NATIONAL PARK SERVICE: Planning for resilience: Understanding the connections between forests, fire, water, and drought

By the National Park Service

You may know Yosemite National Park for its spectacular scenery, waterfalls, granite rock faces, or ancient sequoias. But do you know about the park’s rich fire-management history and distinction as one of the first national parks to allow lightning-ignited fires to burn when conditions are right? Yosemite National Park, at nearly 748,000 acres in size (~1,200 square miles), includes the 40,000-acre Illilouette Creek Basin. In 1972, park managers decided to allow lightning-ignited fires to burn within the basin when the fires were benefiting the ecosystem and as long as park visitors and infrastructure were safe. Managers realized fire’s vital role in the maintenance and health of park ecosystems and began a prescribed-fire and managed-wildfire program. Because of this decision, the Illilouette Creek Basin experienced frequent fires of varying intensity over the past 50 years that led to reductions in the severity and extent of individual fires (Collins et al. 2009) and numerous ecological benefits (Stephens et al. 2021).

However, in many areas across the Sierra Nevada, including some in Yosemite, forests are dense due to decades of fire suppression that began in the mid-1800s with European colonization. That period also led to a decrease in fires that had been used by Indigenous peoples for millennia to promote healthy forests in Yosemite and elsewhere. Although trees clearly provide important benefits to ecosystems and people, there is a limit on how many trees a landscape like Yosemite can sustain in the long term given the amount of water available and high wildfire risk. Low- and moderate-severity fires play a key role in maintaining dry forests within the park—opening the canopy and allowing water and sunlight through, and reducing the hazardous accumulation of dead, woody debris. Fire also allows for the recycling of nutrients to the soil, which encourages the germination and regrowth of plants.

A recent study by researchers at the Desert Research Institute and Yosemite National Park took advantage of the park’s rich fire history by developing planning tools to make Yosemite forests more resilient in the face of dense forest conditions and climate change. Specifically, researchers identified areas most likely to benefit from low-intensity fires (benefits can include reduced drought vulnerability, less intense future fire, and increased streamflow). Focusing on areas where the greatest benefits lie is important because of Yosemite’s large size and limited resources to conduct prescribed burns and fuel treatments.

How did Researchers Conduct the Study?

In 2022–2023, researchers used a combination of new field observations, remote sensing, and computer modeling to carry out the study.

• New field observations included measurements of soil moisture, soil water repellency, and infiltration rates. Researchers chose field sites to include a broad range of topography, time since fire, past fire severity, and vegetation density, as well as to span a large portion of western Yosemite National Park. They used statistical models to relate each of the soil property values to topography, vegetation, and fire history. This field work resulted in a unique dataset of soil properties in the park for this and future work.
• Researchers used remote sensing to determine recent vegetation cover. Remote sensing is a surveying method used to collect data about an object from a distance—in this case, the Earth’s surface. They obtained remote sensing information from Lidar and cameras on airplanes.
• Researchers also made use of a multi-year (existing) dataset of snow depth and soil moisture. The dataset included locations under the forest canopy and in adjacent canopy gaps—all along an elevational transect within the park. See Roche et al. (2019) for a description of the dataset.
• Computer modeling—Researchers used models to calculate spatial variations in drought vulnerability and potential fire activity (such as, flame lengths and rate of spread) across the entire western portion of Yosemite. The results were turned into maps identifying areas critical in reducing these threats through management actions. While some models were created explicitly from field observations described above, other simulations of fire, drought stress, and streamflow were provided by Roger Bales at the Center for Ecosystem Climate Solutions (CECS) using models built from satellite data.

Linking Forest Condition and Water Models

It is important to bring the study and management of forests and water together because they are fundamentally connected. Decisions that affect vegetation growth (or removal) will influence how water moves through the landscape (such as how much precipitation evaporates, moves quickly into streams, or seeps into the soil to be available for roots or sustaining streamflows). In turn, these water dynamics will determine how a landscape responds to vegetation changes. Some forested areas might regrow as shrubs following a fire, while others might transition to meadow vegetation or return to forest, depending on how much water is stored in the soil during the growing season. These interactions can be complex, but accounting for them can lead to a more sustainable landscape that better withstands droughts and high-intensity wildfires.

Through its fire management history, Illilouette Creek Basin shows that allowing natural fires can affect the availability of water for plants, wildlife, and people, while reducing drought stress and the intensity of future fires. A more varied landscape (from more frequent fires) also benefits wildlife and plants by providing a wider array of habitats.

What did Researchers Learn?

