Peter S. Ashton

Comparisons of Structure Among Mixed Dipterocarp Forests of North-Western Borneo

Relationships between lowland rain-forest structure, dynamics and site conditions were examined by the establishment of plots and profile diagrams within mature-phase forest at 13 sites and with permanent plots at three of these sites where trees have been enumerated over 20 years. Most forest structural measures were intercorrelated but forest stature was uncorrelated with HCl-extractable soil nutrient concentrations and was apparently related to topography, soil depth and soil water. The relationship between diameter and height varied between forests and was correlated with HCl-extractable P and Mg for dry-land sites (...)

The Importance of Demographic Niches to Tree Diversity

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.

Assessing Evidence for a Pervasive Alteration in Tropical Tree Communities

In Amazonian tropical forests, recent studies have reported increases in aboveground biomass and in primary productivity, as well as shifts in plant species composition favouring fast-growing species over slow-growing ones. This pervasive alteration of mature tropical forests was attributed to global environmental change, such as an increase in atmospheric CO2 concentration, nutrient deposition, temperature, drought frequency, and/or irradiance. We used standardized, repeated measurements of over 2 million trees in ten large (16–52 ha each) forest plots on three continents to evaluate the generality of these findings across tropical forests. Aboveground biomass increased at seven of our ten plots, significantly so at four plots, and showed a large decrease at a single plot. Carbon accumulation pooled across sites was significant (+0.24 MgC ha−1 y−1, 95% confidence intervals [0.07, 0.39] MgC ha−1 y−1), but lower than reported previously for Amazonia. At three sites for which we had data for multiple census intervals, we found no concerted increase in biomass gain, in conflict with the increased productivity hypothesis. Over all ten plots, the fastest-growing quartile of species gained biomass (+0.33 [0.09, 0.55] % y−1) compared with the tree community as a whole (+0.15 % y−1); however, this significant trend was due to a single plot. Biomass of slow-growing species increased significantly when calculated over all plots (+0.21 [0.02, 0.37] % y−1), and in half of our plots when calculated individually. Our results do not support the hypothesis that fast-growing species are consistently increasing in dominance in tropical tree communities. Instead, they suggest that our plots may be simultaneously recovering from past disturbances and affected by changes in resource availability. More long-term studies are necessary to clarify the contribution of global change to the functioning of tropical forests.

DISTURBANCE HISTORY AND HISTORICAL STAND DYNAMICS OF A SEASONAL TROPICAL FOREST IN WESTERN THAILAND

Disturbances influence forest dynamics across a range of spatial and temporal scales. In tropical forests most studies have focused on disturbances occurring at small spatial and temporal scales (i.e., gap dynamics). This is primarily due to the difficulty of reconstructing long-term disturbance histories of forests in which most tree species lack annual growth rings. Consequently, the role of past disturbances in tropical forests is poorly understood. We used a combination of direct and indirect methods to reconstruct the his- torical disturbance regime and stand development patterns in mature and regenerating seasonal dry evergreen forest (SDEF) in the Huai Kha Khaeng Wildlife Sanctuary in western Thailand. Direct estimates of long-term establishment and growth patterns were obtained from 12 tree species that form annual growth rings as a consequence of the regions strong intra-annual rainfall seasonality. Indirect estimates of establishment patterns were obtained from analyses of stand structure and individual tree architecture and application of age- estimation models to 10 dominant canopy-tree species using demographic data from a large- scale, permanent forest-dynamics plot. The combination of direct and indirect methodologies revealed a complex disturbance history in the seasonal evergreen forest over the past 250 years. In the mid-1800s, 200-300 ha of forest were destroyed by a catastrophic disturbance, which led to the synchronous es- tablishment of many of the trees that presently dominate the forest canopy. Since then wide- spread disturbances of variable intensity have occurred at least three times (1910s, 1940s, and 1960s). These disturbances created discrete temporal pulses of establishment in small to large gaps in the forest matrix across several square kilometers. Background mortality and gap formation were evident in every decade since 1790, but these varied in intensity and frequency. The SDEF retains a distinct structural and floristic legacy from the catastrophic dis- turbance of the mid-1800s. The single-age cohort that established after the disturbance has developed a complex three-dimensional structure as a consequence of differences in in- terspecific growth patterns of the canopy-tree species and subsequent disturbances of mod- erate and low intensity. While no single methodological approach provided a complete picture of the disturbance history and stand development patterns of the seasonal evergreen forest, taken together they offered new insights into the long-term dynamics of a primary tropical forest. In particular, the study highlighted the role of disturbance at multiple spatial and temporal scales and varying intensities in determining the structure and composition of a complex, species-rich tropical forest and raises important questions about the role of rare, catastrophic events on tropical forest dynamics.

HABITAT PATTERNS IN TROPICAL RAIN FORESTS: A COMPARISON OF 105 PLOTS IN NORTHWEST BORNEO

Understanding the maintenance of high tropical tree species diversity requires disentangling the effects of habitat vs. geographic distance. Using floristic, topographic, and soil nutrient data from 105 0.6-ha plots in mixed dipterocarp forest throughout Sarawak, Malaysian Borneo, we explore the degree to which floristic patterns are habitat-driven from local to landscape scales. We assess how the floristic influence of geographic distance vs. abiotic factors varies from local to regional scales. We employ several multivariate analytical techniques and perform a hierarchical clustering of the research plots using the Steinhaus index of floristic dissimilarity, as well as Mantel analyses on matrices of floristic, habitat, and geographic distance. These analyses indicate that floristic variation is more strongly correlated with habitat than with geographic distance on the regional scale. On the local- landscape to community scale, we find evidence of a resource threshold above which habitat effects weaken; that is, below the resource threshold floristic similarity between sites is dominated by habitat effects, while above the threshold floristic similarity between sites is dominated by geographic-distance effects. We also find evidence that topography and soil nutrients correlate in part independently with floristics. These results, together with previous studies in the Neotropics, emphasize that tree species distribution and community com- position are variously influenced by the interplay of both habitat and dispersal-driven effects.

