots where thk forest floor had not been massively disturbed in theharvesting process. Safford and Filip (1974) suggest much lower rates of NPP on

\ 4-yr-old plots that were scarified after cutting with a crawler tractor and a rock rake.It seems likely on clear-cut sites with much-surface disturbance.that av!@BeBt@

\’;9 .$+

oT6%re&very would be lower than those presented here.J

Reorganzation: Recovery of Biotic Regulation 141

duci-ng the-first~fmyears indicates a strong autotrophic growth response tothe increase in resource availability resulting from the clear-cut. For theecosystem as a whole. dur ing the course of development. it seems ,,:; ‘I’.probable-that there are few sustained rates of NPP increase that remoteI+. ‘;approach that of the first few -years after clear-cutting. T&J.apid rate of ‘- , :NPP increase and the relatively high absolute rates of primary productionwithin 3 or 1 years after cutting suggest a fairly rapid response of autotrophsto increased resources resulting from heterotrophic processes (Chapter 3).

Longer-Term Response

We have relatively few data on the productive-behavior of the northernhardwood ecosystem after the initial burst of growth that follows-clear-cutting. _Obviously. NPP cannot increase indefinitely. nor does a singleasymptote stretching over 170 years seem likely. According to our bestunderstanding, the ecosystem undergoes a variety of biotic and abioticchanges during this long period. and it seems likely that these changes arereflected in primary production. There are some data to support thiscontention. Marks (1974) reports aboveground NPP of 1076 g/m’-yr for a14-yr-old pin cherry stand. while values of 957 and 792 g/m’-yr weremeasured for a -IO-15- and 45-%I-yr-old stand dominated by sugarmaple, beech, and yellow birch (Table l-l). These data suggest that netpyimary..prqductivity during the Reorganization Phase peaks and _thendexes.

Studies by -C&ngton-and-Abe& (1980) suggest a pattern for primar)productivity during the Reorganization and Aggradation Phases of !~‘: r: r ,northern hardwoods. They studied leaf production in stands of differentages that arose naturally after clear-cutting and which exist within the

p J :-f ;,- ,>(:

lim?tK~~&th in Chapter 1. To these data they added appropriate 1 qleaf-production values from other studies. Leafprod&gn- is not a good - ifindex of changes in NPP during succes= because of the changing Iproportions of NPP invested in photosynthetic and respiratory tissues asdevelopment proceeds (Whittaker and Woodwell, 1967). Leaf production. Lh~eyer,may~~be~cons_idered_as a rough index of gross pr~y~?&~on(GPP). The curve generated by Covington and Aber (Figure 5-3) thusprovides an index of changes in GPP through time. Although the fit of thecurve is largely speculative, there is an underlying logic to it.

The curve suggests a rapid rise in GPP during the earliest years afterclear-cutting. This conforms with the NPP measurements discussed aboveand can be accounted for by the high levels of resource availabilityresulting from the cutting.

There is some question as to how to draw the curve beyond Year 6.Should it gradually rise to a maximum at about Year 60, or dip and thenrise, as Aber and Covington have drawn it? Use of data from differentstands could be misleading; one stand may be simply more productive or

I

.

142

4

Pattern and Process in a Forested Ecosystem

I 1 I I / I I

h

im

I , I I I I ,3 I

1 IO 20 30 40 50 60 200

Age In Years

Figure 5-3. Leaf biomass (0) for a secondary developmental sequence in northernhardwoods. Age is years after clear-cutting. Data are means and 95% confidenceintervals. Additional data are from (h) Hart et al., 1962; (g) Gosz et al., 1972; (m)Marks, 1974: and (w) Whittaker et al., 1974 (Covington and Aber, 1980).

variance may be such that a dip appears because data points are lacking.However, several lines of reasoning suggest that_thedipan&ubser)llent

.(ir7 --

ii’ rise to Year 60 may be justified.

:q The proposed dip in production could result from the interaction of;I-, 1 many factors. First, reduction in the amount of easily decomposable

’ J*.J”F“ organic substrate in the forest floor could @$ish nutrient availabiwy(Chapter 3). The decline in depth of the forest floor means less\ .mineralization and also less storage space for wateLand_ availablenutrients. Second, the rapid development of high rates of-GPx&@lyafter cutting implies increasing competition for water and-nutrients, whichmight differentially affect productivity of exploitive species since they areprobably less able to compete for water and nutrients at low ranges ofavailability. Third, Covington and Aber point out that the transfer ofproductivity from one species to another may not be smooth, and a

temporary drop in production may result. For example. toward the end ofthe Reorganization Phase,.-as pin cherry ~&giis to-die G$, the uptake .ofavailable resources may be diminished,. and -for a time fhe ._otherspeclesmay not take up the slack.

Explanation of the relatively rapid rise after the decrease in productionis even more problematical. Yet the proposed rise in GPP to about Year50 (Figure 5-3) appears reasonable, since other measurements indicate

that the ~ysteti~simultaneously undergoing a rapid -accumulatti!fnet biomass in three of its four compartments: living organisms, fores!

Reorganization: Recovery of Biotic Regulation 143*,-* :‘-‘, .<.:.-. .C ; i. y , ‘i -“<-; Lr,-.,--. . :c:\ f A;s-- _

:,L’ ,floor, -and deadclwood (Figure 2-l). There are several factors that could ‘,I.---- )contribute to a rising GPP. A shift in predominance from species utilizing 1-1 .. - .-the exploitive growth strategy to those utilizing a conservative growth ‘-r ’strategy could lead to greater production through more efficient use ofsite resources. Secondly.+Lial growth seems to be primarily supported i ’ ,by nutrients drawn largely from the forest floor and the upper few O,centimeters of the mineral soil by shallow-rooted species. Gradual -’ ---occupancy of the site by deeper-rooted species would allow utilization ofnutrient and water resources deeper in the mineral soil. Although cation

‘, ,

- - ’exchange per -unit weight. of soil in the B-horizons of White Mountain ._soils is low in comparison to that of the forest floor. the total amounts ares&tantial owing to greater thickness and bulk density. Similarly, morethan 90% of the soil’s water-holding capacity is found in the mineral soil,(Hoyle, 1973).

