Influence of a common palm, Oenocarpus mapora, on seedling establishment in a tropical moist forest in Panama

Abstract: Plants often modify microsite conditions important for seedling establishment. In tropical moist forest, advance regeneration in the form of shade-suppressed seedlings is a major component of regrowth in new gaps. Tree seedlings may be filtered by the composition and structure of the forest understorey. In a lowland forest in Central Panama, we examined light availability, Utter accumulation and the seedling community (abundance, diversity and composition) under and away from the canopies of a common subcanopy, clonal palm, Oenocarpus mapora, and tested whether seedling abundance varies as a function of changes in understorey light conditions and leaf litter depth. We found evidence that leaf litter was five times deeper and light availability 2 7% lower under O. mapora canopy than where it was absent. The probability of finding a seedling in plots under O. mapora canopies was 33% lower than in plots without the palm. Plots under O. mapora canopies also had 49% fewer species of seedlings and those seedlings came from significantly larger seeds than seedlings in plots not under the palm. Overall, seedling density was positively correlated with light availability and negatively correlated with leaf litter depth. A transplant experiment confirmed that for at least one large-seeded species of dicot, seedlings are negatively affected by the reduction in light availability and increase in leaf litter. Under O. mapora, seedlings of Gustavia superba (Lecythidaceae) had lower survivorship and biomass accumulation than away from the palm. Thus, site conditions produced by large, clonal, multi-layered palms such as O. mapora may reduce establishment of small-seeded and shade-intolerant species, potentially affecting species composition in the understorey seedling pool. Key Words: leaf litter, light availability, palms, seed size, shade tolerance INTRODUCTION distribution of seedlings present when treefalls provide opportunity for growth into the overstorey (Denslow In mesic forests, seedlings and saplings established in the 1987, George & Bazzaz 1999, Hubbell & Foster 1986, understorey are a major component of the regrowth in Kobe et al. 1995, Montgomery & Chazdon 2001, Pacala new gaps (Ashton 1992, Brokaw 1985, Morin 1994, etal. 1996). Uhl etal. 1988). In the understorey these seedlings often In tropical forests, light availability in the understorey are suppressed and their growth and survival are often is typically below 2% of that above the canopy and low unless a tree-fall gap or other disturbance creates is likely to be one of the most limiting resources for more favourable light conditions (Canham et al. 1994, seedling establishment and growth (Chazdon & Fetcher Denslow et al. 1990, Fraver et al. 1998, Pereira de 1984,Kabakoff&Chazdon 1996).However,understorey Souza & Vallo 2001, Poulson & Platt 1989, 1996). As light is spatially variable and this variability may be a result, environmental conditions in the understorey attributed in large part to the distribution of understorey may influence the composition, relative abundance and trees and shrubs (Canham et al. 1994, Denslow et al. 1991, George & Bazzaz 1999, Kabakoff & Chazdon 1996, Montgomery & Chazdon 2001). Species-specific iCorrespondingautlior, 180 SuttonRd.Bryson City, NC 28731, USA differences in crown geometry and tree architecture Email: kristalopez@yahoo.com have been shown to infiuence spatial distribution of