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

Abstract

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.

Keywords:

• alpine tundra
• Betula pubescens ssp. tortuosa
• birch saplings
• changed grazing regime
• climate change
• experimental warming
• growth response
• open-top chambers

Acknowledgements

We would like to thank Tord Bretten and Frode Thorsteinsen for their help with establishing the field site; Brita Hytteborn, Sølvi Wehn, and Andreas Forberg for field assistance; and Johan Olofsson and two anonymous reviewers for helpful comments on the manuscript. We are grateful for the logistic support by the Kongsvold Biological Station and for the financial support from the European Commission (ENV4-CT97-0586) for the period 1999–2002, and NTNU, NINA and the Research Council of Norway (IPY grant 176065/S30 to “PPS Arctic”) in the period after 2002.