Item

Treeline dynamics: Pattern and process at multiple spatial scales

Harsch, Melanie Ann
Date
2010
Type
Thesis
Fields of Research
ANZSRC::060207 Population Ecology , ANZSRC::050101 Ecological Impacts of Climate Change
Abstract
The primary hypothesis of treeline formation, low growing season temperature limitation of growth, predicts that treeline position will track climatic changes. These hypotheses were generated from broad treeline patterns, which may overlook critical local variability. To assess the hypothesis that all treelines are limited by low temperature and will respond in kind, treeline response over the last 100 years was evaluated at 166 treeline sites in a meta-analysis. Treeline advance was variable and not related to climate warming in the way expected. Treelines that experienced strong winter warming were more likely to have advanced and treelines with a diffuse form were more likely to have advanced than those with an abrupt, Krummholz or island form. Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints. The results suggest that mechanisms other than growing season temperature, such as winter dieback and recruitment failure, may also determine treeline position and dynamics. As treeline responses worldwide confirm a close link between form and dynamics, variability in treeline response may be explained by identifying the mechanisms controlling treeline form. The varying dominance of three mechanisms affecting tree performance - growth limitation, seedling mortality and dieback – modified by species traits, local climatic conditions, stressors and neighbour interactions is proposed to result in different treeline forms and the expected response of treelines to climatic change. The proposed mechanisms controlling treeline form and expected responses to climate warming were subsequently tested at the abrupt Nothofagus treeline in New Zealand. The role of growth, mortality (across all size classes) and recruitment in controlling treeline dynamics were evaluated using long-term data collected along seven abrupt Nothofagus treeline transects in the South Island, New Zealand. Demographic parameters were modelled over two periods, 1991-2002 and 2002-2008 within a Bayesian framework. Stem number increased above treeline over the 15-year study duration but stem distribution above treeline did not change; 90% of all stems and of new recruits occurred within 10 m of the treeline edge. Modelled growth, mortality and recruitment rates varied by period, transect and stem size. Results do not provide clear evidence of treeline advance but do indicate that recruitment is ultimately limiting advance. The causes of recruitment limitation were then tested through transplanted Nothofagus solandri var. cliffortioides and Pinus contorta seedlings along a 200 m vertical transect starting 50 m below treeline and with passive warming. Survival and growth of seedlings transplanted 150 m beyond the Nothofagus treeline did not decrease with distance from the treeline edge or improve with passive warming (repeated measures ANOVA, p > 0.05). Survival varied by species; P. contorta exhibited a greater overall probability of mortality than N. solandri. Relative growth rates did not significantly differ between species but pine exhibited a net increase in height whereas N. solandri exhibited a net decrease in height. At the seedling stage, low temperature is not limiting and N. solandri does not appear to be less well adapted to treeline conditions than northern hemisphere conifer species. The role of facilitation was subsequently tested by removing vegetation around N. solandri seedlings. Vegetation removal negatively affected N. solandri seedling survival. No effect of passive warming was observed. The results confirm that N. solandri can survive beyond their present limit but that growth and survival are limited to facilitative microsites. Treeline advance at the Nothofagus treeline in New Zealand is proposed to be limited by germination ability in dense vegetation and intolerance in the early life stages to sky exposure. Positive feedback, whereby established trees create ideal microsites for germination and seedling establishment, is proposed to be critical in determining recruitment patterns and the relative inertia to climatic change observed at the treeline. The results overall indicate that, globally, treeline response to climate change will be highly variable and not necessarily directly related to climate warming. Treeline form is a good indicator of the mechanisms controlling treeline dynamics and the potential response by treeline to climatic change.
Source DOI
Rights
https://researcharchive.lincoln.ac.nz/pages/rights
Creative Commons Rights
Access Rights