Yosemite National Park is famous for its waterfalls, granite walls and giant sequoias. It is also one of the first national parks to let lightning-caused fires burn when conditions are right — a history that turned out to be a natural experiment in how fire, forests and water fit together.
Yosemite Valley from Tunnel View. Credit: National Park Service.
Fifty years of letting fire work
Yosemite covers nearly 748,000 acres (about 1,200 square miles), including the 40,000-acre Illilouette Creek Basin. In 1972, park managers decided to let lightning fires burn there when they benefited the ecosystem and visitors and infrastructure were safe, and began a prescribed-fire and managed-wildfire program. Over the next 50 years, frequent fires of varying intensity reduced the severity and extent of later fires and brought many ecological benefits.
Elsewhere in the Sierra Nevada — including parts of Yosemite — the story is different. Fire suppression beginning in the mid-1800s, with European colonization, also curbed the burning Indigenous peoples had used for millennia to keep forests healthy. The result is forests far denser than the landscape's water can sustain.
| Dense, fire-suppressed forest | Forest kept open by low- and moderate-severity fire |
|---|---|
| Heavy fuel loads, prone to high-severity fire | Less dead, woody debris |
| High water demand and drought stress | Open canopy lets water and sunlight through |
| Nutrients locked in debris | Nutrients recycled to the soil, spurring regrowth |
The study
Researchers from the Desert Research Institute and Yosemite set out to identify where low-intensity fire would do the most good — reducing drought vulnerability, taming future fires and increasing streamflow — because the park is huge and money for prescribed burns and fuel treatments is limited. In 2022–2023 they combined:
- Field measurements of soil moisture, soil water repellency and how fast water soaks in, at sites across western Yosemite chosen to span terrain, time since fire, past fire severity and vegetation density — producing a unique soil dataset for the park.
- Remote sensing — lidar and aerial cameras — to map current vegetation.
- Existing records of snow depth and soil moisture, under the canopy and in nearby gaps, along an elevation gradient.
- Computer models of drought vulnerability and potential fire behavior across western Yosemite, plus simulations of fire, drought stress and streamflow from the Center for Ecosystem Climate Solutions (CECS), built on satellite data.
NPS field technicians collecting data on vegetation and fuels along vegetation monitoring transects at Yosemite National Park. NPS / Chad Anderson.
A low-intensity prescribed fire in an area of Yosemite that is burned frequently. Credit: National Park Service / Chad Anderson.
Why forests and water must be planned together
Whatever changes vegetation changes how water moves — how much evaporates, runs off into streams, or soaks in for roots and later streamflow. And the water, in turn, shapes what grows back: a burned area may return as shrubs, meadow or forest depending on how much water the soil holds through the growing season.
What they found
Thinner forest keeps snow longer. Mapping canopy cover in 2019, researchers found the densest forest (75–100% cover) mostly in the southwest of the park — the areas with the most to gain from thinning, where appropriate, by prescribed fire. Snowpack acts as a reservoir, storing winter precipitation and releasing it as it melts. A thinner canopy traps less snow in the trees, and fewer dark trunks radiate heat to melt it. Observations confirmed snow lasted longer in open areas than under trees, and time-lapse cameras showed snow generally melting earlier in forest than in meadows or shrub fields. The Illilouette Creek Basin had little of the densest cover — it has probably reached healthy forest levels for snowpack.
Prescribed fire would ease drought stress and add runoff. Drought stress is the water plants need that rain, snow and soil don't supply; stressed trees make less food and become vulnerable to insects and disease. Models of every burn unit — the areas managers use to plan prescribed fires — showed that burning them would reduce drought stress and increase runoff, by amounts that varied from place to place, consistent with earlier work in the Illilouette basin. In the 2015 drought, satellite-based data showed higher plant stress in the southwest of the park, especially where there had been no fire for more than 20 years.
Soils recover — and long fire-free periods hurt them too. Fire made soils more water-repellent at first, but that began to fall after the first year and bottomed out about 5–10 years after the fire. Surprisingly, soils unburned for more than 20 years were also highly water-repellent. Repeat fires were good for water soaking in, and managing forests to keep snow longer also helps soil moisture and drought resistance.
Why it matters
The forests are known to be denser than they once were; this work puts numbers on what that costs in water. Less tree cover in western Yosemite generally means more snowpack and less drought stress, and more fire on the landscape could ease both soil damage and drought. The models can now predict how different management choices would play out in different parts of the park. Other research in Yosemite shows that just two fires can reduce stress on forests.
Climate change can feel beyond reach, but keeping snow on the ground longer and cutting forests' water needs can protect scarce water. Used as a tool, fire can yield more water and more resilient forests, for plants, wildlife and people alike.
A California spotted owl (Strix occidentalis occidentalis) perched on a burned stump in Yosemite National Park. Spotted owls benefit from low- to medium-severity fires that result in forests with large, live trees and relatively closed canopies. NPS / Paul Bannick.
Sources
Based on "Planning for Resilience: Understanding the Connections Between Forests, Fire, Water, and Drought," National Park Service; a work of the United States government in the public domain. The article summarizes Boisramé, G., C. Anderson and C. Fong, Assessing Landscape Vulnerability to Drought and Megafires using Linked Forest Condition-Water Models to Guide Resilience Planning (Desert Research Institute and National Park Service, 2024), funded in part by the NPS Natural Resource Condition Assessment Program. Pictures an earlier version of this page left out were restored on 2026-09-26.
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