Climate change and tree-ring relationships of Nothofagus menziesii tree-line forests

To assess the sensitivity of New Zealand tree lines to climate warming, we compared the tree-ring growth characteristics and temperature relationships of silver beech (Nothofagus menziesii (Hook. f.) Oerst) at two elevations, ca. 1200 m (tree line) and ca. 1100 m. Modelled relationships between climate series and tree rings indicated that the main climatic control on tree growth was current summer temperatures. Nevertheless, temperatures during earlier seasons can influence tree growth, pointing to a complex relationship between radial growth and climate at tree line. Overall, the similarity in the growth-temperature relationships for trees at both elevations indicated that high-altitude N. menziesii forests should be useful for examining the impact of climate warming on tree growth. However, the level of common growth variation was greater in the below tree line chronologies, suggesting that other factors, including natural disturbance, may affect or compete with the influence of temperature on tree-ring growth at tree line. Despite the importance of summer temperatures for tree growth at or near tree line and the reported increase in summer temperatures since 1950 in New Zealand, ring widths have not increased in recent decades. We conclude, therefore, that in these N. menziesii tree-line forests there has been no detectable tree-ring growth response to climate warming.