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, with little known about woody temperate systems. Further, due to the difficulties of manipulating snow, the effects of precipitation intensity in cold climates is poorly understood. We tested the effects of increased precipitation intensity in a temperate 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. This range of precipitation event sizes are meant to reflect how increases in temperature from 1 to 10 °C would influence event size. We measured above- and belowground vegetation and soil moisture responses to these treatments.
Results/Conclusions
Larger precipitation event sizes increased soil water availability, sagebrush (Artemisia tridentata) stem radius, and canopy greenness (i.e., NDVI), decreased new root growth in shallow soils, and had no effect on herbaceous plant cover. Our treatments spanned a large range of precipitation intensities, outside historical and expected values, and woody plant growth increased across all treatments. This positive effect of precipitation intensity on woody plant growth was surprisingly consistent among treatment levels and supported by NDVI measurements. Results were consistent with the hypothesis that larger precipitation events increase woody plant growth by decreasing interception and evaporation, increasing water availability, and ‘pushing’ water deeper into the soil. These results are important because they extend inference about the role of precipitation intensity on woody growth from sub-tropical to temperate ecosystems, and systems receiving a significant amount of precipitation as snowfall. While many factors will likely be important, results suggest that greater precipitation intensity can contribute to shrub encroachment. Continued atmospheric warming is likely to exaggerate this effect in the future, but additional research will be needed to further constrain the soil type and slope conditions under which greater precipitation intensity increases woody plant growth.