Soil Enrichment by Neotropical Savanna Trees

Soil Enrichment by Neotropical Savanna TreesAuthor(s): Martin KellmanSource: Journal of Ecology, Vol. 67, No. 2 (Jul., 1979), pp. 565-577Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2259112 .

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Journal of Ecology (1979), 67, 565-577

SOIL ENRICHMENT BY NEOTROPICAL SAVANNA TREES

MARTIN KELLMAN

Department of Geography, York University, Downsview, Ontario, Canada M3J 1P3

SUMMARY

(1) The surface soils beneath trees of five species native to Neotropical savannas have been analysed for exchangeable calcium, magnesium, potassium and sodium, available phosphorus and total nitrogen.

(2) All trees showed preferential enrichment of the soil about them, in some cases to levels approaching or exceeding those found in nearby rain-forest soil.

(3) Enrichment has been achieved without deep rooting by the trees, indicating that the capture of precipitation inputs has been the major mineral-nutrient source.

(4) Neither increased cation-exchange-capacity nor increased moisture-retention- capacity in the soil around the trees can satisfactorily explain their more efficient capture of precipitation inputs. The gradual accumulation of mineral nutrients by persistent genets, and the incorporation of these into an enlarged plant-litter-soil nutrient cycle appears to offer the best explanation of the mechanism responsible.

(5) The creation of such enriched microsites may provide nuclei for the invasion of infertile savannas by rain-forest trees that appears to have recurred sporadically during Quaternary climatic oscillations.

INTRODUCTION

Recent evidence from palaeoecological, palaeoenvironmental and biogeographical sources indicates that many parts of the Neotropics have oscillated between rain forest and savanna during the Quaternary (Wymstra & Van der Hammen 1966; Haffer 1969; Nota 1969; Damuth & Fairbridge 1970; Vuilleumier 1971; Van der Hammen 1974). The work of Tsukada (1966) suggests that in the Peten area of Guatemala rain-forest invasion of savanna has been a very recent phenomenon, postdating Mayan occupation of that area, between 3000 and 1300 B.P. The extent of these transgressions suggests that widespread invasion of savannas by rain-forest, trees (as distinct from gradual advance of forest edges) must have occurred periodically during the Quaternary.

Tropical forests and savannas differ in a number of fundamental ways, including the quantities of mineral nutrients required by, and stored within, each. Forests charac- teristically contain large quantities of nutrients in living biomass, and occupy soils whose superficial horizons are moderately fertile. In contrast, many fewer nutrients are sequestered in the small biomass of savannas, and these communities often occupy soils of very low fertility. The processes by which nutrient-demanding forest trees can have repeatedly invaded these infertile savannas, and accumulated their appreciable nutrient capital, remain an enigma.

0022-0477/79/0700-0565$02.00 ?1979 Blackwell Scientific Publications



566 Soil enrichment by savanna trees

The Mountain Pine Ridge savanna of Belize (latitude 17?N, longitude 89?W) occupies highly weathered and exceptionally infertile ultisols derived from granite and meta- sedimentary parent materials (Romney 1959; Johnson & Chaffey 1973). In the western portion of the savanna, rain forest is restricted to the more fertile soils on adjacent limestones and to narrow strips of alluvial soil along the largest rivers. Fire, acting synergistically with low soil fertilities, may have maintained this savanna for at least 11 000 years (Kellman 1975). Within the last 25 years fire has been artificially excluded from the western portion of the savanna to ensure improved establishment by the commercially valuable Pinus caribaea Morelet. The area has thus become potentially habitable to many fire-sensitive rain-forest trees, and provides a suitable situation in which to observe the earliest stages of a rain-forest invasion of savanna.

The savanna consists of a dominant stratum of fire-tolerant grasses and sedges, within which occur scattered populations of broad-leaved trees and shrubs and two species of pine (P. caribaea and P. oocarpa Schiede). The broad-leaved trees and shrubs comprise a restricted flora, some members of which are widespread in Neotropical savannas. All are fire-tolerant, surviving fires either as large stems protected by thick bark or as buried rootstocks from which rapid re-sprouting occurs. Pine exists in the area as a post-fire opportunist (Hutchinson 1977). In 1936 Lundell (1940) reported that the area was occupied by open savanna with scattered trees. However, fire suppression in recent decades has been followed by a heavy invasion of young pine, as well as by an increased population density among the broad-leaved trees and shrubs (Kellman 1976).

