Implications of a predicted shift from upland oaks to red maple on forest hydrology and nutrient availability

Fire suppression has facilitated the spread of red maple (Acer rubrum L.), a fire-sensitive, yet highly adaptable species, in historically oak-dominated forests of the eastern United States. Here, we address whether a shift from upland oaks to red maple could influence forest hydrology and nutrient availability because of species-specific effects on precipitation distribution and inorganic nitrogen (N) cycling. In eastern Kentucky, we measured seasonal variations in red maple, chestnut oak (Quercus montana Willd.), and scarlet oak (Quercus coccinea Münchh.) throughfall and stemflow quantity and quality following discrete precipitation events, and we assessed net N mineralization rates in underlying soils over a 2-year period (2006-2008). Throughfall was 3%-9% lower underneath red maple than both oaks, but red maple generated 2-3× more stemflow. Consequently, NH₄⁺ throughfall deposition was less under red maple than chestnut oak, whereas stemflow-derived nutrient inputs were substantially larger for red maple than both oaks. Soils underlying red maple had 5-13× greater winter net nitrification rates than soils under both oaks and 20%-30% greater rates of seasonal net ammonification than soils under chestnut oak. These findings suggest a spatial redistribution of water and nutrients via precipitation as red maple dominance increases and point to stemflow as an important mechanism that may foster red maple competitive success, further bolstering the mesophication process in the United States.