A forest vulnerability index based on drought and high temperatures

Abstract

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 and mortality. The aim of this research is to develop and apply a new forest vulnerability index (FVI) associated with drought and high temperatures across the Pacific Northwest region (PNW; Oregon and Washington) 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 continuous MODIS land surface temperature (LST) and evapotranspiration (ET), and with 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 normalized monthly mean LST to calculate a monthly forest stress index (FSI). We then extract the pixel-specific (800-m resolution) 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.

Satellite-Based Run-Off Model for Monitoring Drought in Peninsular Malaysia

“…Recent studies have linked rainfall deficits, soil thermal stress and vegetation growth and mortality to drought [2,3,[5][6][7]9,11,12,15,20,25,[27][28][29][30][31][32]36,[50][51][52], highlighting the need for operational drought monitoring systems that can show drought intensity, location and extent [16]. In response, this study devised an operational procedure and algorithm, a run-off model, from TMPA rainfall and MODIS evapotranspiration products for spatiotemporal drought assessment (Figures 3 and 4).…”

Review of broad-scale drought monitoring of forests: Toward an integrated data mining approach

“…For instance, the Palmer Modified Drought Index (PMDI) is less volatile than the PDSI, and can more accurately capture a linear combination of temperature and precipitation effects across broad geographic regions (Heddinghaus and Sabol, 1991). The Standardized Precipitation Index (SPI) uses only precipitation data to characterize moisture conditions during multiple, well-defined time windows (McKee et al, 1993); unfortunately, this could be a limitation in the face of increasing recognition that high temperatures magnify drought impacts on trees, and that these impacts are likely to worsen due to climate change (Breshears et al, 2005;McDowell et al, 2008;Allen et al, 2010Allen et al, , 2015Williams et al, 2013;Mitchell et al, 2014;Mildrexler et al, 2016). The Standardized Precipitation Evapotranspiration Index (SPEI) is calculated for multiple time windows like the SPI, but also incorporates PET estimates (VicenteSerrano et al, 2010).…”

“…Alternatively, remote-sensing-based measures track short-term drought responses by exploiting known differences in reflected radiation between stressed and unstressed vegetation (Peters et al, 1991(Peters et al, , 2002Zhang et al, 2013;Mildrexler et al, 2016). However, short-term stress may not be a sufficient indicator of drought impacts that sometimes take years to emerge (Pasho et al, 2011;Mendivelso et al, 2014;Park et al, 2016).…”

Assessing Landscape Vulnerability to Wildfire in the USA

“…To date, a few, relatively course scale studies have assessed reduced productivity as a result of wildfire and the recovery to a pre-fire level (e.g., [77]). The recent development of the LandTrendr tool [78] provides an example of a methodology both for assessing fire impacts on ecosystem services across landscapes at a higher resolution for management (i.e., Landsat) and also for developing true a priori vulnerability indices (e.g., [79]). LandTrendr-based analyses of forest resilience have primarily focused on carbon stocks and non-fire related forest productivity [1,80] but hold considerable potential for resilience analysis assessing ecosystem goods and services more broadly.…”