Martin Kellman

Maintenance of rain forest diversity in riparian forests of tropical savannas: implications for species conservation during Pleistocene drought

Structural and floristic attributes of riparian forest patches in a Neotropical savanna were studied in order to evaluate their potential to maintain rain forest plant diversity during periods of drought. Tree density was significantly greater, and basal area significantly lower, than pantropical means for these variables, thus these riparian forests were characterized as low biomass, many-stemmed communities. A total of 292 species ? 0.5 m high were found in an aggregate vegetation sample of 1.6 ha. Most species were typical rain forest taxa, indicating that this flora is not specialized to the riparian environment. Isolation of these patches was found to have some negative effects on species composition, since dioecious and mammal dispersed species were more poorly represented in riparian than in continuous forests. While this riparian system is not floristically depauperate, species densi- ties in individual plots indicate that its richness is comparable to that of continuous forests in Central America. These results suggest that tropical riparian forest fragments have the poten- tial, albeit limited, to maintain large numbers of rain forest species and, thus, that analogous forest fragments may have played an important role as safe sites for tropical rain forest floras during periods of drought in the Pleistocene.

The understorey light regime and patterns of tree seedlings in tropical riparian forest patches

The understorey light regime and its influence on forest tree seedling distribution was examined in isolated riparian forest patches where large numbers of co-existing species have been found. Understorey light intensity was highest at the forest edge, approaching 13% of full sunlight, but rapidly decreased towards the forest interior. By 7- 12 m, light levels had stabilized at <2.0%, comparable to light levels in the interior of continuous tropical forest. Total seedling densities were not correlated with variations in understorey light intensity, suggesting that rates of germination and establishment were similar throughout the forest. However, examination of seedling light sensitivity among six common riparian tree species revealed differenc- es in distribution: five species had higher seedling abundance in the darker forest interior or at the brighter forest-patch edge while one species was indifferent to light variation. These results suggest that the riparian forest light regime has sufficient variation to support several regenera- tion strategies, despite the small patch sizes.

Temporal Variability in Atmospheric Nutrient Influx to a Tropical Ecosystem

The daily input of plant macro-nutrients in bulk precipitation was measured over a 16-month period at Siguatepeque, Honduras. Total annual inputs of all elements were small relative to those recorded elsewhere in the tropics, a difference which is attributed to the low rainfall of the area, its elevation, and its remoteness from the ocean. Input patterns were highly irregular with most of the annual influx of each element taking place on only a few rain days. A major influx of all elements was recorded at the start of each wet season, and one period of exceptionally high cation input during the wet season is tentatively attributed to volcanic activity. Despite the irregularity of nutrient influx, calculation of a daily water balance for the area shows that a large proportion of elements received can normally be retained temporarily in soil-moisture storage. However, effective capture of these by ecosystems probably requires rapid plant uptake. We suggest that plants occupying infertile tropical soils may be strongly selected for the ability to absorb irregular pulses of atmospheric nutrients.

The effects of forest fragmentation on woodlot tree biotas in Southern Ontario

Certain predictions of island biogeographic theory were tested in Southern Ontario woodlots. The tree populations (> 4 cm DBH) of ten old growth woodlots were censused and each species in every woodlot assigned to one of three demographic classes (persisting, invading, going extinct). The domi- nant pattern of flux in all woodlots was that of species extinction, as predicted by island biogeographic theory. However, those species going extinct were mainly shade-intolerant, suggesting that succession, rather than forest fragmen- tation, may have been responsible for their disappearance. This was confirmed by multiple correlation analyses that showed no effects of area or isolation upon the numbers of species going extinct or upon the number of species persisting in woodlots. Instead, the results indicated that woodlots containing more species were losing more and that this loss was most severe in topographically diverse woodlots. The latter effect was interpreted to be the result of decreased illumi- nation in these woodlots, due to mutual shading of adjacent slopes by canopy trees. The results suggest that disturbance is an important species enrichment mechanism in these forest communities.

Preliminary Seed Budgets for Two Plant Communities in Coastal British Columbia

Production, movement and storage of seed in an old growth coniferous forest and adjacent secondary community in British Columbia, Canada, were monitored for 3 years. The old growth forest possessed a small seed budget that included some seed of secondary species, characteristic of disturbed situations. The secondary community possessed a far larger seed budget, dominated by locally-growing secondary species. Small quantities of seed of secondary species were able to infiltrate appreciable distances into the old growth forest. However, the store of seed of these species within the forest was insufficient to account for the large populations that soon appear at logged sites, unless initial populations deriving from this source had undergone very rapid expansion thereafter. The development of rotational tree harvesting in the area is likely to promote an expanding seed budget for secondary species, and a diminishing one for the primary species, characteristic of old growth forests.

