Data from: Patterns and thresholds of grazing-induced changes in community structure and ecosystem functioning: species-level responses and the critical role of species traits

Overgrazing has resulted in widespread decline in biodiversity and ecosystem functioning in grasslands worldwide in recent decades. However, few studies have examined the patterns and thresholds of grazing-induced changes in community structure and ecosystem functioning along a grazing gradient and based on species-level responses and plant functional traits. To identify the thresholds of grazing intensity at both species and community levels, we conducted a grazing manipulation experiment with seven levels of grazing intensity (0–9 sheep ha-1) and two topographies (flat versus slope) in a typical steppe. Four plant functional traits were measured, including specific leaf area (SLA), plant height, leaf nitrogen content (LNC) and stem: leaf ratio (SLR). The threshold of grazing intensity that significantly altered community composition was at 3.75 sheep ha-1 for the flat system and 3.0 sheep ha-1 for the slope system. For both flat and slope systems, the threshold grazing intensity for changes in primary productivity was at 3.0 sheep ha-1, beyond which the productivity decreased substantially. At species level, the abundances of common species, most of which are perennial grasses, declined at moderate grazing intensities (3.0–4.5 sheep ha-1). The abundances of most rare species, which are perennial forbs, declined at low grazing intensities (1.5–3.0 sheep ha-1). Specific leaf area and leaf nitrogen content are good predictors of species-level responses to grazing. Low SLA and high LNC species are negatively affected by high grazing intensity, while high SLA and low LNC species are little affected by grazing. The negative effect of grazing intensity on species abundance was greater in the slope system than in the flat system. Synthesis and applications. Our results indicate that the structural and functioning thresholds of grazing intensity depend on plant traits and species composition, which is mediated by topographic location. These findings, integrating plant functional traits and threshold approaches, have important implications for determining sustainable grazing intensity in grassland management and biodiversity conservation in semi-arid regions.近数十年来，过度放牧已导致全球草原的生物多样性与生态系统功能普遍衰退。然而，现有研究极少基于物种水平响应与植物功能性状（plant functional traits），沿放牧梯度探究放牧引发的群落结构与生态系统功能变化的模式及阈值。 为明确物种与群落水平上的放牧强度阈值，我们在典型草原（typical steppe）设置了7个放牧强度梯度（0~9只羊·公顷⁻¹）与2种地形（平地与坡地）的放牧控制实验。测定了4种植物功能性状：比叶面积（specific leaf area, SLA）、株高、叶氮含量（leaf nitrogen content, LNC）以及茎叶比（stem: leaf ratio, SLR）。 显著改变群落组成的放牧强度阈值，平地系统为3.75只羊·公顷⁻¹，坡地系统为3.0只羊·公顷⁻¹。对于平地与坡地两种系统，初级生产力变化的放牧强度阈值均为3.0只羊·公顷⁻¹，超过该阈值后生产力会大幅下降。 在物种水平上，多数为多年生禾草的常见物种种群丰度在中度放牧强度（3.0~4.5只羊·公顷⁻¹）下出现下降；而多数为多年生杂类草的稀有种群丰度则在低放牧强度（1.5~3.0只羊·公顷⁻¹）下便出现下降。比叶面积与叶氮含量是物种水平放牧响应的良好预测因子。低比叶面积、高叶氮含量的物种会受到高强度放牧的负面影响，而高比叶面积、低叶氮含量的物种则几乎不受放牧影响。放牧强度对物种种群丰度的负面影响在坡地系统中显著强于平地系统。 综合与应用。本研究结果表明，放牧强度的结构与功能阈值取决于植物性状与物种组成，且受地形条件介导。本研究整合植物功能性状与阈值分析方法的发现，对半干旱区草原管理与生物多样性保护中合理确定可持续放牧强度具有重要指导意义。