# Monitoring Changes in Water Use Efficiency to Understand Drought Induced Tree Mortality
## 2 Materials and Methods
### The Most Severe Drought on Record  
In California, the 2012-2016 drought was the most severe on record. With forested areas representing 43% of the state's natural area (Figure 1), multiple years of extended severe drought ([PERSON] et al., 2015; Figure 2) make California an important case study to evaluate the use of remote sensing products to understand large-scale patterns in tree mortality. The self-calibrated Palmer drought severity index (scPDSI; Table 1; Figure 2) was used to quantify drought severity and to calculate the frequency of severe drought (2002-2015). The scPDSI is an improved version of the Palmer Drought Severity Index, which has been widely used to monitor drought conditions [4; 36; 37; 38; 39; 40; 41; 42; 43]. Monthly scPDSI data was available through the Western Regional Climate Center ([[https://www.wrcc.dri.edu/](https://www.wrcc.dri.edu/)]([https://www.wrcc.dri.edu/](https://www.wrcc.dri.edu/))). The scPDSI ranges from \(-10\) to \(10\), with the number indicating the magnitude and the sign denoting (\(+\)) wetter than average or (\(-\)) drier than average conditions for a location based on historical climate and sensitivity to changes in water availability [38]. Values of scPDSI between \(-0.4\) and \(0.4\) denote average conditions and absolute values greater than \(3\) are indicative of severe conditions. Severe conditions occur based on the history of the location and are not determined relative to a default location [38]. The scPDSI has a similar range of variability in diverse climates, allowing for more  
Figure 1: California’s distinct combinations of climate, elevation, and soil types create a unique mix of ecoregions and forest types. Forested areas are shown in grey and white areas represent non-forested land cover that was not included as part of this study. MODIS land cover type (MCD12Q1) in 2012 was used to identify forested areas [35]. The climate ecoregion layer was produced by the Forest Service Ecological Information and Mapping (ECOMAP) Team ([[https://data.fs.usda.gov/geodata](https://data.fs.usda.gov/geodata)]([https://data.fs.usda.gov/geodata](https://data.fs.usda.gov/geodata))).  
exact comparisons between locations and times [38; 44]. The scPDSI has been widely used for drought characterization [44; 45; 46].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Variables** & **Description** & **Study Period** & **Resolution** & **Source** \\ \hline Forest Cover & MODIS land cover type (MCD12Q1) & 2012 & 500 m & NASA \\ \hline scPDSI & Self-calibrated Palmer drought & 1900–2016 & 4 km & USDA \\  & severity index & & & \\ \hline GPP & MODIS gross primary productivity & 2002–2014 & 1 km & NASA \\  & (MOD17; g C m\({}^{-2}\) year\({}^{-1}\)) & 2002–2014 & 1 km & NASA \\ \hline ET & MODIS evapotranspiration (MOD16; ET mm year\({}^{-1}\)) & 2002–2014 & 1 km & NASA \\ \hline Tree Mortality & Aerial detection surveys (TPH) & 2013–2016 & 1 km & USFS \\ \hline FIA Plots &
\begin{tabular}{c} FIA stand dynamics for forested \\ ecosystems \\ \end{tabular} & 2011–2016 & & USFS \\ \hline \hline \end{tabular}
\end{table}
Table 1: Federally funded datasets were used to evaluate patterns in tree mortality across California forests. Data was obtained from the National Aeronautics and Space Administration (NASA), the United States Department of Agriculture (USDA), and the United States Forest Service (USFS).  
Figure 2: Mean monthly scPDSI for all forested areas within California by latitude. The 2012–2016 drought was the most severe on record in terms of drought severity and duration.