Variation of Pentaclethra macroloba natural history and genetic structure along an elevational gradient in lowland Costa Rica
Pentaclethra macroloba (Willd.) Kuntze (Fabaceae), a tropical canopy tree, breaks the archetype of high species diversity and low species density by being one of the most abundant tree species found in lowland forests of northeastern Costa Rica (Wright 2002, Clark 1999, Gaddis et al. 2014). P. macroloba sampled near La Selva Biological Reserve shows low genetic diversity, possibly due to vegetative reproduction influenced by the swampy soil type of the area (Hall 1994). Vegetative reproduction is likely to occur in forest types where resources necessary for success of seedlings resulting from sexual reproduction are not abundant, such as swampy areas or areas with low light exposure (Putz 1989). Although the semi-monodominance and low genetic diversity of P. macroloba are relatively well known, there has been little to no research to date on what factors contribute to making this species so common with a simple population-level genetic structure. In this study, we investigated the connection between the species’ mortality rate, genetic diversity and natural history by statistically comparing soil type and rainfall data to mortality rates found in P. macroloba along an elevational gradient in lowland Costa Rican neotropical wet forests.
Our data show significant relationships between soil type and the mortality rate of P. macroloba saplings (Spearman's rho= 1, d.f.= 382, p-value= 0.0467) as well as between annual average rainfall and mortality rate (Generalized Linear Model, type II, d.f.= 1, Pr(>Chisq)= 0.001312). The relationship between soil type and mortality may be due each of the eight soil types found within the test site contain a unique composition and geologic history. We found the quadratic relationship between average rainfall and the mortality rate to be novel because it is often thought that there is a linear relationship between these two factors when we found evidence that excess rainfall could be detrimental to sapling vitality. An extreme change in rainfall could increase mortality for saplings due to drought or to more frequent changes in forest structure during large storms, which have potential to crush saplings. Research on genetic structure of P. macroloba continues, and we hope to be able to better describe the genetic structure of adult population and its relationship to soil type and elevation. Findings from both portions of this study will provide valuable information regarding the reasons behind the species’ high abundance, low genetic diversity, and high mortality rates.