Theory predicts that higher biodiversity in the tropics is maintained by specialized interactions among plants and their natural enemies that result in conspecific negative density dependence (CNDD). ...By using more than 3000 species and nearly 2.4 million trees across 24 forest plots worldwide, we show that global patterns in tree species diversity reflect not only stronger CNDD at tropical versus temperate latitudes but also a latitudinal shift in the relationship between CNDD and species abundance. CNDD was stronger for rare species at tropical versus temperate latitudes, potentially causing the persistence of greater numbers of rare species in the tropics. Our study reveals fundamental differences in the nature of local-scale biotic interactions that contribute to the maintenance of species diversity across temperate and tropical communities.
The angiosperm flora of Sri Lanka, which has been comparatively well studied by many eminent botanists for over two centuries, has a rich endemic component and also exhibits strong affinities to that ...of peninsular India. Some of the 15 floristic regions recognized in the island have exceptionally rich, highly localized floras. But increased forest disturbances over the years have taken a heavy toll. A plea is made to conserve at least one sufficiently large forest reserve in each floristic region. A meaningful conservation program has been launched in the Sinharaja MAB Reserve; similar programs are urgently needed for other protected areas of the island.
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.
QUESTION: What strategies are most appropriate for restoring tree seedling recruitment whilst avoiding the spread of invasive plant species in high conservation value tropical forests disturbed by ...planting a shade‐demanding crop? LOCATION: Knuckles Forest Reserve, Sri Lanka (7°21′–7°24′ N, 80°45′–80°48′ E). METHODS: An experiment was conducted to test the effects of clipping or removal of established cardamom plants on recruitment of native tree seedlings and spread of non‐native plants in a tropical montane forest with abandoned cardamom stands in the understorey. The number and composition of tree seedling emergents, the cover of herbaceous plants and the recovery of cardamom were assessed for 3 yr. RESULTS: Tree seedling recruitment was higher in plots from which above‐ground cardamom biomass had been removed through slashing (mean ± SE per 5 m²; 28.9 ± 2.70) and those where cardamom plants had been removed completely through uprooting (32.2 ± 3.17), or when dead cardamom leaves and stems were removed with small‐scale extraction of pods (22.5 ± 2.16), than in unmanipulated control plots (16.6 ± 1.13) over 15 mo. The species composition of tree seedling emergents did not differ in response to removal of cardamom. However, the cover of herbaceous plants, including the non‐native invasive Ageratina riparia, increased in response to removal of cardamom. Recovery of cardamom was higher when the plants had been slashed than when entire plants were uprooted and removed. CONCLUSION: Slashing or uprooting cardamom plants is a potential strategy for restoring tree seedling recruitment in forests with abandoned cardamom stands in the understorey, but these interventions would need to be repeated annually over many years to be successful, and they risk promoting expansion of the cover of herbaceous plants, including non‐native species. Hence, this approach would be labour‐intensive and costly. An alternative approach to promoting tree seedling emergence and establishment is to clear dead cardamom leaves and stems, and to encourage small‐scale extraction of pods from the residual cardamom plants. Harvesting pods reduces the likelihood that the crop will be sustained in situ through natural regeneration, and supplies an income to local communities, which would enhance the social acceptability of the intervention.
The relationship between β-diversity and latitude still remains to be a core question in ecology because of the lack of consensus between studies. One hypothesis for the lack of consensus between ...studies is that spatial scale changes the relationship between latitude and β-diversity. Here, we test this hypothesis using tree data from 15 large-scale forest plots (greater than or equal to 15 ha, diameter at breast height ≥ 1 cm) across a latitudinal gradient (3–30o) in the Asia-Pacific region. We found that the observed β-diversity decreased with increasing latitude when sampling local tree communities at small spatial scale (grain size ≤0.1 ha), but the observed β-diversity did not change with latitude when sampling at large spatial scales (greater than or equal to 0.25 ha). Differences in latitudinal β-diversity gradients across spatial scales were caused by pooled species richness (γ-diversity), which influenced observed β-diversity values at small spatial scales, but not at large spatial scales. Therefore, spatial scale changes the relationship between β-diversity, γ-diversity and latitude, and improving sample representativeness avoids the γ-dependence of β-diversity.
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.
► We investigated the effects of cardamom cultivation on high conservation value montane rain forests in central Sri Lanka. ► The density of trees, saplings and seedlings were lower in abandoned ...cardamom plantations than adjacent natural forests. ► Cardamom cultivation resulted in a shift in tree species composition with the addition of pioneer species. ► Cultivation increased concentrations of total P and exchangeable K in soil, but lowered total N. ► Cardamom cultivation has persistent impacts on forest that need to be mitigated.
The cultivation of cash crops in the understorey of tropical forests is an ancient practice, but the effects of cultivation on forest ecosystem processes are poorly understood. We assessed the effects of planting the high-value spice crop cardamom (Elettaria cardamomum) on forest structure, tree species composition, and soil properties in the montane forests of the Knuckles Forest Reserve in central Sri Lanka, where cardamom cultivation has been banned since 1985 because of the high conservation value of this site. Vegetation and soil were sampled in forest under-planted with cardamom and adjacent natural forests without planted cardamom. The densities of woody plants (⩾5cmdbh), saplings (<5cmdbh and ⩾1.5m tall), and seedlings (<1.5m tall) were lower in cardamom plantations than natural forests, while stand basal area was marginally higher in cardamom plantations. Canopy openness at 1.3m height was higher in the cardamom plantations (mean±SEM: 35±8%) than in the natural forests (19±3%). Pioneer tree species such as Macaranga sp. increased in abundance in cardamom plantations, and this contributed to the emergence of a difference in species composition between cardamom plantations and adjacent natural forests. Species richness of trees ⩾5cmdbh per plot was higher in natural forests than cardamom plantations, while species diversity was higher in cardamom plantations. The concentration of total N in top-soil was higher in natural forests, while concentrations of total P and exchangeable K were higher in the cardamom plantations. We conclude that cardamom cultivation results in a net loss of tree stems through weeding and opening of the canopy to promote cardamom production. The higher concentrations of total P and exchangeable K in the soil of cardamom plantations may be associated with the application of fertilizer, while total N concentration may have been higher in the natural forests because residual uncultivated forest occurs at a slightly higher elevation than the majority of cardamom plantations and/or because of elevated denitrification rates in the cardamom plantation. Since cardamom cultivation has affected forest structure and soil properties, management interventions may be required to mitigate these effects in high conservation value forests where cardamom cultivation has been banned.
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