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A forest vulnerability index based on drought and high temperatures

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https://ir.library.oregonstate.edu/concern/articles/73666618m

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  • Increasing forest stress and tree mortality has been directly linked to combinations of drought and high temperatures. The climatic changes expected during the next decades – large increases in mean temperature, increased heat waves, and significant long-term regional drying in the western USA – will likely increase chronic forest stress andmortality. The aimof this research is to develop and apply a newforest vulnerability index (FVI) associatedwith drought and high temperatures across the Pacific Northwest region (PNW; Oregon andWashington) of the USA during the MODIS Aqua era (since 2003). Our technique incorporates the alterations to canopy water and energy exchange processes caused by drought and high temperatures with spatially continuousMODIS land surface temperature (LST) and evapotranspiration (ET), andwith Parameter-elevation Relationships on Independent Slopes Model (PRISM) precipitation (P) data.With P and ET, we calculate a monthly water balance variable for each individual pixel normalized by forest type group (FTG), and then difference the water balance with the corresponding normalizedmonthlymean LST to calculate amonthly forest stress index (FSI).We then extract the pixel-specific (800-mresolution) statistically significant temporal trends of the FSI from 2003 to 2012 by month (April to October). The FVI is the slope of the monthly FSI across years, such that there is a FVI for each month. Statistically significant positive slopes indicate interannual increases in stress leading to expected forest vulnerability (positive FVI) for a given month. Positive FVI values were concentrated in the months of August and September, with peak vulnerability occurring at different times for different FTGs. Overall, increased vulnerability rates were the highest in drier FTGs such as Ponderosa Pine, Juniper, and Lodgepole Pine. Western Larch and Fir/Spruce/Mountain Hemlock groups occupy moister sites but also had relatively high proportion of positive FVI values. The Douglas-fir group had the second largest total area of increased vulnerability due to its large areal extent in the study area. Based on an analysis using imagery viewed in Google Earth, we confirm that areas with increased vulnerability are associated with greater amounts of stress and mortality. The FVI is a new way to conceptualize and monitor forest vulnerability based on first-order principles and has the potential to be generalized to other geographical areas.
  • KEYWORDS: Evapotranspiration, Aqua MODIS land surface temperature, Stress, climate change, Water deficit, Drought, Forest vulnerability index
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  • Mildrexler, D., Yang, Z., Cohen, W. B., & Bell, D. M. (2016). A forest vulnerability index based on drought and high temperatures. Remote Sensing of Environment, 173, 314-325. doi:10.1016/j.rse.2015.11.024
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  • 173
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  • This work was supported by the USDA Forest Service, Region 6 Climate Change Program, and the Graduate School and the Department of Forest Ecosystems and Society at Oregon State University.
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