The paucity of data on productivity beyond about Year 5.5 makes y’; , . /speculation difficult. However, in an old aggrading forest, dominated by b;,,rilarge old trees. there would be a high ratio of respiratory to photo- L.synthetic tissue (Whittaker and Woodwell, 1967). and this could lead to 1relatively low net primary productivity, since GPP minus green plant -’respiration equals NPP. ;,,. ,j 3

RECOVERY OF BIOTIC REGULATION OVERECOSYSTEM EXPORT

How quickly does the revegetating ecosystem establish biotic regulationover export patterns? We have sought answers to this question in severalways: by following the recovery of the deforested watershed (W2) atHubbard Brook, by comparing stream-water chemistry from revegetating,commercially clear-cut forests and adjacent aggrading forests, and bydrawing on information from the literature.

Recovery After Enforced Devegetation

The sequence of revegetation of the deforested watershed (W2) at K -Hubbard Brook is shown in Figure 5-4.nce_revegelatio~~sallo~w~o .I* .-’ I

o$cur,the.rate.ofrecovery of net primary productivity-was initiallymch,;

-! 1’.-lower (Reiners e t a l . , 1981) than in areas where natural revegetation (go-’ ‘1

immediately followed clear-cutting (Figure 5-5). On W2&e was a 2-yr /<Iwiod-ofvery low growth, as compared to a single year of low growth 0% ~<LP~~’the commercially cut watersheds. NPP increased more or less linearly my& - iafter the second year of recovery on W2, but the rate of increase from LYear 2 to Year 5 was about 27% lower than that of immediate-growth

F ’ ’ ;Iivs

systems. By Year 5, average aboveground NPP on immediate-growth u;,,‘i 1 1’4

systems had risen to over 1000 g/m’. while on W2 it was about 45% lower.,I

_I,, o ‘& “( ” i!

e-r-nU,-li--....-L-


I I I I I I

0/

Years Of Regrowth

Figure 5-5. Aboveground regrowth (NPP) after (0-O) clear-cutting and enforceddevegetation (W2, Reiners et al., 1981) and (C---S) clear-cutting followed

immediately by natural revegetation (from Figure 5-2).

These fundamental differences in productive behavior plus 3 years ofenforced devegetation suggest that biogeochemical data for the recoveryperiod from W2 may not precisely portray the reestablishment of bioticregulation in commercially clear-cut systems that are allowed to revege-tate immediately. Nevertheless. hydrologic and biogeochemical data

from W2 are the most complete in existence, and they do reveal insights5 ?’ into basic patterns of ecosystem recovery.

,dpi*,,. Annual streamflow on W2 increased between 24 and 35cm during the

1’ ’.’ devegetation period. These values represent 26 to 41% greater stream-, _‘. flow than would have occurred if the forest had been left uncut (Chapter

I. 3; Figure i-6). Variation in streamflow during this period was due to.

,.i” differences in amount and timing of precipitation (Hornbeck, 1975).Almost all of these increases occurred during the growing season and

Y’ were coincident with the elimination of transpiration by cutting. Stream-,TC ,‘I

flow decreased markedly during the first 4 years of revegetation (Figurec’ “A 5-6). Although much of that decrease was probably due to increases in

I transpiration related to revegetation, some proportion was apparentlyCfC’ related to chance variations in amount of precipitation, since streamflow

,J- rose again during the last 3 water-years reported here. 1974-75 to 1976-77._ ‘.I-,< Thus after 7 years. despite moderately high levels of primary productivity.

,:a : the export of liquid water remained significantly higher than that

expected from a 60-yr-old aggrading forest (J. Hornbeck. personalcommunication).

Reorganization: Recovery of Biotic Regulation I 47

Treat-

/+ Calibration+/ Lmentl Regrowth -+/

-lo’ I '2'3'4'5'6'7'8'9'10'11'12'13'14'15'16'17'l6'

Water YearsFigure 5-6. Deviations from regression of annual streamflow for Watershed 2 onannual streamflow for adjacent uncut reference Watershed 3. Period A representsprecutting calibration period; Period B is the enforced devegetation period, andPeriod C is the regrowth period. Water-years are from 1 June to 31 May and begin1 June 1958 (from Hornbeck, 1975, and J. Hornbeck, personal communication).

Concentrations of dissolved substances ~(T.able S-l) and the&-export r : ’ “’ I- - -~-- ~-~froamigure_31])ddecreased markedly during -the first 3 years of %e P-fl’f,recovery period. The- explanation for this decrease is-not-entirely -clear,but nutrient uptake by vegetation was not solely- responsible. The decline

d’ ’ ‘, .*’ ’ ‘- ‘-~-

in annual concentrations of nitrate, potassium, and calcium in stream : : ,-water began during the third year of devegetation in the absence of any .:vegetation. Earlier (Chapter 3) we suggested this might be due to theexhaustion of easily decomposable substrates. A modest decline in nutri-

,,-.> <

ent concentrations in stream water was recorded during the first year $2 1,-

of revegetation, followed by a sharp decline in the second and third fl~‘-‘“‘~”Year. Primary production during the second year was relatively low, yet ,;I:@“the decreases in stream-water concentrations were among the sharpest Cc+recorded (Table 5-l). Several interpretations are possible, for example:

:I; LI.”

(l)-c&-aastion of decomposable -substrate may have continued to be a\oyrvTi’.

major facto~~(~)~GiZGi~~small amounts of vegetation growing along %’ ’ b “7 .dr_a_inage.channels may have had a major effect in d&e&ining&am-water chemistry; (3) nitrification may have been inhibited by living plants.BY the third year of the regrowth period (Table 5-l), nitrate concen-

J ifl ‘1 /I<% T :.i ,-.: L /r I .h ..LreL* -


Table 5-i. Annual Weighed Concentrations of Nitrate. Cai-cium, and Potassium ions in Stream Water fromReference Watershed 6 (W6) and the Experimen-tally Devegetated Watershed 2 (W2)a

Nitrate Calcium Potassium

Year W6 w2 W6 w2 W6 w2

Precutting period1963-1964 -b -b 1.5 , 2.1 0.3 0.31964-1965 1.0 1.3 1.0 1.4 0.2 0.21965-1966 0.9 0.9 1.4 1.x 0.2 0.2

Devegetated period1966-1967 0.7 38.1 1.3 6.5 0.2 1.91967-1968 1.3 52.9 1.3 7.6 0.3 3.01968-1969 1.3 40.4 1.3 6.0 0.3 2.9

Regrowth period1969-1970 3.2 37.6 1.6 6.2 0.3 2.81970-1971 2.5 18.8 1.5 4.0 0.2 1.61971-1972 2.3 2.7 1.4 2.2 0.2 0.71972-1973 1.8 0.3 1.4 1.8 0.2 0.5197s1971 2.3 0.1 1.4 1.7 0.2 0.5197-L-1975 2.4 0.1 1.4 1.7 0.2 0.41975-1976 2.2 0.1 1.3 1.6 0.2 0.3

aValues are in milligrams per liter.bAb data.

trations were indistinguishable from those of an approximately 60-yr-oldaggrading system, and beyond that time they were lower. Calciumconcentrations after the third year of regrowth were about the same asthose found in the 60-yr-old forest, but potassium concentrationsremained relatively high.

