The Science of Resilient Forests
Building the perfect campfire requires the right mix of ingredients: plenty of kindling, a spark to ignite it, and large, dry logs to keep the fire burning strong. Unfortunately, fire suppression strategies adopted long ago—combined more recently with severe droughts and climate change—have created this same mixture writ large across many of the dry forests of the western United States, such as those in Yosemite National Park and elsewhere in the Sierra Nevada. Over the past several years, these conditions have led to disastrous, headline-grabbing fires that threaten human communities, ecosystems, and the very survival of our forests.
Despite their destructive power, fires are natural phenomena in many forests, where they are essential to the biomes’ long-term health. Decades of field-based studies have built the field of fire ecology and have informed nuanced views of fire as both a threat and a restorative process. However, the expense of such fieldwork has meant that relatively small portions of forests—and their relation to fire—have been studied in detail. Even extensive field studies involving hundreds of forest plots may cumulatively measure conditions over only dozens to hundreds of hectares, yet because of the limited data available, these samples are taken to represent highly varied conditions over millions of hectares.
Today, with help from remote sensing technologies, fire ecologists are more often examining continuous forest landscapes to understand their conditions before and after fires. In particular, they are using high-resolution laser imaging measurements gathered by lidar instruments aboard planes to map conditions from the treetops to the ground. Lidar allows us, for the first time, to quantify forest structure directly—that is, to determine tree heights, canopy densities, and the distribution of branches and leaves throughout the canopy—a feat previously possible only by painstaking field measurements. Lidar-based studies are beginning to enrich our understanding of wildfires historically, and they are providing forest managers with new tools to use in planning forest restorations and thus to improve forests’ resilience to future fires.
A New Understanding of Fire in Forests
For much of the 19th and 20th centuries, forest management efforts in the United States were focused on fire suppression. The rationale was that by preventing fires, forest management agencies could protect natural resources and wildlife, drive economic growth in the timber industry, and safeguard the lives and livelihoods of those living nearby.
However, in the 1960s, fire ecology research at the University of California, Berkeley shined a light on the connection between regular fires and forest health for many forests in the arid western United States. In this work, researchers found that areas burned by fires under nonextreme weather conditions ultimately became more resilient and resistant to future burning. With less flammable material for subsequent fires to burn, these fires were prevented from burning as intensely and moving as rapidly over the landscape as they otherwise would have.
As a result of this research, in the early 1970s, forest managers in Sequoia and Yosemite national parks in the Sierra Nevada of California were among the first to introduce prescribed burns and to allow lightning-sparked wildfires to burn in their jurisdictions as part of a fire benefit program. In doing so, they sought to return these forests to a healthy cycle involving frequent fires that had existed for centuries before managers first sought to suppress all fires in the 19th century.
Much of this early fire ecology knowledge was gained through field studies. Research teams measured conditions on the ground, then extrapolated from these small plots to estimate likely conditions across vast reaches of parkland as they developed management plans. From there, forest managers embarked on strategic thinning initiatives or set managed, prescribed fires to improve forest health and resilience.
The success of this approach became especially evident during the 2013 Rim Fire, which started in California’s Stanislaus National Forest but quickly spread to neighboring Yosemite. The fire caused less damage in Yosemite where it entered forests that had been subjected previously to lower-severity burns. In these areas, there was less undergrowth and thus smaller fuel loads, which resulted in lower-intensity fires that burned along the ground, rather than laddering up into the crowns of large, old-growth trees.
Measuring the Whole Forest
Scientists’ ability to study Yosemite’s forests both on a broader scale and in more detail began to change in 2010. From 2010 to 2011, Watershed Science (now NV5 Geospatial) used its airborne lidar instruments to image and measure a total of 64,800 acres (26,200 hectares) of Yosemite National Park’s forests, in research initiated by James Lutz, now at Utah State University, and Malcolm North of the U.S. Forest Service’s Pacific Southwest Research Station. These lidar data—collected at a high density of about 100,000 measurements per acre (247,000 per hectare) across the full study area—provided a census of the 3D structure of vegetation and the ground below.
These data sets, supplemented by a larger lidar acquisition in 2013 following the Rim Fire that year, enabled numerous and varied studies focusing on the overall effects of fires, their impacts on habitat for critical species and on hydrology, and guidelines for managers seeking to improve the resilience of other Sierra Nevada forests to wildfire [Kane et al., 2013, 2014, 2015].
