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

• Published in: Journal of biogeography 2012-11, Vol.39 (11), p.2061-2071
• Main Authors: Harsch, Melanie A., Buxton, Rowan, Duncan, Richard P., Hulme, Philip E., Wardle, Peter, Wilmshurst, Janet
• Format: Article
• Language: English
• Subjects: Biogeography; Censuses; Climate change; Climate models; demographic model; Demography; dispersal; growth; Modeling; Mortality; Mortality and recruitment; Nothofagus; Nothofagus menziesii; Nothofagus solandri; recruitment; Seedlings; Simulations; Southern Alps; Timberlines; tree line
• ISSN: 0305-0270; 1365-2699
• DOI: 10.1111/j.1365-2699.2012.02763.x

Aim: Abrupt tree lines are relatively rare world-wide and are poorly understood. We studied demographic processes at abrupt tree lines to better understand the factors governing their dynamics. Location: Five Nothofagus-dominated tree line sites across the South Island, New Zealand. Methods: All Nothofagus stems at or above the tree line were 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 demographic model to project stem numbers into the future. Results: Stem numbers had increased above the tree line at most sites over the last 15 years, but with little evidence of tree line advance. Growth, mortality and recruitment rates, modelled as a function of stem size, showed variation through time and among sites. Using a demographic model to project the numbers of stems above tree line 15 years into the future suggests that stem number will continue to increase, but that these tree lines are 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 line was most strongly related to variation in recruitment rates. Main conclusions: Forest expansion at the abrupt Nothofagus tree line in 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 microsites for seedling establishment. Based on past demographic rates, our results suggest that these tree lines are relatively unresponsive to recent climate warming and are unlikely to show substantial upslope movement if past demographic trends continue.