1. This article reviews the application of some summary statistics from current theory of spatial point processes for extracting information from spatial patterns of plants. Theoretical measures and ...issues connected with their estimation are described. Results are illustrated in the context of specific ecological questions about spatial patterns of trees in two forests. 2. The pair correlation function, related to Ripley's K function, provides a formal measure of the density of neighbouring plants and makes precise the general notion of a 'plant's-eye' view of a community. The pair correlation function can also be used to describe spatial relationships of neighbouring plants with different qualitative properties, such as species identity and size class. 3. The mark correlation function can be used to describe the spatial relationships of quantitative measures (e.g. biomass). We discuss two types of correlation function for quantitative marks. Applying these functions to the distribution of biomass in a temperate forest, it is shown that the spatial pattern of biomass is uncoupled from the spatial pattern of plant locations. 4. The inhomogeneous pair correlation function enables first-order heterogeneity in the environment to be removed from second-order spatial statistics. We illustrate this for a tree species in a forest of high topographic heterogeneity and show that spatial aggregation remains after allowing for spatial variation in density. An alternative method, the master function, takes a weighted average of homogeneous pair correlation functions computed in subareas; when applied to the same data and compared with the former method, the spatial aggregations are smaller in size. 5. Synthesis. These spatial statistics, especially those derived from pair densities, will help ecologists to extract important ecological information from intricate spatially correlated plants in populations and communities.
Importance of Demographic Niches to Tree Diversity Condit, Richard; Ashton, Peter; Bunyavejchewin, Sarayudh ...
Science (American Association for the Advancement of Science),
07/2006, Volume:
313, Issue:
5783
Journal Article
Peer reviewed
Open access
Most ecological hypotheses about species coexistence hinge on species differences, but quantifying trait differences across species in diverse communities is often unfeasible. We examined the ...variation of demographic traits using a global tropical forest data set covering 4500 species in 10 large-scale tree inventories. With a hierarchical Bayesian approach, we quantified the distribution of mortality and growth rates of all tree species at each site. This allowed us to test the prediction that demographic differences facilitate species richness, as suggested by the theory that a tradeoff between high growth and high survival allows species to coexist. Contrary to the prediction, the most diverse forests had the least demographic variation. Although demographic differences may foster coexistence, they do not explain any of the 16-fold variation in tree species richness observed across the tropics.
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.
•A review of experimental and observational studies on native tree species beneath pine.•Pine plantations serve as successional surrogates for native species recruitment and growth.•Dipterocarp tree ...species demonstrate high survival and growth beneath pine.•Forest restoration is a viable economic strategy compared to tea cultivation.
In the wet forest regions of southwest Sri Lanka and the Western Ghats, India, Pinus caribaea was a common tree species for reforestation on public lands that were originally cleared of native forest for agriculture but subsequently succeeded to fire tolerant grasses and ferns. Much of this reforestation occurred during the 1970s and 1980s. The ecological literature suggests that in many temperate broadleaf forest regions pine is an important component of early succession on old fields, beneath which second growth hardwood can establish and eventually dominate. More recent studies have demonstrated the establishment of native rain forest regeneration beneath a variety of exotic tree plantations. We review nearly twenty-five years of research on this topic for South Asia through a series of studies done in southwest Sri Lanka. Results demonstrate that native species recruitment of both pioneers and site generalist late-successional trees grow well beneath exotic pine plantations. Diversity and density increase from plantation interior to edge. Protection from groundstory fire is the single most important component of promoting regeneration recruitment beneath pine. Establishing late-successional site and dispersal restricted species requires planting beneath pine rather than reliance on natural establishment. Best establishment and growth occur in openings where rows of canopy pine have been removed. Species considered for planting comprise the major late-successional canopy tree species of the forest in the genera Dipterocarpus, Shorea (Dipterocarpaceae) and Mesua (Clusiaceae). Native species that produce non-timber forest products (NTFP) need to be planted with best results in canopy openings. The most valuable NTFP’s comprise a sugar palm (Caryota urens), rattan (Calamus spp.) and a medicinal liana (Coscinium fenestratum). Financial analyses reveal that pine plantations that are enrichment planted and cultivated with rain forest timber and non-timber species can provide superior economic benefits as compared to land cultivated singly for tea.