Earlier work in the area has shown that most broad-leaved trees and shrubs have surprisingly high foliar-element levels (Kellman 1976). This led to the postulate that these plants may be preferentially enriching the soil beneath them, and thus providing nuclei for invasion of the savanna by rain-forest tree seedlings. The research described here is concerned with the first part of this hypothesis. It examines the degree to which some of the commoner savanna tree and shrub species are enriching the soil about them, and explores the sources of these nutrients.

METHODS

Five of the commoner broad-leaved species present in the Mountain Pine Ridge savanna were selected for analysis: the trees Byrsonima crassifolia (L.) Kunth, Clethra hondurensis Britton, Quercus schippii Standl. and Q. oleoides Cham. & Schlecht., and the melastomataceous shrub Miconia albicans (Sw.) Triana, which can ultimately reach small-tree proportions. Miconia and Clethra form compact single- or multi-stemmed plants. Byrsonima occurs either as individual stems or as a close aggregation of stems arising from an enlarged rootstock. Both Quercus spp. tend to spread clonally by root- suckering and to form small copses.

In an area of open savanna on a granite interfluve approximately 1 km distant from the nearest rain forest, a single isolated genet of each species was selected, and trunk diameter(s) and height measured (Table 1). Soil conditions beneath the Clethra, Miconia and two Quercus spp. were sampled along transects 6-7 m long, running from open savanna beyond the canopy to the main stem of each tree, or centre of each clone. Along this transect five to eight equally-spaced sample points were established. At each point a 0. 1-M2 sample of ground vegetation and litter was removed, dried at 60 ?C and weighed. Beneath this a 25-cm2 sample of the upper 5 cm of mineral soil was taken. At opposite ends of the transects associated with the three tree species, a soil pit was



MARTIN KELLMAN 567

TABLE 1. Trunk diameter (cm) and height (m) of the individual genets of the five tree species sampled

Trunk diameter Height

Quercus schippii* 4.5, 8.3, 9.2, 13.7 7 Q. oleoides* 4.8, 5.8, 5.4, 5.7, 8.6 7 Clethra hondurensis 24.5 12 Miconia albicans 2.9 2 Byrsonima crassifolia 2.7, 3.8, 4.5, 6-4, 10.8 5

* Stems within 2 m of sample transect only.

dug and samples taken at 5-15, 15-25 and 40-50 cm depths. Soil conditions beneath the chosen Byrsonima genet were sampled with a grid of forty-two points spaced 1.5 m apart (Fig. 3), and encompassing both open savanna and Byrsonima cover; some young Pinus caribaea saplings were included within the grid. At each sampling point, ground herbage + litter and surface soil samples were collected as before, and deeper soil samples were also taken from pits in the open savanna and at the base of the Byrsonima. At all sites after soil sampling, the main root system of each tree was excavated manually to establish rooting patterns and depths.

For comparison, a set of surface and sub-surface soil samples was collected at two rain-forest sites on valley alluvium within the savanna. One set of four samples was collected on a low terrace supporting well-developed rain forest. A similar set was collected beneath less well-developed forest on an adjacent high colluvial terrace down- slope of open savanna.

Soil samples were air-dried before being brought back to the laboratory, where each sample was homogenized, passed through a 2-mm sieve, and analysed for pH, organic carbon, total nitrogen, cation-exchange-capacity, exchangeable calcium, magnesium, potassium and sodium, available phosphorus and moisture retention at field capacity. Measurements of pH were made with a glass electrode using a soil to water ratio of 1: 2. Organic carbon was determined by the chromic acid method of Walkley & Black (Black 1965), and total nitrogen by the micro-Kjeldahl technique with salicylic acid modification to include nitrate and nitrite. Available phosphorus was extracted by Truog's procedure using a soil to extractant ratio of 1:100 (Jackson 1962), and the extracted phosphorus was determined colorimetrically. Exchangeable metallic cations were displaced with 1 N ammonium acetate at pH 7, and Ca, Mg, K and Na in the filtrate estimated by atomic-absorption spectrophotometry. Cation-exchange-capacity was determined by saturating the exchange complex with Na, using a sodium acetate solu- tion of pH 8.2, then extracting Na with ammonium acetate as described above. Per- centage moisture content at field capacity was determined gravimetrically, on samples brought to equilibrium on a semi-permeable ceramic plate maintained at a pressure differential of 1/3 bar.