NUTRIENT FLUX AND RETENTION IN A TROPICAL SAND-DUNE SUCCESSION

SUMMARY (1) The input to soils of nutrients in solution, and the output from them in percolating water, have been computed over a three-month period in four stages of a sand-dune succession in Veracruz, Mexico. The first three stages occur upon recent sand and represent a succession from pioneer communities to forest. The fourth is forest on a much older fossil dune. (2) The quantity of nutrients entering the soil increased from the pioneer to the forest community on recent sand, but outputs from these systems increased even more rapidly. This declining effectiveness of retention is attributed to mobilization of cations by increased quantities of bicarbonate anions beneath the larger biomass communities. (3) On the fossil dune, nutrient outputs in percolating water were much lower despite large inputs and comparable water fluxes. This is tentatively attributed to soil changes induced by weathering, especially lower pH and the development of an anion exchange capacity. (4) These data suggest that biological mechanisms of nutrient retention may be relatively ineffective at conserving nutrients unless coupled to a soil adsorption system that temporarily suppresses acute leaching in percolating water.

The effect of water supply on tree seed germination and seedling survival in a tropical seasonal forest in Veracruz, Mexico

In tropical seasonal forests, mass seedling death may incidentally occur early in the rainy season following untimely germination provoked by an isolated heavy rainfall. We hypothesized that, in a soil with little moisture retention capacity, irregular rainfall patterns would result in large fluctuations of water supply and, in the early wet season, drought episodes could cause seedling death. This hypothesis was tested in a seasonal tropical forest in Mexico where seedling density is low and the soil consists of almost pure sand with a low water retention capacity. Various patterns of simulated rainfall were applied in the forest to seeds and seedlings of three common tree species. (...)

Disturbance and tree species coexistence in tropical riparian forest fragments

Patterns of past fire incursions and tree falls were documented in a tropical riparian forest. Fire-scarred trees were concentrated on gentler slopes near the savanna:forest boundary, while tree fall sites were of highest frequency on convex upper valley slopes. Most tree falls were in a down-slope direction while the remainder were concentrated in directions that suggested an origin in extreme winds of atypical orientation. Tree species response to these two forms of disturbance was evaluated by assuming that the zones identified were inherently more disturbanceprone, and comparing the tree populations within and beyond the areas identified. Fire incursions had a larger measurable effect in augmenting species richness: 19% of species evaluated were significantly more abundant in burned zones, while only 8% of those evaluated were more abundant in tree fall zones. This difference is tentatively attributed to fire having a larger proximal effect on tree species recruitment as well as a spatially more persistent pattern of occurrence. Observations made at sites of recent fire incursion indicated that this caused little tree death or canopy opening but eliminated seedlings, saplings, litter and root mats: it thus created new seedbed conditions without the high light levels that would promote herbaceous establishment. Both disturbances play an augmentative, rather than exclusive, role in promoting species coexistence in these forests: they provide a varied micro-environment for seedling establishment but their patchy distribution ensures that disturbance-sensitive species can persist elsewhere in the forest patch.

Seasonal Pattern of Fine Root Proliferation in a Tropical Dry Forest

The growth of fine roots (≤ 2 mm diameter) in a tropical dry forest on recent dune sand has been monitored for 2 years during the dry season-wet season transition, using root-ingrowth bags buried for short time intervals. In both years, most root growth was concentrated in one brief period after the beginning of rains, and it is estimated that the fine root system of the forest can be fully re-deployed before the start of water percolation through the soil. In an accompanying field experiment, root-ingrowth bags were protected from rainfall and treated with distilled water or simulated throughfall solution (...)

Nutrient retention by savanna ecosystems ii. retention after fire

(1) The daily patterns of precipitation, soil water movement, and the concentration of nutrient elements in soil solutions at five depths were examined beneath three types of savanna in Belize, Central America, after burning. (2) Large increases in the concentration of some elements were recorded in the rooting zones in all types, but these disappeared within 1 week. Despite much percolation, no comparably large increases were recorded in solution concentrations deeper in the soil. (3) A comparison of the mean element concentrations after this post-fire peak with concentrations before the burn, and during an equivalent period 1 year earlier. showed many significant increases in concentration in the topsoil, but few increases in deeper soil. Ca and P appear to have been most effectively retained, Mg and K less so, and Na the least. (4) The results confirm the resistance of the three savanna types to acute losses of nutrients due to leaching after fire, and it is suggested that greater losses are likely to result from surface run-off during rainstorms. However, repeated burning has not significantly reduced the fertility of the surface soil, but has significantlv increased the amounts of Ca and Mg in the soil exchange complex.