Trees are struggling in the Sierra Nevada.
Ponderosa pines, sugar pines, white firs and incense cedars that typically thrive at elevations of 4,500 to 6,000 feet have been dying at higher-than-average rates, leaving russet-colored patches of dead trees. Severe drought stress and beetle damage combined to push many conifers past their ability to recover.
Giant sequoia trees, some of the planet’s largest and oldest living organisms, have fared better but still show signs of stress, said Koren Nydick, a scientist at Sequoia and Kings Canyon national parks.
“So far the sequoias have actually coped quite well,” Nydick said, but in 2014, she and other researchers noticed dead foliage on nearly half of the giant sequoias they surveyed. That was unusual.
Those initial sequoia surveys expanded into the Leaf to Landscape project, a collaborative effort that combines remote sensing data with on-the-ground surveys to map which parts of the Sierra Nevada forests are most vulnerable to drought, and why. The project collects information about forests across California and pays specific attention to the iconic sequoias.
Funding from the Southwest Climate Science Center, which is based in the Institute of the Environment at the University of Arizona, allowed the Leaf to Landscape project to continue taking measurements in 2016 and 2017.
With multiple scales of analysis and the use of remote sensing, Nydick said the Leaf to Landscape project is “paving the way” for new methods of understanding vulnerability.
Leaf to Landscape’s new models will show land managers the areas where trees are most sensitive, which can inform decisions about forest treatments like prescribed burns. “We designed this project to take advantage of the drought, to ask the trees themselves how they’re faring during that very stressful time,” Nydick said.
Leaf to Landscape documents tree responses at multiple scales, from a single plant’s physiology to the ecology of the forest. Why were trees doing well in some areas, while others showed signs of more severe stress?
To address this question at the leaf level, research team members from the University of California at Berkeley began climbing trees. By clipping samples of scaly sequoia leaves from the canopy, the research team could measure the pull of water into the leaves to understand how individual trees respond to drought.
Researchers also surveyed and monitored forest plots to understand stress responses at the scale of populations. U.S. Geological Survey researchers have carried out this monitoring at specific sites in the Sierra Nevada for more than three decades, creating a useful baseline of information about tree populations.
Giant sequoias, however, do not often appear in the forest outside of concentrated groves, so at the height of the drought, research team member and U.S. Geological Survey ecologist Nathan Stephenson organized a survey specifically for sequoia populations. His work revealed that sequoias were losing their foliage, probably to prevent water loss amid dry conditions.
Individual tree samples and plot surveys helped answer questions about drought response at the local level. At the regional scale, however, researchers needed different tools.
Leaf to Landscape researchers began working with Carnegie Airborne Observatory to remotely sense the spectrum of light reflected up from the canopy. This includes visible light but also ultraviolet and infrared wavelengths. Scientists can analyze spectral signatures to determine water content in the canopy.
Now, researchers are beginning to combine data from the leaves and the landscape to calibrate models of tree health across the forest. “We can really study individual tree canopies and compare what we see from the sky to the result on the ground,” Nydick said.
After above-average precipitation across California this past winter, the Leaf to Landscape team hopes to extend data collection to understand how individual trees and forests recover—or don’t—from severe drought.