Are Northeastern U.S. forests vulnerable to extreme drought?

Abstract

In the Northeastern U.S., drought is expected to increase in frequency over the next century, and therefore, the responses of trees to drought are important to understand. There is recent debate about whether land-use change or moisture availability is the primary driver of changes in forest species composition in this region. Some argue that fire suppression from the early twentieth century to present has resulted in an increase in shade-tolerant and pyrophobic tree species that are drought intolerant, while others suggest precipitation variability as a major driver of species composition. From this debate, an emerging hypothesis is that mesophication and increases in the abundance of mesophytic genera (e.g., Acer, Betula, and Fagus) resulted in forests that are more vulnerable to drought. This review examines the published literature and factors that contribute to drought vulnerability of Northeastern U.S. forests. We assessed two key concepts related to drought vulnerability, including drought tolerance (ability to survive drought) and sensitivity (short-term responses to drought), with a focus on Northeastern U.S. species. We assessed drought-tolerance classifications for species, which revealed both consistencies and inconsistencies, as well as contradictions when compared to actual observations, such as higher mortality for drought-tolerant species. Related to drought sensitivity, recent work has focused on isohydric/ anisohydric regulation of leaf water potential. However, based on the review of the literature, we conclude that drought sensitivity should be viewed in terms of multiple variables, including leaf abscission, stomatal sensitivity, turgor pressure, and dynamics of non-structural carbohydrates. Genera considered drought sensitive (e.g., Acer, Betula, and Liriodendron) may actually be less prone to drought-induced mortality and dieback than previously considered because stomatal regulation and leaf abscission in these species are effective at preventing water potential from reaching critical thresholds during extreme drought. Independent of drought-tolerance classification, trees are prone to dieback and mortality when additional stressors are involved such as insect defoliation, calcium and magnesium deficiency, nitrogen saturation, and freeze-thaw events. Overall, our literature review shows that multiple traits associated with drought sensitivity and tolerance are important as species may rely on different mechanisms to prevent hydraulic failure and depleted carbon reserves that may lead to mortality.

The role of understory phenology and productivity in the carbon dynamics of longleaf pine savannas

“…Even though the xeric site is chronically exposed to lower soil moisture conditions, we found that understory NDVI was significantly lower for all levels of PDSI compared to the mesic site, showing that understory species were not able to thrive at the level of that of the mesic site under drought conditions. However, the magnitude of NDVI change (i.e., slope of the regression) was lower at the xeric site when severity of drought increased, indicating that plant functional types at the site could buffer drought conditions more effectively (Coble et al 2017, Granda et al 2018, such that understory NDVI decreased by half as much as at the mesic site. In addition, overall ecosystem productivity at xeric sites was shown to recover more rapidly from drought (Starr et al 2016), compared to more mesic sites, which was the result of drought-resistant plant species in the over-and understory (Wright et al 2013).…”

“…However, the magnitude of NDVI change (i.e., slope of the regression) was lower at the xeric site when severity of drought increased, indicating that plant functional types at the site could buffer drought conditions more effectively (Coble et al. , Granda et al. ), such that understory NDVI decreased by half as much as at the mesic site.…”

Temperate tree seedlings have similar drought vulnerability despite having different hydraulic drought responses in adults

“…It has relatively narrow xylem conduits (i.e., diffuseporous xylem; Ellmore et al 2006) and has been shown to exhibit isohydric stomatal behavior in response to drought [24]. In contrast, Q. rubra is moderately shade-tolerant [60] and moderately drought-tolerant [59]. Trees of this species have large xylem conduits (i.e., ring-porous xylem; Ellmore et al 2006) and exhibit anisohydric stomatal behavior in response to drought [8].…”

Differential and dynamic water regulation responses to El Niño for monospecific and mixed species planted forests

“…On the basis of the theoretical framework outlined in by Martinez‐Vilalta, Poyatos, Aguade, Retana and Mencuccini (2014), we calculated the slope of the linear line for each of the treatments for normal year conditions (2015) and ENSO conditions (2016). The slopes (σ) of the lines loosely represent isohydric and anisohydric behaviour (Coble et al, 2017; Martinez‐Vilalta et al, 2014). In this framework, four distinct behaviours exist: strict isohydric (σ = 0), partial isohydric (0 < σ < 1), strict anisohydric (σ = 1) and extreme anisohydric (σ > 1).…”