Ecological significance of the patches dominated by pioneer trees for the regeneration of dipterocarps in a Bornean logged-over secondary forest

Abstract

Logging has created mosaics of Macaranga-dominated patches in mixed dipterocarp forests throughout Borneo. We questioned if the recruitment of dipterocarp seedlings is retarded within such Macaranga patches versus non-Macaranga patches, and if so, what mechanisms are involved. We determined the spatial pattern of dipterocarp saplings in a 4-ha plot established in a 12-year old logged-over forest, where Macaranga patches admix with non-Macaranga patches. The Iwao’s ω index demonstrated that the extant dipterocarp saplings were distributed exclusively with Macaranga patches at the beginning of our survey. We also monitored the survival of naturally dispersed dipterocarp seeds after a masting event. Seed supplies and germination occurred throughout the plot. The survival of naturally recruited seedlings decreased in Macaranga patches compared with non-Macaranga patches based on a GLMM analysis. Herbivory on the first pair of leaves did not affect the seedling survival, suggesting no contribution of herbivores to the decreased survival of seedlings in Macaranga patches. Net N mineralization rate and the concentration of bicarbonate soil Pi and Po were significantly lower in Macaranga patches than in non-Macaranga patches, while other abiotic factors such as soil water content, soil bulk density, and relative light intensity were not significantly different with each other. These results suggest that Macaranga patches limit the recruitment of dipterocarp seedlings through the impoverishment of soil nutrients. The lower N and P availability in Macaranga patches was possibly caused by the slow accumulation of soil organic matter under Macaranga trees because of the rapid decomposition of their litter.