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Drought’s legacy: multiyear hydraulic deterioration underlies widespread aspen forest die-off and portends increased future risk
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Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and car- bon-cycle feedbacks. Recent drought-induced, widespread forest die-offs highlight that climate change could acceler- ate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and pre- cludes mechanistic modeling and prediction of forest die-off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die-off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumu- lated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavita- tion, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deteri- oration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vul- nerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate–vege- tation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress- induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.
Keywords: biosphere–atmosphere interactions, climate change, ecosystem shift, forest mortality, vegetation model, xylem cavitation, dieoff
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Climate Science Documents
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A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests
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Of particular concern are potential increases in tree mortality associated with climate- induced physiological stress and interactions with other climate-mediated processes such as insect outbreaks and wildfire. Despite this risk, existing projections of tree mortality are based on models that lack functionally realistic mortality mechanisms, and there has been no attempt to track observations of climate-driven tree mortality globally. Here we present the first global assessment of recent tree mortality attributed to drought and heat stress. Although episodic mortality occurs in the absence of climate change, studies compiled here suggest that at least some of the world’s forested ecosystems already may be responding to climate change and raise concern that forests may become increasingly vulnerable to higher background tree mortality rates and die-off in response to future warming and drought, even in environments that are not normally considered water-limited. This further suggests risks to ecosystem services, including the loss of sequestered forest carbon and associated atmospheric feedbacks. Our review also identifies key information gaps and scientific uncertainties that currently hinder our ability to predict tree mortality in response to climate change and emphasizes the need for a globally coordinated observation system. Overall, our review reveals the potential for amplified tree mortality due to drought and heat in forests worldwide.
heat, temperature, drought, tree mortality, forest dieoff
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Climate Science Documents
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Afforestation Effects on Soil Carbon Storage in the United States: A Synthesis
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Afforestation (tree establishment on nonforested land) is a management option for increasing terrestrial C sequestration and mitigating rising atmo- spheric carbon dioxide because, compared to nonforested land uses, afforestation increases C storage in aboveground pools. However, because terrestrial ecosystems typically store most of their C in soils, afforestation impacts on soil organic carbon (SOC) storage are critical components of eco- system C budgets. We applied synthesis methods to identify the magnitude and drivers of afforestation impacts on SOC, and the temporal and verti- cal distributions of SOC change during afforestation in the United States. Meta-analysis of 39 papers from 1957 to 2010 indicated that previous land use drives afforestation impacts on SOC in mineral soils (overall average = +21%), but mined and other industrial lands (+173%) and wildlands (+31%) were the only groups that specifically showed categorically significant increases. Temporal patterns of SOC increase were statistically significant on former industrial and agricultural lands (assessed by continuous meta- analysis), and suggested that meaningful SOC increases require ≥15 and 30 yr of afforestation, respectively. Meta-analysis of 13C data demonstrated the greatest SOC changes occur at the surface soil of the profile, although par- tial replacement of C stocks derived from previous land uses was frequently detectable below 1 m. A geospatial analysis of 409 profiles from the National Soil Carbon Network database supported 13C meta-analysis results, indicating that transition from cultivation to forest increased A horizon SOC by 32%. In sum, our findings demonstrate that afforestation has significant, positive effects on SOC sequestration in the United States, although these effects require decades to manifest and primarily occur in the uppermost (and per- haps most vulnerable) portion of the mineral soil profile.
Abbreviations: BD, bulk density; CI, confidence interval; MAP, mean annual precipitation; MAT, mean annual temperature; SOC, soil organic carbon.
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Climate Science Documents
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Agenda - March 11, 2015 Workshop
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Urban Woodlands Conservation and Management Workshop. Organized and facilitated by the National Park Service to identify and create opportunities for greater collaboration among urban woodland researchers and managers working to restore and manage urban woodland ecosystems.
To view the goals and objectives of the workshop, please open the workshop agenda.
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Cultural Resources
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Urban Conservation
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Urban Woodlands Conservation and Restoration
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Assessing Future Energy Development across the Appalachian LCC. Final Report
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In this study funded by the Appalachian LCC, The Nature Conservancy assessed current and future energy development across the entire region. The research combined multiple layers of data on energy development trends and important natural resource and ecosystem services to give a comprehensive picture of what future energy development could look like in the Appalachians. It also shows where likely energy development areas will intersect with other significant values like intact forests, important streams, and vital ecological services such as drinking water supplies.
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Tools & Resources
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Assessing Future Energy Development
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Products and Tools for Energy Modelling
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Models of wind, shale gas, and coal development for the entire study area have been created to predict potential future energy development and impacts to natural resources within the Appalachians. Models and data from all development projections populate a web-based mapping tool to help inform regional landscape planning decisions.
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Tools & Resources
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Assessing Future Energy Development
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AMJV Partnership Receives $8 Million RCPP Award to Enhance Cerulean Habitat
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A project proposal from the Appalachian Mountains Joint Venture (AMJV) Partnership was one of 115 high-impact projects to receive in total more than $370 million as part of the new Regional Conservation Partnership Program (RCPP), Agriculture Secretary Tom Vilsack announced today.
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News & Events
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Online course and conference offered to Virginia forest landowners
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Virginia forest landowners looking to gain an understanding of how to keep their woods healthy and productive can do so in the comfort of their own home.
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News & Events
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Managing Urban Forests in a Changing Climate
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Register now to learn how you can help your urban forest and your community prepare for the effects of climate change at The Morton Arboretum Urban Tree Conference, “Managing Urban Forests in a Changing Climate,” November 18 and 19, 2014, in Lisle, Illinois.
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President Recognizes Role of Private Forests in Climate Action Plan
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We are writing to you, as members of the Forest‐Climate Working Group, to thank you for the central role that you have created for U.S. forests and forest products in your Climate Action Plan and the new natural resources policy proposals that you have announced this week.
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