What deters plant colonization in a tropical pine plantation?

Abstract

Pine plantations in the tropics are often employed to recondition eroded slopes from mudslides, as the Pinus caribaea plantation that shields the Universidad Simón Bolívar campus in Caracas (Venezuela). However, mismanagement of this plantation has led to its rapid degradation. The best option to maintain the protective service is to restore the plantation and direct its successional trajectory towards the neighbouring montane forest. Through experimental manipulation, we aimed to determine which factors block secondary succession and to investigate their effects. Within the experimental constraints imposed by the plantation small area, we analysed the effects of light and fertility limitation, litter accumulation and access to seed on plantation restoration. Light availability was manipulated by clearing and thinning three 800 m2 main plots. Fertilization and litter removal was applied to sub-plots within the light plots. Soils were analysed, microclimate was monitored and, for four years, stem density, species richness and basal area were tallied. Our results showed that light accessibility was the main factor deterring 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 (triplicating stem density and basal area and >20 times higher species richness). The main colonizers were Croton megalodendron, Ocotea fendleri, and Clusia spp. all dominant trees in the nearby native forest. We concluded that the results of this pioneer study, showed that small clearings, repeated in 3-4 year cycles are appropriate for similar restoration schemes. This procedure would create a mosaic of vegetation patches at different successional stages while protecting the slopes from erosion and increasing local biodiversity.