Determinants of tree mortality in mixed old-growth Nothofagus forest

Rates and spatial patterns of tree mortality were examined using long-term data from old-growth, mixed-species forests of the Maruia Valley, South Island, New Zealand. The aim of the study was to investigate patterns of tree mortality in two common, co-occurring species, Nothofagus fusca (Hook. f.) Oerst. and Nothofagus menziesii (Hook. f.) Oerst. The dynamics of three old-growth stands were followed over a 23-year period, using plots sized 0.8-1.0 ha. In total the fates of 1138 individual N. fusca and 1611 N. menziesii were recorded, which had annual mortality rates of 0.016 and 0.0089 per year, respectively. Differing spatial and size-related patterns of mortality were found between species. For both species, individual-based logistic models showed that slower growing trees were more likely to die than faster growing trees. N. fusca trees growing in previously disturbed stands were also more likely to die than those in undisturbed stands. Spatial point process analysis showed that dead N. fusca trees were spatially aggregated, and were segregated from living trees, a pattern that was consistent across both small and large trees. Dead N. menziesii were spatially aggregated, but were not segregated from living trees. Aggregated mortality of N. fusca trees should favor regeneration of this light-demanding species in large canopy gaps, perpetuating its aggregated distribution, whereas the lower mortality of shade-tolerant N. menziesii allows this species to persist. Our results demonstrate that performance differences in coexisting tree species can be manifested spatially. Between species, different mortality patterns may have implications for sustainable forest management. (C) 2012 Elsevier B.V. All rights reserved.