EVIDENCE OF WIDESPREAD, SYNCHRONOUS, DISTURBANCE-INITIATED FOREST ESTABLISHMENT IN WESTLAND, NEW-ZEALAND

Mature stands of trees dominate Westland forests and young stands are relatively scarce. To examine this pattern, we collate previously-gath ered data on the age and diameter of trees in 60 forest stands from th roughout Westland, and use these data to estimate the dates at which a pproximately even-aged cohorts of trees were initiated by major distur bances. The ages of the oldest tree in each of 80 cohorts were combine d to give a picture of the timing of cohort establishment and the hist ory of major disturbance in the Westland region. We identified two per iods of greatly increased cohort establishment in the last 600 years, at c. 200-300 and 500-550 years ago. During these periods over 70 % of the cohorts we examined were initiated. These establishment periods w ere consistent across all six canopy species in the study, throughout north, central, end south Westland, and across a range of landforms. O ur results suggest there have been two major pulses of tree regenerati on in Westland over the last 600 years, although the longer and most r ecent pulse may reflect two separate pulses. Many cohorts contained in dependent evidence of a catastrophic origin following major disturbanc es such as floods and landslips, and we suggest that the pulses of coh ort establishment reflect periods when there was a marked increase in the frequency or intensity of major natural disturbances throughout We stland. Adjusting for the time taken for trees to grow to coring heigh t and for a delay in colonisation following disturbance, we estimate t hat there were episodes of major natural disturbance throughout Westla nd around 250-350 and 550-600 years ago. These dates closely coincide with estimates of the timing of two of the three last major movements of the Alpine Fault, and less closely coincide with inferred nationwid e periods of increased storminess.