Edaphic and human effects on landscape-scale distributions of tropical rain forest palms

Ecology, Dec, 1995 by Deborah A. Clark, David B. Clark, Rosa Sandoval M., Marco Vinicio Castro C.

INTRODUCTION

To understand many aspects of populations, communities, and ecosystems, a first need is to determine how the distributions and abundances of the organisms vary across the landscape. To develop predictive power further requires understanding the processes, past and present, underlying the spatial heterogeneity thus identified (Levin 1992). Although these ideas are generally recognized by most ecologists, current ecological studies often lack the underpinnings of a clear understanding of the important scales of spatial variation within study sites.

In the wet tropics a growing number of studies at small-to-large scales within old-growth forest have demonstrated strong effects of edaphic heterogeneity on floristic composition (Ashton 1969, Newbery and Proctor 1984 and cited references, Lescure and Boulet 1985, Lieberman et al. 1985, Basnet 1992, Johnston 1992, Ruokolainen and Tuomisto 1993, Gentry and Ortiz 1993, Oliveira-Filho et al. 1994). Much of the Amazon Basin is a highly interdigitated mosaic of forest types produced by the erosion/deposition cycles of the major rivers (Salo and Rasanen 1989, Foster 1990) and marked variation in terra firme parent material and geochemistry (Jordan 1985, Chauvel et al. 1987, Guillaumet 1987). Another potential source of small-to-medium scale spatial heterogeneity within tropical wet forests is the historical or current impact of local human activity, from silviculture to selective harvesting, even within stands considered to be old-growth (Gordon 1982, Gomez-Pompa and Kaus 1990, Anderson 1990, Bush and Colinvaux 1994).

Understanding the patterns and causes of spatial variability in the community structure of tropical wet forests can contribute to the resolution of important questions about these ecosystems. Is small-to-medium scale edaphic heterogeneity more characteristic of these forests than of temperate stands? Could this be a factor in the maintenance of high species richness in tropical rain forests? Are wet tropical forests characterized by major internal spatial heterogeneity in eco-system-level processes such as primary productivity? Is variability in site conditions and forest communities likely to have major impacts for efforts in tropical forest conservation and restoration?

Our goal in this study was to assess landscape-scale spatial heterogeneity within an old-growth neotropical rain forest, as reflected in the distribution and abundance patterns of the large palms. An unprecedented set of research tools for landscape-level studies of tropical rain forest (a reserve-wide grid system, soil map, and Geographical Information System [GIS]) is newly available at the study site. We used them to ask the following questions, at the scale of 500 ha of contiguous forest: (1) How do the abundance, size distribution, and local species diversity of the guild of sub-canopy and canopy palms vary with respect to the land-scape-scale variation in soil and topography? (2) Do the distribution and abundance of the individual species vary with edaphic conditions, and do these patterns differ among species? (3) Is there evidence of human impact on the distributions of large palms in the old-growth forest?

STUDY SITE AND SPECIES

The La Selva Biological Station of the Organization for Tropical Studies (OTS) is a 1550-ha reserve in the Atlantic lowlands of Costa Rica (10 [degrees] 26[minutes] N, 84 [degrees] 00[minutes] W; elevation 37-150 m). It is classified in the Holdridge life zone system as tropical wet forest (Hartshorn and Hammel 1994). Mean annual rainfall is 3962 mm, with every month averaging at least 100 mm of rain (Sanford et al. 1994).

The flora includes 323 tree species (Hartshorn and Hammel 1994) and is rich in palms (31 native species, Chazdon 1985; 25% of all woody stems [greater than or equal to]10 cm in diameter, Lieherman et al. 1985). Although not subject to large-scale disturbances such as hurricanes, the forest is very dynamic. Stem turnover is high (2.0-2.3%/ yr for trees [greater than or equal to]10 cm dbh; Lieberman et al. 1990), as is the frequency of gap formation (Hartshorn 1978, Sanford et al. 1986).

La Selva's soils range from relatively fertile entisols and inceptisols to infertile ultisols (Sollins et al. 1994). The 1:10 000 soils map of La Selva (Sancho and Mata 1987) demarcates 23 consociations (mapping units within which [greater than or equal to]75% of the area is the described soil type) and one complex.

Our study species are the seven subcanopy to canopy palms found in old-growth at La Selva (Table 1). Henceforth all except the Astrocaryum species are designated by genus name only. Four species have been locally harvested (for heart-of-palm, construction wood, thatching) in recent history (Table 1).

METHODS

To evaluate landscape-scale distributions of the large palms in upland old-growth forest, we used the original La Selva reserve ("Original La Selva"; McDade and Hartshorn 1994), after excluding swamp, secondary forest, disturbed habitats, and the 12.4 ha of restricted access plots. Within the resulting study area (568 ha; [ILLUSTRATION FOR FIGURE 1 OMITTED]) we sampled at intersections of La Selva's reserve-wide grid (posts every 50 m along lines at 100-m intervals; accuracy [ or -]20 cm). In the limited areas of alluvial soils, we sampled at all grid intersections (N = 78). On the nonalluvial soils we sampled at the intersections along every other grid line (thus along lines separated by 200 m; N = 438 grid points). We excluded points that fell within streams or floodable microsites as well as those [less than or equal to]30 m from secondary forest or altered habitat.