Fire plays an important role in maintaining community structure in upland hardwood ecosystems, and after long-term fire suppression in the eastern United States, shade-tolerant, fire-sensitive species have begun to proliferate in forest understories, creating moister and less-flammable conditions (i.e., mesophication). Changes to ecosystem structure and function begin with individual trees. For example, redistribution of rainwater by the forest canopy into throughfall, stemflow, and interception is determined by tree species traits (canopy and bark storage capacity; bark roughness; bark thickness), and may consequently impact forest flammability. This species-level control on the delivery of water and nutrients to the forest floor is also manifested in soil moisture and nutrient composition. To better understand how shifting forest composition could impact rainwater distribution and the subsequent impact of prescribed fire, we measured (1) throughfall and stemflow, (2) soil moisture, (3) soil respiration, and (4) soil nutrient availability beneath oak and non-oak species (e.g., hickory, maple, sweetgum).
Results/Conclusions
Non-oak species had denser canopies than oaks and intercepted more rainwater, reducing throughfall by > 15%. Conversely, stemflow water inputs were +4 times greater beneath non-oak species, which had smoother bark compared to rough-barked oak species. No differences were observed in throughfall chemistry between species, but dissolved organic carbon in stemflow was +4 times greater from oaks compared to non-oaks. Differences in canopy fluxes were manifested in soil moisture, which was significantly higher beneath non-oak species. Following a dormant season prescribed fire, C:N spiked in soils around southern red oak (p=0.005) and was overall reduced in soils around sweetgum (p=0.020) . Heterotrophic respiration was also greater from burned than un-burned soils for all species (p=0.051), with the largest increase observed in soils around southern red oak. Results of inorganic nitrogen availability are still pending. The results of this study indicate the importance of oak canopy and foliar traits to forest hydrology, biogeochemistry and overall forest health and imply that a shift in forest composition could influence future ecosystem function in southeastern upland oak-hickory forests.