RESULTS

Although it had been hoped that selection of trees in close proximity to each other would ensure that all five sites had similar soil profiles, this proved not to be the case. The soils beneath the two Quercus spp. and Miconia were structureless, of coarse quartz sand with a few iron concretions at 50 cm depth. Beneath the remaining species, a sandy clay surface soil overlay a heavier, mottled, sandy clay B horizon at c. 25 cm depth.




6 - Co Opene- --Conopy

0 6- K Open4- --Canopy

0 0 04 2 0 -

.5~~~~~~~~~~~~~~~~~~~~~~~O

'E 2- 0-2A

3 g , 00- N

0 2 40 0604 - 4

toa irgni uraesi ln h forsapetnec.O=Qursshii,

E E~ 0-02-

oL 01

15 P F04 0*3-N

10 0-2-

z 0 a-

5 0i-

0L

0

0 2 4 6 0 2 4 6

Distance along transect (in)

FIG. 1. Changes in exchangeable Ca, K, Mg and Na, and in available phosphorus and total nitrogen in surface soil along the four sample transects. 0 = Quercus schippii, O = Q. oleoides, A = Clethra hondurensis, V = Miconia albicans. Values for the low

terrace rain-forest site (D and the high terrace rain-forest site ? are also shown.



MARTIN KELLMAN 569 Because of these differences, 'treeless' v. 'tree-covered' soil properties could be compared only within the set of samples taken around each tree.

The results of soil analyses are presented in Figs 1-5 and Table 2. The data for herbage + litter weights and for soil pH, which are not included in these Figures and Tables, are tabulated in the Appendix.

Quercus schipp/i (m-equiv. per 1OOg) (8g g'l)

0 0-1 02 030 01 0-20 005 0*1 0 2 4

0- O _

20- Ca m g K P04

40-

Quercus oleoides

0 04 08 0 0.5 0 0.1 02 0 5

0- I

I /l I

20 - (Ca VIMg K P04

40- 4 Clethra hondurensis

0-

2 0 2 4 0 2 4 0 0 4 4

40- Rain-forest sites

0 2 4 6 0 1 0 0.2 0 4 0 10 20

L a MgK 0 20

-

40k

FIG. 2. Changes in mineral-nutrient concentrations with depth in the soils around three savanna trees and at the two rain-forest sites; , soil beneath open savanna; * *, soil beneath tree canopy; 0 0, low terrace rain-forest site; A A, high terrace

rain-forest site.




Soil enrichment

The concentration of mineral nutrients in surface soils along the four sample transects is shown in Fig. 1, together with the concentrations recorded in surface soil at the two rain-forest sites. In every instance, soil enrichment was recorded beneath the tree canopies. Calcium was increased 2-9-fold, magnesium 3-6-fold, potassium 2-5-fold, sodium 1.5-fold, phosphorus 2-fold and nitrogen 2-3-fold. Mineral-nutrient concentrations rose- from exceptionally low levels in open savanna to concentrations beneath trees that, in some instances, approached or even exceeded those of rain-forest soil. There were differences in the degree of enrichment achieved beneath different species. Quercus schippii had proportionately little effect, while Clethra hondurensis had a dramatic effect upon calcium and magnesium. This latter species occurred at a site with

r| T T T T 010 014 0|44 0 6 0 28 0 20 017

I- -I-c -, - -i- + >+ o-4- 012 026 047 0 67 0-25 0-52 0-18

0.15 020 0 30 0-50

I- ?.~' + ' 03 0 17\ 074 1 025 09 1

10 0 50 0 30 0-20

'0 I - t?' 0 i_, 10 018 043 35 019 0 18

L "'-4- '1' + -I-

-4-

022 018 028 0.19

0 27

02i

009

1 6 @8 3 4 2 34 2 - 015 014 020 015 | 024 017

(c) (d 2 7

1-8 23 3-7 5-3 3J: 3 1 0-15 0-24 0-23 0 29 0 20 035 0-20

2.0 3 0 5-0 020 0.30

.5 2i 5 8 0 4 38 24 0 19 0.15 0,29 0 47 0 26 028 0.17

1 5 21i 36 55 3 8 24 2-6 0.10 0-16 0.32 035 0 -46 0 23 019

0-30 0 20

20 2 2 9 2 4 1 29 2-2 0- 22 0 22 026 0 19 0 25 0.26 012

28 20 22 23 3.0 3 9 3 9 0-18 ~ 0-21 0 17 0 20 0 .24 0 26 0-241

0 3 6

m

FIG. 3. Maps of (a) canopy cover, (b) exchangeable calcium, (c) exchangeable magnesium and (d) 00 base saturation in the surface soil at the Byrsonima site. Values for Ca and Mg are m-equiv. per 100 g. Shaded area, Byrsonima canopy; pecked lines, pine sapling canopies. The high concentrations in the N.E. quadrant occurred at a small ants' nest.



