Tree mortality during California’s drought

Last summer, after four years of extreme drought, more than 21 million trees died in California.

This figure is based on mortality surveys performed by the U.S. Forest Service, which every summer for the past 10 years has flown a small aircraft over most of the forested area of California and recorded the locations of dead trees. The mortality observed in 2015 was by far the worst ever recorded. Mortality was especially intense in the Southern Sierra Nevada, where, across many large landscapes, the majority of the conifers died.

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Tree mortality in the Southern Sierra. Photos: U.S. Forest Service

I’ve been working with the U.S. Forest Service aerial mortality monitoring program and UC Davis and Yale colleagues Jens Stevens, Mason Earles, and Andrew Latimer to analyze the mortality patterns recorded by the Forest Service and to understand the factors that lead certain forests to suffer more during drought. The first step of the analysis was to take the set of polygons that the aerial observers drew around dead trees (here’s an example of those polygons from the Southern Sierra Foothills that you can view in Google Earth) and convert them into a regular grid in which each cell is assigned a value representing the number of dead trees observed inside it. Here’s the resulting grid for the mortality throughout the state in 2015, highlighting the serious situation in the southern Sierra:

View larger map

The map shows the mortality amount observed in each grid cell (adjusted proportionally for mortality patches that overlap multiple grid cells and/or only partially overlap a given grid cell). The map only includes grid cells that fell completely within the plane’s field of view (the Forest Service also reports their flight lines and approximate observation distance) in order to avoid bias in our subsequent analysis of mortality patterns–this explains the linear gaps in the mortality grid.

Our next step was to see how tree mortality rates changed with time, year after year, as the drought progressed. To do this, we converted the mortality survey data from each year into a grid the same way we did for the 2015 data. We can visualize the mortality over time as an animation:

FigS5_animation

The map makes it painfully clear that although there is always some amount of mortality each year, the mortality in 2015–particularly in the southern Sierra–is far greater than that observed during other years in recent history. It is interesting to note that 2105, the year in which mortality spiked, was the fourth year of extreme drought in the state. This observation highlights the fact that tree mortality can take several years to respond to drought. Such a delayed response is often observed in studies of drought stress, and the existence of this delayed response hints that we are likely to observe high mortality well into 2016 and potentially beyond, especially in Southern California, where the severe drought continues for a fifth year.

My colleagues and I have used the Forest Service aerial mortality survey data, combined with other sources of environmental data–including long-term climate–to evaluate the factors that predispose forests to experience high mortality during drought. Our analysis is currently in review at an academic journal–stay tuned for a description of what we found!

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