Seasonal Precipitation and Soil Moisture Relationships Across Forests and Woodlands in the Southwestern United States

Abstract

Precipitation [P: mm] controls forest and woodland dynamics in the southwestern United States (SWUS) by altering soil moisture [θ: mm3 mm−3] availability, but the influence of P on θ is complex, varying across space and time. We evaluated seasonal P and θ relationships at shallow (0–20 cm) and intermediate (50 cm) soil depths for nine semiarid forest and woodland sites (56 total years), which comprised three elevation gradients in the SWUS. We developed time-varying definitions of winter (snow accumulation), spring (moisture recharge), and summer (moisture deficit), and determined how these sites exhibited similar P influence on θ across depths in the soil profile, between seasons, and in seasons with above- and below-average P. Higher elevation sites (>2,800 m) experienced greater winter P, longer springs, and shorter summers compared to lower elevation sites (<2,500 m). Seasons with above- and below-average P reduced elevation-associated differences. θ at 0–20 cm was generally decoupled from θ at 50 cm in seasons with average and below-average P, imparted by differences in spring and summer rainfall versus winter snowfall. Notably, across-season influence of θ (e.g., a season’s similarity to subsequent seasons) was high when the first season experienced above- or below-average P, and the subsequent season experienced average P, illustrating an important temporal connection initiated by wet and dry conditions. These results illustrate similarities in P-θ relationships across widely differing ecosystems in the SWUS, and elucidate how these relationships may be altered in a changing climate.