MARnN KEaLMAN 571

ii _;- C'X<r*

a I W-A w

I~~~~~~~~ A

Pi L.(a) RootsystemsattheByrsoniasite.Thesmooth-barked stemsare of Byrsonima, and the platy-barked stems are of Pinus. Notice the absence of tap roots, preponderance of shallow laterals, and large, partially decayed, rootstocks in Byrsonima. The single massive tap root and the collar of shallow lateral roots that was characteristic of all the Pinus individuals can be seen at the right. (b) Partially excavated root system at the Quercus oleoldes site. Notice the dense mass of small roots that arises from, and overlies, the main laterals, below which few small roots are found; the small feeder roots are concentrated in the decaying litter and the uppermost layer of the mineral soil. (Photo-

graphs by M. Keilman.)




intrinsically higher soil fertility levels, and the existence of an ants' nest at the base of the trunk may have augmented nutrient accumulation there. Soil analyses from deeper soil horizons at opposite ends of the three 'tree transects' (Fig. 2) showed that any enrichment was essentially superficial. The results of comparable analyses at the two rain-forest sites are also shown.

Similar results were found at the Byrsonima site. Here surface-soil enrichment by calcium and magnesium was so consistent that it could be mapped (Fig. 3). At this site there were sufficient numbers of sample points beneath the three cover-types (open savanna, Byrsonima, pine) to allow significance levels to be assigned to the differences between means (Table 2). These comparisons showed that the soil beneath the Byrsonima cover had significantly (P < 0.05) higher concentrations of calcium, magnesium and potassium than did the soil of open savanna. However, the soil beneath Pinus caribaea saplings showed no significant differences from open savanna soil.

TABLE 2. Mean values (? S.E.) of chemical and physical properties of surface soils beneath savanna, Byrsonima and Pinus caribaea covers; significance of differences between savanna and Byrsonima, and savanna and Pinus, using a 2-tailed t test, are given (* = P < 0O05, ** = P < 0.01, NS = not significant)

Savanna Byrsonima Pinus (n =13) (n =6) (n = 9)

Exchangeable cations (m-equiv. per 100 g) Ca 0.21 ? 0.03 0.74 + 0.16** 0.19 ? 0.02NS Mg 0.20 ? 0.02 0.35 + 0.03** 0.20 ? 0.02Ns K 0.08 ? 0.01 0*10 + 0*004** 0*08 ? 0.01NS Na 0.035 + 0.003 0*033 + 0.01NS 0-037 ? 0.005NS

Available P04 (,tg per g) 2.40 ? 0.03 2.58 + 0.17NS 2.64 ? 0.28NS Cation-exchange-capacity

(m-equiv. per 100 g) 21.1 ? 0*81 22.6 + 0.73NS 19.9 ? 0.79NS Base-saturation (%y) 2.5 ? 9.22 5.3 + 0.69** 26 + 0.22NS Organic carbon (%y) 2.76 ? 0.10 3*24 + 0.22NS 2.66 + 0.19NS Moisture content at 1/3 bar (%4) 23*3 ? 089 23*5 + 0.72NS 20.5 ? 1.78NS

Nutrient sources It had originally been postulated that any preferential enrich-ment around savanna

trees, with the exception of nitrogen, would have been derived from the weathering zone at the base of the solum and been translocated to the surface by a deep root system. However, the excavation of twelve root systems (six pine, six broad-leaved tree) showed no sign of deep rooting in any of the broad-leaved trees. Only pine possessed large tap roots combined with a collar of shallow roots. Broad-leaved tree roots were concentrated at very shallow depth, and were rarely found below 30 cm (Plate 1(a)). In the Quercus oleoides clone, most large roots gave rise to an upper layer of small feeder roots concentrated in the decomposing litter (Plate l(b)). Lateral spread of root systems was also limited; an occasional long root spread into open savanna, but most were concentrated beneath the tree canopy.

