California Woodland and Forest Change

Drought and wildfire accelerate tree cover loss, studies reveal

By Earth Resources Observation and Science (EROS) Center February 18, 2025

The U.S. Geological Survey’s land cover products can be used to monitor changes in forests and woodlands over time. One clear example is in California, where various environmental factors have contributed to significant changes in tree cover over the past few decades. Long-term droughts, extreme heat events and severe wildfires are occurring with greater frequency in California (Westerling 2016).

The 2012-2016 multi-year drought was one of the hottest in more than a century (Lund and others 2018). With that in mind, land cover scientists at the USGS Earth Resources Observation and Science (EROS) Center conducted two studies in California, one focused on forests and the other on woodlands, or forests with an open canopy, to assess changes in tree cover over time, especially during the 2012-2016 drought.

They used the land cover and land cover change component from the USGS Land Change Monitoring, Assessment and Projection (LCMAP) datasets, which are based on Landsat satellite data (Brown and others 2020; Zhu and Woodcock 2014).

California blue oak

A leafless tree with widespread branches on the slope of a brown grassy hill against a blue sky

California Blue Oak Tree

First Blue Oaks, then More Trees across California

The first study (Dwomoh and others 2021) focused on the woodlands dominated by California’s blue oaks (Quercus douglasii), the largest and most diverse ecosystem in California (McDonald 1990; Reiner and Craig 2011; Stahle and others 2001). These woodlands are of particular concern because they are threatened by environmental stressors such as drought, wildfire and changing precipitation patterns (Cartwright 2019; Morueta-Holme and others 2010). A follow-on study looked at forests and woodlands across California with respect to how these stressors influenced tree cover over time (Dwomoh and others 2023).

The studies compared the timing and location of tree death and decline in relation to environmental disturbances. For both studies, scientists merged multiple land cover maps from the annual LCMAP v1.0 and v1.1 datasets with publicly available environmental-related data including vapor pressure deficit, wildfires, minimum and maximum temperature and total precipitation. Vapor pressure deficit is a measure of atmospheric dryness and an indicator of drought severity (Juang and others 2022; Restaino and others 2019).

Drought conditions between 2012-2016 were characterized by hotter temperatures, higher vapor pressure deficit and lower precipitation.

Dive into the Studies

Trends in Tree Cover Change in California

Drought Escalates Declines in California Blue Oaks

Map of the Study Areas

A map of California and surrounding areas, with portions outlined and designated in various colors and a legend

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The boundary of blue oak study area (Dwomoh and others 2021) is outlined in red. It essentially surrounds the Central Valley, which includes the San Juaquin Valley in the south and the Sacramento Valley in the north. Blue oaks are found on the foothills of the Sierra Nevada mountain range, the Tehachapi Mountains (south of the San Juaquin Valley), and the Coast Ranges that run from the northern border to the southern border on the western edge of the state (McDonald 1990; Waddell and Barrett 2005).

The boundary of the second study, based on ecoregions (Dwomoh and others 2023), is outlined in blue. Within these ecoregions are the blue oak study area and all other forests surrounding it. The ecoregion study area is based on 8 Level III ecoregions, which are geographic areas with similar climate, vegetation and landforms, as defined by the U.S. Environmental Protection Agency (2013). The ecoregions selected for this study represent tree-dominated ecosystems.

The wildfire outlines (magenta) are of the 20 largest wildfires in California between 1985 to 2020, taken from Monitoring Trends in Burn Severity (MTBS) fire severity data (Eidenshink and others 2007). The location of the Wragg fire that occurred in 2015 is found north of San Francisco in the blue oak study area.

The Northern (Nor) California (CA) animation outline is in black, located in northeastern California. It is the outline of two counties, Butte and Plumas.

What the Studies Found

The goal of these two studies was to see how environmental disturbances affected forests and woodlands. Both studies found that there was a loss of tree cover over time, especially during the 2012-2016 drought.

The study of all forests and woodlands throughout California suggested that higher temperatures and frequent droughts accelerated tree cover loss across the state. Tree cover loss through medium to high severity fires was especially large in exceptionally dry and hot years.

Additionally, an upward trend in minimum temperature is evident with warmer nights. These conditions allow fires to burn longer into the night, usually at a time when fires lose energy and slow down, ultimately increasing fire spread (Balch and others 2022).

LCMAP land cover data identified areas where trees died and other areas where the health of the trees declined, highlighting both tree cover loss and change in condition. Overall, California experienced significant tree cover loss between 1986 and 2019, with the rate of loss increasing in the later years of the study period.

Visualizing Tree Losses and Temperatures

A slider on the map below shows LCMAP Primary Land Cover for California for the years 1985 and 2021. The green represents forests (tree cover). Comparing the 1985 image to the 2021 image shows dramatic changes in the tree cover over time.

