Data from: The effects of timing of grazing on plant and arthropod communities in high-elevation grasslands

Livestock grazing can be used as a key management tool for maintaining healthy ecosystems. However, the effectiveness of using grazing to modify habitat for species of conservation concern depends on how the grazing regime is implemented. Timing of grazing is one grazing regime component that is less understood than grazing intensity and grazer identity, but is predicted to have important implications for plant and higher trophic level responses. We experimentally assessed how timing of cattle grazing affected plant and arthropod communities in high-elevation grasslands of southwest Montana to better evaluate its use as a tool for multi-trophic level management. We manipulated timing of grazing, with one grazing treatment beginning in mid-June and the other in mid-July, in two experiments conducted in different grassland habitat types (i.e., wet meadow and upland) in 2011 and 2012. In the upland grassland experiment, we found that both early and late grazing treatments reduced forb biomass, whereas graminoid biomass was only reduced with late grazing. Grazing earlier in the growing season versus later did not result in greater recovery of graminoid or forb biomass as expected. In addition, the density of the most ubiquitous grassland arthropod order (Hemiptera) was reduced by both grazing treatments in upland grasslands. A comparison of end-of-season plant responses to grazing in upland versus wet meadow grasslands revealed that grazing reduced graminoid biomass in the wet meadow and forb biomass in the upland, irrespective of timing of grazing. Both grazing treatments also reduced end-of-season total arthropod and Hemiptera densities and Hemiptera biomass in both grassland habitat types. Our results indicate that both early and late season herbivory affect many plant and arthropod characteristics in a similar manner, but grazing earlier may negatively impact species of conservation concern requiring forage earlier in the growing season.畜牧放牧可作为维持生态系统健康的核心管理手段。然而，利用放牧调控具有保护价值的物种的栖息地的效果，取决于放牧制度（grazing regime）的具体实施方式。放牧时间作为放牧制度的组成部分之一，其研究认知程度不及放牧强度（grazing intensity）与牧食者类群（grazer identity），但已有研究预测其对植物及更高营养级（trophic level）生物的响应具有重要影响。本研究针对美国蒙大拿州西南部的高海拔草原开展牛放牧（cattle grazing）时间调控实验，以明确其对植物与节肢动物（arthropod）群落的调控效应，进而更科学地评估放牧作为多营养级管理工具的应用潜力。本研究于2011年与2012年分别在两种不同的草原生境类型（湿草甸与旱地草原）中开展两组实验，设置两种放牧处理：分别于6月中旬与7月中旬启动放牧，以此调控放牧时间。在旱地草原实验中，本研究发现早期与晚期两种放牧处理均会降低非禾本科草本植物（forb）生物量，而禾草类（graminoid）生物量仅在晚期放牧处理下出现下降。与预期不符的是，生长季早期放牧并未比晚期放牧更能促进禾草类或非禾本科草本植物生物量的恢复。此外，两种放牧处理均降低了旱地草原中分布最广泛的节肢动物类群——半翅目（Hemiptera）的种群密度。对比旱地草原与湿草甸草原在生长季末期对放牧的响应发现，无论放牧时间如何，放牧均会降低湿草甸中的禾草类生物量以及旱地草原中的非禾本科草本植物生物量。两种放牧处理还同时降低了两种草原生境下生长季末期的节肢动物总密度、半翅目种群密度及其生物量。本研究结果表明，生长季早期与晚期的牧食作用对多数植物与节肢动物特征的影响模式相似，但早期放牧可能会对那些在生长季早期需要取食的具有保护价值的物种产生负面影响。