Fire in the Jemez Mountains

The Jemez Mountains are a diverse mountain range located in north-central New Mexico. The mountains were formed by volcanic eruptions, the remnants of which can be seen as the Valles Caldera. The Jemez Mountains landscape contains Bandelier National Monument, Santa Fe National Forest, Native American Pueblo lands, and Valles Caldera National Preserve. This beautiful ecosystem is also home to many plants and wildlife that have evolved with fire, including the endangered Jemez Mountains salamander that is only found here.

Click through the slides to learn more about wildlife in the Jemez Mountains.

For thousands of years, frequent fire has been an important process for ecosystems in the Jemez Mountains; we know this through analysis of charcoal from deep sediment cores and fire scars recorded in tree-ring samples. The effects of lightning fires or indigenous burning varied across the different ecosystems along the elevation gradient, but played a key role in local ecosystems. For example, repeated, low-severity fire in the ponderosa pine and mixed conifer forests maintained these ecosystems by reducing fuels, like dead leaves, needles, branches, and grasses. This kept fire intensity low, so most large trees survived. This also kept the forests open through the thinning of small trees, which promoted grass and forbs in the spaces between trees.

There are two common ignition sources of local wildfires. Click through the slides below to learn more about how wildfires start.

If fires have burned in the Jemez for centuries to millennia, then why are we seeing huge modern fires that are wiping out entire forests? To understand modern fire, we must first understand the history of fire and that humans have excluded fire for the last 120 years.

With the local arrival of railroads in the late 1800s, immense numbers of grazing animals such as cows, sheep, and goats were introduced into our ecosystems to feed on the then abundant grasses. These grasses are what historically carried frequent and widespread, low-intensity, surface fires, because grasses burn easily at low temperatures. This supported large fires that burned up fallen branches, leaves, and needles, but were not hot enough to kill most mature trees. Once overgrazed by the introduced domestic livestock by about 1900, there was no longer sufficient grass cover to support extensive surface fires.

Click to move the slider arrow side to side over the repeat photos from 1935 (overgrazed) and 2001 (no grazing) to see the influence of overgrazing.

Without fire, forest densities and fuel loads have increased, as fire removes the buildup of these fuels. Ladder fuels, like over-abundant young trees that carry surface fires into the canopies, have also increased without regular burns, resulting in large crown fires that kill most trees across large areas.

The introduction of fire suppression and Smokey Bear turned the public against wildland fires and created a stigma around forest fires, even though natural frequent fires are ecologically beneficial. Fires were suppressed as soon as they were discovered. The consequence of keeping fires small resulted in a buildup of fuels that were available to burn during the hottest and driest conditions. In these conditions, fires spread quickly, are hard to stop, and do the most damage. This failed logic has contributed to where we are today, with historically-unprecedented large, high-severity fires killing large swaths of forest.

Up to four years after a wildfire we see a high probability of post-fire debris flows, which are powerful mixtures of water, soil, rocks, and logs. Trees hold soil in place with their roots, while live and dead plant cover slows the flow of precipitation to the ground, allowing the water to be absorbed. When severe fires consume vegetation cover there is nothing left to hold the soil in place. In addition, the intense heat from the fire makes the soil surface impermeable to rain, increasing the chance of intense flooding events. Check out the video of flooding in Santa Clara Pueblo after the Las Conchas Fire.

To learn more about the post-Las Conchas Fire flooding in Santa Clara, check out  the PBS news hour story .

After modern high-severity fires, ponderosa pine forests are being replaced by Gambel oak and New Mexico locust. This conversion is taking place on a large scale due to the high tree mortality that comes with large, hot modern fires. Rising global temperatures also means that areas once suitable for these forests are no longer hospitable for conifer regeneration, and thus likely will remain shrubland for the foreseeable future -- not just in our lifetimes but possibly for a century or more.

Have fires in the Jemez Mountains always been this large and hot? By studying tree rings, scientists can look at historical fires and compare them to modern fires. Reconstructing historical fire regimes helps us understand how prevalent fire was in the area before fire suppression.

The Southwest is a great place to study tree rings. In a wet year a tree will put on a large ring because the tree has a lot of resources (mainly water) to grow. In a dry year, a tree will put on a small, thin ring because the tree is stressed for resources. In a particularly stressful year, a tree might not put on a ring at all. Knowing drought years and the local tree-ring pattern created by dry and wet years allows the tree rings in logs and stumps to be precisely dated to the year.

Some trees record fires in their tree rings. This allows tree ring scientists (dendrochronologists) to date when and where prehistoric fires occurred. Although most mature trees are protected by thick bark and avoided injury from historical low-intensity surface fires, young thin-barked trees can be damaged by fires. After a tree is scarred, subsequent fires will continue to scar the tree and be recorded in the tree rings, creating a record of fire extending back hundreds of years. Scientists collect samples from these fire-scarred trees and date them to learn more about the fire history of an area.

The cross-section below was sampled from a large ponderosa pine in upper Frijoles Canyon in Bandelier and shows 15 different years with dated fire scars. See if you can count them all!

In the Jemez Mountains, fire-scarred trees have been sampled from > 100 different sites across the mountain range over the last 40 years. This is the largest tree-ring fire-scar network in North America. Because fire scars record the year of past fires, this network can be used to look at the size of historical fires and then make comparisons with modern fires.

Fire in the Jemez used to burn more frequently and across larger areas than we are seeing now, but many old trees survived repeated fires. One tree survived and recorded 32 fires as tree-ring fire scars. How is this possible?

Pre-1900 fire intervals of 5-15 years in ponderosa pine sites meant that fires were burning at lower temperatures since there was not a large buildup of fuels. This allowed most mature trees to survive repeated surface fires and kept the forest open so that the existing trees had room to grow.

Comparing historical fires to modern day fires, we find that the large size of modern megafires is not as unusual as we once thought. But importantly, the large size of recent high-severity, tree-killing fire patches does seem historically unprecedented in many local forest types.