Erosion and transpqflation of particulate matter were-veryxespansiye.*,: -/ ~_~___.. ~~‘r * .ta?Gegetatlon (Figure 5-7). Toward the end of the devegetation period^‘“‘-IJ and during the first year of vegetation regrowth, the export of particulate

&,,AA,!’ b’ 1 matter was increasing exponentially. This trend was reversed during the

. ? ‘ second and third year of recovery; erosion decreased sharply, coincidentr s / with a relatively small amount of vegetation growth. With *Lsmall,^_ :Y’; amounts of primary production (Figure 54), regulation of particulate

export obviously did not come from reestablishment of the forest floorthrough litterfall. Observations suggest that a concentration of plantgrowth in and around stream banks might have been responsible for, thestabilization of stream banks and reestablishment of organic-debris d&s.Stream banks, by virtue of direct exposure to streamflow, are more subjectto the erosive force of flowing water than any other single area of theecosystem.

+bas

z”


~Q&o&gd TDeforestedt Recovery ,-.

I !

90 .c 1 1

Kg/

k‘wi,

Calcium60- tt\

ha I ‘\30- /I 0 ,

0- -_ /- 5 - -

Potassium3 0 - \

\2 0 - \

Kg/haIO-

If04-u6 0 0 - Nitrate

Kg/ha400-

2 0 0 -/

o- w z z : ‘- 2 _“\n _ - -- - --“--4 0 0 b

1963-64 I966- 67 1969-70 1972-73 1975-76

Figure 5-7. Export patterns of dissolved substances (calcium, potassium andnitrate) and particulate matter in stream water from (O--O) the experimentallyclear-cut watershed (W2) and (H) the forested reference watershed (W6)(modified from Likens et al., 1978).

Information from W2 contributed in the following ways to ourunderstanding of the recovery of biotic regulation of export patterns inclear-cut ecosystems:

1. T&L& massive loss of nutrients from W2 resultedin. part from-cheXdelay in coupling primary productivity and the hydrologic and

,,~$~~%%irnent that occurred immediately after disturbance.Not only- did enforced devegetation. prevent growth-and q~c@c--~__m--- -~~.tive strategies from taking effect-immediately, but it .qeduced the- -~ .effectiveness of these strategies once regrowth_ took.. place. Therelatively sluggish growth once revegetation began was probablydue, in part, to the loss of nutrients during the period of enforceddevegetation.

150 Pattern and Process in a Forested Ecosystem Reorganization: Recovery of Biotic Regulation 151

.-,,*,,‘j ! ‘-.x These points emphasize the importance of effective and early

5. IPi. {9, . coupling of forest floor decomposition and nutrient uptake by newvegetation. Any harvesting practice that tends to uncouple decom-

,.\I! >: position and nutrient uptake by disrupting the forest floor mayr ,-LA ,- ! contribute to unnecessary nutrient export and a reduction in the rate

: ,.;.i of recovery of primary productivity.2. Despite 3 years of enforced devegetation and substantial loss of

nutrients from the ecosystem, there were. coincident with revege-

-7 : : tation, rapid initial declines in the concentrations of_--dissolved. ;’ substances in stream water and the export of liquid water, dissolved

substances, and particulate matter. Steep initial declines were.?‘%,I 1) associated with relatively small increases in vegetation growth,

/ : I- >.,\,.. / suggesting the possibility that living plants, particularly alongI t- ,f drainage channels, have a great effect in establishing early regulation

; ! (d: :< I’.I \ over export.

3. After 7 years of recovery. even though net primary productivity hadrisen to substantial levels (Figure 5-4) streamflow and the net exportof calcium and potassium were still greater than precutting levels(Figures 5-6, S-7). -Nitrogen, on the other hand. after the fourth-yearof the regrowth period, showed a net gain (i.e., meteorologic inputminus gross export gave a positive value), indicating- that losses-of dissolved nitrogen are rapidly brought under control by-therevegetating system and that a net accumulation of nitrogenmayhave begun. Caution is advised here because we don’t know the netbalance for gaseous nitrogenous substances. These results indicatethat each species of dissolved ion had a different recovery pattern(Likens et al., 1978).

Recovery With immediate Revegetation

Extensive data from throughout the eastern deciduous forest suggest thatthe export of particulate matter after clear-cutting is minimal if the siteis rapidly revegetated and not excessively erosion-prone and if road con-struction and cutting are carried out in a careful manner (Panic, 1976b,1977). Our findings for the northern hardwood forest are in accordwith these data. In Chapter 2 we elaborated on a number of bioticmechanisms, relating to both living and dead biomass, that exerciseregulation over the erodibility of the aggrading forest. _o_uI: experimental

_&tudy of the-de-vegetated ecosystem (W2) demonstr-res- that--mechanisms-_ controlling erodibility of the ecosystem can fuuction at high_leve_ls .fa2

years in the absence of any green plant growth as long asthe Forrest-floor%-not excessively disturbed (Chapter 3). -De- recovery_offtheded_--.ecosystem indicates that relatively small amounts.of.vegetation.regrowthcan rapidly reduce export of particulate-matter~A&of~these~factors@&entogether indicate that, with acceptable harvesting practices, biotic regula-tion of particulate-matter export after clear-cutting is only moderatelydiminished and recovers rapidly with reestablishment of vegetation.

