Many perennial plants in semiarid rangelands have experienced population declines, and understanding the ecological and demographic processes behind these declines is important to slowing or reversing them. Although anthropogenic disturbances drive many declines, other sorts of environmental variability, such as the differences in solar radiation with aspect, may impact population success locally. We experimentally assessed the role of solar radiation in driving an apparent decline in a common perennial bunchgrass, bluebunch wheatgrass (Pseudoroegneria spicata), on south-facing slopes at a site in the Columbia Basin of North America. Across three separate experiments on south-facing slopes, we observed dramatically (5−36 ×) higher seedling success in plots shaded to approximate the solar radiation of north-facing slopes relative to adjacent open (unshaded) plots. When we applied the rates of seedling success from these experiments to demographic models from this site, we found that seedling success in unshaded plots was often too low to allow a stable population on south-facing slopes, but that seedling success in shaded plots was often high enough to allow a stable or increasing population on north-facing slopes. We originally hypothesized that the primary mechanism driving this effect was water stress caused by greater evaporation from hotter open plots. However, despite soils in open plots being consistently hotter than soils in shaded plots, we did not observe a clear pattern of greater soil moisture in shaded plots. Therefore, it appears that higher solar radiation or higher temperatures on south-facing slopes may be sufficient to dramatically reduce seedling survival, and that the higher density of bluebunch wheatgrass on north-facing slopes relative to south-facings slopes may be driven primarily by this low survival of seedlings. As climate warms, the reduced seedling survival that threatens our bluebunch wheatgrass population may be expected to threaten many other species of perennials in similar rangelands.

半干旱牧场的许多多年生植物都经历了种群数量下降，了解这些下降背后的生态和人口过程对于减缓或逆转它们非常重要。尽管人为干扰导致了许多种群数量的下降，但其他类型的环境变化，例如太阳辐射与方位的差异，可能会影响当地种群的成功。我们通过实验评估了太阳辐射在导致北美哥伦比亚盆地朝南斜坡上常见的多年生束草——蓝束小麦草（Pseudoroegneria spicata）明显减少中的作用。在朝南斜坡上的三个独立实验中，我们观察到相对于邻近的开放（无阴影）地块，在遮挡以近似朝北斜坡太阳辐射的地块中，幼苗成功率显着提高（5−36 ×）。当我们将这些实验的幼苗成功率应用到该地点的人口统计模型时，我们发现，无阴影地块中的幼苗成功率通常太低，无法在朝南的斜坡上形成稳定的种群，但阴影地块中的幼苗成功率往往很高。足够高以允许朝北斜坡上的人口稳定或增加。我们最初假设，驱动这种效应的主要机制是由于较热的开阔地块蒸发量更大而引起的水分胁迫。然而，尽管开放地块中的土壤始终比阴影地块中的土壤热，但我们没有观察到阴影地块中土壤湿度较高的明显模式。因此，朝南斜坡上较高的太阳辐射或较高的温度可能足以显着降低幼苗的存活率，并且相对于朝南的斜坡，朝北的斜坡上蓝束小麦草的密度较高可能主要是由于这个原因。幼苗成活率低。随着气候变暖，威胁蓝束小麦草种群的幼苗存活率下降，预计也会威胁到类似牧场中的许多其他多年生植物物种。