Neutral and niche theories give contrasting explanations for the maintenance of tropical tree species diversity. Both have some empirical support, but methods to disentangle their effects have not ...yet been developed. We applied a statistical measure of spatial structure to data from 14 large tropical forest plots to test a prediction of niche theory that is incompatible with neutral theory: that species in heterogeneous environments should separate out in space according to their niche preferences. We chose plots across a range of topographic heterogeneity, and tested whether pairwise spatial associations among species were more variable in more heterogeneous sites. We found strong support for this prediction, based on a strong positive relationship between variance in the spatial structure of species pairs and topographic heterogeneity across sites. We interpret this pattern as evidence of pervasive niche differentiation, which increases in importance with increasing environmental heterogeneity.
Among the local processes that determine species diversity in ecological communities, fluctuation‐dependent mechanisms that are mediated by temporal variability in the abundances of species ...populations have received significant attention. Higher temporal variability in the abundances of species populations can increase the strength of temporal niche partitioning but can also increase the risk of species extinctions, such that the net effect on species coexistence is not clear. We quantified this temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. A fitted mechanistic model showed that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal variability in the abundances of species populations has no clear negative or positive contribution to the latitudinal gradient in tree species richness.
We quantified temporal population variability for tree species in 21 large forest plots and found much greater variability for higher latitude plots with fewer tree species. In addition, we used a fitted mechanistic model to show that among the forest plots, the net effect of temporal population variability on tree species coexistence was usually negative, but sometimes positive or negligible. Therefore, our results suggest that temporal population variability has no clear negative or positive contribution to the latitudinal gradient in tree species richness.
Most of the world's forest has been cleared, cultivated, and then often abandoned. In many instances these areas have changed to successionally arrested grasslands, shrublands, or fernlands ...maintained by frequent fires and high herbivore populations. Many studies have shown that various herbaceous, nitrogen‐fixing legumes can protect soil surfaces, retain soil moisture, improve soil fertility, and retard ground fires. Our objective was to ascertain if some of these species can potentially inhibit herbivory and satisfactorily establish in these arrested grassland areas to serve as sites for reforestation. We evaluated the potential for four species of nitrogen‐fixing legumes (Calapogonium mucunoides, Centrosema pubescens, Desmodium ovalifolium, and Pueraria phaseoloides) to establish on exposed soil within successionally arrested grasslands of Panicum maximum and Cymbopogon nardus in the central hills of Sri Lanka. Four different sites within rectangular grassland areas were cleared of graminoids and sown with seed of each legume. Half of each clearing was protected from browsing rabbits and porcupines, and half was not protected. After 6 months, certain plots were destructively sampled to determine dry biomass gain for each species and treatment. Analyses of variance were performed to test for differences among sites, treatments, and species. All three factors revealed differences, indicating that species must be matched to site. On sites with high amounts of herbivory, D. ovalifolium had the greatest dry biomass gain after 6 months of growth, possibly because of its relatively low nitrogen and moisture content. Where herbivory was absent, P. phaseoloides and C. muconoides had the greatest dry biomass gain. Dry biomass gain of all four legume ground covers was low on sites with lowest pH and nutrient concentrations. Under conditions of low relative fertility and low pH, establishment of the tested legumes failed. Though soil moisture availability was not measured, we speculate that these low fertility sites were also prone to drought. Findings support the site‐specific establishment of legume species for purposes of reforestation and watershed protection in central Sri Lanka. This work is applicable to other regions particularly dominated by successionally arrested grasslands with similar circumstances in other parts of south and southeast Asia.
In this study four species of the genus Shorea section Doom were investigated. All occur together as canopy trees in the Sinaraja rainforest of south-west Sri Lanka. Partitioning of the regeneration ...niche can be one explanation for the co-existence of ecologically similar canopy tree species within a forest. Seedlings were planted in plots located in five zones that represent a range of forest groundstorey microenvironments found adjacent to and across canopy openings of three sites – valley, midslope, ridgetop. Experiments were designed to monitor survíval and growth of planted seedlings for two years. At the end of two years percentage survival was calculated, height increment recorded and destructive samples taken to measure dry mass gain of root, stem and leaves. Comparisons were made of establishment and growth performance of seedlings planted in the different plots and sites. Results demonstrated clear differences in survival and growth among species. These differences appeared related to availability of soil moisture and groundstorey radiation regimes. Disturbance patterns that determine species co-existence are suggested.
Performance of seedlings of seven rain forest, canopy dominant Shorea species was studied in a transplant experiment in forest sites at three different elevations (low, mid and high) within the humid ...zone of southwest Sri Lanka. Five species generally inhabit low- to mid-elevations, one at mid- and lower montane elevations, and one exclusively at lower montane elevations. Temperature, rainfall and cloudiness varied with elevation. For each site seedlings were grown in pots under partial shade conditions using similar soils and evermoist conditions. All growth measures showed differences among elevation sites, among species and in the interaction between species and elevation sites. Performances of species collectively showed (i) decline in height and leaf number with increase in elevation, (ii) higher dry mass at low- and mid-elevation sites compared to that at high-elevation and (iii) a higher mass of single leaves at the mid-elevation site than at the high-elevation site. Rank order of species changed across elevations for both height and dry mass. Dry mass declined with elevation in four of the seven species studied. S. gardneri, the only exclusively lower montane species, increased dry mass with elevation. Height declined with elevation for six of the species with only S. gardneri showing no change. Changes with elevation in the rank order of species for total leaf number and mass of single leaves were small. However, total leaf number and masses of single leaves differed among species and among elevations. S. megistophylla and S. disticha had a few leaves with high individual masses, while S. gardneri, S. affinis and S. trapezifolia had many leaves with less mass per individual leaf. One group of species showed relatively little change in leaf number per seedling and large changes in mass of single leaves. The other group varied more in leaf number but mass of individual leaves remained constant. Growth allocation to leaf production versus individual leaf size appears related to the successional division of Shorea section Doona. Also all species grew better at the low-elevation site irrespective of their natural ranges except S. gardneri, whose natural range is restricted to high elevations, and exhibits markedly lower growth responsiveness than the other wide ranging species.
