Seasonal patterns of net primary production, decomposition, and resource availability in a high rainfall everwet tropical forest

The effect of high mean annual precipitation (MAP) on tropical forests is poorly known despite clear indications of its potentially negative effect on ecosystem net primary production (NPP). High precipitation has been related to low soil fertility, caused by high nutrient leaching, low nutrient mineralization rates, and to low light availability due to increased cloudiness. Our objective was to quantify NPP in a lowland tropical everwet forest exposed to a MAP of 6700 mm. We also wanted to determine the availability of both light and nutrients as potential limiting factors to NPP. To do so, we established a one-hectare permanent plot in a mature undisturbed forest on the Island of Gorgona, Colombia (2º58’10’’N and 78º11’05’’W). Measurements of above canopy light and canopy transmittance indicated that light was likely to be limiting gross primary production during the rainiest months. Moreover, litter production peaked during the short dry season (4.01 g/m²/d) and declined during the wettest season (2.48 g/m²/d). Litter d₁₅N and N:P ratios suggested that both nitrogen and phosphorous could be co-limiting forest production. However, only P additions stimulated fine root production in root cores, while N addition alone had no effect. Litter decomposition was nearly halted during the wettest months (only 22.2% of the initial biomass decomposed after 4 months) suggesting that rainfall had an important effect on decomposition rates and thus nutrient mineralization. Total NPP was lower than in mesic tropical forests elsewhere, suggesting that rainfall had an important suppressive effect on carbon cycling through a reduction in both nutrient and light availability.