Treefall gap size effects on above- and below-ground processes in a tropical wet forest

1 We examined the effects of variation in gap size on above- and below-ground light and nutrient processes in a tropical wet forest in Costa Rica. 2 Trees were felled to create canopy openings ranging in size from 65 to 611 m(2). Following treefall, we measured initial litter mass in the crown zone of six gaps. During the subsequent year, we measured litter decomposition rate and light levels as well as NH4-N, NO3-N and PO4-P levels in surface soils, soil moisture and fine root mass. We also measured growth rates of fertilized and non-fertilized plants of four species of Miconia (Melastomataceae) to assess nutrient limitation to plant growth in large gaps. 3 Light levels in the centres of gaps were significantly related to size of the canopy opening. After 1 year, light levels near the ground in larger openings (35-40% full sunlight immediately after treefall) declined to levels similar to those in smaller gaps (10-20%). 4 Although canopy opening had only slight effects on soil NH4-N, NO3-N pools were significantly greater in gaps than in understorey at both sites. The effect was positively correlated with gap size. Extractable PO4-P was also greater in gaps than in adjacent understorey, although the difference did not vary as a function of gap size. In three of six gaps, tine root biomass was less in gaps than in adjacent understorey. 5 Of four species of Miconia, only M. affinis, a small tree common in early secondary forests, grew significantly faster in fertilized than in non-fertilized treatments in three large gaps. 6 Our data suggest that higher nutrient pools in surface soils of treefall gaps may result from decomposition and mineralization of the large mass of fresh litter from the fallen tree. While growth rates of shade-tolerant rain forest trees and shrubs are not much affected by the increased nutrient availability even at high light levels, growth rates of pioneer or high-light demanding species may be enhanced by increased above- and below-ground resources.