Ecological factors, not climate warming, explain variability in treeline patterns
Current vegetation models predict that as climate warms, forests will replace large areas of tundra. However, observed treeline response to climate warming has been highly variable across the alpine and subarctic. Site-level data can provide local-scale information but has limited use for understanding the influence of broad-scale factors (e.g., extent of site warming) in mediating change.
What are the most important climatic and ecological factors influencing treeline variability? Is the extent of site warming useful for predicting treeline response?
We synthesized 25 species-specific case studies from 10 regions across circumpolar treeline. Tree growth form and age structure data from life history stages (seedling, sapling and adult) for each case study were compared between zones across the treeline gradient. Generalized Linear Mixed Models (GLMM) were used to determine what ecological and climate factors are good predictors of treeline growth form and treeline response.
Growth form relationships for trees could not be explained by the extent of treeline warming, although shifts in growth form across treeline were detected. We showed that ecological factors were better at predicting treeline advance and responses to these factors depended on the life history stage (seedling, sapling or adult trees) of the tree species. We found that the best predictor of whether a treeline would respond positively was treeline form (diffuse or persistent) and how close the treeline was to the ocean. Our findings also suggest that treelines with a propensity for vegetative reproduction were more likely to respond positively. However, direct estimates of climate warming (summer and winter mean temperature, growing degree-days) were not significant in predicting treeline response. Despite the assertion that treeline position is controlled by temperature, ecological factors play a significant role in predicting treeline patterns.
Our results show that ecological factors, in the context of climate change, best explain treeline patterns though continued climate warming will play a significant role in treeline migration.