Jordan Samuel E, Palmquist Kyle A, Burke Ingrid C, Lauenroth William K
School of the Environment, Yale University, New Haven, Connecticut, USA.
Department of Biological Sciences, Marshall University, Huntington, West Virginia, USA.
Ecol Appl. 2022 Dec;32(8):e2693. doi: 10.1002/eap.2693. Epub 2022 Jul 25.
Livestock grazing is a globally important land use and has the potential to significantly influence plant community structure and ecosystem function, yet several critical knowledge gaps remain on the direction and magnitude of grazing impacts. Furthermore, much of our understanding of the long-term effects on plant community composition and structure are based on grazer exclusion experiments, which explicitly avoid characterizing effects along grazing intensity gradients. We sampled big sagebrush plant communities using 68 plots located along grazing intensity gradients to determine how grazing intensity influences multiple aspects of plant community structure over time. This was accomplished by sampling plant communities at different distances from 17 artificial watering sources, using distance from water and cow dung density as proxies for grazing intensity at individual plots. Total vegetation cover and total grass cover were negatively related to grazing intensity, and cover of annual forbs, exotic cover, and exotic richness were positively related to grazing intensity. In contrast, species richness and composition, bunchgrass biomass, shrub density and size, percentage cover of bare ground, litter, and biological soil crusts did not vary along our grazing intensity gradients, in spite of our expectations to the contrary. Our results suggest that the effects of livestock grazing over multiple decades (mean = 46 years) in our sites are relatively small, especially for native perennial species, and that the big sagebrush plant communities we sampled are somewhat resistant to livestock grazing. Collectively, our findings are consistent with existing evidence that indicates the stability of the big sagebrush plant functional type composition under current grazing management regimes.
牲畜放牧是一种全球重要的土地利用方式，有可能显著影响植物群落结构和生态系统功能，但在放牧影响的方向和程度方面仍存在一些关键的知识空白。此外，我们对植物群落组成和结构长期影响的许多理解是基于放牧排除实验，这些实验明确避免了对沿放牧强度梯度的影响进行表征。我们沿着放牧强度梯度设置了68个样地，对大艾草植物群落进行采样，以确定放牧强度如何随时间影响植物群落结构的多个方面。这是通过在距离17个人工水源不同距离处对植物群落进行采样来实现的，利用与水源的距离和牛粪密度作为各个样地放牧强度的指标。总植被覆盖度和总草覆盖度与放牧强度呈负相关，一年生草本植物覆盖度、外来植物覆盖度和外来植物丰富度与放牧强度呈正相关。相比之下，物种丰富度和组成、丛生禾本科植物生物量、灌木密度和大小、裸地、凋落物和生物土壤结皮的覆盖百分比在我们的放牧强度梯度上并没有变化，尽管我们原本预期会相反。我们的结果表明，在我们研究的地点，几十年（平均 = 46年）的牲畜放牧影响相对较小，特别是对本地多年生物种而言，而且我们采样的大艾草植物群落对牲畜放牧有一定的抵抗力。总体而言，我们的研究结果与现有证据一致，这些证据表明在当前放牧管理制度下大艾草植物功能型组成的稳定性。
