Evidence of widespread, synchronous, disturbance‐initiated forest establishment in Westland, New Zealand.

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Abstract
Mature stands of trees dominate Westland forests and young stands are relatively scarce. To examine this pattern, we collate previously‐gathered data on the age and diameter of trees in 60 forest stands from throughout Westland, and use these data to estimate the dates at which approximately even‐aged cohorts of trees were initiated by major disturbances. The ages of the oldest tree in each of 80 cohorts were combined to give a picture of the timing of cohort establishment and the history of major disturbance in the Westland region. We identified two periods 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 were consistent across all six canopy species in the study, throughout north, central, and south Westland, and across a range of landforms. Our results suggest there have been two major pulses of tree regeneration in Westland over the last 600 years, although the longer and most recent pulse may reflect two separate pulses. Many cohorts contained independent evidence of a catastrophic origin following major disturbances such as floods and landslips, and we suggest that the pulses of cohort establishment reflect periods when there was a marked increase in the frequency or intensity of major natural disturbances throughout Westland. Adjusting for the time taken for trees to grow to coring height and for a delay in colonisation following disturbance, we estimate that there were episodes of major natural disturbance throughout Westland 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 nationwide periods of increased storminess.