Yep. It’s Getting Hotter and Drier Out There
“Hotter droughts,” which are severe droughts associated with human-caused climate change, are an emerging but poorly understood threat to forests worldwide. As climate change drives much of the nation into hotter, drier conditions, forest managers and scientists are not able to rely on historical patterns of temperature and precipitation for planning and decision making. Yet it is critical to identify forests and tree species most at risk.
Thus, USGS scientists and their collaborators are using California’s recent hotter drought (2012-2015) as a preview of the future, gaining the information needed to help forest managers adapt to a warming world.
Drought is More Than a Low Water Supply – And Why That Really Matters
We usually think of droughts as periods of low water supply caused by less rain or snow, but often overlook the other side of the equation: the drying power of the atmosphere, or atmospheric water demand. For example, if we look only at precipitation records, California’s recent drought would rank as severe but not unprecedented; comparable periods of low precipitation occurred during the Dust Bowl era of the 1920s and 1930s. However, compared to the Dust Bowl era, temperatures during the 2012-2015 California drought averaged about 1º C (about 2º F) warmer. Even though this may not seem like much, it significantly increased the atmospheric water demand and easily made this the most severe drought in California’s 120-year instrumental record, and perhaps much longer.
Additionally, water supplies for California’s cities, agriculture, industry and forests all depend on the accumulation of a thick mountain snowpack each winter, which then melts and slowly releases water during the otherwise dry summer months. But the higher temperatures of the recent drought meant that virtually no snow accumulated during the winter, and the little bit that did accumulate melted far earlier than usual in the spring.
Extreme Effects of Hotter Drought on Forests
The effects of California’s drought on forests, particularly in the Sierra Nevada mountain range, have been extreme. U.S. Forest Service experts estimated that, by the summer of 2015, tens of millions of trees had died, most of them in lower-elevation forests. Virtually no species was spared, with very high numbers of deaths recorded in pines, firs, incense cedars and oaks.
Shedding Leaves to Save the Tree?
The glaring exception? The iconic giant sequoias.
Only a handful of sequoias died in the hotter drought, and those that did usually already had fire scar damage at their bases, which likely reduced their ability to transport water to their crowns. But even though the large majority of sequoias survived the drought, many coped with it through unprecedented foliage die-back. By systematically shedding their older leaves, sequoias were able to conserve water by reducing the leaf area exposed to the drying power of the atmosphere.
Time to Triage
If Earth continues to warm as even the most conservative climate models project, forests will undoubtedly experience hotter droughts that are both more frequent and more severe. Fortunately, managers are not helpless in the face of such changes; they can increase a forest’s ability to survive hotter droughts. For example, forests can be thinned – usually by prescribed fire or mechanical thinning that selectively removes smaller trees – to reduce competition for water among the remaining trees.
But the task is so vast that forest managers must be smart about where on the landscape their limited resources can best be applied. They must, in short, conduct forest triage. What forests are more likely to die anyway with or without management intervention? What ones are likely to live anyway with or without intervention? And what ones might die without intervention, but live with adequate and timely intervention?
To do this, managers need reliable forest vulnerability maps to help them strategically target their treatments, and also to help them plan appropriate responses to future forest die-offs in highly vulnerable areas that they are unable to treat.
The Leaf to Landscape Project
Using California’s hotter drought as a potential preview of the future of many forests worldwide, USGS Climate Research & Development Program andEcosystems Program scientists, working with a number of collaborators and stakeholders, have helped catalyze the Leaf to Landscape project. Leaf to Landscape has two broad, complementary goals. First, it aims to provide maps of forest vulnerability to hotter droughts for large parts of California, letting the trees themselves reveal which parts of the landscape are most and least vulnerable. Second, it aims to improve our basic understanding of forest vulnerability to hotter droughts, providing the grist for scientific models that can be applied in forests elsewhere around the world
First, Really Get to Know Your Tree
To reach these ends, Leaf to Landscape has three main components, which zoom down to the scale of tree leaves and out to entire forest landscapes.
The first component is to really unravel what happens physiologically to trees during drought. To do this, professional tree climbers ascend individual trees of about 10 dominant species, and cut small branches from high in their crowns.
Back on the ground, scientists immediately insert a branch in a pressure chamber and record how much pressure is required to squeeze water out of the branch – a measure of the degree of drought stress experienced by the tree.
Remaining branches are transported to a laboratory where researchers measure leaf water content and other chemical factors which, combined, can provide insight into a tree’s level of drought stress.
Second, Look at Whole Patches of Forest
To understand how drought affects forests, you can’t just look at an individual tree, so Leaf to Landscape scientists also monitor tree populations. USGS maintains the world’s longest annual-resolution forest dynamics dataset, having annually tracked the health and fate of each of some 30,000 trees in California’s Sierra Nevada mountain range for up to 34 years.
These data provide an invaluable long-term baseline of forest health, dynamics, and causes of tree deaths during “normal” forest conditions. By continuing this monitoring throughout the drought, scientists are unraveling key information on the timing and causes of tree mortality. Adding to these data, foliage dieback in giant sequoias is now monitored annually.
Third, Use Eyes in the Sky — Remote Sensing Over Broad Landscapes
From the individual tree to the population, scientists then use remote sensing to map forest drought responses over broad landscapes. High-resolution LiDAR is used to create a detailed three-dimensional map of tree crowns for millions of trees.
The spectral signature of each tree crown is also recorded, allowing identification to species and measurement of whole-crown water content and chemical signatures (such as nitrogen and non-structural carbohydrates) that may be related to tree health and drought stress.
Giant Sequoias Hint of Forest Findings to Come
Researchers with the Leaf to Landscape project have completed all three of these data collection components. Researchers are now analyzing data, but early results from giant sequoias may give hints of findings to come. Remote sensing reveals substantial variation in sequoias’ whole-crown water contents. In contrast, direct ground-based measurements show very little variation in sequoias’ leaf water content or drought stress.
So what might be going on?
The apparent paradox might be explained by tree-to-tree variation in whole-crown leaf area; that is, sequoias might maintain a more favorable water status by adjusting their total leaf area. A lower leaf area could reflect the trees’ responses to acute drought (that is, when faced with drought sequoias abruptly reduce their leaf area by shedding older foliage) and/or long-term adjustments to those sites with chronically low water supplies (that is, sequoias never develop a large total leaf area).
Much work remains to be done, but, at least for sequoias, the total water content in their crowns during the current drought might well prove to be an indicator of how vulnerable different sites will be to the hotter droughts of the future.
Fortunately, because of an El Niño weather pattern, California is getting a little relief from its crippling drought. Even if the current El Niño is followed by several wet years, thus signaling the end of the drought, the Leaf to Landscape project’s efforts to understand forest vulnerability to hotter droughts will continue, especially to document and understand the long-term drought effects and forest recovery. For example, anecdotal accounts suggest that tree mortality remains elevated for one or more years following drought. Leaf to Landscape will quantify the magnitude and duration of lagged tree mortality, and will determine the factors contributing to such deaths, such as insects and disease. The project will also document how fast and how well tree crowns recover in terms of leaf area, water content and chemistry and how that relates to tree growth and mortality.
The team also plans to complete maps of forest vulnerability for selected sections of California’s forests, providing and helping interpret that information for land managers. The Leaf to Landscape project promises to improve our basic understanding of hotter droughts and their effects on the world’s forests.
Reflecting the interdisciplinary nature of the work, funding and in-kind contributions came from USGS, the National Park Service, the U.S. Forest Service, Stanford University/Carnegie Institute of Science, and the University of California, Berkeley.