US Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon, USA.
Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, USA.
Ecol Appl. 2024 Jun;34(4):e2953. doi: 10.1002/eap.2953. Epub 2024 Apr 1.
Exotic annual grass invasion is a widespread threat to the integrity of sagebrush ecosystems in Western North America. Although many predictors of annual grass prevalence and native perennial vegetation have been identified, there remains substantial uncertainty about how regional-scale and local-scale predictors interact to determine vegetation heterogeneity, and how associations between vegetation and cattle grazing vary with environmental context. Here, we conducted a regionally extensive, one-season field survey across burned and unburned, grazed, public lands in Oregon and Idaho, with plots stratified by aspect and distance to water within pastures to capture variation in environmental context and grazing intensity. We analyzed regional-scale and local-scale patterns of annual grass, perennial grass, and shrub cover, and examined to what extent plot-level variation was contingent on pasture-level predictions of site favorability. Annual grasses were widespread at burned and unburned sites alike, contrary to assumptions of annual grasses depending on fire, and more common at lower elevations and higher temperatures regionally, as well as on warmer slopes locally. Pasture-level grazing pressure interacted with temperature such that annual grass cover was associated positively with grazing pressure at higher temperatures but associated negatively with grazing pressure at lower temperatures. This suggests that pasture-level temperature and grazing relationships with annual grass abundance are complex and context dependent, although the causality of this relationship deserves further examination. At the plot-level within pastures, annual grass cover did not vary with grazing metrics, but perennial cover did; perennial grasses, for example, had lower cover closer to water sources, but higher cover at higher dung counts within a pasture, suggesting contrasting interpretations of these two grazing proxies. Importantly for predictions of ecosystem response to temperature change, we found that pasture-level and plot-level favorability interacted: perennial grasses had a higher plot-level cover on cooler slopes, and this difference across topography was starkest in pastures that were less favorable for perennial grasses regionally. Understanding the mechanisms behind cross-scale interactions and contingent responses of vegetation to grazing in these increasingly invaded ecosystems will be critical to land management in a changing world.
外来一年生草本植物的入侵是北美西部山艾树生态系统完整性的一个普遍威胁。尽管已经确定了许多预测一年生草本植物流行程度和本地多年生植被的因素，但对于区域尺度和局部尺度的预测因素如何相互作用来确定植被异质性，以及植被与牛放牧的关系如何随环境背景而变化，仍存在很大的不确定性。在这里，我们在俄勒冈州和爱达荷州的燃烧和未燃烧、放牧的公共土地上进行了一次广泛的季节性实地调查，在牧场内按朝向和距离水的位置划分了样方，以捕捉环境背景和放牧强度的变化。我们分析了一年生草本植物、多年生草本植物和灌木覆盖的区域尺度和局部尺度模式，并研究了样方水平的变化在多大程度上取决于牧场水平对地点适宜性的预测。与依赖火灾的假设相反，一年生草本植物在燃烧和未燃烧的地点都很普遍，而且在较低的海拔和较高的温度地区更为常见，在较温暖的斜坡上也更为常见。牧场水平的放牧压力与温度相互作用，使得一年生草本植物的覆盖率与较高温度下的放牧压力呈正相关，但与较低温度下的放牧压力呈负相关。这表明，尽管这种关系的因果关系值得进一步研究，但牧场水平的温度和与一年生草本植物丰度的放牧关系是复杂的，并且取决于背景。在牧场内的样方水平上，一年生草本植物的覆盖率与放牧指标无关，但多年生植物的覆盖率却有关；例如，靠近水源的多年生草本植物的覆盖率较低，但在牧场内的粪便数量较高的地方，多年生草本植物的覆盖率较高，这表明这两个放牧指标的解释相反。重要的是，对于预测生态系统对气候变化的反应，我们发现牧场水平和样方水平的适宜性相互作用：在较凉爽的斜坡上，多年生草本植物的样方水平覆盖率较高，而在区域上对多年生草本植物不太有利的牧场中，这种地形差异最为明显。了解这些日益受到入侵的生态系统中植被对放牧的跨尺度相互作用和有条件响应的机制，对于在不断变化的世界中进行土地管理将是至关重要的。