Apparently partial biatir_regulationc&streamfl~ia-transpimtionbegins rapidly with-regrowth,-but establishment oft full regulation-maytake several decades. For example, the recovery of a clear-cut forest inNorth Carolina indicated a rapid initial decline in streamflow, but after23 years streamflow was still significantly greater than that expected if thewatershed had not been cut (Figure 5-8). Similar data from West Virginiaindicated that more than 10 years were required for streamflow to returnto precutting levels (Lull and Reinhart, 1967). These data indicate thatdecline in streamflow is not a simple function of recovery of NPP. Otherfactors such as height and dispersion of the canopy (Douglass, 1967)percentage of canopy in evergreens (Swank and Douglass, 1974)percentage of soil occupied by roots (Lull and Reinhart. 1967) andchanges in the water-holding capacity of the forest floor also may bedeterminants.Jnclear-c_u! northern~ hardwood ecosystems, it~.seems likely ’that recoverv of full biotic regulation of streamflow takes 10 to 20 years, or ’_ -~__-- - -~~to about the end of the-proppsed..Reorganization.Phase...__._~-~~~-~ ~~

To determine the effect of immediate revegetation on stream-waterchemistry, we made a comparative study of water from nine smallcommercially clear-cut watersheds and from nearby uncut stands (Pierceet al., 1972; three stands are not included here because one had aninadequate reference watershed, another was dominated by spruce andfir. and the third had a record of inadequate length). Stream-water

Calibration

bZd kp Regrowth ~~4

4 0E0 1.: 3 0.-.0>2 2 030

~/II

E IOH

CT

0Water -Years

Figure 5-8. The decline in streamflow associated with the regrowth of a clear-cutforest at Coweeta, North Carolina. Decline in streamflow is calculated as deviationfrom expected streamfiow. The zero line represents expected streamflow if theforest had not been cut. There is a 3-yr calibration period prior to cutting and a23-Yr period of regrowth (after Hibbert, 1967).

,’


concentrations from the uncut stands were very similar to those reportedfor aggrading forests at Hubbard Brook (Chapter 2, Likens et al., 1977).

Although biotic regulation over stream-water chemistry was substan-,tially- weakened, it was not changed to the extent shown in W2 (the

(?.A:.I,4

devegetated system, Table 5-2). For the six commercially clear-cut

:7 -” watersheds. annual concentrations (unweighted for volume) of nitrate-/ .,;: 3. nitrogen, potassium, and calcium averaged for the first 2 years after cutting

1’ showed increases by factors of 1,.3 3 and 3, respectively (Table 5-2). Thisy ‘---‘-’j. < :’ compares with increases by factors of 41, 11, and 5 for the devegetated

t t. ; watershed (W2) when compared to the average of all uncut watersheds atHubbard Brook (Likens et al., 1970).

.:’ In contrast to the devegetated system at Hubbard Brook, only onei /: +\!’ commercially clear-cut watershed showed stream-water concentrations to

Y,4d be highest in the second year after cutting. Thr_ee-commercially clear-cut

watersheds had highest concentrations in the first year, and the patternwas not clear for the-remaining two. In only two of the six watersheds didnitrate concentrations fall to precutting levels within 3 years after regrowthbegan, as happened on W2. Trends in the other four watershedssuggested that such a reduction would take 4 or 5 years after cutting.Calcium concentrations in two watersheds and potassium concentrationsin five watersheds remained above reference levels throughout themeasurement period of 4 years.

2..jY!i i- We draw two important conclusions from these results. Immediate

revegetation reduces the magnitude of increases ins concentrations-&,P., .

; ddissolved substances in drainage water, but it does not prevent-significant

.:i ,‘J increases from occurring. This indicates that coupling between mineraliza-;, I

,_ ., 1’ tion processes (accelerated by clear-cutting) and nutrient uptake byregrowing vegetation is far from perfect and that other nutrient sinkswithin the ecosystein are incapable of retaining all nutrients not taken up

, ‘, ”J’. ’ by vegetation.ri-

., .’ .,r( ,’,’

Table 5-2. Ratio of Stream-Water Concentrations of Commercially Clear-Cut Forests to that of Reference Streams Draining nearbyUncut Forests in the White Mountains of New Hampshire’

Clear-cut/Uncut RatioYears After No. of Sites

Cutting Sampled Nitrate Calcium Potassium

1 4 14.0 !I 5.0 2.4r0.1 3.0 2 0.7

2 6 9.723.8 2.1 20.4 2.6 ?I 0.3

3 6 3.8 i 1.6 1.620.3 2.120.3

4 2 1.12 1.0 1.4 t 0.4 2.5FO.l

‘Values expressed as means= SEM. Ratios are based on unweightedaverages (Pierce et al., 1972. and R. Pierce, W. Martin, and A. Federer.unpublished data).


We conclude that reestablishment of biotic regulation over export ofnutrients from comfnercially cut forests is similar to that shown for theexperimentally devegetated system. Nutrient export in stream water is afunction of concentration and amount of streamflow. In the northe+ 3 >;hardwood system. regulation of stream-water concentrations is estabi -8” -’lished fairly rapidly for nitrate (1 to 5 years) and more slo~y!yforcations (5 to / ‘..lg~years) and regulation of streamflow occurs still more slowly (10 to 20 ,l’?; ”years). Considering concentration and streamflow together, it is apparent’ ’that biotic regulation (over large increases in nutrient export) is achievedfairly r’apidly. i.e., within 3 to 5 years after cutting, but that one or twodecades is required before precutting levels of export are reestablished. - ’ ’Substantial quantities of nutrients are lost from the clear-cut ecosystembefore full control is reestablished.

,.. r,-:; ” ‘j

COUPLING OF MINERALIZATION ANDSTORAGE PROCESSES

A major consideration in the analysis of ecosystem dynamics afterdisturbance is the efficiency with which the ecosystem-b-able to storenutrients. Storage may. consist of the transfer of nutrients to newvegetation or to other sinks within the ecosystem. For example, followingclear-cutting in a Douglas fir forest, a very d8erent kind of ecosystem,little export of nutrients was reported, but major transfers from upper tolower horizons are thought to occur (Cole and Gessel, 1965; Gessel et al.,1973).

Covington (1976) has pointed out that during the ReorganizatiogJhase_ -../more nitrogen- is._released_ from the -forest floor than can be accountedfor by a~_~e~~.~~rnu!ation in living biomass plus that- lost as dissolvedsubstances in stream water.