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The animations below are from Butte and Plumas counties in northeastern California from 1985 to 2021. In the LCMAP land cover on the left, green represents forests (tree cover). The Landsat satellite images are true color. In both the LCMAP land cover and the Landsat images, gradual decline in tree cover over time can be seen, as well as an abrupt loss of tree cover due to wildfires that frequent this area.

Animation of two maps, side by side, showing the same area of land in different ways

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Below are outlines of animation areas. The scale bar for Butte and Plumas counties in California shows 125 kilometers, and the scale bar for the Wragge Fire shows 5 kilometers.

White background with thin black outlines of two irregular shapes labeled and with a scale bar below each

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The animations below are from the Wragg fire that occurred in 2015 north of San Francisco in the blue oak study area. In both the LCMAP land cover and in the Landsat images, gradual change in tree cover over time can be seen, as well as the abrupt loss of tree cover due to the Wragg wildfire.

Animation of two views of changing land cover in the same location, one a graphic and one a satellite image

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The map below shows the annual time series of standardized minimum temperature anomalies, which indicate how much nighttime low temperatures vary from the historical average across California from 1986 to 2019 (Dwomoh and others 2023). Each year highlights where the warmest nighttime temperatures were happening across the state.

Animation of a map of California with changing colors over time and a legend of the colors — Source of map data: The Gridded Surface Meteorological (gridMET) Dataset.

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A hilly area of California with a mix of woodland and forest

Tree Loss, Tree Gain and Tree Cover Change in California Blue Oak Woodlands from 1986 to 2016

A bar chart with three bar categories and % tree cover area on the y axis, plus years on the x axis — Graphing tree loss, tree gain and tree cover change in condition over time shows the variability in the amount of tree cover change across blue oak woodlands from 1986 to 2016. Tree cover change was derived from LCMAP products. Percent tree cover area on y-axis is the area of change expressed as a percentage of the original tree cover area in 1985. (Dwomoh and others 2021)

Precipitation and Temperature Anomalies for California Blue Oak Woodlands from 1986 to 2016

A line graph with four lines, standardized climate anomaly on the y axis and years from 1986 to 2016 on the x axis — The highlighted area on this graph of vapor pressure deficit and temperature, with standardized climate anomaly on the y-axis, represents how much these conditions differed from the historical average (at 0). This shows the prolonged drying and warming that occurred 2012-2016 (Dwomoh and others 2021). Sources of data for this graph: Parameter-elevation Regression on Independent Slopes Model (PRISM), POLARIS and the North American Land Data Assimilation System (NLDAS2).

Usefulness of the Studies

Even with geographic variability in tree cover trends across the ecoregions, these studies illustrate a strong relationship between fire-related tree cover loss, vapor pressure deficit and minimum temperature. The observed trends indicate drier and warmer conditions, which have played a role in tree cover loss and may continue to shape future patterns (Madakumbura and others 2020).

Findings of the blue oak woodlands study were referenced by the California Wildlife Foundation’s recommendations for revisions to the state’s Joint Policy on Hardwoods Range Management in Advisory Committee of the California State Board of Forestry & Fire Protection.

Furthermore, the publications for both studies have been included in the bibliography database of the California Office of Environmental Health Hazard Assessment (OEHHA). The two papers are listed among the recent research on land change to support their reporting on indicators of environmental variability in California.

Hills covered with grassland in greens and browns, plus some trees lacking leaves

Burned and drought-stressed woodland area in California

Tree mortality during California’s extreme drought

Current Generation of Land Cover Mapping

Annual NLCD (2023) for the conterminous United States, showing different major land cover types in the background

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Although LCMAP as an operational mapping effort has ended, an improved and updated USGS National Land Cover Database (NLCD) known as Annual NLCD debuted in 2024.

Annual NLCD continues the year-by-year land change beginning in 1985 that was offered by LCMAP but features more detailed thematic land cover classes, potentially allowing for the same analyses to yield results with enhanced spatial and thematic detail.

Notably, the three forest-type classes available in the Annual NLCD—deciduous, evergreen and mixed forests—will enable future studies to achieve greater precision and nuance when leveraging the updated data to assess environmental disturbances.

