Quantifying the effects of stand age on components of forest evapotranspiration

Forest age and species composition varies widely in the southern Appalachian Mountains, due to legacies of land use and disturbance such as repeat harvesting, fire exclusion, and invasive pests. In this highly productive region, leaf area recovers rapidly after harvesting (typically within five to ten years), while biomass accumulates more slowly (reaching peak basal area between 25 and 80 years). Although tree-level water use should generally increase with size, age-related differences in sap flow are unknown for many species. To address this knowledge gap, we measured sap flow, throughfall, and subcanopy evapotranspiration in a chronosequence of mesic cove forest stands: a young stand, 15-years postharvest; a 35-year postharvest stand; and a ~200-year-old stand. Tulip poplar, black birch, and red maple had greater sap flow velocities and greater sapwood to basal area ratio than oak species, resulting in greater tree-level transpiration. Intraspecific variability in sap velocity did not show consistent trends with tree size and only declined with age in oaks. Because the proportion of oak species increased with stand age, stand-level transpiration was lower in the ~200-year-old stand compared to the younger stands. Annual subcanopy evapotranspiration declined with stand age and accounted for <10% of precipitation. In contrast interception increased with stand age by an order of magnitude from the youngest to oldest stand, largely offsetting differences in canopy transpiration in the 35- and 200-year-old stands. These results suggest that predicted increases in the atmospheric demand for water and observed trends showing a decreasing proportion of oaks in this region could lead to greater forest water use in the future.