Does the herb Acanthus pubescens and/or elephants suppress tree regeneration in disturbed Afrotropical forest?

Abstract

As a result of extensive deforestation, the survival of many tropical forest species may depend on disturbed forests. However, recent studies demonstrate that tree regeneration following logging can be slow, decreasing the conservation value of disturbed forests. Here we test whether the dominating herb, Acanthus pubescens, in the understory of logged forest, suppresses tree regeneration in Kibale National Park, Uganda. We compare the establishment, growth, and survival of tree seedlings over 2.5 years between sites cleared of A. pubescens and controls. We also consider the role of elephants ( Loxodonta africana) in inhibiting tree regeneration. Seedling abundance and species richness was greater in cleared than control plots at the start of the study. Seedling abundance decreased over the study, but remained greater in cleared than control plots. Species richness did not vary over the study. However, species composition of the cleared plots was marginally different from control plots at the start, but converged on the composition of control plots by the end of the study. This suggests a common regeneration assemblage derived from reorganisation of species, rather than recruitment of new species. There was no difference in recruitment rates between cleared and control plots. These findings suggest no strong direct effects of the herb on tree regeneration in this disturbed forest. However, when we modelled (i.e., removed) the effect of elephants on regeneration, the abundance of seedlings increased in both plot types and remained reasonably constant over time. The decline in density of regenerating stems over the study was most likely caused by elephants visiting the plots. We conclude that elephants have a far greater effect in suppressing tree regeneration at Kibale than the herb stratum and are ultimately responsible for arresting succession in disturbed forest. However, as intensive logging creates conditions favourable for A. pubescens, which is eaten by elephants, the best management intervention is to constrain logging intensities to below levels above which widespread establishment of A. pubescens is promoted, thus limiting elephant activity in regenerating areas.