Cluster Analysis of Spatial Patterns in Malaysian Tree Species

Tree species in tropical rain forests exhibit a rich panoply of spatial patterns that beg ecological explanation. The analysis of tropical census data typically relies on spatial statistics, which quantify the average aggregation tendency of a species. In this article we develop a cluster‐based approach that complements traditional spatial statistics in the exploration and analysis of ecological hypotheses for spatial pattern. We apply this technique to six study species within a fully mapped 50‐ha forest census in peninsular Malaysia. For each species we identify the scale(s) of spatial aggregation and the corresponding tree clusters. We study the correlation between cluster locations and abiotic variables such as topography. We find that the distribution of cluster sizes exhibits equilibrium and nonequilibrium behavior depending on species life history. The distribution of tree diameters within clusters also varies according to species life history. At different spatial scales, we find evidence for both niche‐based and dispersal‐limited processes producing spatial pattern. Our methodology for identifying scales of aggregation and clusters is general; we discuss the method’s applicability to spatial problems outside of tropical plant ecology.

Individual-based simulation models for forest succession and management

Individual-based forest models simulate forest dynamics on the basis of establishment, growth and death of individual trees. This paper attempts to review and compare two major types of individual-based forest models: growth-yield and gap models. Although the two types of models share some similar features, they differ in model structure and data requirements and play several complementary roles. Growth-yield models are used by foresters to assist timber production and evaluate growth and yield of one to several commercial timber species in managed forests, while gap models are generally developed by ecologists to explore ecological mechanisms and patterns of structure and functional dynamics in natural forest ecosystems. Site-specific environmental and species information is necessary for constructing growth-yield models while gap models require species-specific biological information of individual trees and site-specific environmental data. The growth-yield models are more diverse in terms of model structure. In contrast, gap models belong to the same genealogy and later gap models are ultimately derived from the earliest one although they may vary in detail. In the future, we expect to see more individualbased hybrid models which integrate gap models with growth-yield models and ecophysiological models. As computer technology advances explosively, individual-based models could be much more efficient and user-friendly. There have existed disproportionately few individual-based models for biodiverse forests such as evergreen tropical rain forests. Development of individual-based models for tropical rain forests is more challenging and is being stimulated by new international conservation efforts.

Species–habitat associations in a Sri Lankan dipterocarp forest

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.

Phylogeny of the tropical tree family Dipterocarpaceae based on nucleotide sequences of the chloroplast RBCL gene

The Dipterocarpaceae, well-known trees of the Asian rain forests, have been variously assigned to Malvales and Theales. The family, if the Monotoideae of Africa (30 species) and South America and the Pakaraimoideae of South America (one species) are included, comprises over 500 species. Despite the high diversity and ecological dominance of the Dipterocarpaceae, phylogenetic relationships within the family as well as between dipterocarps and other angiosperm families remain poorly defined. We conducted parsimony analyses on rbcL sequences from 35 species to reconstruct the phylogeny of the Dipterocarpaceae. The consensus tree resulting from these analyses shows that the members of Dipterocarpaceae, including Monotes and Pakaraimaea, form a monophyletic group closely related to the family Sarcolaenaceae and are allied to Malvales. The present generic and higher taxon circumscriptions of Dipterocarpaceae are mostly in agreement with this molecular phylogeny with the exception of the genus Hopea, which forms a clade with Shorea sections Anthoshorea and Doona. Phylogenetic placement of Dipterocarpus and Dryobalanops remains unresolved. Further studies involving representative taxa from Cistaceae, Elaeocarpaceae, Hopea, Shorea, Dipterocarpus, and Dryobalanops will be necessary for a comprehensive understanding of the phylogeny and generic limits of the Dipterocarpaceae.

An estimate of the number of tropical tree species

Significance People are fascinated by the amazing diversity of tropical forests and will be surprised to learn that robust estimates of the number of tropical tree species are lacking. We show that there are at least 40,000, but possibly more than 53,000, tree species in the tropics, in contrast to only 124 across temperate Europe. Almost all tropical tree species are restricted to their respective continents, and the Indo-Pacific region appears to be as species-rich as tropical America, with each of these two regions being almost five times as rich in tree species as African tropical forests. Our study shows that most tree species are extremely rare, meaning that they may be under serious risk of extinction at current deforestation rates. The high species richness of tropical forests has long been recognized, yet there remains substantial uncertainty regarding the actual number of tropical tree species. Using a pantropical tree inventory database from closed canopy forests, consisting of 657,630 trees belonging to 11,371 species, we use a fitted value of Fisher’s alpha and an approximate pantropical stem total to estimate the minimum number of tropical forest tree species to fall between ∼40,000 and ∼53,000, i.e., at the high end of previous estimates. Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼19,000–25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼4,500–6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions. We provide a methodological framework for estimating species richness in trees that may help refine species richness estimates of tree-dependent taxa.