Laser These

These laser scans demonstrate how fires have affected Yosemite’s forests

A forest can take many different shapes: prickly with oak undergrowth, dim and mossy, or sunlit and full of soft grass. Those structures affect the animals that live there, the amount of carbon the ecosystem can store, and how a wildfire will move through the landscape. But unless a casual hiker knows what to look…

A forest can come in many forms: it could be prickly, with oak undergrowth, dim, and mossy or sunny and full of softgrass. These structures have an impact on the wildlife that lives there, how much carbon the ecosystem can store and how wildfires will move through the landscape. It can be difficult to spot these landscape-scale patterns if a casual hiker doesn’t know what to look for.

A set of remarkable LIDAR scans of Yosemite National Park in California, published by forest ecologists at the University of Washington and the remote imaging company NV5 Geospatial in EOS this month, offers a glimpse into the subtle distinctions in forests–and the huge consequences for wildfire-across an area of 100 square miles.

[Related: The American West is primed for a summer of fire]

The project was started as part the US Geological Survey’s 3D Elevation Program ,, which creates topographic maps across the country. NV5 collects elevation data by flying a plane over Yosemite and then casting a laser on the ground below. LIDAR technology can be used to map the surface of the earth by measuring how long it takes for the laser to bounce back at the plane. It can even detect individual trees.

To make a topographical mapping, NV5 needs to determine where the laser reached the ground beneath the trees. The LIDAR captures exact detail about the trees and the undergrowth above ground. “[Light] continues to descend through the canopy – some of it gets reflected and some of that keeps going until it reaches the ground,” Andrew Brenner, program director at NV5.

Multicolored trees from Yosemite's forests mapped from a lateral view on black
LIDAR scans of Yosemite can show both ground elevation, and trees and shrubs (colored to make it easier to tell apart individuals). Courtesy NV5 Geospatial

Using scans of Yosemite taken between 2010 and 2019, forest ecologists at the University of Washington were able to map how fires change the fabric of a landscape.

Before the wholesale adoption of fire suppression by the US Forest Service in the early 1900s, most North American landscapes burned regularly, including much of Yosemite. Ecologists know that forests that have been burned look different than those that have not. Repeated fires created a patchwork of mixed forest and open meadows in Yosemite’s mixed pine-fir ecosystem.

Unburned Yosemite forest mapped in blue, green, and yellow
Moderately burned Yosemite forest mapped in blue, green, and yellow
An unburned forest, on the left, is dense, and more likely to experience severe fires. On the right is a moderately burned forest, with a patchwork of “natural firebreaks.” Courtesy NV5 Geospatial

Those “natural firebreaks” in a patchwork forest mean that fires will usually be less intense: They’ll race through the grass and underbrush rather than “torching” whole stands of trees. This not only reduces fire danger, but can also help local ecosystems by providing habitat for sun-loving wildflowers and edible plants as well as birds.

The open structure was “key to forests thriving in a regime of frequent fires,” the authors write in EOS.

Moderately burned Yosemite forest mapped in blue, green, and yellow
Highly burned Yosemite forest mapped in blue, green, and yellow
Compared to a moderately burned forest again, the aftermath of a high-intensity fire, shown on the right, a forest can transform into grassland. Courtesy NV5 Geospatial

Intense fires, fueled by thick forest and drought-stressed trees, can set off an ecological cascade. They can incinerate seedslings and scorch soils, which makes it difficult for forests to recover from milder fires. In the West, open grasslands are replacing forested areas in the wake fires.

Over the last 50 years, however, Yosemite’s forest managers have attempted to reintroduce regular fires, both by setting prescribed burns and leaving ro

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