Data from: Indirect effects of global change accumulate to alter plant diversity but not ecosystem function in alpine tundra

1. Environmental change can affect species directly by altering their physical environment and indirectly by altering the abundance of interacting species. A key challenge at the interface of community ecology and conservation biology is to predict how direct and indirect effects combine to influence response in a changing environment. In particular, little is known about how direct and indirect effects on biodiversity develop over time or their potential to influence ecosystem function. 2. We studied how nitrogen (N), winter precipitation (snow), and warming influenced diversity and ecosystem function over six years in alpine tundra. We used path analyses to partition direct effects of environmental manipulations from indirect effects due to changes in the abundance of two dominant plants. We hypothesize that 1) indirect effects will develop more slowly but will become stronger than direct effects over time, and 2) after six years, indirect effects will more strongly influence diversity while direct effects will influence ecosystem function. 3. Indirect effects of N on diversity were consistently stronger than direct effects and actually developed quickly, prior to direct effects. Direct effects of snow on diversity were detected in year two but then subsequently were reversed, while indirect effects were detected in year four and grew stronger over time. Overall in year six, indirect effects were much stronger than direct effects. 4. Direct effects predominated for three of four ecosystem functions we measured (productivity, N mineralization, winter N availability). The only indirect effects we found were that N and snow indirectly affected microbial biomass N by influencing Geum abundance. Across all four ecosystem measures, indirect effects were infrequent and weaker than direct effects. 5. Synthesis. Increasing indirect effects on diversity over time indicate that short-term experiments or monitoring of natural systems may underestimate the full magnitude of global change effects on plant communities. Explicitly accounting for changes in dominant plant abundance may be necessary for forecasting plant community response to environmental change. Conversely, weak indirect effects for ecosystem processes suggest that predicting ecosystem function without knowledge of plant responses to global change may be possible.

1. 环境变化可通过改变物种的物理环境对其产生直接影响，也可通过改变互作物种的丰度产生间接影响。在群落生态学（community ecology）与保护生物学（conservation biology）的交叉领域，一项核心挑战是预测直接与间接效应如何共同作用，驱动生物在环境变化下的响应。目前，学界对生物多样性所受的直接与间接效应如何随时间演变，以及二者影响生态系统功能的潜力仍知之甚少。
2. 本研究在高山苔原（alpine tundra）生境中开展了为期6年的定位实验，探究氮（N）、冬季降水（降雪）与增温如何影响生物多样性与生态系统功能。研究采用路径分析（path analysis）方法，将环境操控实验的直接效应与由两种优势植物丰度变化所介导的间接效应进行拆解。我们提出两项假说：其一，间接效应的形成速度慢于直接效应，但随时间推移其强度会超过直接效应；其二，在实验开展6年后，间接效应将对生物多样性产生更强的调控作用，而直接效应则主导生态系统功能的变化。
3. 氮对生物多样性的间接效应始终强于直接效应，且其形成速度实际上快于直接效应。降雪对生物多样性的直接效应在实验第2年被观测到，但后续出现了反转；而其间接效应则在第4年被检测到，并随时间推移不断增强。在实验第6年整体来看，间接效应的强度远高于直接效应。
4. 在本研究测定的4项生态系统功能指标中，有3项以直接效应为主导（生产力、氮矿化（N mineralization）、冬季氮有效性）。本研究仅观测到一类间接效应：氮与降雪通过改变水杨梅属（Geum）植物的丰度，间接影响了微生物生物量氮（microbial biomass N）。综合4项生态系统指标来看，间接效应较为少见，且强度均弱于直接效应。
5. 研究综合：生物多样性所受间接效应随时间不断增强，这一结果表明，短期实验或自然系统监测可能会低估全球变化对植物群落的整体影响。若要精准预测植物群落对环境变化的响应，或许需要明确考量优势植物丰度的变化。与之相对，生态系统过程所受间接效应较弱，这意味着即便不了解植物对全球变化的响应，也有可能实现对生态系统功能的预测。