Ecosystem Response to Throughfall Exclusion at Hubbard Brook Experimental Forest is Dominated by Shallow-soil Processes

Climate change is likely to affect Northeastern U.S. forests through an increase in the frequency and severity of drought events. Humid temperate forest tree species may be especially vulnerable given the historical rarity of drought in the region. However our understanding of tree species response to moisture stress is understudied in mesic climates. We installed a prototype forest drought simulation as part of the International Drought Experiment, excluding 50% of throughfall on two 225m2 plots from 2015-2018, with exclusion increased to over 90% in 2019 before treatments ended, followed by a natural drought in 2020. We monitored soil moisture and soil water potential (10-50 cm depth), leaf water potential, photosynthetic rates, stomatal conductance, foliar 13C, canopy tree sap flow, foliar production, fine root production (0-20cm depth) leaf litter decomposition, standardized substrate (teabag) decomposition, and soil respiration, among other metrics. Overall, we found few significant responses to the four years of partial throughfall exclusion, but processes that occur predominantly in the shallow soil (soil respiration, litter decomposition, fine root production, photosynthesis and water use of tree seedlings) were dramatically slowed under the near-total precipitation exclusion treatment in 2019. Aboveground processes (e.g. tree growth and water use) showed relatively little response to either level of treatment, implying that trees had access to sufficient water either via deep roots or roots extending outside the treatment plots. We conclude that our prototype throughfall manipulation successfully captured the sensitivity of belowground processes to short-term severe drought; however, the exclusion structure design should be modified in future experiments to minimize water subsidies to trees from outside the treatment plots.