What deters plant colonization in a tropical pine plantation?

Abstract

A forty year old Pinus caribaea plantation, surrounding the Universidad Simón Bolívar campus in Caracas (Venezuela), provides effective protection from erosion and mudslides. Owing primarily to past mismanagement, mainly lack of thinning, this exotic pine plantation is senescing rapidly. Assisted restoration aiming to attain a successional trajectory towards the neighbouring montane forest is the most realistic option for maintaining the plantation’s protective service and increase local biodiversity. Within the experimental constraints imposed by the small area of the plantation and its central protective role, we describe and analyse the effects of light and fertility limitation, litter accumulation and access to seed on plantation restoration. This experimental restoration trial is the first for a Neotropical pine plantation. Light availability was manipulated by clearing and thinning three 800 m 2 main plots. Fertilization and needle litter removal (by fire and raking) was applied to sub-plots within the light plots. Soils were analysed, microclimate was monitored and, across 4 years, stem density, species richness and basal area were tallied. Light accessibility was the main factor predicting the successional trajectory of the plots, with varying grades of interaction with the sub-treatments. By the end of the fourth year, the cleared plot showed the largest responses in all traits (three times higher stem density and basal area and up to twenty times higher species richness) as compared to the thinned and control plots. Although difficult to establish, unimpeded access to seed dispersers probably contributed to this result. The main colonizers were Croton megalodendron, Ocotea fendleri, and Clusia spp; all dominant trees in the nearby native forest. At this stage, our results permit us to recommend the use of small sized clearings, repeated in 3-4 year cycles, for similar restoration schemes. They would generate a mosaic of patches at different successional stages maintaining the protective role of the vegetation cover while increasing local biodiversity.