Causes of tree line stability: stem growth, recruitment and mortality rates over 15 years at New Zealand Nothofagus tree lines

Aim Abrupt tree linesare relatively rare world-wide and are poorly understood. We studied demographicprocesses at abrupt tree linesto better understand the factors governing their dynamics. Location Five Nothofagus-dominated tree linesites across the South Island, New Zealand. Methods All Nothofagusstems at or above the tree linewere tagged and their spatial locations and heights recorded on three occasions over 15 years at each site. We estimated rates of stem height growth, recruitment and mortality, and used these data to develop a demographicmodel to project stem numbers into the future. Results Stem numbers had increased above the tree lineat most sites over the last 15 years, but with little evidence of tree lineadvance. Growth, mortality and recruitment rates, modelled as a function of stem size, showed variation through time and among sites. Using a demographicmodel to project the numbers of stems above tree line15 years into the future suggests that stem number will continue to increase, but that these tree linesare unlikely to advance at the rate predicted by recent climate warming. Across sites, variation in the rate of change in the number of stems above the tree linewas most strongly related to variation in recruitment rates. Main conclusions Forest expansion at the abrupt Nothofagus tree linein New Zealand appears to be limited primarily by a lack of recruitment of new stems, which may be due to a paucity of suitable sheltered micrositesfor seedling establishment. Based on past demographicrates, our results suggest that these tree linesare relatively unresponsive to recent climate warming and are unlikely to show substantial upslope movement if past demographictrends continue.