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

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.

Potential influence of wildfire in modulating climate-induced forest redistribution in a central Rocky Mountain landscape

Introduction:Climate change is expected to impose significant tension on the geographic distribution of tree species.Yet,tree species range shifts may be delayed by their long life spans,capacity to withstand long periods of physiological stress,and dispersal limitations.Wildfire could theoretically break this biological inertia by killing forest canopies and facilitating species redistribution under changing climate.We investigated the capacity of wildfire to modulate climate-induced tree redistribution across a montane landscape in the central Rocky Mountains under three climate scenarios(contemporary and two warmer future climates)and three wildfire scenarios(representing historical,suppressed,and future fire regimes).Methods:Distributions of four common tree species were projected over 90 years by pairing a climate niche model with a forest landscape simulation model that simulates species dispersal,establishment,and mortality under alternative disturbance regimes and climate scenarios.Results:Three species(Douglas-fir,lodgepole pine,subalpine fir)declined in abundance over time,due to climate-driven contraction in area suitable for establishment,while one species(ponderosa pine)was unable to exploit climate-driven expansion of area suitable for establishment.Increased fire frequency accelerated declines in area occupied by Douglas-fir,lodgepole pine,and subalpine fir,and it maintained local abundance but not range expansion of ponderosa pine.Conclusions:Wildfire may play a larger role in eliminating these conifer species along trailing edges of their distributions than facilitating establishment along leading edges,in part due to dispersal limitations and interspecific competition,and future populations may increasingly depend on persistence in locations unfavorable for their establishment.