Spatial and demographic structure of tara stands (Caesalpinia spinosa) in Peru: Influence of present and past forest management

Abstract

Tropical dry forests are highly endangered ecosystems that have been scarcely studied. Many species within these forests suffer regeneration problems due to unsustainable management regimes. In particular, a regeneration problem has been detected in a forest of tara (Caesalpinia spinosa), a neotropical tree of high ecological and economic value, in Atiquipa (Peru). The study of the spatial patterns and population structure of forests can help us understand their dynamics and evaluate the effects of management. In this article, we analyse the stand demographic structure, spatial distribution and patterns of plant interactions in tara forests. We evaluate whether the regeneration problem in Atiquipa is strictly local or a problem of general concern and investigate the most probable causes. Four tara stands were selected at different localities in Peru. Two stands (Andurco and Polán) had a reverse J-shaped diametric structure, typical of stable self-replacing forests, although Polán had a low number of young trees, indicating an incipient regeneration problem. The Lloque histogram was skewed (with a maximum in seedlings 61 cm), indicating over-exploitation in the past and present forest regeneration. Maguey had a low number of regenerates, with peaks in some intermediate diametric classes, which may indicate natural regeneration problems or some past management. Spatial distribution of tara trees did not depart from the null model (�random distribution), typical of trees dispersed by zoochory. Maguey was an exception, showing a regular pattern at short distances, possibly associated with past management (like selective cuttings and/or plantations). These results suggest that in most of the analysed stands the current forest management (i.e. excessive seed collection or grazing) limits tara forest regeneration. However, the only stand with a protected status presented a clear tendency toward population increase. Bivariate analyses revealed an aggregated pattern between seedlings and adult trees. Moreover, the plant-plant interaction study showed that seedlings were associated with woody vegetation. These positive associations highlight a facilitative effect that ameliorates stressful microclimatic characteristics and/or protects tara seedlings from herbivory. The results of this study support some recommendations for sustainable management, such as controlled stocking rate, limited seed collection and promotion of bush cover.