Climate change is likely to affect Northeastern U.S. forests through an increase in the frequency and severity of drought events. These temperate forest tree species may be especially vulnerable given the historical rarity of drought in the region; however, our understanding of tree responses to moisture stress is limited. We conducted two ~50% throughfall removal experiments at sites in the Northeastern U.S. with contrasting climate and vegetation: the Hubbard Brook Experimental Forest and the University of New Hampshire’s Thompson Farm. These treatments were superimposed on the severe 2016 natural drought that occurred across the region. We assessed the physiological responses of the dominant tree species (Pinus strobus, Quercus rubra, Acer rubrum)to drought by measuring pre- and post-treatment sap flow using both thermal dissipation and heat pulse probes continuously during the growing season. Additionally, we measured leaf water potential and gas exchange on excised leaves using a Scholander pressure chamber and LI-6400 (LICOR, Inc.), respectively. Soil respiration (LI-COR) and fine root dynamics (ingrowth cores) were assessed across all plots, and stem increment growth was measured on the dominant tree species using dendrometer bands. Microclimate data (e.g., soil moisture, air temperature and relative humidity) were also monitored continuously during the growing season.
We observed differences among forest types and dominant tree species in their responses to the simulated drought treatment and natural drought event. During the height of the drought (late August to early September 2016), P. strobus exhibited high sensitivity to moisture stress, as indicated by the almost complete cessation of transpiration for a period of several days, which was associated with relatively high water use efficiency and more positive mid-day water potential. In contrast, Q. rubra trees continued transpiring even when soils were very dry, and maintained relatively low water use efficiency and more negative mid-day water potential. In contrast, transpiration and water use efficiency of A. rubrum trees did not vary among treatments. These findings are consistent with northeastern tree species exhibiting a range of adaptive strategies to moisture stress including relatively drought-sensitive (P. strobus) to drought-tolerant (Q. rubra, A. rubrum) responses. Continuation of the throughfall removal treatments and field measurements will allow for assessment of potential time lags and cumulative effects of multi-year drought on these species’ growth and physiological responses. Long-term precipitation manipulation studies that push temperate forests beyond ecological thresholds are critical to enhancing understanding of their resilience and vulnerability to future climate change.