Plant growth responses to resources may be an important mechanism that influences species distributions, coexistence, and community structure. Irradiance is considered to be the most important resource for seedling growth in the understory of wet tropical forests, but a broad array of soil resources and range of species have yet to be evaluated. To identify the resources that best predicted Neotropical tree seedling growth, we measured irradiance and soil nutrients over a seedling-appropriate spatial scale (1 m²) in five sites of old-growth wet tropical forests at La Selva Biological Station, Costa Rica.  Sites were established on three volcanic and two alluvial soils. We characterized irradiance as percent canopy openness, estimated from hemispherical canopy photos that were taken 6 times during the study. We also measured an array of soil nutrients including nitrate (NO₃), ammonium (NH₄), total nitrogen (total N), phosphate (PO₄), potassium (K), magnesium (Mg), and calcium (Ca) sampled once during the study.  We developed height growth models as functions of measured resources for 94 Neotropical woody species (including lianas and canopy, subcanopy, and understory trees) that had sample sizes >20 individuals.

Results/Conclusions

Among the 94 tested species, the effects of soil nutrients were more prevalent than the effects of irradiance and at least some soil resources had a similar or greater magnitude of effect. Irradiance was a significant predictor of growth in 52% of the species, but in most of these cases at least one soil resource also was correlated with growth.  Among the soil resources,  total N was correlated with growth in almost half (46%) of the species, sum of base cations in 39%, NO₃ and NH₄ in 33% and 35%, respectively, and PO₄, commonly considered the main limiting nutrient in many tropical systems, was significant in only 30%.  In about 47% of species, growth was significantly correlated with only soil resources; the growth of 46% of species was correlated with both soil resources and irradiance.  In addition to prevalence, the magnitude of the effect of most soil resources was similar to that of irradiance. Among species that responded to both irradiance and soil resources, irradiance and soil resources each had a greater magnitude of effect in about half of the species. In this broad survey of species and soil resources, the nearly ubiquitous effects of soil nutrients on seedling growth challenge the idea that soil resources are not important in the light-limited understory of wet tropical forests.