The Drought Response of Eastern US Oaks in the Context of Their Declining Abundance

Novick, Kimberly A.; Jo, Insu; D’Orangeville, Loïc; Benson, Michael; Au, Tsun Fung; Barnes, Mallory L.; Denham, S. O.; Fei, Songlin; Heilman, K.; Hwang, Taewon; Keyser, Tara L.; Maxwell, Justin T.; Miniat, Chelcy Ford; McLachlan, J. S.; Pederson, Neil; Wang, Lixin; Wood, J. D.; Phillips, Richard P.

doi:10.1093/biosci/biab135

Cited by 18 publications

(10 citation statements)

References 96 publications

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“…Third, despite the lower agreement in the eastern United States, our analysis points to at least one predominantly eastern forest type group—oak/hickory—that makes up a substantial portion of regions where an increase in drought exposure is expected. Although past research has indicated that oaks are relatively drought‐tolerant, recent evidence suggests that the ability of oak trees to survive drought in the future may be more limited or at best uncertain as vapor pressure deficits rise (Novick et al, 2022).…”

Section: Discussionmentioning

confidence: 99%

“…In some of the forest ecosystems expected to be most exposed to drought in the future and that are known to be sensitive to drought, including ponderosa pine and pinyon pine forests, thinning to lower stand density may be one management action for mitigating future drought impacts and helping those forests adapt (Bottero et al, 2017; Bradford et al, 2022; Bradford & Bell, 2017; van Mantgem et al, 2020). In other forest ecosystems, such as oak‐hickory forests, drought exposure could increase substantially, but future drought impacts are less certain (Novick et al, 2022), so further investigations of future drought impacts may be needed. By providing this mid‐century drought outlook for forest ecosystems, we hope it can help facilitate adaptation to ensure forests are resilient to future climate change.…”

Section: Discussionmentioning

confidence: 99%

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Future exposure of forest ecosystems to multi‐year drought in the United States

2023

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As the future climate becomes hotter or drier, forests may be exposed to more frequent or severe droughts. To inform efforts to ensure resilient forests, it is critical to know which forests may be most exposed to future drought and where. Longer duration droughts lasting 2–3 years or more are especially important to quantify because forests are likely to experience impacts. We summarized exposure to 36‐month drought for forests across the conterminous United States using the Standardized Precipitation‐Evapotranspiration Index (SPEI) overlaid on forest inventory plot locations. Exposure was quantified under 10 scenarios that combined five modeled climates and two Representative Concentration Pathways (RCPs, 4.5 and 8.5) through 2070. Future projections indicate a tripling of the monthly spatial extent of forests exposed to severe or extreme drought—38% of forests were exposed on average by mid‐century as opposed to 11% during 1991–2020 (2041–2070). Increases in drought exposure were greatest under hotter (HadGEM2‐ES), drier (IPSL‐CM5A‐MR), and middle (NorESM1‐M) climate models, under either RCP. Projections agreed that forests in portions of the western United States, especially the southwestern United States, could face high levels of exposure. Forest types including pinyon/juniper, woodland hardwoods, and ponderosa pine were projected to be exposed to drought more than 50% of the time on average across all scenarios by mid‐century, when no forest type was exposed more than 25% of the time under any scenario during the recent period. Projections agreed less for the eastern United States, but in some scenarios, particularly under RCP 8.5, large portions of the East could be exposed to drought nearly as often as parts of the West. Moreover, a substantial portion of oak/hickory forests occur in eastern regions, where projections agree on increased drought exposure. This study provides novel insights about the changing conditions forests face in both the eastern and western United States. Our results can be combined with information about the sensitivities and adaptive capacities of forest ecosystems to prioritize drought adaptation efforts.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Future exposure of forest ecosystems to multi‐year drought in the United States

2023

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“…Because functional traits exert control over various ecosystem processes and functions, these changes in species composition have the potential to negatively impact a wide array of provisioning (e.g., timber), regulating (e.g., climate; Mushinski et al, 2019), supporting (e.g., nutrient cycling; Alexander, 2010), and cultural (Pezzarossi, 2014) services. The maintenance of xerophytic species across the landscape is particularly important given their tendency to be more resilient to mortality and/or reductions in growth during drought (Abrams, 1990;Kaproth and Cavender-Bares, 2016;Novick et al, 2022); a disturbance forecasted to increase in extent and severity over the next 50 years (Vose et al, 2016). Additionally, there is growing concern that this largescale transition may be a self-reinforcing process-termed mesophication (Nowacki and Abrams, 2008)-whereby this compositional shift creates conditions in the forest understory that promote the continued establishment and persistence of functionally similar mesic species.…”

