Large-tree growth and mortality rates in forests of the central North Island, New Zealand

Large trees are a significant structural component of old-growth forests and are important as habitat for epiphytic biodiversity; as substantial stores of biomass, carbon and nutrient; as seed trees; and as engineers of large gap sites for regeneration. Their low density across the landscape is an impediment to accurately measuring growth and mortality, especially as infrequent tree deaths are rarely captured without long periods of monitoring. Here we present large-tree (≥ 30 cm in diameter at breast height) growth and mortality rates for six common New Zealand tree species over a 42-year period from 28 large permanent plots (0.4–0.8 ha) in the central North Island. Our goal was to examine how rates of growth and mortality varied with tree size and species. In total we sampled 1933 large trees across 11.6 ha, corresponding to a large-tree density of 167 trees ha^–1, of which we used 1542 as our six study species. Mean annual mortality rates varied more than 10-fold among species being least in Dacrydium cupressinum (0.16%) and greatest in Weinmannia racemosa (2.21%). Diameter growth rates were less variable among species and ranged from 1.8 mm yr^–1 in Ixerba brexioides to 3.3 mm yr^–1 in D. cupressinum. Tree size influenced the rate of mortality in Beilschmiedia tawa, I. brexioides and W. racemosa but there was no support for including tree size in models of the remaining three species. Likewise, tree size influenced growth rates in I. brexioides and Nothofagus menziesii but not the remaining four species. These data provide robust size- and species-specific estimates of large-tree demographic rates that can be used as baselines for monitoring the impacts of management and global change in old-growth forests.