Mechanisms of plant species coexistence in a semi-deciduous tropical forest in Panama
Niche-differentiation has been proposed to explain the coexistence of large numbers of tree species in tropical forests. Previous studies on established individuals have found little evidence for this. However, niche-differentiation may be apparent at the seed and seedling stages. This study investigated the effects of environmental variation in germination and seedling establishment for a range of Central Panamanian species. Firstly, topography has a large impact on soil water availability; slopes are wetter than nearby plateaux. The potential impact on seedling emergence and survival was studied by monitoring seedling emergence and survival over 15 months. This revealed that small-seeded species are unlikely to establish on plateaux, presumably because of the rider conditions. Secondly, the germination of four pioneer species in response to four environmental variables, related to canopy gap size, was investigated. Species exhibited consistent patterns of response suggesting adaptation for germination in particular gap sizes; Piper marginatum responded to nitrate, had a low base potential for germination and tolerated large temperature fluctuations, suggesting adaptation for germination in comparatively large gaps. Conversely, Pipe peltatum exhibited responses, which suggested adaptations for germination in small gaps. Finally, germination and seedling mortality for four pioneer species in two sizes of canopy gap was predicted using computer modelling. This indicated that large-seeded pioneers can germinate and establish in large gaps. Conversely, percentage germination and survival of small-seeded species was low in large gaps. However, small-seeded species produce greater quantities of seed than large-seeded species. Consequently they may "win by default" in large gaps, because of the absence of seeds of larger species. These results indicate that there are a number of axes of environmental variation along with nice-differentiation can occur. Furthermore, the response is variable between species. Thus niche-differentiation is important for fostering species coexistence, although density-dependent mortality and dispersal-limitation also contribute.