Forest Density and Snowpack

Researchers produced many maps from this work, including Figure 1 (below), which shows forest canopy cover in 2019. The map shows four categories of forest canopy cover, with 75–100% (red in the figure) being the greatest/densest, followed by 50–75% (orange). One can see that areas of densest forest cover are especially in the southwestern portions of the map, but scientists and park managers see something more. They see that the areas in red are the areas with the greatest potential for a reduction in canopy cover (where appropriate), such as through prescribed fire, to achieve multiple benefits.

One such benefit is increased snowpack persistence. Snowpack persistence is important because an area’s snowpack stores water as snow during the colder months and gradually releases it as it melts during the warmer months—essentially acting as a reservoir (California Cooperative Snow Surveys Program 2024). Areas with dense forest cover would likely experience long-term snowpack retention benefits with a reduction in canopy cover, because 1) less snow would be trapped in a thinner forest canopy, and 2) less/slower melting would occur due to longwave radiation emitting from dark tree surfaces like tree trunks.

Note that the Illilouette Creek Basin (outlined in light blue on the right side of the map) has very little area mapped in red; it has probably achieved relatively healthy levels of forest cover in terms of snowpack.

Drought Stress and Surface Water Runoff

According to model results, researchers found that using prescribed fire across all of Yosemite’s burn units would reduce drought stress and increase surface water runoff (Figure 3); however, the degree of the effects would vary by location. This finding is consistent with previously published work in the Illilouette Creek Basin (Boisramé et al. 2017, 2019). A burn unit is an organizational unit that park managers use to plan for prescribed fires.

Drought stress (or plant water stress) is the amount of water that plants need that is not met by precipitation and stored soil water. Eventually during a drought, trees photosynthesize less (make less food), and this leads to a lack of needed nutrients for survival (Texas A&M Forest Service 2024). When trees do not have the water they need, they become stressed and vulnerable to insect pests, disease, and other threats.

Surface water runoff is precipitation that runs off the land. A common concern about wildfires is that they will result in increased soil water repellence (or hydrophobicity), and that this will cause high levels of surface runoff and erosion that will result in flooding and poor water quality. Measurements from the current study did show increased soil water repellence following fire, but it began decreasing after the first post-fire year and fell to a minimum about 5–10 years post-fire. Importantly, researchers also found high soil water repellence in areas that had not burned for over 20 years, showing that long periods without fire might also have unwanted impacts on soil properties. Therefore, even though recently burned water-repellent soils can see reduced infiltration and soil moisture in the short term, soils appear able to recover, and even improve, their ability to infiltrate and retain moisture over multiple years following fire.

Other Important Takeaways of the Study

• During the drought year of 2015, the CECS dataset, which uses satellite imagery and a water balance model to calculate plant water stress, showed higher plant water stress in the southwest part of Yosemite National Park, especially in areas that had not experienced fire in over 20 years.
• Researchers used multiple datasets of snowpack observations to verify that snow persisted longer in open areas compared to under tree canopies within the study area.
• A network of time lapse cameras showed that snow generally melted earlier in forested areas (compared to meadows or shrub fields) in the locations measured.
• Forest management to increase snow retention can also help soil moisture and, therefore, drought resistance.
• Multiple repeat fires are good for water infiltration.

Why are these Findings Important?

While it is widely acknowledged that the forests of Yosemite National Park are denser than they were historically, it can be difficult to quantify how this change has impacted water and forest sustainability. This assessment shows that areas with less tree cover in western Yosemite National Park generally have greater snowpack storage and less drought stress. The analyses suggest that increasing the presence of wildfire on the landscape could reduce the negative impacts of fire on soil properties and alleviate drought stress through increased soil moisture. The CECS models also show the potential to increase streamflow, reduce fire intensity, and reduce drought stress through prescribed fire. The models employed in this study can be used to predict how various management scenarios are most likely to affect different areas in Yosemite National Park.

At times, it may seem like there is little we can do regarding climate change, including its effects on precipitation. But keeping snow on the ground longer and reducing the forests’ water needs can help protect our scarce water resources. By using fire as a tool we can manage for more water and encourage more resilient forests, benefitting the entire system, including plants, wildlife, and people. Other research in the park shows that just two fires can help to reduce stress on forests (Hankin and Anderson 2022).

For More Information

• Indigenous Fire Practices Shape our Land: An article about Indigenous peoples’ use of fire for millennia.
• Forest Restoration at Yosemite National Park
• Fire Management at Yosemite National Park overview, with links to related topics, such as Fire Ecology and Monitoring and Fire History.

NOTE:  This article was first published by the National Park Service.  Additional resources available at that page.