Questions: An advance of high-altitude treelines has been reported in response to warming climate throughout the globe. Understanding local controls on the establishment of seedlings above the treeline is essential for predicting wider patterns of treeline response from a process-based perspective. Here we investigate patterns of seedling establishment in relation to microsite conditions in an advancing treeline ecotone in the subtropics, using climate data recorded at the plant-relevant scale. We sought to determine which temperature factors were of importance, if sheltering plays an important role in seedling establishment and if the response varied with seedling age. Location:The Abies kawakamii treelines of the Central Mountain Range, Taiwan, 2800 to 3275 m a.s.l.Methods: Seedlings were monitored in plots covering a range of treeline structural forms over a period of 2 yrs. Temperatures were recorded at plant-relevant height (5 cm) above ground and depth below ground with data loggers. Microtopographic sheltering (at a 10-m scale) and surrounding vegetation were measured. The influence of the above variables on seedling number and growth was investigated using generalized linear models and linear mixed effect models, respectively.Results: Soil temperatures had more influence on seedling number than air temperature, whilst air temperature was positively associated with subsequent seedling growth. Establishment patterns were found to have a strong relationship with microtopographic sheltering, with more sheltered areas having elevated seedling numbers. Early growth may have significant implications for subsequent plant performance since smaller seedlings were more sensitive to both temperature and microtopography than larger seedlings.Conclusions: Air and soil temperatures and microtopography determine spatial patterns of seedling establishment. Our results suggest that establishment above the treeline is likely to continue as the climate warms, although advance will not be spatially uniform due to the modifying influence of topography. This variability has important implications for the persistence and extinction of alpine plant communities occurring above treeline in topographically complex systems.

Aim Climate change is causing shifts in the range of species worldwide. In high-altitude areas forests are often observed to be shifting their upper limits to higher altitudes in response to warming temperatures. Although this phenomenon is well described, the possible consequences of this for the wider forest community have not been fully considered. In this study, we used epiphytic macro-lichens to investigate the impacts of tree line advance on associated organisms. We hypothesized that the rate of forest advance should influence the species richness and composition of associated communities.Location The Hehuanshan area, Central Mountain Range, Taiwan (24°08-09Methods Lichen communities were sampled on trees occurring at 33 tree line sites displaying varying degrees of advance. Habitat variables were recorded. ANOVA, GLMM, nestedness analysis and indicator species analysis were used to investigate patterns of species richness and community composition and their association with tree line advance and habitat variables.Results Species richness was lower in tree lines exhibiting rapid advance; reductions were related to tree size (considered here as a proxy for age) and the distance over which advance had occurred. Community composition varied with tree line form and forest position. Only a subset of species found in slowly advancing or stable forest edges occurred in rapidly advancing tree lines.Main conclusions Differential migration rates between co-occurring species and differences in habitat structure associated with tree line advance can result in community change, but this depends, amongst other factors, on the speed of tree line advance. The importance of advance rate in determining the response suggests that reductions in species richness at tree line could be transitory. However, this will depend on whether advance continues, and on the changes in habitat associated with advance. Given the complexity of tree line behaviour, the findings that we report represent an essential step in understanding community responses to climate change. This understanding is of importance for biodiversity and conservation, especially given the high rate of endemism reported for this and other alpine regions.