The first of these studies, led by one of the coauthors of this article, used the lidar data to examine the effects of fire across several forest types [Kane et al., 2013]. These forest types included stands (groups of trees) that had experienced a range of fire histories, from stands where fires had been suppressed for a century to others burned as many as three times under the restored fire regime enacted by the park’s managers since the 1970s.
Key to these studies was the concept of the resulting burn severity. Wildfires naturally burn at different intensities over different areas. Burn severity describes the effects of fire on soils and vegetation and is commonly classified as low (only material on the ground burned), moderate (a portion of trees were also killed), or high (most to all trees were killed). Burn severity also has varying impacts on soil structure, permeability, organic matter, and ability to support regeneration of the forest. Generally, these severities are estimated from multispectral images collected by satellites, such as NASA’s Landsat, by analyzing ratios of spectral bands in the images that indicate values corresponding to the removal of green vegetation and organic matter from soils.
The lidar-based studies added nuance and breadth to prior research and observations by ecologists and forest managers on the effects of fire. For example, previous fieldwork had suggested that low-severity fires removed fuels primarily on the surface but caused little change to the structure of the forest canopies above. However, the lidar measurements indicated that low-severity fires did a better job at thinning both underbrush and dead and unhealthy trees than had been thought, suggesting these burns may be effective at improving forest resilience.
Forests that have survived one or more fires tend to be more resilient to subsequent fires. A key trait of these forests is that previous fires leave a pattern of surviving individual trees or small clumps of trees interspersed with openings and gaps. Reconstructions of forests from more than 100 years ago, before managers began suppressing fires, have shown that these conditions were widespread among the U.S. West’s drier forests, including in Yosemite, and were key to forests thriving in a regime of frequent fires [Collins et al., 2011; Larson and Churchill, 2012; North et al., 2022]. This pattern reduced the fuel load available and created a web of natural firebreaks, increasing the probability of lower-severity fires. However, fire suppression over many decades allowed trees to fill in the openings, creating dense stands prone to intense fires that threaten forest survival.
The historic tree clump and opening patterns were created by natural fires burning occasionally over centuries. Could fires burning today in vastly changed forests re-create these key patterns? With the lidar data collected in the early 2010s, we mapped patterns of trees and openings in Yosemite’s forests. The results revealed that where multiple fires had burned the same locations—reflecting a successful restoration of the frequent fire regime—these key patterns were present. Unexpectedly, we also found that even the first fire to burn a location in a century, if it burned under moderate weather and caused low to moderate burn severity, could re-create clump and opening patterns reminiscent of historic fire-resilient forests. This finding strongly supports the idea that using prescribed fires when weather conditions are not too dry can help restore forests and make them more resilient to future fires.
The Next Generation of Lidar-Based Studies
Until 2019, only about a third of Yosemite had been measured with lidar. In 2019, NV5 Geospatial, funded by the U.S. Geological Survey (USGS) and the Yosemite Conservancy, conducted a comprehensive aerial survey across all of Yosemite and some adjacent areas. This survey, collected using the latest generation of airborne commercial lidar technology, provides more detailed measurements, particularly of vegetation structure, than the earlier surveys did. The project was completed as part of the USGS’s 3D Elevation Program, the objective of which is to meet growing needs for high-quality topographic data and for a wide range of other 3D representations of the nation’s natural and constructed features. The new data, for example, offer clearer looks at canopy structure and enable better mapping of fuel ladders (fuel that carries fire from low-growing vegetation into the tree canopy) and snags (dead trees). The new survey showed that forests in Yosemite have changed considerably even since they were first measured with lidar less than a decade earlier. Between 2013 and 2019, 354 fires burned 132,205 acres (53,500 hectares) of its forests—sometimes for the second or third time.
More important, the recent data document how the recent devastating multiyear drought resulted in the death of a large fraction of the park’s trees in several key forest types like ponderosa pine and mixed-conifer forests, changing the character of these forests. The dead trees constitute a huge, unprecedented pulse of fuel that will feed future fires. Instead of being restorative, future fires fed by this massive fuel load could be devastating, akin to the 2020 Creek Fire that burned under similar conditions farther south in the Sierra Nevada.
The new lidar data have been processed and recently been made available to Yosemite’s managers and to researchers. We are beginning to work with the park’s managers to apply the new measurements to assess conditions across the entire park. Data from the earlier lidar flights will provide an important historic sample that will allow us to examine the intervening changes in detail.