To gain a more detailed insight into mineralization-storage processes innorthern hardwoods, we estimated nutrient losses from the forest floor,nutrient gains by regrowing vegetation, and net nutrient export ofdissolved substances in stream water for an average stand over a 4-yrperiod after commercial clear-cutting (Figures 5-9, 5-10, and 5-11). Theseaverage data were obtained by compositing data collected in differentstands at different times and thus should be considered as a firstapproximation. More precise data are needed, and a simultaneous studyof all of these parameters should be conducted in a single stand over thesame time period. Nevertheless, the present analysis suggests severalinteresting and to some degree startling conclusions.

1. In immediately revegetating clear-cut ecosystem>, the SQwzth__~ -.-~~response of newvegetation is not sufficient to prevent sigr$icantlosses of dissolved substances. However, within a fewyears aftercutting,-. annual increases of nutrients stored in biomass wereapproximately equal to or sweater than annual decreases in stream-~-

i


8 0 -

2 0 -

O-

Nitrogen

Years after clear-cuttingFigure 5-9. An estimated nitrogen budget for a commercially clear-cut forest. Netdissolved losses in stream water, storage in living biomass, and total losses fromthe forest floor are shown for 4 years after cutting. Forest floor data are for anaverage site and are estimated using nutrient concentrations and an equation forpredicting changes in forest floor depth (Covington, 1976). Accumulation in living

biomass is estimated from Marks (1974) and Likens and Bormann (1970), and theestimate is probably high. Stream-export data are calculated with the stream-waterconcentrations and estimated streamflow (Table 5-2) for six commercially clear-cutwatersheds in the White Mountains. Unaccounted loss from the forest floorrepresents the difference between total loss (TL) from the forest floor and biomassstorage (8s) plus net stream-water losses (NSL) i.e., FL- (BS+ NSL)].

Ir7’-. ‘y ,l.,;, water output of nutrients (Figures 5-9, 5-10, and ‘i-11). It would~ ,,.) ” that with time after disturbance a direct relationship< -‘-J’ appear

rtr I,‘?

, , 2’ : 1’develops between nutrient storage in living biomass and stream-

i, ‘., 4

water chemistry. However, this relationship may not be-simpleor~,~ ,2?'.' / !<‘C'

‘bi :.I

direct; for examp~@r&ing vegetation m_ay take up ML* directly[(I:-,, ,-’ and hence limit the process of nitrification, which in turn -affects

J I ,n .@l4, (?. cation exchange, and so forth (Chapter 3).

‘62. A large amount of nitrogen, presumably lost from the forest floor

,L i0. after clear-cutting, is not accounted for by stream-water loss or plant

I;; * <‘ uptake and storage (Figure 5-9). If our estimate of the rate of

I ’; .<{I ( nitrogen loss from the forest floor is correct, about 60% of the

6Cl-

50

4 cl-

2\T? 30

-2z

20

IO

0


Calcium

Potassium

I I I IA I I I II 2 3 4v I 2 3 4

Years After Clear-CuttingFigure 5-10. Estimated average biomass storage and net stream-water loss ofpotassium and calcium for a 4-yr period after commercial clear-cutting. See Figure5-9 for methods of calculation.

predicted loss is unaccounted for by vegetation uptake and/or loss in ;J?, ,,drainage water. Covington (1976) suggests, for the Reorganization ;J i-, .:.-Phase as a whole, that some of the “missing” nitrogq-may be tied. +,j-up in decomposing logs left over from the .clearcgJti-ng. However, IY-.-,__-----~ - --~_.~ -.decomposing logs would be an unimportant “sink” during the first : T,~~,,.4 years after cutting.

_Nitrogen lo.sSfrom_thg-for_e_St floors m a y b e acco_untedfoL-ty_ / ‘?‘= c-traslocation_to_deeper-mineral-horizons. or-volatilization by_cbemi- .&‘kGT> ,1_,

‘7: cal or biological denitrifica~j~~~~processes. The latter pathway wouldincrease the already significant losses of nitrogen (i.e., dissolved

4- .-:

nitrogen) from the ecosystem by a factor of five. It is interesting tonote that Dominski (1971) also estimated nitrogen loss from theentire soil profile of W2 after 3 years of devegetation to be about 35%greater than stream-water losses. He suggested volatilization as anexplanation, but his data were inconclusive. -

These results indicate that there is much we do not know aboutthe nitrogen cycle in forest ecosystems. The evaluation of denitti-cation under field conditions represents a major research challenge.It also should be borne in mind that a finding of significantdenitrification would automatically entail still-greater amounts of


6 0

5 0

4 0

0c

g 3 0

2z

2 0

IC

C

)-

I-

Calcium

Potassium

-‘//i, Forest Floor /,Q///,*

2 3 4

Years After Clear-Cutting

Figure 5-l 1. Flux of calcium and potassium in the ecosystem during the 4 yearsafter commercial clear-cutting. Cations stored in biomass plus those flushed fromthe ecosystem cannot be wholly accounted for by losses from the forest floor. It isassumed the difference comes from the mineral soil. See Figure 5-9 for methods ofcalculation.

biological nitrogen fixation during the Aggradation Phase, to makeup the additional losses.

3$stimated losses of calcium and potassium from the forest-goorcannot account for that takenup and stored in living biomass-plus

--that lost in stream water (Figures 5-10 and 5-11). This suggeststhat cations are being withdrawn from the mineral soil as well asfrom the forest floor. Most of the calcium and potassium lost from

r

#Ji’,-

,T : y’the forest floor may be taken up by the shallow-rooted vegetation

I that typifies early revegetation (Table 4-7), while most of the dis-JC

4solved ions exported from the ecosystem may be drawn from the

, , >,)? deeper mineral soil. This is not an unreasonable assumption, sincewater filtering through the lower horizons is probably enriched in

,’ 1 hydrogen ions resulting from the accelerated nitrification that, *I/: follows clear-cutting, while lower horizons contain adequate ex-

I7,\<T‘ ‘-.

changeable cations (Hoyle, 1973).i


This discussion on nitrogen and cations allows us to comment on thebehavior of the mineral-soil compartment during ecosystem development.In Chapter 1. we assumed that the organic content of the mineral soilremained unchanged during the development of the ecosystem afterclear-cutting. We made that assumption because of our inability tomeasure changes accurately. The small amount of direct data we have(Dominski. 1971) suggests that the mineral soil loses organic matter andnitrogen immediately after clear-cutting, while the large amount ofunaccounted-for nitrogen loss (Figure 5-9) suggests the possibility that themineral soil may accumulate some nitrogen from the forest floor. Theseconflicting data reinforce our original decision to consider organic matterin the mineral soil as relatively unchanging. New research must be doneto bridge this important gap in our knowledge.

