Increasing Ring Porous Species Composition Improves Forest Water Yield and Stabilizes Productivity

ABSTRACT

Global evaporative demand is increasing and may alter plant transpiration rates, with consequences for streamflow and vegetation productivity. Tree water use can vary fourfold among species, dependent on sapwood area and xylem anatomy, and in eastern US forests, the ongoing shift from more water‐conservative oaks ( Quercus ) and hickories ( Carya ) (ring‐porous xylem) to water‐demanding maples ( Acer ) and tulip‐poplar ( Liriodendron tulipifera ) (diffuse‐porous xylem) has already reduced water yields. How the relative abundance and dominance of species' water use strategies interact with climate to shape forest growth and water use remains uncertain. Therefore, we simulated how a gradient of species compositions (10%–90% diffuse‐porous) determines streamflow, evapotranspiration (ET) and net ecosystem exchange (NEE) in two Southern Appalachian watersheds. Under a future hot climate scenario (HadGEM2‐ES, RCP 8.5), forest water use increased, productivity to respiration increased, and streamflow declined relative to a no‐change baseline. Among species composition scenarios, diffuse‐porous–dominated landscapes had 15%–22% higher ET and 17%–19% lower streamflow than ring‐porous–dominated landscapes. In contrast, the scenario dominated by ring‐porous species had the greatest productivity and the lowest interannual variability in the carbon balance. Diffuse‐porous–dominated scenarios were also more variable from year to year, with more years functioning as net carbon sources to the atmosphere rather than sinks (38% vs. 15% of years for 90% vs. 10% diffuse‐porous under a hot climate scenario). While ecosystem processes were largely driven by climate, management to favour ring‐porous species may enhance ecosystem services and resilience to warmer climate conditions.