Spatial distribution patterns of the dominant canopy dipterocarp species in a seasonal dry evergreen forest in western Thailand

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Abstract

Population structure and spatial patterns were examined for four species of canopy dipterocarps (Anisoptera costata, Dipterocarpus alatus, Hopea odorata, Vatica cinerea) in a 50 ha plot in seasonal dry evergreen forest at the Huai Kha Khaeng Wildlife Sanctuary in western Thailand. Spatial dispersion was assessed with Morisita’s index for quadrat sizes ranging from 61 m2 to 25 ha; spatial attraction and repulsion between species and size classes were measured with Iwao’s index. Only Vatica had a negative exponential diameter distribution suggesting continuous recruitment. The other species had either normal (Hopea) or irregular diameter distributions with peaks in the large size classes (Anisoptera, Dipterocarpus). All four species were significantly clumped at most quadrat sizes. At the local scale, saplings and poles of Hopea and Anisoptera were negatively associated with adults at quadrat sizes <1000–5000 m2, while the distributions of Dipterocarpus and Vatica saplings and poles were independent of adult trees. In general, saplings and poles were always positively associated with each other. Spatial segregation among species may imply habitat specialization. A torus-translation analysis of habitat association for each of the dipterocarp species revealed both positive and negative species-specific associations. At HKK, most of the dipterocarps’ spatial distributions were independent of each other and there was no evidence of strong spatial segregation among species. The irregular diameter distributions, clumping at large spatial scales, and lack of positive association between juvenile and adult stems suggest that many of the dipterocarps at the 50 ha plot may have established after a large-scale catastrophic disturbance. In the absence of catastrophic disturbance, we hypothesize that the Hopea and Anisoptera populations will eventually disappear from the plot because of a lack of suitable recruitment conditions.

Introduction

In recent decades much ecological research has focused on identifying potential mechanisms for the maintenance of diversity in species-rich communities (Ashton, 1998). In the study of tropical forests two questions have received particular attention: (1) are tree species narrowly specialized for a certain habitat (e.g., Ashton, 1969, Hubbell, 1979)? and (2) is the distribution of juvenile trees a function of the distribution of adult trees (e.g., Janzen, 1970, Connell, 1971, Condit et al., 1992)?

The relationship between the local distribution of a species and topography in tropical forests has been studied in many regions (Hubbell and Foster, 1986, Basnet, 1992, Itoh, 1995, Yamada et al., 1997). Nonetheless, the relative importance of habitat specialization in structuring species-rich forest communities remains unclear. Some studies, particularly in the aseasonal evergreen forests of southeast Asia, have suggested that tree species may be habitat specialists for particular edaphic or topographic conditions (Ashton, 1964, Ashton, 1976, Ashton and Hall, 1992, Richards, 1996, Yamada et al., 1997). Other studies have not found strong evidence of habitat specialization. For example, Hubbell and Foster (1986) found that the majority of species in a semi-evergreen neotropical forest were habitat generalists with respect to topography. However, studies in Asian forests have been mostly restricted to aseasonal forests; the relationship between spatial distribution and topography in seasonal forests in continental Asia has not yet been studied.

The spatial distribution of tropical tree populations has been a major source of interest among tropical ecologists because of its potential role in explaining the coexistence of tree species in species-rich forests. Janzen (1970) and Connell (1971) first proposed that the probability of mortality and survival of juveniles may be a function of the density of conspecific adults in the surrounding area. The intensity of predation by insects and other herbivores (Janzen, 1970, Burkey, 1994) and the probability of infection by fungal pathogens (Augspurger, 1983, Augspurger and Kelly, 1984) are expected to be higher where accumulations of seeds or seedlings are denser, and lower where seeds and seedlings are sparse. Given the dispersal limitations of most tropical forest tree species, seed and seedling density is typically highest directly below the crown of a mother tree and decreases exponentially with increasing distance from the mother tree. Yet, as with habitat specialization, the extent to which density-dependence influences community diversity patterns remains uncertain. For example, in an analysis of a large-scale permanent forest dynamics plot in Panama, Condit et al. (1992) found that few species showed signs of density-dependence. In a later paper (Condit et al., 1994), they suggested that the role of density-dependence may only be important among those species with the highest population densities. More recently, however, Wills et al. (1997), using more robust statistical techniques to reanalyze the Panama plot data, showed that density-dependence was much more common than originally believed. As with habitat specialization, most studies of spatial association among tropical forest species have been either in aseasonal or neotropical forests. No such studies exist for the seasonal forests of tropical southeast Asia.

