As the atmosphere warms, precipitation events become larger, but less frequent. Yet, there is fundamental disagreement about how increased precipitation intensity will affect vegetation. Walter's two-layer hypothesis and experiments testing it have demonstrated that precipitation intensity can increase woody plant growth. Observational studies have found the opposite pattern. Not only are the patterns contradictory, but inference is largely limited to grasslands and savannas. We tested the effects of increased precipitation intensity in a shrub-steppe ecosystem that receives >30% of its precipitation as snow. We used 11 (8 m x 8 m) shelters to collect and redeposit rain and snow as larger, more intense events. Total annual precipitation was the same in all plots, but each plot received different precipitation event sizes ranging from 1 mm to 18 mm. Over three growing seasons, larger precipitation event sizes increased soil water availability, sagebrush (Artemisia tridentata) stem radius, and canopy greenness, decreased new root growth in shallow soils, and had no effect on herbaceous plant cover. Thus, we found that increased precipitation intensity can increase soil water availability and woody plant growth in a cold semi-arid system. Assuming that stem growth is positively correlated with shrub reproduction, establishment and spread, results suggest that increasing precipitation intensity may have contributed to the woody plant encroachment observed around the world in the past 50 years. Further, continuing increases in precipitation intensity caused by atmospheric warming are likely to continue to contribute to shrub encroachment in the future.

中文翻译：

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冷半干旱系统木本植物生长随降水强度增加

随着大气变暖，降水事件变得更大，但频率降低。然而，关于降水强度增加将如何影响植被存在根本分歧。Walter 的两层假设和测试它的实验已经证明降水强度可以增加木本植物的生长。观察性研究发现了相反的模式。不仅模式相互矛盾，而且推断主要限于草原和稀树草原。我们测试了在一个灌木草原生态系统中增加降水强度的影响，该生态系统接收了超过 30% 的降水作为降雪。我们使用了 11 个 (8 mx 8 m) 避难所来收集雨雪并将其重新储存为更大、更激烈的事件。所有地块的年总降水量相同，但每个地块都收到了不同的降水事件大小，范围从 1 毫米到 18 毫米。在三个生长季节中，较大的降水事件规模增加了土壤水分可用性、山艾 (Artemisia tridentata) 茎半径和冠层绿度，减少了浅层土壤中新根的生长，并且对草本植物盖度没有影响。因此，我们发现在寒冷的半干旱系统中，降水强度的增加可以增加土壤水分的有效性和木本植物的生长。假设茎的生长与灌木的繁殖、建立和蔓延呈正相关，结果表明降水强度的增加可能导致了过去 50 年世界各地观察到的木本植物侵占。更多，