We investigated the nature of nitrogen, phosphorus and potassium limitation of growth on over 1,800 tropical tree seedlings with a factorial experiment conducted over an eight-year period in lowland tropical forest at the Barro Colorado Island Nature Monument in Panama. The five study species were Heisteria concinna, Tetragastris panamensis, Sorocea affinis, Alseis blackiana, and Desmopsis panamensis. Growth was assessed as annual or bi-annual height growth increments normalized by initial height and expressed as relative growth rate. Repeated measures analysis of variance was used to determine the effects of nutrient addition on seedling growth.

Results/Conclusions

The effect of year was always highly significant, indicating that some years are clearly better for seedling growth than are other years. In single species analyses over all years, potassium addition increased growth in Heisteria and had marginally significant effects on Alseis and Tetragastris, whereas nitrogen had a marginally significant effect on Tetragastris and the effect of phosphorus was insignificant for all five species. Because two-way nutrient-species interactions were all insignificant, we dropped the two-way nutrient-species interactions from the model and did a final single repeated measures analysis of variance that included all five species. This analysis indicated that seedling growth increased with nitrogen (p = 0.014) and potassium (p = 0.001) addition and there was a significant nitrogen-phosphorus interaction (p = 0.011). These data indicate the potential for alterations of nutrient cycles to influence biological diversity and productivity, even in the light-limited tropical forest understorey.