The calcium and potassium data (Figure j-11), on the other hand. seemmore definitive; these data suggest it is unlikely that there was a netincrease in exchangeable cations in the lower mineral soil after cutting ofnorthern hardwoods such as that reported when Douglas fir was clear-cut(Cole and Gessel, 1965). Rather, th:whole profile under~noIt.hM-woods seems to lose exchangeable cations.

REPLACEMENT OF LOST NUTRIENT CAPITAL

As discussed above, restoration of&%icregulationmdrologwti -exp_ort f nutriintsisnot-synchronous. Apparently, full biotic regulation Iover export is achieved 10 to 20 years after clear-cutting. roughly coincident L n ’ ’with the end of the Reorganization Phase in our model of ecosystem ‘?-development (Figure l-7). Thereafter, the ecosystem acquires avaiIable J ‘- ’ -)nutrients from various meteorological inputs (including nitrogen fixation) * I..and from weathering, as discussed in Chapter 2.

ECOSYSTEM REGULATION

The Aggradation Phase

Biogeochemistry_. Our evidence indicates that various processes in the- -aggrading ecosystem are closely regulated. The most direct evidencecomes from biogeochemical data. Of all the phases in the developmentalsequence, the Aggradation Phase has rhe greatest degree of biogeo-chemical predictability and constancy. Despite highly variable precipita-tion chemistry (Chapter 2), concentrations of most dissolved substances instream water have narrowly defined limits and are predictably related toflow rates or seasonal factors (Chapter 2). We find that the aggradingecosystem is characterized by a relatively small range in amount of watervapor produced by evapotranspiration, despite a two-fold range in amount


f-s”P,’ ’

“’ d\ of precipitation (Figure 2-6), a small range in net export of dissolved

.T ,’ ; substances, and highly controlled erosion with low export of eroded materialI,(Chapter 2). We take constancy of export and biogeochemical.p.I-edictability

as evidence of a highly regulated system. The regulation of concentrations ofdissolved substances is not determined simply by physical factors but resultsfrom the interactions between tightly coupled biotic and abiotic subsystems(Figure 3-5).

Total Biomass Accumulation. A striking feature of the Aggradation Phaseof ecosystem development is the strong tendency to accumulate biomass;i.e .,~lJ%mass~ accumulation (living plus dead) occurs when grossprimary productivity exceeds ecosystem respiration. The rate and amountof biomass accumulation is determined by the initial conditions of the siteand the interactions between the biota and the external environment.

-P Regulation of the rate of biomass accumulation might be thought of;rr -‘P -‘: as resulting from factors both external and internal to the ecosystem.2 z 5’,’,-( External factors like solar energy, amount of precipitation, nutrient input,

1 and temperature set theoretical limits on rate of biomass accumulation.,.t( f If we consider external variables as constant, we might observe some

maximum rate of biomass accumulation. In nature, however, internal> .i

\P 7 ,.’factors such as ~i.~g_spe_cieS_compositiqn, competitive _reLaJiQn&Fs,

,*:Y5 and insect- defoliations- cause the actual trend of biomass accumulanon

,e;--b r to depart significantly from any such theoretical maximum rate (Figure’ A;,. 1 ’ 5-12).

’ .;.‘I,, cc. Changes in rate of biomass accumulation, whether it is due to a series

Time -

Figure 512. Hypothetical trends of biomass accumulation. (A)Ir!3nd_of~maximum,potentialbiomass accumulation during the Aggradation Phase under constant and

ideal growing conditions; the curve shows the decline in net biomass storagetoward the end of the phase. (8)~Trend.s in-biomass accumulation as affected bythe following departures from ideal conditions: (a) a series of good gro%iTtiars,(b) drought periods, (c) a period of subnormal temperatures, and (d) an outbreak Ofdefoliating insects.


of good or poor growing years. variable internal factors, or the inherentdecrease with time in the storage of living and/or dead biomass within theecosystem, must be reflected in the biogeochemistry of the ecosystem.Thus, even though our concept of ecosystem development suggests ahighly predictable and fairly constant biogeochemistry during the Aggra-dation Phase, some variation>-to-be_.expected.--~~ ~-

-The Relationship benyeen Productivity and Resource Availability. One ofthe factors that most affects the biogeochemical predictability and con-stancy of the Aggradation Phase is the generally close connection betweenpriyary productivity, resource availability, and nutrient. regeneration.Thus, although major trends in GPP vary with time (Chapter 5) variationsabout the trend line itself tend to be relatively small. Aspects of thisrelationship are self-regulating; for example, competition among treesmay contribute to the cybernetic regulation of primary productivity in away analogous to that described for phytophagous insects by Mattsonand Addy (1975). For example, strong competitors (intra- or interspecific)affect the physiological status of nearby but less competitive individuals,whose primary productivity is temporarily reduced. Demands of lesscompetitive individuals on resource availability are relaxed; their resis-tance to predators and parasites is weakened and eventually they die. Fora time, litterfall is increased, space in the canopy is surrendered, andresources previously used go unused. Canopy openings also may resultin increased decomposition and nutrient regeneration. All of these re-sponses contribute to increased availability of light, water, and nutrientsto the surviving trees surrounding the opening. The stronger competitorsrespond with increased productivity, which is once again followed byincreased competition, and the cycle is repeated.

If the sum of local increases in resource availability within theecosystem was relatively great, while corresponding increases in produc-tivity were relatively slow, one would expect increases in concentrationsof nutrients in stream water, together with increased streamflow. Thatlarge oscillations in these biogeochemical parameters do not occur duringthe Aggradation Phase is probably related to the even-agedness of theaggrading forest. During the early part of the Aggradation Phase,___.__~ .- ~~. .<.Icompetition between the 1argeJ even-aged trees results in death oi=y ,,?c I”;’ .-- - - - - ------___-individuals, but gaps in the canopy are relatively small since trees are of ‘,psmall or modest size (Chapter 4). These small gaps probably have little

.,f !i I i

affect on increasing decomposition rates and are rather rapidly closed -I”,’ ~by ingrowth of surrounding individuals. Thus, the feedbackrel&ionship ;-.’

