The speed at which a forest recovers from disturbances can foretell that forest’s untimely demise. In a paper published today in Nature Climate Change, researchers tracked via satellite the vitality of California’s forests during the recent prolonged droughts and developed an early-warning signal for forest death. The new signal can detect a forest’s death spiral 6–19 months ahead of time.
Statistical and empirical formulas for predicting forest mortality “can change over time, especially as climate in the future will be outside the regime of historical climate,” said lead researcher Yanlan Liu, an environmental scientist at Stanford University in California. “This method…directly monitors the dynamics of vegetation from remote sensing, meaning that it’s bridging the gap between climate and vegetation.”
Complexities of Modeling Mortal Forests
“Every year, generally, [a forest’s] biomass increases during the green season and reduces in the dormant season,” explained coauthor Mukesh Kumar, a hydrologist at the University of Alabama in Tuscaloosa. “When a tree is stressed, its physiological functions are impaired. The rate of the recovery of the vegetation with respect to its normal cycle gets slower.”
“Forest managers are constantly trying to predict the fate of forest stands in the face of increasing stressors so that they can prescribe management tools to try and avert forest loss and transition to other vegetation types,” said Heather Alexander, a professor of forest biology at Mississippi State University in Mississippi State who was not involved with this research. “However, by the time forests show obvious signs of failing, like browning leaves or leaf loss, it’s usually too late.”
Predicting a forest’s time of death is challenging for models of vegetation dynamics, Kumar said, because mortality is a complicated process on tree and ecosystem scales: A forest sometimes can adapt to challenging conditions for a time, estimates of carbon and water budgets can be off, and data on forest dynamics can be at a too low spatial resolution.
Identifying the Start of a Death Spiral
To address these challenges, the researchers analyzed high-resolution images of California’s forests taken by the Landsat 7 satellite from 1999 to 2015. That time span encompasses two periods of intense drought in California, one during 2007–2009 and the other in 2012–2015. Forest managers estimate that the entire drought, which lasted until 2017, killed nearly 150 million trees.
With the images, “we’re using the normalized difference vegetation index, NDVI, which is a measurement of greenness from satellite data,” Liu said. “It’s proportional to biomass.”
The researchers used NDVI to track the ebb and flow of the forests’ life signs in different regions and to spot whether and when recovery began to slow down. By comparing these calculations to a map of forests that eventually died, they found that the loss of a forest’s resilience was an early-warning signal for its death.
“We found that 75% of the cases exhibited an early-warning signal more than 6 months before mortality,” Kumar said. “In 25% of the cases, it showed more than 19 months before mortality.” In some regions, this signal appeared before a loss of greenness indicated a forest’s imminent demise. But in some areas, the signal for the forest as a whole was very muddled.
“The breakthrough point came when I separated out the early-warning signal by species distribution, meaning that I applied the relationship for pines and oaks separately,” Liu said. “And suddenly, the relationship became very clear. So that means that the resilience signal is species specific.”
The species dependence was initially surprising, both Liu and Kumar said, but it made sense in hindsight. “Each species of tree responds differently,” Kumar explained. “Some trees can handle stress for a long time before shedding their leaves or dying. Others are more susceptible or shed their leaves much earlier.” They found that oaks were more likely to survive under low-resilience conditions than were spruces and pines.
“The key point is that we need to separate the species rather than put them together, because they all have different translations, so to say, between low resilience and mortality,” Liu added.
Saving a Forest’s Life
The researchers emphasized that there’s still a lot of work to be done before this method can be used to predict forest death in other areas of the world. “Caution needs to be taken, because the relationships between low resilience and mortality can change between species and also across climate regions,” Liu said.
Satellite data availability over tropical forests can be spotty because of clouds, and “another thing is that we need accurate [tree] species distribution maps, which I’m not currently aware of for many regions,” Liu added.
Alexander thinks this method has promise. “The early-warning signal tool offered by Liu et al. could provide an exciting new way to predict forest resiliency to stressors like drought many months before the forests show obvious signs of decline,” she said. “This would give managers more time to implement management strategies to alleviate stressors and hopefully restore forest health.”
Doug Miller, an environmental informatics researcher at Pennsylvania State University in University Park who was not involved with this study, added that this research provides “the type of tools that land management agencies are really going to need to manage resources under a changing climate.”
“We do think that this lead time will allow us to do something like prescribed burnings or removal of infested trees, or maybe even do variable density thinning,” Kumar said. Other factors like the economics and logistics of forest management, he added, also need to line up to determine whether this signal gives enough warning to keep a forest alive.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer