stem growth, recruitment and mortality rates over 15 years at New Zealand Nothofagus tree lines

ABSTRACTAim Abrupt tree lines are relatively rare world-wide and are poorlyunderstood. We studied demographic processes at abrupt tree lines to betterunderstand 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 theirspatial locations and heights recorded on three occasions over 15 years at eachsite. We estimated rates of stem height growth, recruitment and mortality, andused these data to develop a demographic model to project stem numbers intothe future.Results Stem numbers had increased above the tree line at most sites over thelast 15 years, but with little evidence of tree line advance. Growth, mortalityand recruitment rates, modelled as a function of stem size, showed variationthrough time and among sites. Using a demographic model to project thenumbers of stems above tree line 15 years into the future suggests that stemnumber will continue to increase, but that these tree lines are unlikely toadvance at the rate predicted by recent climate warming. Across sites, variationin the rate of change in the number of stems above the tree line was moststrongly related to variation in recruitment rates.Main conclusions Forest expansion at the abrupt Nothofagus tree line inNew Zealand appears to be limited primarily by a lack of recruitment of newstems, which may be due to a paucity of suitable sheltered microsites for seed-ling establishment. Based on past demographic rates, our results suggest thatthese tree lines are relatively unresponsive to recent climate warming and areunlikely to show substantial upslope movement if past demographic trendscontinue.KeywordsClimate change, demographic model, dispersal, growth, mortality, Nothofagusmenziesii, Nothofagus solandri, recruitment, Southern Alps, tree line.INTRODUCTIONTree lines are broadly defined as the transition zone from theupper boundary of the closed forest to the upper boundary ofkrummholz or trees of at least 2 m in height. Trees at the treeline are known to be sensitive to temperature and are thusparticularly responsive to climate warming. Consequently,there is considerable interest in monitoring changes in treeline position and exploring the mechanisms controlling treeline dynamics. At a broad scale, studies have focused onidentifying a mechanism that can explain the variation inglobal tree line position, leading to the hypothesis that, at thisscale, tree line position is constrained by low growing seasontemperatures that limit tree growth, with the resultantexpectation that tree line position should shift in response toclimate warming (Ko¨rner, 1998; Ko¨rner & Paulsen, 2004).