Rangeland ecosystems worldwide are characterized by a high degree of uncertainty in precipitation, both within and across years. Such uncertainty creates challenges for livestock managers seeking to match herbivore numbers with forage availability to prevent vegetation degradation and optimize livestock production. Here, we assess variation in annual large herbivore production (LHP, kg/ha) across multiple herbivore densities over a 78-yr period (1940-2018) in a semiarid rangeland ecosystem (shortgrass steppe of eastern Colorado, USA) that has experienced several phase changes in global-level sea surface temperature (SST) anomalies, as measured by the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO). We examined the influence of prevailing PDO phase, magnitude of late winter (February-April) ENSO, prior growing-season precipitation (prior April to prior September) and precipitation during the six months (prior October to current April) preceding the growing season on LHP. All of these are known prior to the start of the growing season in the shortgrass steppe and could potentially be used by livestock managers to adjust herbivore densities. Annual LHP was greater during warm PDO irrespective of herbivore density, while variance in LHP increased by 69% (moderate density) and 91% (high density) under cold-phase compared to warm-phase PDO. No differences in LHP attributed to PDO phase were observed with low herbivore density. ENSO effects on LHP, specifically La Niña, were more pronounced during cold-phase PDO years. High herbivore density increased LHP at a greater rate than at moderate and low densities with increasing fall and winter precipitation. Differential gain, a weighted measure of LHP under higher relative to lower herbivore densities, was sensitive to prevailing PDO phase, ENSO magnitude, and precipitation amounts from the prior growing season and current fall-winter season. Temporal hierarchical approaches using PDO, ENSO, and local-scale precipitation can enhance decision-making for flexible herbivore densities. Herbivore densities could be increased above recommended levels with lowered risk of negative returns for managers during warm-phase PDO to result in greater LHP and less variability. Conversely, during cold-phase PDO, managers should be cognizant of the additional influences of ENSO and prior fall-winter precipitation, which can help predict when to reduce herbivore densities and minimize risk of forage shortages.

中文翻译：

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对大型草食动物生产力的大规模和局部气候控制：对适应性牧场管理的意义。

全世界范围内的牧场生态系统的特点是多年内和跨年降水高度不确定。这种不确定性给寻求将草食动物的数量与牧草的数量相匹配以防止植被退化和优化畜牧生产的畜牧管理者带来了挑战。在这里，我们评估了半干旱牧场生态系统（美国科罗拉多州东部的短草草原）经历了几次干旱之后的78年（1940-2018年）内多种草食动物密度的年度大型草食动物产量（LHP，kg / ha）的变化。由太平洋年代际涛动（PDO）和厄尔尼诺-南方涛动（ENSO）测得的全球海平面温度（SST）异常的相位变化。我们研究了目前的PDO阶段，冬末（2-4月）ENSO的影响，LHP的生长季节之前的生长季节降水（4月之前至9月之前）和生长季节之前的六个月（10月之前至当前的4月之前）的降水。所有这些在矮草草原生长期开始之前都是已知的，并且可能被牲畜管理者用来调节草食动物的密度。与草食动物密度无关，在温暖的PDO期间，年LHP较大，而冷态下LHP的变化分别增加了69％（中等密度）和91％（高密度）。在低草食动物密度下，未观察到归因于PDO相的LHP差异。ENSO对LHP（特别是LaNiña）的影响在PDO寒冷时期尤为明显。随着中秋和冬季降水的增加，高草食动物密度增加的LHP比中低密度的增加。相对于较低的草食动物密度而言，差分增益是LHP的加权度量，它对普遍的PDO阶段，ENSO量级以及先前生长季节和当前秋冬季的降水量敏感。使用PDO，ENSO和局部降水的时间分层方法可以增强针对灵活的草食动物密度的决策。草食动物的密度可以增加到建议水平以上，从而降低经理人在暖期PDO期间产生负回报的风险，从而导致更大的LHP和更少的变异性。相反，在冷态PDO期间，管理人员应意识到ENSO和以前的秋冬降水的额外影响，