Comparing warming and grazing effects on birch growth in an alpine environment - a 10-year experiment

Background: Tree encroachment of arctic tundra and alpine vegetation is a generally predicted response to climate warming. However, herbivory plays an important role in structuring these ecosystems and their responses to warming. Aims: To experimentally test how grazing and increased growing season temperature influence growth, physiognomy and stature of birch in the alpine zone. Methods: Trait responses of naturally regenerated birch saplings to warming (open-top chambers), and changed grazing regime (exclosures) were compared with those growing in unmanipulated conditions over a 10-year period (1999-2008). The effect of treatment over time and differences between treatments were analysed with repeated measures GLM (Generalised Linear Model) and simple contrasts in GLM. Results: Warming alone had no major effect on trait responses, however, significantly smaller leaves and an increased number of short shoots indicated warming-related growth constraints. Grazing showed a strong controlling effect on most traits, conserving low stature sapling stage characterised by fewer shoots and larger leaves, compared with non-grazed treatments. Conclusions: Although derived from one experimental site, the results point to a grazing-controlled response to environmental change, with climate (warming) as a secondary driver. This herbivore-driven masking of expected climate-driven tree expansion emphasises the necessity to consider changes in grazing regimes along with climate change, in order to avoid misleading interpretations regarding climate-driven tundra encroachment.