•A review of a history of non-timber forest product use within tropical forests.•A set of ecological principles for categorizing the ecology and growth of NTFP’s.•A framework for incorporating ...non-timber products into forest restoration.•A synthesis on the ecology and silviculture of non-timber forest products.
Rural communities have traditionally valued forests for a diversity of products and services, with timber serving a minor role. No-where has this diversity been greater than in tropical South Asia, and in particular south India and Sri Lanka. As economies advance towards full development and populations become increasingly urbanized, forests become increasingly valued for their services. National development generally occurs at differing rates in different regions, with rural forest dependent communities falling behind and pockets of poverty long remaining. The demand for ‘non-timber forest products’ (NTFPs) therefore changes from subsistence to monetary based values. Overall, though, forests have suffered an unprecedented decline with development in the tropics, especially in Asia. This necessitates restoration which takes account of the enrichment of economy, wellbeing and culture which forest products provide. Methods for such restoration, and the fundamental principles upon which these must rest, are presented for species yielding NTFP’s. In this paper we first review the history of NTFP species use within south India and Sri Lanka. Second we provide a description of the broad regional characterizations of the forest formations within this region in relation to their affiliated patterns of NTFP use and exploitation. We consider seven guilds as a way to categorize NTFP’s into autecological groups for application in restoration silviculture, and use it as a framework to suggest restoration protocols for South Asian forests. We use examples of scenarios based on experimental studies of NTFP’s in reforestation trials which take account of different social values and land tenures. We conclude with a call for further research.
Understanding the ecological mechanisms that constrain forest succession in tropical degraded anthropogenic grasslands is a prerequisite for the design of techniques for restoring biodiversity and ...ecosystem processes. In this context, succession on post-agricultural lands may be arrested by a variety of site-specific biotic and abiotic factors. Here we synthesise our research on the effects of five biotic factors (seed dispersal, development of a soil seed bank, seedling emergence, herbivory, competition) and five abiotic factors (fire, microclimatic conditions, soil nutrients, water availability, disturbance) as constraints to forest succession on degraded anthropogenic grasslands in a tropical lower montane forest landscape in Sri Lanka. The aim of this research was to deduce ecologically and socially acceptable restoration techniques to accelerate forest recovery. Colonisation of grasslands by trees is constrained by limited seed dispersal from adjacent remnant forest patches and their incorporation into grassland soil seed banks. For the few tree seeds that are dispersed into grasslands, a combination of vertebrate herbivory and annual dry season fires reduces the likelihood that they emerge as seedlings. Removal of the grass canopy by clipping or tilling increases the emergence of woody plant seedlings close to the boundaries of forest patches, but has no effect beyond 20m into the established grassland. Our research shows that isolation of seedling root systems from those of competing grasses increases the growth and survival of tree seedlings transplanted directly into grassland swards, while above-ground competition and exclusion of vertebrate herbivores has no effects on seedling growth and survival. These experiments identified that the early-successional species Macaranga indica Wight and Symplocos cochinchinensis (Lour.) S. Moore are potential candidates for use in reforestation programmes on abandoned grasslands. We propose a strategy for a model forest restoration programme based on the creation of vegetation islands using early-successional native tree species, the application of a tilling treatment around remnant forest patches, creation of fire breaks around vegetation islands, and the protection of isolated individual trees and tree patches within established grasslands. We highlight the importance of further research on the ecology and biology of seed dispersers and seed predators, and expansion of knowledge on the regeneration traits of native tree species, for future refinements of this restoration strategy.
