Five Years of Hourly Soil Water Potential Monitoring Demonstrates Forest Thinning Benefits in the North American Southwest

Abstract

North American Southwest semi-arid forests are experiencing unprecedented stress due to the combination of the 21st century megadrought and abnormally dense, young forest stands. Restoration thinning is being widely implemented across the region with the aim of restoring historical stand structures, improving forest health and decreasing the risk of unnaturally severe wildfire. While restoration thinning likely affects soil moisture as well, it is unknown how significant or long-lasting such effects are. Especially little is known about the influence of thinning on root-zone soil moisture used by mature trees. In this study, we used 5 years of data from 126 soil water potential sensors to examine patterns in root-zone (25–100 cm) soil moisture in thinned and non-thinned dense ponderosa pine (Pinus ponderosa) forests as well as the edge areas (boundary) between them during 1–6 years post-thinning. We focused on the spring dry season and calculated three metrics: mean soil water potential, days to onset of soil drying and days spent under a critical drying threshold beyond which ponderosa pine experiences physiological drought stress. We found that thinned areas were consistently significantly wetter and spent less time under critical drying conditions than either non-thinned edge or non-thinned dense forest. Importantly, the thinned forest also experienced more consistent water availability compared to non-thinned forest, regardless of year-to-year precipitation variability. South-facing non-thinned edge areas dried earlier than either of the other treatments and may be especially vulnerable to drought. Our results strongly suggest that restoration thinning significantly improves forest resilience to climate change.