An important factor that must be taken into consideration is the relative increase in disturbance intensity with increasing seasonality in tropical Asia. The occurrence of catastrophic drought, fire, and cyclones increases with increasing distance from the equator (Whitmore, 1984). Spatial patterns of tree species may provide indirect evidence of the relative influence of large- and small-scale disturbances in structuring forest communities (Duncan and Stewart, 1991).

Trees of the family Dipterocarpaceae dominate forests across much of south and southeast Asia (Wyatt-Smith, 1963, Champion and Seth, 1968, Ashton, 1982, Whitmore, 1984). Dipterocarps are typically canopy trees or emergents and reach considerable dimensions throughout forests of the region. Consequently, they are both ecologically and economically important. Developing a better understanding of how dipterocarp populations are maintained within a forest is critical to advancing forest management and silviculture in the seasonal tropics.

In this study, we examine patterns of spatial association and habitat specialization within and among four dominant dipterocarp species in a 50 ha permanent forest dynamics plot located in seasonal dry evergreen forest in western Thailand. Specifically, we test the following hypotheses:

  • 1.

    Spatial dispersion of each species is random with respect to size class; that is, saplings and poles are neither positively or negatively associated with adult trees.

  • 2.

    Spatial dispersion of each dipterocarp species is independent of all other dipterocarp species.

  • 3.

    Spatial dispersion of each dipterocarp species is random with respect to habitat.

We evaluate the spatial patterns of each species within the context of the current stand structure and consider the processes that may have created these patterns and their role in forest development in the seasonal tropics of southeast Asia.

Section snippets

Study area

The study area is located in the Huai Kha Khaeng Wildlife Sanctuary (15°40′N, 99°10′E) in Uthai Thani Province, western Thailand (Fig. 1). The sanctuary encompasses approximately 2780 km2. Three main forest types, seasonal dry evergreen forest, deciduous dipterocarp forest and dry mixed deciduous forest, form a mosaic across most of the sanctuary, with a few high altitude sites (<5% of the total sanctuary area) occupied by lower montane forest. Rainfall is highly seasonal with a 4–6-month dry

Size structure

Vatica was the most abundant dipterocarp with 2085 individuals. Hopea, Dipterocarpus, and Anisoptera had 332, 297, and 155 trees, respectively. Of the four species all but Vatica had individuals with diameters >100 cm. The diameter distributions of the four dipterocarp species are shown in Fig. 2. Only one of the four species, Vatica, is described by the reverse-J shaped frequency distribution expected of self-replacing populations. The frequency distribution for Hopea is approximately normal

Size structure

Only Vatica exhibited a size structure with the reverse-J shape associated with continuously regenerating populations. Normally distributed diameter distributions, particularly for shade-intolerant species, have been used as indirect evidence of a single cohort age structure in temperate forests (Lorimer and Krug, 1983). The diameter distributions of Hopea and the adults of Anisoptera were consistent with the presence of a single age cohort. The cohorts of Hopea and adult Anisoptera may have

Acknowledgements

The HKK FDP is a cooperative research project of the Royal Forest Department and the Center for Tropical Forest Science of the Smithsonian Tropical Research Institute and has been funded in part through grants from the US National Science Foundation, the John D. and Catherine T. MacArthur Foundation, USAID/WWF, Conservation Food and Health, Inc., and the Merck Foundation. Many people have contributed to the success of the HKK FDP; to all of them we are deeply grateful. We wish to thank the

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