Lowland and mid-elevational rain forests of Sri Lanka harbor a relict, endemic-rich flora which is also rich in timber and non-timber forest resources. These forests supply nearly half the total wood ...requirements of the country and are dwindling rapidly; management of the forests to conserve the biological richness and maintain environmental services is therefore a difficult proposition. This predicament is further compounded by the dependency of rural people on a range of non-timber forest resources for their subsistence and income. A forestry master plan recently prepared for Sri Lanka has not given adequate recognition to the important role played by non-timber forest resources in rural livelihood. This oversight is primarily due to the lack of quantitative information to justify the role of non-timber forest resources in forestry sector development. Forestry policies that ignore these resources often anger local people, leading the villagers to vent their disapproval through destructive actions such as burning timber plantations. Long-term interdisciplinary research in ecology, reproductive and soil biology, ethnobiology, silviculture, rural sociology and resource economics in progress at Sinharaja attempts to address these questions of sustainable development of forest resources in an integrated mode. While research oriented toward conservation investigates both short- and long-term ecosystem dynamics in natural and modified forest stands, utilization-oriented research probes the impact of increased human disturbance, particularly the impact of rural communities on dwindling forest resources, as well as the effect of forest conservation on rural livelihood. These studies continue to seek alternative methods of forest management which are socially acceptable, economically viable, and ecologically sustainable for multiple uses. These methods will assist in the refinement of current forest policies, forestry planning, and the implementation of new policies and plans in Sri Lanka.
A comparative study of litter decomposition in a tropical lowland rain forest and an adjacent fernland using leaf litter from a dominant forest tree species, Cullenia ceylanica (Bombacaceae), and ...frond litter of the most abundant fernland species, Dicranopteris linearis (Gleicheniaceae), revealed significant differences in percentage mass lost between the two litter types and between the two sites: C. ceylanica leaf litter was reduced by 63 percent in one year in the forest vs 41.7 percent in the fernland, whereas D. linearis frond litter was reduced by 24.6 percent during the same period in the forest vs 15.7 percent in the fernland. A positive correlation with the amount of rainfall was observed for the mass loss of C. ceylanica litter in both sites; litter of D. linearis showed no such correlation with rainfall. Nutrient changes examined over one year indicated immobilization of nitrogen in both litter types in both sites and that of phosphorus only in D. linearis. Concentration and mass of K, Mg, and Ca decreased with decomposition in all four experimental treatments. A relatively slow rate of decomposition and an accumulation of organic matter in the fernland contribute to the conservation of nutrients in this aggrading ecosystem.
Data on the reproductive biology of the Annonaceae are rather fragmentary, particularly for paleotropical species. The pollination ecology and breeding system of the Sri Lankan endemicXylopia ...championii(Annonaceae) are described in detail. The pollination ecology was investigated using a diverse range of approaches, including (1) observations of flower‐level and population‐level phenology, (2) assessments of floral visitors and effective pollinators, (3) monitoring of floral temperaturein situusing a digital data logger, and (4) analysis of scent chemistry using solid‐phase microextraction sampling and gas chromatography–mass spectrometry identification of volatiles. The breeding system was evaluated using pollen/ovule ratios and field‐based controlled‐pollination experiments. Intrafloral dichogamy (protogyny) occurs over a 2‐d period, with a reproductively inactive phase between the pistillate and staminate phases, although there is no evidence of interfloral dichogamy. The inner petals close to form a pollination chamber during the reproductively active phases. The flowers are pollinated by a species ofEndaeusweevil (Coleoptera: Curculionidae). Floral chamber temperatures are elevated to 8°C above ambient levels. The floral scent contains a combination of volatiles that have previously been observed in fruits and other flowers and that possibly mimic insect pheromones.Xylopia championiihas an essentially xenogamous breeding system, promoted by protogyny. AlthoughX. championiipossesses numerous clear adaptations for cantharophily, there is no evidence for a species‐specific interaction. The beetles are attracted to the flowers by strong scents; rewards offered to the beetles include heat energy and protection from predators. Low levels of fruit set in natural conditions suggest that pollinator availability may be a limiting factor.
Nitrogenase activity in the surface soil and litter layers from a tropical lowland rain forest and an adjacent fernland in Sri Lanka was measured by the acetylene reduction method. The total amount ...of acetylene reduced, a measure of biological nitrogen fixation, was greater in the fernland (38.4 µmol m−2 h−1) than in the forest (15.3–29.2 µmol m−2 h−1). In both ecosystems the nitrogenase activity was greater (59–85%) in the soil than in the litter layer. The rates of acetylene reduction in soil and litter showed an inverse relationship with the decay constants of mixed litter and a positive relationship was found between the rate of acetylene reduction and the C:extractable nitrogen ratio of the soil. The relatively greater rate of nitro genase activity coupled with the slower rate of litter decomposition in the fernland indicates that accumulation of nitrogen may result in the soil and litter layers of the deforested areas during their aggrading phases.
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