Benefits Beyond Forests
The utility of lidar extends beyond fire management applications. For example, the earlier round of Yosemite lidar data was also used in a study of California spotted owl habitat. A key question that field studies had not been able to resolve was whether these threatened birds require a high density of canopy cover—a condition that would encourage more severe fires—throughout their ranges to survive. When we combined the Yosemite lidar data with lidar data from other Sierra Nevada forests, we showed that these owls require dense canopy cover only around their immediate nesting sites [North et al., 2017]. This finding can help forest managers safely thin forests farther from owls’ nests, thereby improving the forests’ resilience to future fires and drought while maintaining safe habitat for the owls.
Droughts and fires not only jeopardize forest health and wildlife habitat; they also stress water resources for residents in the western United States. The depth and location of snowpack often affect water availability, which in turn can create shortages for residents. Lidar can help water managers in the Sierra Nevada, the Rocky Mountains, and other snowpack-influenced regions measure snow depth across large areas, and when it is combined with airborne hyperspectral imagery that gives information about the reflectivity of the snow (albedo), the combined data set provides information about the quantity of water stored in the snowpack. By comparing changes in snowpack over time within watersheds, rates of snowmelt can be estimated.
These data help managers regulate water releases from reservoirs that provide water to urbanized areas in California, Colorado, and elsewhere. If water managers underestimate snowmelt and retain more water, their reservoirs could be overtopped, requiring rapid releases to avoid catastrophic damage. Alternatively, if they overestimate snowmelt and release too much water, they may not have enough to supply communities during drier times of the year.
Seeing the Forest and the Trees
Forests play many vital and stabilizing roles on our planet, including in mitigating climate change by moderating temperature and humidity and as prominent parts of Earth’s carbon and water cycles. They are also home to diverse species of animals and plants, they contribute to economies through timber production and tourism, and they are widely used for recreation.
Understanding forest structure and responses to fire is more important than ever, considering how the incidence and intensity of forest fires are rising across much of the planet. Improving our understanding will help us to ensure the health of these important resources, prevent out-of-control fires that threaten lives and livelihoods, and preserve endangered wildlife habitat.
Lidar’s capabilities to measure vegetation structure in detail across wide areas are shifting the paradigm of how forests are analyzed, and the technology is now being adopted as a foundational data collection method for forest management in the same way aerial photography was more than half a century ago.
Since lidar’s initial use to study Yosemite just over a decade ago, lidar data have already revealed many new ecological insights that are changing not just how forestry practices are implemented but also how we see the forest. Scientists and forest managers are looking at how individual trees, rather than management units (i.e., stands of trees spread across 5–100 acres (2–40 hectares)), respond to and interact with climate and their local environment. This granular level of detail tells us much more about the processes occurring in forests and will help us make sustainable and wise decisions about resources that are essential for our long-term survival.
Collins, B. M., R. G. Everett, and S. L. Stephens (2011), Impacts of fire exclusion and recent managed fire on forest structure in old growth Sierra Nevada mixed-conifer forests, Ecosphere, 2, 51, https://doi.org/10.1890/ES11-00026.1.
Kane, V. R., et al. (2013), Landscape-scale effects of fire severity on mixed-conifer and red fir forest structure in Yosemite National Park, For. Ecol. Manage., 287, 17–31, https://doi.org/10.1016/j.foreco.2012.08.044.
Kane, V. R., et al. (2014), Assessing fire effects on forest spatial structure using a fusion of Landsat and airborne LiDAR data in Yosemite National Park, Remote Sens. Environ., 151, 89–101, https://doi.org/10.1016/j.rse.2013.07.041.
Kane, V. R., et al. (2015), Water balance and topography predict fire and forest structure patterns, For. Ecol. Manage., 338, 1–13, https://doi.org/10.1016/j.foreco.2014.10.038.
Larson, A. J., and D. Churchill (2012), Tree spatial patterns in fire-frequent forests of western North America, including mechanisms of pattern formation and implications for designing fuel reduction and restoration treatments, For. Ecol. Manage., 267, 74–92, https://doi.org/10.1016/j.foreco.2011.11.038.
North, M. P., et al. (2017), Cover of tall trees best predicts California spotted owl habitat, For. Ecol. Manage., 405, 166–178, https://doi.org/10.1016/j.foreco.2017.09.019.
North, M. P., et al. (2022), Operational resilience in western US frequent-fire forests, For. Ecol. Manage., 507, 120004, https://doi.org/10.1016/j.foreco.2021.120004.
Van R. Kane and Liz Van Wagtendonk, Forest Resilience Laboratory, University of Washington, Seattle; and Andrew Brenner (andrew.brenner@NV5.com), NV5 Geospatial, Ann Arbor, Mich.