From these observations it is concluded that the mineral nutrients concentrated beneath the trees cannot have been derived from weathering at depth, and are unlikely to have been drawn in from the soil below the surrounding open savanna. They must, therefore, have been derived from the capture of precipitation inputs.



MARTIN KELLMAN 573

Mechanisms of nutrient capture

Two mechanisms by which precipitation inputs could be preferentially captured at tree sites were examined: an increased cation-exchange-capacity in the soil beneath trees, and an increased soil moisture-retention-capacity at these sites.

A large proportion of the cation-exchange-capacity in the surface horizons of highly weathered tropical soils derives from organic colloids (Nye & Greenland 1960). Heavier litter falls beneath savanna trees could give rise to an increased cation-exchange-capacity there, leading to a greater adsorptive capacity for cations derived from precipitation. Changes in cation-exchange-capacity along the four transects sampled are shown in Fig. 4, together with percentage base saturation of the same samples. Although cation- exchange-capacities increased beneath trees, base saturation also increased there, most notably beneath Clethra. Moreover, base saturation, even beneath trees, remained at low levels. At the Byrsonima site, cation-exchange-capacity beneath the tree canopy was not significantly higher than that in open savanna, although percentage base saturation was (Table 2, Fig. 3).

Open.- -Canopy Open- -.Canopy 40 40e

0 0

l- 30 302

0) ~~~~~~~~~~~0

5 20- 0 20-

0

-0 10 10

0

0 2 4 6 0 2 4 6

Distance along transect (mi)

FIG. 4. Changes in cation-exchange-capacity and Y. base saturation in surface soil along the four sample transects. 0 = Quercus schippii; E = Q. oleoides; A = Clethra hondurensis; V = Miconia albicans. Values for the low terrace rain-forest site T and

the high terrace rain-forest site T are also shown.

From these data it is concluded that an increased cation-exchange-capacity beneath savanna trees is not a significant mechanism in mineral-nutrient capture. The soil of open savanna contains a small, but unsaturated, exchange complex; its failure to capture precipitation inputs effectively cannot be ascribed to an insufficient adsorptive capacity. The enrichment recorded beneath savanna trees has involved the filling of an already available, but unsaturated, exchange complex.




Open - Canopy

Open- -Canopy

6- 30-

0

C V~~~~~~~~~~~~~~~~~~C

0 ~ ~ ~ ~ ~ ~ ~ ~ ~~1

o 0

0 _ , I , . I I I 0 ? I I I '' I''

0 2 4 6 0 2 4 6

Distance along transect (m)

FIG. 5. Changes in organic carbon and moisture content at field capacity in surface soil along the four transects. 0 = Quercus schippil; E = Q. oleoides; A = Clethra hondurensis; V = Miconia albicans. Values for the low terrace rain-forest site (D and

the high terrace rain-forest site (2) are also shown.

Increased moisture-retention-capacity produced by higher organic-matter contents in the soils beneath trees could provide a further capture mechanism. In Fig. 5 levels of organic carbon and soil moisture retention at field capacity (1/3 bar) are shown along the sample transects. Although the percentage of organic carbon increased beneath the trees to levels comparable to those beneath rain forest, soil moisture-retention-capacity showed no distinct trend. At the Byrsonima site neither organic carbon nor moisture retention capacity showed a significant increase beneath the tree cover (Table 2).

Organic-matter contents are apparently too low in all instances to have a significant effect upon moisture-retention-capacity. These data indicate that the preferential capture of precipitation nutrient inputs beneath savanna trees cannot be ascribed to an improved soil moisture-retention-capacity there.

DISCUSSION

The mechanism responsible for the preferential nutrient capture by savanna trees remains unspecified. I suggest that this can be ascribed to the ability of these trees gradually to establish an enlarged plant-litter-soil nutrient cycle within the savanna as a result of prolonged persistence at one site. Savanna trees, although slow-growing, can ultimately achieve a biomass far in excess of herbaceous savanna plants. Insofar as litter and herbage weights increase beneath each tree (Appendix), and foliar nutrient contents are high (Kellman 1976), an appreciable sequestering of nutrients must be achieved in these compartments. This is dramatically demonstrated after savanna fires, when the thick ash layers around savanna trees (notably Clethra) contrast with the almost ashless residue of burned open savanna. The trees may also be mycorrhizal and be capable of efficient nutrient reabsorption. Given sufficient time, an enriched soil microsite may be the inevitable result of these biological accumulations of mineral nutrients.



