Effects of interannual climate variability and wildfire on the patterns of tree cover decline in California, 1985-2016

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Francis K. Dwomoh, ASRC Federal Data Solutions, Contractor to the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD, Roger F. Auch and Heather J. Tollerud, U.S. Geological Survey Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD, Jesslyn F. Brown, ], U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center, Sioux Falls, SD

Background/Question/Methods
California state (CA) suffers from frequent drought events, hotter temperatures and other disruptions to the climate system whose negative impacts on ecosystems have been widely reported in the last several decades. Dozens of studies have documented adverse impacts on vegetation communities as a result of the changing climate and changing wildfire regimes. Nonetheless, most of the studies were confined to specific vegetation groups or few species, specific drought incidents, or analysis over a relatively short annual time scales; thus, limiting our understanding of the broad-scale impacts on tree cover as well as the spatio-temporal variability of impacts among ecoregions across CA. Here, we leveraged multiple annual land cover and land surface change products from the Land Change Monitoring, Assessment and Projection (LCMAP) dataset to assess patterns in tree cover change in tree-dominated ecoregions in CA from 1985 to 2016. Furthermore, we combined LCMAP-derived tree cover change maps with climate and wildfire datasets to assess effects of fire and climate on tree cover loss and identified hotspots of drought-related tree cover loss and fire-induced tree cover declines across the study area.
Results/Conclusions
Across CA, we found increasing trends in annual tree cover loss and tree cover conditional, which denotes a change in tree cover condition without a thematic change. Within the state, rates of tree cover loss and tree cover conditional change were 15.8 km2/year and 7.7 km2/year, respectively. Likewise, there were increasing trends in fire and non-fire related tree cover loss, but trends differed among ecoregions. Tree cover change area estimates were based on pixel counts from the maps and not statistically-based sampling. Patterns of tree cover loss were strongly linked to interannual climate fluctuations with higher losses coincident with drought and warmer years. A prolonged warmer drought during 2012 – 2016 were associated with record tree cover loss in most ecoregions. We found hotspots of drought-related tree cover declines that were geographically and temporally varied across ecoregions. Our findings suggest that frequent hotter and drier climate in the future may trigger more rapid tree cover loss in the study area, thereby endangering the ecosystem goods and services therefrom.