More about Annual NLCD

References

^{Balch, J. K., Abatzoglou, J. T., Joseph, M. B., Koontz, M. J., Mahood, A. L., McGlinchy, J., ... Williams, A. P. (2022). Warming weakens the night-time barrier to global fire. Nature, 602(7897), 442-448. doi:10.1038/s41586-021-04325-1}

^{Brown, J. F., Tollerud, H. J., Barber, C. P., Zhou, Q., Dwyer, J. L., Vogelmann, J. E., and others (2020). Lessons learned implementing an operational continuous United States national land change monitoring capability: The Land Change Monitoring, Assessment, and Projection (LCMAP) approach. Remote Sens. Environ. 238:111356. doi: 10.1016/j.rse.2019.111356}

^{Cartwright, J. (2019). Ecological islands: conserving biodiversity hotspots in a changing climate. Frontiers in Ecology and the Environment, 17(6), 331-339. doi:10.1002/fee.2058}

^{Dwomoh, F. K., Auch, R. F., Brown, J. F., & Tollerud, H. J. (2023). Trends in tree cover change over three decades related to interannual climate variability and wildfire in California. Environmental Research Letters, 18(2), 024007. doi:10.1088/1748-9326/acad15}

^{Dwomoh, F. K., Brown, J. F., Tollerud, H. J., & Auch, R. F. (2021). Hotter drought escalates tree cover declines in Blue oak woodlands of California. Frontiers in Climate, 3(67). doi:10.3389/fclim.2021.689945}

^{Eidenshink, J., Schwind, B., Brewer, K., Zhu, Z.-L., Quayle, B., & Howard, S. (2007). A project for monitoring trends in burn severity. Fire Ecology, 3(1), 3-21. doi:10.4996/fireecology.0301003}

^{Higuera P.E. and Abatzoglou J.T. (2021). Record-setting climate enabled the extraordinary 2020 fire season in the western United States Glob. Change Biol. 27 1–2}

^{Juang, C. S., Williams, A. P., Abatzoglou, J. T., Balch, J. K., Hurteau, M. D., & Moritz, M. A. (2022). Rapid growth of large forest fires drives the exponential response of annual forest-fire area to aridity in the western United States. Geophysical Research Letters, 49(5), e2021GL097131. doi: https://doi.org/10.1029/2021GL097131 }

^{Lund, J., Medellin-Azuara, J., Durand, J., & Stone, K. (2018). Lessons from California’s 2012–2016 drought. Journal of Water Resources Planning and Management, 144(10), 04018067. doi:10.1061/(ASCE)WR.1943-5452.0000984}

^{Madakumbura, G. D., Goulden, M. L., Hall, A., Fu, R., Moritz, M. A., Koven, C. D., . . . Randerson, J. T. (2020). Recent California tree mortality portends future increase in drought-driven forest die-off. Environmental Research Letters, 15(12), 124040. doi:10.1088/1748-9326/abc719}

^{McDonald, P. M. (1990). “Quercus douglasii Hook & Arn. Blue oak,” in Silvics of North America, eds R. M. Burns, and B. H. Honkala (Washington, DC: U.S. Department of Agriculture, Forest Service), 631–639.}

^{Morueta-Holme, N., Fløjgaard, C., & Svenning, J.-C. (2010). Climate change risks and conservation implications for a threatened small-range mammal species. PLoS ONE, 5(4), e10360. doi:10.1371/journal.pone.0010360}

^{Reiner, R., and Craig, A. (2011). Conservation easements in California blue oak woodlands: testing the assumption of livestock grazing as a compatible use. Nat. Areas J. 31, 408–413. doi: 10.3375/043.031.0411}

^{Restaino, C., Young, D. J. N., Estes, B., Gross, S., Wuenschel, A., Meyer, M., & Safford, H. (2019). Forest structure and climate mediate drought-induced tree mortality in forests of the Sierra Nevada, USA. Ecological Applications, 29(4), e01902. doi:10.1002/eap.1902}

^{Stahle, D. W., Therrell, M. D., Cleaveland, M. K., Cayan, D. R., Dettinger, M. D., and Knowles, N. (2001). Ancient blue oak reveal human impact on San Francisco Bay salinity. Eos. Trans. Am. Geophys. Union 82, 141–145. doi: 10.1029/EO082i012p00141}

^{U.S. Environmental Protection Agency (2013). Level III ecoregions of the continental United States: Corvallis, Oregon, US EPA–National Health and Environmental Effects Research Laboratory, map scale 1: 7,500,000.}

^{Waddell, K. L., and Barrett, T. M. (2005). Oak Woodlands and Other Hardwood Forests of California, 1990s. Portland, OR: US Department of Agriculture, Forest Service, Pacific Northwest Research Station, 245. doi: 10.2737/PNW-RB-245}

^{Westerling, A. L. (2016). Increasing western US forest wildfire activity: sensitivity to changes in the timing of spring. Philos. Trans. R. Soc. Lond., B, Biol. Sci. 371:20150178. doi: 10.1098/rstb.2015.0178}

^{Zhu, Z., & Woodcock, C. E. (2014). Continuous change detection and classification of land cover using all available Landsat data. Remote Sensing of Environment, 144(0), 152-171. doi:10.1016/j.rse.2014.01.011}

Learn more about the California tree cover studies.

January 17, 2025

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California Woodland and Forest Change

Drought and wildfire accelerate tree cover loss, studies reveal