MARTIN KELLMAN 575

The enrichment measured in surface soil about these trees thus represents only one component of the total site enrichment. Although elements sequestered in the litter and standing biomass compartments would not be immediately available to any establishing rain-forest tree seedlings, they could ultimately become so, as litter decay progressed and saplings overtopped and ultimately suppressed their savanna nurse trees.

The results suggest that precipitation may provide an important source of nutrients to initially infertile ecosystems. While the annual input from this source may be small, it is presumably relatively constant and in a form readily available for plant uptake. Such an effect would be augmented if isolated trees were also able to intercept greater than average quantities of precipitation (cf. Glover, Glover & Gwynne 1962). Harcombe (1977) has drawn similar conclusions on the basis of work on secondary succession in Costa Rica. However, it is clear that not all plants can capture these inputs efficiently. Within the savanna community, grasses and sedges were ineffective in this capacity, as were young pine. In contrast, the savanna trees and shrubs represented a unique group, capable both of withstanding the rigours of the savanna environment and of acting as nutrient sinks. Provided that such species are available to act as a nurse crop, there is no reason to suppose that the nutrient capital of rain forest cannot ultimately be re-accumulated upon infertile savanna soils.

The time required for significant nutrient accumulation in the soil about these trees cannot be reliably estimated. Tree and shrub genets are probably much older than any of their extant stems, but one can assign no absolute date to these. Excavated rootstocks showed signs of concomitant growth and decay, precluding the use of 14C to date these. Harcombe (1977) has measured rainfall inputs in Costa Rica, and estimated that the nutrient capital of a rain forest could be accumulated in 250 years if complete capture was achieved. This suggests that accumulation may be quite rapid if capture of precipitation is efficient.

The sites around savanna trees may possess conditions conducive to rain-forest tree establishment additional to those provided by soil enrichment. The trees are likely to be preferential resting and nesting sites for animals dispersing rain-forest seed into savanna, and the areas beneath the trees may thus receive preferential seed inputs. Seed and seedling predation may be lower in isolated savanna clumps than in closed forest, while the savanna trees may act as mycorrhizal sources to establishing seedlings. In combination with improved mineral nutrient levels, these effects may contribute to widespread diffuse invasions of savanna by rain-forest plants at the nuclei provided by savanna trees.

Confirmation that the enriched soils about savanna trees provide nuclei for rain- forest tree seedling invasion will require both the demonstration that these seedlings are sensitive to the soil fertility gradients measured, and that preferential establishment is actually taking place beneath them. While the first condition is currently being examined experimentally, the second will require field observations after longer periods of fire suppression than have yet elapsed in this area. However, a concentration of seedlings of one small tree, Xylopia frutescens Aubl., was observed around many savanna trees during the course of soil sampling. The species is characteristic of the thicket forest that occurs on the edges of savanna, and appears to be transitional to rain forest. It possesses high foliar contents of calcium and is presumably nutrient-demanding; its fruit is a red berry and the seed is presumably bird-dispersed. This behaviour of Xylopia is suggestive of the rain-forest invasion model postulated.




ACKNOWLEDGMENTS

Assistance in the field by Ian Napier and other members of the Belize Forestry Depart- ment is gratefully acknowledged. Soil chemical analyses were performed by Kandiah Sanmugadas of York University, whose advice on technical procedures and patience in carrying out 837 analyses has been much appreciated. The research has been supported, in part, by the National Research Council of Canada.

REFERENCES Black, C. A. (Ed.) (1965). Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties.

American Society of Agronomy, Madison. Damuth, J. E. & Fairbridge, R. W. (1970). Equatorial Atlantic deep-sea arkosic sands and ice-age

aridity in tropical South America. Bulletin of the Geological Society of America, 81, 189-206. Furley, P. A. (1974). Soil-slope-plant relationships in the Northern Maya Mountains, Belize, Central

America. II. The sequence over phyllites and granites. Journal of Biogeography, 1, 263-279. Glover, P. E., Glover, J. & Gwynne, M. D. (1962). Light rainfall and plant survival in East Africa. II

Dry grassland vegetation. Journal of Ecology, 50, 199-206. Haffer, J. (1969). Speciation in Amazonian forest birds. Science, New York, 165, 131-137. Harcombe, P. A. (1977). Nutrient accumulation by vegetation during the first year of recovery of a

tropical forest ecosystem. Recovery and Restoration of Damaged Ecosystems (Ed. by J. Cairns & K. L. Dickson), pp. 347-378. University Press of Virginia, Charlottesville.