1,;

wide oscillations, and the more or less continuous canopy leads to closeregulation of energy flow, hydrology, and biogeochemistry. Toward theend of the Aggradation Phase, competition and other factors that causethe death of trees result in larger gaps in the canopy, since the

r f*’f ,, :

predominantly even-aged trees are now of larger size. These large gaps it;-“: ;


may foster local@ increased decompositionand nitrification and markedlocal changes in productivity causing the equilibrium between produc-tivity and resource availability for the ecosystem as a whole to exhibitsomewhat wider oscillations. This in turn may result in slightly increasedoscillations in the export of nutrients and water. The effect of decliningbiomass storage on biogeochemistry will be discussed in Chapter 6.

The Reorganization Phase

The Aggradation Phase, with its fairly constant curve of biomassaccumulation and its highly predictable biogeochemistry represents asystem under strong regulation. In a sense it also represents a period of

%l~~~q~~~&ence in our mode1 of ecosystem development, a period withrelatively little endogenous disturbance. In contrast, the ReorganizationPhase begins with a major exogenous perturbation--clear-cutting. Clear-

, ,‘r’_, ,\.i cutting-sets--in motion a series of unusually severe stresses whose_---, I initial effect is to reduce markedly the capacity of the ecosystem to- _m

regulate the flow of both radiant and mechanical energy, water, and

j i ..-. ._ nutrients in ways that diminish destructive effects. In a sense, thes

I ;,-I’ ” disturbance opens the system to those potentially degradingforces that

I.;,;.

’ are ever-present in its environment: rain, running water, wind, heat,~and

I2 ’ ,;<-gravity. The longer these forces operate in an uncontrolled way, the more

\ I . ‘7 ( ’ the ecosystem is degraded and the longer the time .necessary for the_.( ecosystem to recover to predisturbance conditions, if indeed recovery is

I ; ,& possible.For many humid terrestrial ecosystems, particularly those on slopes

or with readily erodible substrates, emion of particulate matter after

,&/i/c; ; ) disturbance presents great-potential danger (Stone, 1973; Bormann et al.,, ,:I

,, ‘,; 1974). The-physical removal of organic and inorganic-materials not- onlyI,,,&.“ results in loss of exchangeable cations attached to exchange surfaces but

1” in the removal of the surfaces themselves. These losses in nutrient capital/ ., i;(^:‘. , and exchange capacity in turn affect the productive capacity of the

ecosystem and its ability to regain predisturbance levc!s of regulation overthe flow of energy, water, and nutrients. In instances of severe erosion,an ecosystem might require a very long time to attain previous

levels of production. Not only would productive capacity be sharplyreduced from predisturbance levels, but redevelopment would involve theslow formation of new humic and clay surfaces and the sequestration ofsubstantial amounts of nutrient capital. Unless checked by biological activi-ties. accelerated erosion might continue until the ecosystem achieved anew relationship with the physical forces impinging on it; and this wouldoccur at a much lower level of productivity and with far less ecosystem,regulation of the physical environment (Figure 6-10). ,~~

_ _L- -’

A Hypothesis of Homeostasis. We propose that the F?Fre stress initi-ated by clear-cutting not only accelerates the activity of some of the~~ ____~~ --~~


mechanisms responsible for biotic regulation during the AggradationPhase but also calls into action another set of mechanisms largely

quiescent during that phase. This idea is based, in part, on the set ofrelationships discussed in Chapter 3 and summarized in Figure 5-l.

_A_vailabilit~of~.~many~resaurcesincreases~ as-2Esul~f-clea~cutting++There is a greater availability of soil moisture throughout the total profilecaused by severely reduced transpiration. Removal of the canopy causesradiant-energy reaching the forest floor to be increased__b_y ~se?l_eraJ&rsof magnitude. Concentrations of dissolved substances in-the--soil solutionare increased about an order of magnitude owing to accelerated decom-position and mineralization resulting from warmer and moister soil.There is also an in~~_s_qin.-the. act_ivity oft nitrifyingorganisms ~related tothe removal of vegetation. H_elerotrophic processes may for a time exhibitpositive feedback (as shown in Figure 3-5) that raises levels of. de&n-position to a maximum the !&t-year after cutting, while mineralizationreaches its maximum~during the second year.

The cutover ecosystem responds to these conditions of increased 4resource availability by a burst-in primary-production (Figure 5-Z), not bbi’t- 7only by species that characterize the precutting forest but also by a group j G,-of species not part of the predisturbance forest (e.g.. buried-seed species) : : i;..that may have evolved specifically tofill..a_n~c~e_cr_eatebby thistype ,rT.3-..of disturbance (Chapter 3). Compared to the aggrading forest, primary 4, , .:prz&c:ivity for the recently cutover ecosystem swings through at least Jo ?,,

one very wide oscillation. Cutting reduces productivity to near-zero, but L,,_productivity rapidly rises and for a few years may exceed that of the uncut

J’i

aggrading forest. Thus, productivity is rapidly restored. and within a fewyears hydrologic and biogeochemical functions begin to approach thepredisturbance levels discussed in Chapter 2. r-, -.

&--contrast to the rapid changes in available nurri:ntK and waterin :rY2*4t& soil. increases in erodibility of the ecosystem are reJ3-tiyely slow.~o -~” -’--_.- ~- .~ .---develop (Chapter 3). Processes related to dead biomass maintain a high ‘r--’ 2,level of regulation over erodibility for about 2 years. The clear-cutting-and b -‘---.~.. -enforced-devegetation -experiment showed that these mechanisms begin +X-J zj : .

$~~~aken seriously only in the third growing season. In well-designed %clear-cuts with immediate revegetation, accelerated productivity prevents ’ FLserious erosion from ever occurring.

The c?gplicg -of. heterotrophic processes to autotrophic processes isfar from perfect. Considerable leakage of nutrients from the ecosystemoccurs during the first few years. even in immediately revegetatingclear-cut systems (Chapter 5). Such losses might be even higher if therelease of gaseous nitrogen were included. msuggests..that the-rapid*se-in ~productiyjty _$~a[ follo.ws disturbance is a relatively inefficienta~y~ and is costly in terms of nutrient and biomass storage-withinthe ecosystem, Ho~~,el-&;-, the sncr13ce of efjciemy results in wceieruted~Prodlrctiotz kc,hiclz may be considered m effectilie strrrtrgy of ecosystem sta-bib .siwe it fore.yta1l.r CI sfill-gr’euter- sc~t$ce irr biotic regdntion of erosion.

