Earthquake impacts in old-growth Nothofagus forests in New Zealand

Six stands located on different land forms in mixed old-growth Nothofagus f orests in the Matiri Valley (northwest of South Island. New Zealand) were s ampled to examine the effects of two recent large earthquakes on tree estab lishment and tree-ring growth, and how these varied across land forms. 50 t rees were cored in each stand to determine age structure and the cores were cross-dated to precisely date unusual periods of radial growth. The 1968 e arthquake (M = 7.1, epicentre 35 km from the study area) had no discernible impact on the sampled stands. The impact of the 1929 earthquake (M = 7.7, epicentre 20 kin from the study area) varied between stands, depending on w hether or not they had been damaged by soil or rock movement. In all stands , the age structures showed a pulse of N. fusca establishment following the 1929 earthquake, with this species dominating establishment in large gaps created by landslides. Smaller gaps, created by branch or tree death, were closed by both N. fusca and N. menziesii. The long period of releases (1929 -1945) indicates that direct earthquake damage was not the only cause of tr ee death, and that many trees died subsequently most likely of pathogen att ack or a drought in the early 1930s. The impacts of the 1929 earthquake are compared to a storm in 1905 and a drought in 1974-1978 which also affected forests in the region. Our results confirm that earthquakes are an importa nt factor driving forest dynamics in this tectonically active region, and t hat the diversity of earthquake impacts is a major source of heterogeneity in forest structure and regeneration.