Museum of Natural History and Archaeology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway.
Ecology. 2010 Nov;91(11):3414-20. doi: 10.1890/09-2300.1.
The treeline ecotone divides forest from open alpine or arctic vegetation states. Treelines are generally perceived to be temperature limited. The role of herbivores in limiting the treeline is more controversial, as experimental evidence from relevant large scales is lacking. Here we quantify the impact of different experimentally controlled herbivore densities on the recruitment and survival of birch Betula pubescens tortuosa along an altitudinal gradient in the mountains of southern Norway. After eight years of summer grazing in large-scale enclosures at densities of 0, 25, and 80 sheep/km2, birch recruited within the whole altitudinal range of ungrazed enclosures, but recruitment was rarer in enclosures with low-density sheep and was largely limited to within the treeline in enclosures with high-density sheep. In contrast, the distribution of saplings (birch older than the experiment) did not differ between grazing treatments, suggesting that grazing sheep primarily limit the establishment of new tree recruits rather than decrease the survival of existing individuals. This study provides direct experimental evidence that herbivores can limit the treeline below its potential at the landscape scale and even at low herbivore densities in this climatic zone. Land use changes should thus be considered in addition to climatic changes as potential drivers of ecotone shifts.
林线交错带将森林与开阔的高山或北极植被带分开。一般认为林线受温度限制。食草动物在限制林线方面的作用更具争议性，因为缺乏相关大尺度的实验证据。在这里，我们量化了不同实验控制的食草动物密度对挪威南部山区沿海拔梯度生长的桦树（Betula pubescens tortuosa）繁殖和存活的影响。经过八年在大尺度围栏内进行的夏季放牧，围栏内的桦树在未放牧围栏的整个海拔范围内繁殖，但在低密度绵羊围栏内繁殖较少，而在高密度绵羊围栏内则主要局限于林线内。相比之下，在放牧处理之间，幼树（比实验年龄更大的桦树）的分布没有差异，这表明放牧绵羊主要限制了新树繁殖的建立，而不是降低了现有个体的存活率。这项研究提供了直接的实验证据，表明食草动物可以在景观尺度上限制林线低于其潜在范围，即使在该气候带的低食草动物密度下也是如此。因此，除了气候变化外，土地利用变化也应被视为生态交错带变化的潜在驱动因素。