---- ~_- -_ i

{ c \ r f. ; ‘3,Ji.., :


The Forest Floor as a Regulator of Ecosystem Acticiry. T& forest floorws

a major role in-ecosystem homeostasis. Nutrient and energy costs duringthe critical period that immediately follows disturbance are paid bydrawing on nutrients and energy stored in dead biomass, primarily in theforest floor. In a sense. the forest floor functions as a “regulator” for-theecosystem, with a net storage of energy and nutrients during the first thirdof the Aggradation Phase and a net liberation of energy and nutrientsduring the Reorganization Phase. This would be an important corollary to-.._____~~ ~. _ _Margalef’s dictum (1968) that “biomass is the keeper of organtzatton,”except that in this case dead biomass serves this function.

Not all northern hardwood ecosystems have a forest floor of the mortype common to northern New Hampshire. Some have a characteristicmull-type soil, where most leaf litter that falls on the mineral soil israpidly decomposed, and the remainder is incorporated into the mineralsoil (i.e., the Al-horizon) by fauna1 activity (Lutz and Chandler, 1946;Bormann and Buell, 1964); mixing is due in large part to extraordinarilylarge populations of earthworms (Eaton and Chandler, 1942). Usually,mulls occur on richer sites and have a higher level of fertility than morsoils and are thought to sustain a high level of nitrification (Melillo, 1977).A basic question arises as to the behavior of organic matter in mull soils:Does it act as a regulator of ecosystem activity in the same way as organicmatter in mor-type soils?

The large amount of dead wood on the forest floor immediately after aclear-cut may function as a nutrient reservoir in a manner somewhatsimilar to the forest floor. Dead wood is probably far-lessimp.ort~antduring the critical years immediately after clear-cttting because -ofresistance to rapid decay. It seems likely that the principal importance-ofdead wood may be realized toward the middle and end of the-Reorgan-ization Phase, when the decomposition of slash would provide amodestsource of nutrients for new growth. Perhaps of more~I~~_qrtan_ce~e-~d.

- wood_prcvides a substrate for the nitrogen-fixing organisms-discussed inChapter 2.

The role of dead wood in ecosystem dynamics needs careful study,particularly since new whole-tree harvesting techniques completelyremove all aboveground wood from the clear-cut stand.

Linkage between Primary Productitlity a n d Heterotrophy. T h e r a p i d

L response of the ecosystem to clear-cutting may be considered in terms of! @erotrophic processes and primary-productivity linked by a number of

feedback-channels. Perturbation favors an increase in heterotrophy, and,m-fact, for a time vari,oussheterotrophic_ processes ~may be linked by

-positive feedback. Primary productivity is positively linked to hetero-trophic end products as well as to soil moisture and energy condrtronsresulting from destruction of the original vegetation. As the rate

ofi primary productivity increases. however, it exercises a negative-feedback

-*i -- control on heterotrophic processes. For example, we would expect:^ available soil moisture to drop as primary productivity and transpiration,


correspondingly, are increased and would expect nitrification to belimited as the vegetation reestablishes itself. Eventually, we would expectthe rate of primary productivity to decline as the special growth condi-tions that immediately follow disturbance give way to conditions thatcharacterize the intact aggrading system.

SUMMARY

1. Primary productivity of northern hardwood ecosystems recoversrapidly after clear-cutting. This rapid increase in productivity is largelybased on the growth response of exploitive species utilizing resourcesmade available by destruction of the living vegetation and accelerateddecomposition of the forest floor.

2. Biotic regulation of large losses of water and dissolved nutrients is ;-= s’established in&o-JIears after cutting, but losses may exceed those of ‘Yuncut forests for 10 to 20 years, or to about the end of the Reorganization

-. SePhase. ,. -, -->_^,3. Coupling between increased amounts of available nutrients after cutting J-. __

and uptake of nutrients by regrowing vegetation is far from complete.4. Comparison of.nutrient uptake plus loss of nutrients in drainage water

and estimated nutrient release from the forest floor suggests that a ij ; =_ ’large amount of nitrogen is unaccounted for and that considerable ?‘,*’cationleaching occurs from~themineral soil. Unaccounted for nitrogenloss from the forest floor may result from unmeasured denitrification

r - -Gcc

losses or from transport to the mineral soil. >$lP I*5. After a rapid early rise, p&ry productivity may decline at 6 or 7years >-.

after cutting but recover and rise for several decades thereafter. The cicc’ N I

rise may be due to a shift from species utilizing inefficient exploitive ,p-- //e,//

growth strategies to those using more efficient conservative strategies, 1/x ,:+.,-’gradual occupancy and utilization of deeper layers of soi1. and an increasein cation-exchange and water-holding capacity associated with the (

; .r -.-’

growth in mass of the forest floor.6. The Aggradation Phase of ecosystem development is the most highly 2,-?,, =.., .j

regulated of all phases in the developmental sequence. Begulatipn hasrlbi,;<, 1:a strong feedback component associated with the close relationslY =,, r

betweenxi-m.ary productivity and resource availability resulting from‘

the even-aged condition of the vegetation.c:/:;cI / T

7. Clear-cutting initiates a strong homeostatic response based on feed- “.:I*” ’back between’;.fa&drs controlling resource availability and those Tc, ~,I, I /

controlling primary productivity. This response brings exports of ijL y;..., I

nutrients and water under control relatively quickly and preventsserious erosion from ever occurring. :!

bE.,/e, I

cosystem recovery -after~Bisturbance-lsexpensive and is powered ‘:2.:>.- ,~

ii Primarily by energy and nutrients from the forest floor. The forest

floor acts as a regulator of ecosystem activity. storing energy andnutrients in times of quiescence and releasing them in times of stress.-

Documents

F. Herbert Bormann Gene E. Likens Pattern and Process in a Forested Ecosystemwebpage.pace.edu/dnabirahni/rahnidocs/law802/Chapter 5... · 2006-06-28 · 140 Pattern and Process in