Initial biomass and biomass change of tundra and forest plants in three herbivore treatments in Norway and Sweden

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures with different mesh sizes that exclude either only reindeer or all mammalian herbivores including voles and lemmings. The exclosures were replicated at three forest and tundra sites at four different locations along a climatic gradient (oceanic to continental) in northern Fennoscandia. Since the last 10 years have been exceptionally warm, we could study how warming has influenced the vegetation in different grazing treatments. Our results show that the abundance of the dominant shrub, Betula nana, has increased during the last decade, but that the increase was more pronounced when herbivores were excluded. Reindeer have the largest effect on shrubs in tundra, while voles and lemmings have a larger effect in the forest. The positive relationship between annual mean temperature and shrub growth in the absence of herbivores and the lack of relationships in grazed controls is another indication that shrub abundance is controlled by an interaction between herbivores and climate. In addition to their effects on taller shrubs (> 0.3 m), reindeer reduced the abundance of lichens, whereas microtine rodents reduced the abundance of dwarf shrubs (< 0.3 m) and mosses. In contrast to short-term responses, competitive interactions between dwarf shrubs and lichens were evident in the long term. These results show that herbivores have to be considered in order to understand how a changing climate will influence tundra ecosystems.

近期泛北极（Pan-Arctic）灌木扩张现象被普遍认为是气候变暖的响应。然而，草食动物同样可调控北极生态系统中灌木的种群丰度。本研究通过追踪过去10年间，芬诺斯堪的亚北部沿气候梯度（从海洋性到大陆性）分布的4处地点的3处森林与苔原样地内，不同网眼尺寸的动物排除围栏内外永久样地的植物群落组成变化，探讨了这两种对立的解释：此类围栏可选择性排除仅驯鹿，或是排除包括田鼠、旅鼠在内的所有哺乳类草食动物，且该实验设置在上述样地中完成了重复。由于过去10年气候异常温暖，本研究得以探讨气候变暖对不同放牧处理下植被的影响。
研究结果显示，作为优势灌木的矮桦（Betula nana）的种群丰度在近十年间有所提升，但在排除草食动物的样地中，这种增长更为显著。驯鹿对苔原生境中的灌木影响最为显著，而田鼠与旅鼠则对森林生境中的灌木影响更大。在无草食动物的样地中，年平均气温与灌木生长呈正相关，而在放牧对照组中则无此关联，这进一步表明灌木丰度受草食动物与气候的交互作用调控。
除对高度大于0.3米的高大灌木产生影响外，驯鹿还会降低地衣的种群丰度；而田鼠类啮齿动物则会降低高度小于0.3米的矮灌木与苔藓的种群丰度。与短期响应不同，矮灌木与地衣之间的竞争关系在长期实验中表现显著。本研究结果表明，若要明晰气候变化对苔原生态系统的影响，必须将草食动物的作用纳入考量范畴。