Determining How Increasing Precipitation Intensity Will Impact Rangelands in Utah.

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 woody plant encroachment observed around the world in the past 50 years may be explained in part by increasing precipitation intensity. Further, continued atmospheric warming that is likely to increase precipitation intensity may also potentially increase shrub encroachment in the future