Size-dependent growth responses to competition and environment in Nothofagus menziesii

Abstract

The increased availability of permanent-plot and environmental data allows examination of the relative influence of environment and competition as controls of individual tree growth at large spatial scales. To understand and predict stem diameter growth of a dominant tree, Nothofagus menziesii (Hook. F.) Oerst., within natural forest in the South Island of New Zealand we tested the relative importance of stem diameter, competition by neighbors, soil water balance and elevation as explanatory variables. The analysis was based on 482 permanent sample plots and GIS data. Because only relatively short trees are susceptible to shading, models were fitted independently for trees smaller and larger than 25cm in stem diameter at breast height, using multilevel log-linear models. N. menziesii exhibited slow diameter growth rates in natural forest, with a median 0.39mmyear−1 and 1.39mmyear−1 for small and large trees, respectively. Growth rates were broadly uniform across 11 sampled regions, but an important fraction of the explained variation was attributed to plot-level growth estimates. Stem diameter, elevation and cross-sectional area of neighbors at the tree top (a refinement of basal area of larger neighbors, a commonly used variable) were important explanatory variables for growth of small stems. The effect of elevation was important for large stems, but size and competition had much weaker effects; there was a clear negative effect of elevation on growth but only moderate statistical support for a humped effect of tree size and for a negative effect of stand density. Thus, our results indicate that whilst asymmetric competition for light controls diameter growth in small trees, the physical environment is relatively more important in large canopy trees.