Department of Integrative Biology and Biodiversity Research, Institute of Wildlife Biology and Game Management, University of Natural Resources and Life Sciences, Vienna, Gregor-Mendel-Straße 33, 1180, Vienna, Austria.
Department of Integrative Biology and Evolution, Research Institute of Wildlife Ecology, University of Veterinary Medicine, Savoyenstraße 1, 1160, Vienna, Austria.
Sci Rep. 2020 Nov 20;10(1):20274. doi: 10.1038/s41598-020-76843-3.
Herbivores are constitutive elements of most terrestrial ecosystems. Understanding effects of herbivory on ecosystem dynamics is thus a major, albeit challenging task in community ecology. Effects of mammals on plant communities are typically explored by comparing plant densities or diversity in exclosure experiments. This might over-estimate long-term herbivore effects at community levels as early life stage mortality is driven by a multitude of factors. Addressing these challenges, we established a set of 100 pairs of ungulate exclosures and unfenced control plots (25 m) in mixed montane forests in the Alps in 1989 covering a forest area of 90 km. Investigations ran until 2013. Analogous to the gap-maker-gap-filler approach, dynamically recording the height of the largest trees per tree species in paired plots with and without exclosures might allow for assessing herbivore impacts on those individuals with a high probability of attaining reproductive stages. We thus tested if recording maximum heights of regenerating trees would better reflect effects of ungulate herbivory on long-term dynamics of tree regeneration than recording of stem density, and if species dominance patterns would shift over time. For quantifying the effects of ungulate herbivory simultaneously at community and species level we used principle response curves (PRC). PRCs yielded traceable results both at community and species level. Trajectories of maximum heights yielded significant results contrary to trajectories of total stem density. Response patterns of tree species were not uniform over time: e.g., both Norway spruce and European larch switched in their response to fencing. Fencing explained about 3% of the variance of maximum tree heights after nine years but increased to about 10% after 24 years thus confirming the importance of long-term surveys. Maximum height dynamics of tree species, addressed in our study, can thus reflect local dominance of tree species via asymmetric plant competition. Such effects, both within and among forest patches, can accrue over time shaping forest structure and composition.
食草动物是大多数陆地生态系统的组成部分。因此，了解食草动物对生态系统动态的影响是群落生态学中的一项主要任务，尽管具有挑战性。通常通过比较围栏实验中的植物密度或多样性来探索哺乳动物对植物群落的影响。由于早期生命阶段的死亡率受到多种因素的驱动，因此这可能会高估社区层面上长期食草动物的影响。为了应对这些挑战，我们于 1989 年在阿尔卑斯山的混合山地森林中建立了一组 100 对有蹄类动物围栏和无围栏对照小区（25 米），覆盖了 90 公里的森林面积。调查一直持续到 2013 年。类似于缺口制造者-缺口填充者方法，在有围栏和无围栏的配对小区中动态记录每个树种最大树木的高度，可能允许评估食草动物对那些有很大可能达到繁殖阶段的个体的影响。因此，我们测试了记录再生树木的最大高度是否比记录茎密度更好地反映有蹄类动物食草对树木再生长期动态的影响，以及物种优势格局是否会随时间推移而变化。为了同时在群落和物种水平上量化有蹄类动物食草的影响，我们使用了主要响应曲线（PRC）。PRC 在群落和物种水平上都产生了可追踪的结果。最大高度轨迹产生了显著的结果，而总茎密度轨迹则没有。随着时间的推移，树种的响应模式并不均匀：例如，挪威云杉和欧洲落叶松在对围栏的反应上都发生了变化。围栏在九年之后解释了最大树木高度方差的约 3%，但在 24 年之后增加到约 10%，从而证实了长期调查的重要性。我们研究中涉及的树种最大高度动态，可以通过不对称的植物竞争反映出树种在当地的优势。这种效应，无论是在森林斑块内还是森林斑块之间，都可以随着时间的推移积累，从而塑造森林结构和组成。
