Browsing interacts with climate to determine tree-ring increment

Abstract

Summary 1. A warming climate has been linked to shifts in plant distribution and growth. The relationship between climate and growth is used to infer past climate conditions within dendrochronological studies. However, browsing may interact with climate to determine growth, yet the impact of large herbivores on tree-ring growth series is largely unknown. 2. Here, we disentangle the interactions between climate and herbivory in determining plant growth at the upper distribution limit of mountain birch (Betula pubescens ssp. czerepanovii). 3. Stem discs of 206 birch were sampled within an altitudinal tree line ecotone in southern Norway, after 9 years of experimental browsing at three sheep densities (unbrowsed, low and high with 0, 25 and 80 sheep km−2). Annual radial growth was measured using digital microscopy and related to summer temperatures, altitude and sheep density. 4. Radial growth was negatively related to altitude and related to summer temperature in a nonlinear fashion, increasing from low temperatures and saturating and decreasing at high temperatures. However, the variation between browsing treatments overrode the influence of interannual temperature in determining growth. Increasing sheep densities limited radial growth and interacted with both temperature and altitude. The temperature of peak radial growth increased at higher densities of sheep, and growth was less limited by altitude at low sheep density. 5. This demonstrates that browsing interacts with temperature to determine the growth of mountain birch at the upper distribution limit and that the variation in growth linked to sheep density was greater than that linked to the 50-year range of interannual temperatures. The ability of browsing to affect radial growth has important implications for the use of dendrochronology to infer past climatic conditions, where samples are taken from regions that have undergone past land-use change.