Induced chemical defences in Colophospermum mopane trees

Abstract

A field experiment was conducted in which Mopane (Colophospermum mopane) trees were subjected to three different canopy treatments:felling of trees at a mean height of 0.7m above ground level, felling at a height of 2m, or pruning of selected branches. These treatments were intended to simulate wood harvesting by local communities, pollarding by elephants, and browsing by herbivores, respectively. The foliar concentrations of total phenol, condensed tannin and protein-precipitating tannin were tracked for three consecutive years after treatments were effected. Coppicing, pollarding and pruning induced chemical defences in Mopane tree foliage. The order of magnitude of the response was positively related with impact severity: coppicing > pollarding > pruning > control. Differences between treatments remained the same for the duration of the study. The effects of canopy treatments on phenol and tannin concentrations were small compared to the seasonal and inter-seasonal fluctuations. The protein precipitation capacity of the secondary metabolites increased significantly with increasing impact. We concluded that secondary metabolite concentrations in Mopane foliage are dynamic and can be explained satisfactorily by season, year and treatment severity, with a high explanatory power (r2 = 0.94–0.98). Findings of this study have consequences for current harvesting, animal production and conservation practices.