Forest structure and species distribution patterns were examined among eight topographically defined habitats for the 205 species with stems ≥ 1 cm dbh inhabiting a 25-ha plot in the Sinharaja rain ...forest, Sri Lanka. The habitats were steep spurs, less-steep spurs, steep gullies and less-steep gullies, all at either lower or upper elevations. Mean stem density was significantly greater on the upper spurs than in the lower, less-steep gullies. Stem density was also higher on spurs than in gullies within each elevation category and in each upper-elevation habitat than in its corresponding lower-elevation habitat. Basal area varied less among habitats, but followed similar trends to stem density. Species richness and Fisher's alpha were lower in the upper-elevation habitats than in the lower-elevation habitats. These differences appeared to be related to the abundances of the dominant species. Of the 125 species subjected to torus-translation tests, 99 species (abundant and less abundant and those in different strata) showed at least one positive or negative association to one or more of the habitats. Species associations were relatively more frequent with the lower-elevation gullies. These and the previous findings on seedling ecophysiology, morphology and anatomy of some of the habitat specialists suggest that edaphic and hydrological variation related to topography, accompanied by canopy disturbances of varying intensity, type and extent along the catenal landscape, plays a major role in habitat partitioning in this forest.
The decline in species richness at higher latitudes is among the most fundamental patterns in ecology. Whether changes in species composition across space (beta-diversity) contribute to this gradient ...of overall species richness (gamma-diversity) remains hotly debated. Previous studies that failed to resolve the issue suffered from a well-known tendency for small samples in areas with high gamma-diversity to have inflated measures of beta-diversity. Here, we provide a novel analytical test, using beta-diversity metrics that correct the gamma-diversity and sampling biases, to compare beta-diversity and species packing across a latitudinal gradient in tree species richness of 21 large forest plots along a large environmental gradient in East Asia. We demonstrate that after accounting for topography and correcting the gamma-diversity bias, tropical forests still have higher beta-diversity than temperate analogues. This suggests that beta-diversity contributes to the latitudinal species richness gradient as a component of gamma-diversity. Moreover, both niche specialization and niche marginality (a measure of niche spacing along an environmental gradient) also increase towards the equator, after controlling for the effect of topographical heterogeneity. This supports the joint importance of tighter species packing and larger niche space in tropical forests while also demonstrating the importance of local processes in controlling beta-diversity.
1. Colonization by woody plants is often very slow or absent on grasslands occupying degraded land in the tropics. Seed dispersal limitation is widely reported, but the constraints to forest ...succession imposed by barriers to seedling establishment are poorly understood. We tested the hypotheses that seedling emergence of woody plants is limited by interactions of anthropogenic fire, vertebrate herbivory and competition with the dominant grass sward in human-induced montane grasslands in Sri Lanka. 2. Seedling emergence was determined fortnightly for 18 months in response to experimental manipulation of fire regimes, access by vertebrate herbivores and competition from the dominant grass canopy at the forest/grassland edge and at 10, 20 and 40 m from the edge into four blocks of grassland. Seedling emergence was also monitored in the absence of any experimental manipulation at 10, 20 and 40 m into adjacent blocks of lower montane rainforest. Emergence of seedlings of woody plants was much lower in the grassland (mean <0·1 seedlings m⁻² year⁻¹) than in natural forest patches (mean 6·0 seedlings m⁻² year⁻¹), but was maximal at the forest/grassland edge (mean 9·5 m⁻² year⁻¹). In the grassland, fire reduced seedling emergence by 36% in fenced vertebrate exclosures but increased seedling emergence by 68% in unfenced plots. Exclusion of vertebrate herbivores had no impact on seedling emergence at the the forest/grassland edge. Removal of the grass canopy by clipping or tilling increased seedling emergence at the edge, but had no effect in the grassland. Seedlings of the woody pioneer species Macaranga indica dominated the community of emergents in the grassland and at the edge. 4. Synthesis and applications. Although dispersal limitation represents the primary constraint to forest succession on degraded montane grasslands in Sri Lanka, management of fire regimes, vertebrate herbivory and competition from the dominant grasses would influence the abundance and composition of tree seedling emergents. Tilling in narrow strips, 2-5 m wide and within 10 m of the forest edge, would facilitate the emergence and establishment of early successional trees in the grassland, but the strips would require protection from fire for at least 2 years. Within the interior of grassland patches, a combination of controlled burning and short-term access to vertebrate dispersers would promote tree seedling emergence, but the burned patches would then require long-term protection from fire and grazing subsequently to allow the emergents to establish and catalyse forest succession.
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