Section: Introductionmentioning

confidence: 99%

Stand and environmental conditions drive functional shifts associated with mesophication in eastern US forests

Woodbridge¹,

Keyser²,

Oswalt³

2022

Front. For. Glob. Change

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There is a growing body of evidence that mesic tree species are increasing in importance across much of the eastern US. This increase is often observed in tandem with a decrease in the abundance and importance of species considered to be better adapted to disturbance and drier conditions (e.g., Quercus species). Concern over this transition is related to several factors, including the potential that this transition is self-reinforcing (termed “mesophication”), will result in decreased resiliency of forests to a variety of disturbances, and may negatively impact ecosystem functioning, timber value, and wildlife habitat. Evidence for shifts in composition provide broad-scale support for mesophication, but we lack information on the fine-scale factors that drive the associated functional changes. Understanding this variability is particularly important as managers work to develop site-and condition-specific management practices to target stands or portions of the landscape where this transition is occurring or is likely to occur in the future. To address this knowledge gap and identify forests that are most susceptible to mesophication (which we evaluate as a functional shift to less drought or fire tolerant, or more shade tolerant, forests), we used data from the USDA Forest Service Forest Inventory and Analysis program to determine what fine-scale factors impact the rate (change through time) and degree (difference between the overstory and midstory) of change in eastern US forests. We found that mesophication varies along stand and environmental gradients, but this relationship depended on the functional trait examined. For example, shade and drought tolerance suggest mesophication is greatest at sites with more acidic soils, while fire tolerance suggests mesophication increases with soil pH. Mesophication was also generally more pronounced in older stands, stands with more variable diameters, and in wetter sites, but plots categorized as “hydric” were often highly variable. Our results provide evidence that stand-scale conditions impact current and potential future changes in trait conditions and composition across eastern US forests. We provide a starting point for managers looking to prioritize portions of the landscape most at risk and developing treatments to address the compositional and functional changes associated with mesophication.

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“…[12]. However, less frequent fire occurrence appears to have contributed to diminishing oak abundance the most [14]. In addition to anthropogenic fire suppression, the demographic shift also contributed to less frequent fire because the maple leaf litter is less flammable, which altered the forests with a cooler and moister microclimate and generated a positive feedback loop that favored the establishment of maples but suppressed the regeneration of other fire-tolerant species, such as oaks and hickory [11].…”

Section: Introductionmentioning

confidence: 99%

“…In the context of increasing drought stress in the eastern US, one of the potential consequences of such a compositional shift from oaks to maples is an alteration of drought impacts on forest productivity, where drought could cause stronger growth reduction of maple-dominant stands, especially when drought occurs in the later growing season [15]. However, the water demand (i.e., potential evapotranspiration) controls oak species dominance differentially; the higher potential evapotranspiration favors dominance of white oak (Quercus alba) over red oak (Quercus rubra) [14].…”

Section: Introductionmentioning

confidence: 99%

Drought Sensitivity and Resilience of Oak–Hickory Stands in the Eastern United States

Maxwell

2022

Forests

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Forest composition in the eastern United States (US) has been shifting from an oak–hickory to maple–beech assemblage, but whether there are species-specific differences within these oak–hickory stands in their responses and recovery from drought remains unclear. Here, we examined drought responses and resilience derived from radial growth of 485 co-occurring Carya ovata and Quercus alba individual trees at 15 forests in the eastern US. Water availability over the growing season (May to August) of the current year controls growth variability of both C. ovata and Q. alba. Drought that occurred in June caused the greatest growth reduction for both species while interspecific differences inof drought-induced growth reduction was found in July, where Q. alba experienced stronger reduction than C. ovata. Both species are resilient to early growing season drought, but late growing season drought caused larger drought legacy effects for Q. alba. The increasing drought frequency and intensity will have a more prominent impact in oak–hickory stands in the eastern US. The species composition of a forest along with species-specific responses and recovery is likely to be a critical control on forest productivity and species abundance.

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The Drought Response of Eastern US Oaks in the Context of Their Declining Abundance

Cited by 18 publications

References 96 publications

Future exposure of forest ecosystems to multi‐year drought in the United States

Future exposure of forest ecosystems to multi‐year drought in the United States

Stand and environmental conditions drive functional shifts associated with mesophication in eastern US forests

Drought Sensitivity and Resilience of Oak–Hickory Stands in the Eastern United States

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