Soil nutrients, particularly phosphorus (P), are known to influence the distribution of tree species in lowland tropical forests, but their effect on productivity remains unclear. Other soil elements (e.g. aluminum, Al) may influence productivity by limiting growth via toxicity at high concentrations. At local scales, however, the precise relationships between the growth of individual species and variations in soil nutrient concentrations (or toxins) remain to be quantified. We used tree census and soil data from a 50 ha lowland tropical forest plot in Panama to investigate the influence of soil nutrients and potential toxins on aboveground tree productivity. Growth was calculated as the increase in tree diameter and the effects of soil variables were quantified using hierarchical, linear, mixed-effects models. The soil variables P, manganese (Mn), Al, potassium, calcium and pH were tested along with topographical variables (slope and convexity) and tree stem density.
Results/Conclusions
For all trees, growth was positively associated with several variables, especially P, with a negative effect on growth due to Mn. However, the strongest growth effect of P occurred only for small trees (< 25 mm at dbh), while trees of intermediate size (25 to 100 mm) did not respond significantly to P. By contrast, small trees responded negatively to increasing Mn concentrations, suggesting a toxic effect of the relatively high concentrations of Mn in these soils. Because most trees in the plot are small, as is also the case in most tropical forests, the population-wide response to soil nutrients was strongly influenced by the response of those small individuals, with comparatively modest increases in diameter of larger trees. These findings indicate widespread P limitation in a lowland tropical tree community, reveal the significance of Mn as a potential toxin, and demonstrate that small-scale spatial variation in soil nutrients can significantly affect tree growth, especially for small trees in the understory.