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Frequent observations of higher mortality in larger trees than in smaller ones during droughtshave sparked an increasing interest in size-dependent drought-induced mortality. However, theunderlying physiological mechanisms are not well understood, with height-associated hydraulicconstraints often being implied as the potential mechanism driving increased droughtvulnerability. We performed a quantitative synthesis on how key traits that drive plant waterand carbon economy change with tree height within species and assessed the implications thatthe different constraints and compensations may have on the interacting mechanisms (hydraulicfailure, carbon starvation and/or biotic-agent attacks) affecting tree vulnerability to drought.While xylem tension increases with tree height, taller trees present a range of structural andfunctional adjustments, including more efficient water use and transport and greater wateruptake and storage capacity, that mitigate the path-length-associated drop in water potential.These adaptations allow taller trees to withstand episodic water stress. Conclusive evidence forheight-dependent increased vulnerability to hydraulic failure and carbon starvation, and theircoupling to defence mechanisms and pest and pathogen dynamics, is still lacking. Furtherresearch is needed, particularly at the intraspecific level, to ascertain the specific conditions andthresholds above which height hinders tree survival under drought.
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Mitigating drought mortality by incorporating topography into variable forest thinning strategies
Abstract Drought-induced productivity reductions and tree mortality have been increasing in recent decades in forests around the globe. Developing adaptation strategies hinges on an adequate understanding of the mechanisms governing the drought vulnerability of forest stands. Prescribed reduction in stand density has been used as a management tool to reduce water stress and wildfire risk, but the processes that modulate fine-scale variations in plant water supply and water demand are largely missing in ecosystem models. We used an ecohydrological model that couples plant hydraulics with groundwater hydrology to examine how within-stand variations in tree spatial arrangements and topography might mitigate forest vulnerability to drought at individual-tree and stand scales. Our results demonstrated thinning generally ameliorated plant hydraulic stress and improved carbon and water fluxes of the remaining trees, although the effectiveness varied by climate and topography. Variable thinning that adjusted thinning intensity based on topography-mediated water availability achieved higher stand productivity and lower mortality risk, compared to evenly-spaced thinning at comparable intensities. The results from numerical experiments provided mechanistic evidence that topography mediates the effectiveness of thinning and highlighted the need for an explicit consideration of within-stand heterogeneity in trees and abiotic environments when designing forest thinning to mitigate drought impacts.
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- Award ID(s):
- 2106030
- PAR ID:
- 10529653
- Publisher / Repository:
- IOPScience
- Date Published:
- Journal Name:
- Environmental Research Letters
- Volume:
- 19
- Issue:
- 3
- ISSN:
- 1748-9326
- Page Range / eLocation ID:
- 034035
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Drought, coupled with rising temperatures, is an emerging threat to many forest types across the globe. At least to a degree, we expect management actions that reduce competition (e.g., thinning, prescribed fire, or both) to improve growth of residual trees during drought. The influences of management actions and drought on individual tree growth may be measured with high precision using tree-rings. Here, we summarize tree-ring-based assessments of the effectiveness of thinning and prescribed fire as drought adaptation tools, with special consideration for how these findings might apply to dry coniferous forests in the southwestern United States. The existing literature suggests that thinning treatments generally improve individual tree growth responses to drought, though the literature specific to southwestern coniferous forests is sparse. Assessments from studies beyond the southwestern United States indicate treatment effectiveness varies by thinning intensity, timing of the drought relative to treatments, and individualistic species responses. Several large-scale studies appear to conflict on specifics of how site aridity influences sensitivity to drought following thinning. Prescribed fire effects in the absence of thinning has received much less attention in terms of subsequent drought response. There are limitations for using tree-ring data to estimate drought responses (e.g., difficulties scaling up observations to stand- and landscape-levels). However, tree-rings describe an important dimension of drought effects for individual trees, and when coupled with additional information, such as stable isotopes, aid our understanding of key physiological mechanisms that underlie forest drought response.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1748-9326/ad29aa
