Changes in soil moisture availability and water yield in response to longleaf pine restoration in southeast Texas

Brett Lawrence , Matthew McBroom

Our study, based in Trinity County, Texas, focused on whether strategic management of longleaf pine forest could promote a less water-intensive land cover type. We modeled soil evapotranspiration (ET) by measuring vertically stratified soil moisture (15-120 cm) across five forest monitoring sites, four of which received restoration treatments, and one served as a control. Forest attributes, including leaf-area-index (LAI) and traditional forest inventory metrics were measured intermittently across each monitoring stand. Modeled ET was used to create measures of pretreatment and post-treatment water yield and determine whether certain forest attributes were associated with increased water availability. Following forest management activities, cumulative soil moisture entering and exiting treatments were higher compared to our control. Where LAI decreased, we observed increases in modeled total soil moisture (TSM) and matric potential at greater depths (75-120 cm), and increases in subsurface flow, suggesting increases in water availability are best detected at greater soil depths. LAI served as a significant and non-linear predictor of annual water yield during both plot-level (R2 = 0.62) and stand-level (R2 = 0.52) analysis, with an estimated 29.35 cm yr-1 increase in water yield in scenarios where forest thinning reduced LAI by 1.7 units (2.65  0.95). The same analysis of wetting and drying periods indicated that most of that increase is attributable to warmer periods where ET is higher. These results support the assertion that strategic forest management can benefit water resources in limited scenarios, especially where forest thinning is applied and an open pine structure is maintained using prescribed fire or other invasive species control. Continued, long-term monitoring of clearcut conversions to open longleaf pine forests, and away from dense, intensively managed loblolly pine, show potential for creating water yield increases.