Disturbance and climate warming infiuences on New Zealand Nothofagus tree-line population dynamics

1. We used forest stand history reconstruction to infer the relative roles of disturbance and climate warming on the population dynamics of Nothofagus menziesii (silver beech) dominated tree lines in north Westland, South Island, New Zealand. 2. Stem recruitment in tree line forests over the last 300 years has been episodic, has tended to occur in small, scattered parches, and has been dominated by the production of new stems from existing trees. Pulses of stem recruitment also coincide with episodes of abrupt decline in the radial growth of established trees. These patterns suggest that infrequent natural disturbances form localized canopy openings, damage trees that survive the event, and initiate the establishment of new trees and the production of new stems by surviving trees that fill these openings. 3. Climate warming in New Zealand since 1950 has had little effect on the recruitment of Nothofagus close to the tree line. There is a large seedling pool within the tree line forests, but recruitment will probably require a disturbance-related canopy opening. 4. Natural disturbances drive the population dynamics of Nothofagus tree lines and may modify their response to climate warming. Unlike many Northern Hemisphere tree lines, there has been no recent upward movement of the tree line or increase in seedling establishment. This difference could reflect the greater importance of natural disturbance for recruitment in the abrupt closed-canopy tree lines formed by light-demanding Nothofagus species in the Southern Hemisphere. However, given the ubiquity of disturbance effects in low-altitude forests in both hemispheres, future studies need to consider their role when investigating any tree line response to climate warming.