As rates of evapotranspiration increase with rising temperatures, moisture stress and drought conditions will occur more frequently if they are not offset by a corresponding increase in precipitation and soil moisture (Hayhoe et al. 2007, Kunkel et al. 2013). Modeling results suggest a much greater potential for more frequent droughts and moisture stress during the growing season when climate models project both much warmer temperatures and reduced summer precipitation in the Northeast. Even when precipitation is not projected to change substantially, warmer temperatures are also predicted to contribute to increased evapotranspiration and physiological stress if increases in precipitation do not correspond to temperature increases (Janowiak et al. 2018, Lynch et al 2016; Peters and Iverson 2019).

Moisture stress can negatively impact forest ecosystems in a variety of ways, and the potential effects will depend on many factors, including the duration and severity of the drought and site-level characteristics of the forest. Prolonged and/or severe drought conditions may contribute to tree dieback and mortality, particularly as these conditions interact with other forest stressors (Clark et al. 2019). Although mature trees are better able to resist increases in temperature and reductions in available moisture, severe or sustained drought can increase tree mortality, open the forest canopy, alter forest growth and composition, and increase susceptibility to other stressors (Clark et al. 2016, Dale et al. 2001, Millers et al. 1989, Pederson et al. 2014). Drought during the past century has been linked to dieback in sugar maple and some species of birch (Auclair et al. 2010), as well as to a decline of oak and ash trees in the Northeast (Millers et al. 1989, Mohan et al. 2009).

Interactions between drought and other disturbances increase the likelihood of tree dieback and mortality (Coble et al. 2017). While purely drought induced mortality events are rather rare in the Northeast, moisture stressed trees are typically more vulnerable to disturbances, particularly insect pests and diseases (Dale et al. 2001, Millar and Stephenson 2015, Ryan and Vose 2012). Additionally, drought conditions can hinder forest recruitment and regeneration. Germination, establishment, and early seedling survival depend on early-season moisture, and can be limited by drought. When moisture is limited over successive years, the fecundity of adult trees may also be significantly reduced, as greater seed production is correlated with high levels of soil moisture (Clark et al. 2019).

Within oak forests, the dominant species are considered to be relatively drought tolerant and may potentially benefit from or at least endure future drought conditions. However, there are complexities that imply these forests may still be vulnerable. The ability of oak forests to cope with drought conditions may be hindered as species composition shifts toward more drought-intolerant species such as beech and maple. Impacts will be species-specific and are likely to vary based on the timing of drought within the growing season; for instance, recent research indicates that Quercus alba may be particularly sensitive to low water availability early in the growing season (Fung Au et al. 2020).

There are still some uncertainties regarding the potential future impact of drought on forests in the Northeast. Approaches to drought risk assessment are variable and in many cases lack future scenario modelling. Additionally, there is a lack of agreement on drought risk assessment methods (Hagenlocher et al. 2019).

In the eastern US, the effects of increasing drought are becoming better understood at the level of individual trees, but this knowledge cannot yet be confidently translated to predictions of changing structure and diversity of forest stands (Clark et al. 2016).