Contrasting responses of two temperate forest species to extreme drought in the Northeastern U.S.

Climate change is likely to affect Northeastern U.S. forests through increased frequency and severity of drought. The region's tree species may be especially vulnerable given the historical rarity of drought; however, our understanding of species' response to moisture stress is limited. We conducted a 50% throughfall exclusion (TFE) experiment in a mature Pinus strobus-Quercus rubra stand in New Hampshire, which coincided with the severe 2016 natural drought. To assess physiological responses to drought and identify thresholds in soil moisture availability, we collected pre-treatment and post-treatment measurements of sap flow during the growing season, in addition to foliar gas exchange, plant water potential, and stem increment growth. In 2016, hydric period (June) sapflow for Q. rubratrees in the TFE plot was significantly lower compared to control trees, then slowly declined during drying (July) and drought (August) periods, until stabilizing near zero during the extreme part of the drought in mid-September. In contrast, June sapflow was similar for P. strobus droughted and control trees until July, after which TFE trees declined sharply through the drought. Both speciesnearly ceased transpiring during peak drought (volumetric water content, VWC = 8%), and exhibited a similar threshold soil moisture whensapflow declined sharply (VWC = 14%). Both species reduced photosynthesis and stomatal conductance and increased water use efficiency during drought. However, while P. strobus sapflowmarkedly increased in response to rain events during the recovery period, Q. rubra's response was much more muted. Finally, P. strobusexhibited greater growth declines (51%) compared to Q. rubra (22%) during the 2016 drought; however, Q. rubrashowed further growth declines in 2017 (35%), while P. strobusshowed modest recovery. Thus, despite maintaining relatively high growth rates during drought, Q. rubra experienced greater post-drought lagged effects and longer recovery times compared to P. strobus. The greater sensitivity of Q. rubra to early season moisture stress, combined with the faster rate of sapflow decline and sensitivity to post-drought rain events in P. strobus, likely reflect differing growth phenologies and adaptive strategies.