Hutchinson, I. (1977). Ecological modelling and the stand dynamics of Pinus caribaea in Mountain Pine Ridge, Belize. Ph.D. thesis, Simon Fraser University, British Columbia.

Jackson, M. L. (1962). Soil Chemical Analysis. Constable, London. Johnson, M. S. & Chaffey, D. R. (1973). A Forest Inventory of Part of the Mountain Pine Ridge,

Belize. Land Resources Study No. 13. Land Resources Division, Surbiton, England. Kellman, M. (1975). Evidence for Late Glacial Age fire in a tropical montane savanna. Journal of

Biogeography, 2, 57-63. Kellman, M. (1976). Broadleaved species interference with Pinus caribaea in a managed pine savanna.

Commonwealth Forestry Review, 55, 229-245. Lundell, C. L. (1940). Botany of the Maya Area: Miscellaneous Papers XIV. The 1936 Michigan-

Carnegie Botanical Expedition to British Honduras. Publication No. 522. Carnegie Institution, Washington, D.C.

Nota, D. J. G. (1969). Geomorphology and sediments of western Surinam shelf; a preliminary note. Geologie en Mijnbouw, 48, 185-188.

Nye, P. H. & Greenland, D. J. (1960). The Soil under Shifting Cultivation. Technical Communication No. 51. Commonwealth Bureau of Soils, Harpenden.

Romney, D. H. (Ed.) (1959). Land in British Honduras. Report of the British Honduras Land Use Survey Team. Colonial Research Publication No. 24. H.M.S.O., London.

Tsukada, M. (1966). The pollen sequence. Memoirs of the Connecticut Academy of Arts and Sciences, 17, 63-66.

Van der Hammen, T. (1974). The Pleistocene changes of vegetation and climate in tropical South America. Journal of Biogeography, 1, 3-26.

Vuilluemier, B. S. (1971). Pleistocene changes in fauna and flora of South America. Science, New York, 173, 771-780.

Wymstra, T. A. & Van der Hammen, T. (1966). Palynological data on the history of tropical savannas in northern South America. Overdruk uit Leidse Geologische Mededelingen, 38, 71-90.

(Received 4 July 1978)

APPENDIX

Data on soil pH and litter + herbage weights (g m2) not included in Figs 1-5 and Table 2. Values for sample points beneath tree or shrub canopies are shown in bold- face, those on canopy margins in parentheses.



MARTIN KELLMAN 577

Surface-soil pH

Quercus schippii 5.25 5.60 5.20 (5.20) 5.15 5.40 4.35 4.50 Q. oleoides 4.95 4.90 5.05 (4.95) 4.70 4.70 4.60 4.60 Clethra hondurensis 5.10 5.10 4.95 (5.40) 5.20 5.10 5.10 Miconia albicans 5.00 5.30 (5.30) 5.35 5.40 5.40 Byrsonima crassifolia 4.90 5.20 5.20 5.00 4.80 5-00 5-00

4.90 5.10 5 20 5.00 (5.00) 4.60 5.20 4.80 (5.20) 5.20 4.90 4.80 5.20 4.90 5.10 5.10 5.20 5.20 4*60 (5.10) 5.10 4.90 5.10 5.00 (4.80) 5.20 5.10 4.90 4.90 5.10 5.00 4.90 5.30 5.20 4.90

Litter + herbage dry weights

Quercus schippii 490 580 790 (860) 930 780 1310 1730 Q. oleoides 320 340 390 (940) 1090 970 900 1000 Clethra hondurensis 380 490 520 (550) 810 1000 1630 Miconia albicans 400 400 (550) 1050 1070 630 Byrsonima crassifolia 840 570 670 840 960 850 690

360 1150 750 1010 (880) 2170 1290 430 (910) 1270 1540 1230 980 1500 490 550 880 1250 1050 (1120) 870 420 970 900 (970) 1860 1590 910

1390 790 490 730 760 930 1180



Documents

Soil Enrichment by Neotropical Savanna Trees