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 limited. We conducted a ~50% throughfall removal experiment in a mature Pinus strobus-Quercus rubra stand in New Hampshire, which was superimposed on the severe 2016 natural drought. Our primary objectives were to (1) Assess the physiological and growth responses of these tree species to drought; (2) Identify thresholds in soil moisture availability leading to nonlinear physiological responses; (3) Compare these species’ sensitivity and vulnerability to drought. We collected pre-treatment and post-treatment measurements of sap flow (heat ratio probes) continuously during the growing season. Additionally, we measured foliar gas exchange using a LICOR 6400 and stem increment growth using dendrometer bands.
Results/Conclusions
We analyzed 2016 sap flow data for four time periods: hydric (6/15-7/14), drying (7/30-8/20), drought (8/23-9/17), recovery (9/22-9/29). Hydric period sapflow for droughted Q. rubra trees was significantly lower compared to control trees, followed by slowly declining sapflow during drying and drought periods, until stabilizing in September. In contrast, hydric period sapflow was similar for droughted P. strobus droughted and control trees until July, after which droughted trees declined sharply during drying and drought periods. Both species nearly ceased transpiring during peak drought (volumetric water content, VWC = 0.8%), and exhibited a similar threshold soil moisture when sapflow declined sharply (VWC = 14%). Both species reduced photosynthesis and stomatal conductance and increased water use efficiency during drought. However, while P. strobus sapflow markedly increased in response to rain events during the recovery period, Q. rubra’s response was much more muted. Finally, P. strobus exhibited greater growth declines (52%) compared to Q. rubra (27%) during the 2016 drought; however, Q. rubra continued to show significant growth declines in 2017 (39%), while P. strobus showed a growth recovery trend. 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, may reflect differing growth phenologies and adaptive strategies of these two species. The contrasting trends in growth response suggest that despite maintaining relatively high growth rates during the 2016 extreme drought, Q. rubra experienced greater post-drought lagged effects and longer recovery times compared to P. strobus.