Does soil moisture availability explain liana seedling distribution across a tropical rainfall gradient?

Abstract

Liana density tends to increase with decreasing rainfall and increasing seasonality. However, the pattern of liana distribution may be due to differences in soil water retention capacity, not rainfall and seasonality per se. We tested the effect of rainfall and soil substrate with respect to the distribution of liana seedlings in six sites across a rainfall gradient from the wet Atlantic to the dry Pacific in central Panama. Soils were either limestone, with low water-holding capacity, or laterite, with higher water-holding capacity. We sampled liana seedlings at each site using three 1 × 100 m transect. We found that relative liana seedling density was higher on limestone soils compared to laterite soils regardless of the amount of rainfall. Furthermore, liana community composition on limestone soils was more similar to dry forest sites than to adjacent wet and moist forest sites. Liana seedling species diversity relative to trees was significantly higher in a low-fertility dry forest site compared to a high-fertility forest, but did not differ from the other sites. Thus, liana seedling density and community structure may be driven more by soil type and thus by soil moisture availability than strictly by mean annual rainfall and the seasonality of rainfall.