Discovering new life in forests after fire

The author’s daughter enjoying the abundant young trees in a large snag forest patch created by the Rim Fire. The Forest Service claims that trees are not growing back here.

You may be living near an amazing ecosystem that few people have explored or even are aware of. Conifer forests in the western U.S. are home to a special type of habitat known as “snag forests” that contain some of the highest levels of animal and plant diversity and abundance of any forest type. These important places are created when patches of intense, high-severity fire occur within larger wildfires. 

This may be surprising to hear. While many people understand that fire is a natural part of western forests, it is often assumed that those fires are only supposed to burn at low severity, rather the higher-severity “crown fires” that cause substantial tree mortality. Crown fires produce the dramatic images that you commonly see in the news, so it would be easy to get the impression that most forest fires are now burning at high-severity. The reality is that forest fires naturally burn with a mixture of severities. Most fires consist mainly of low- and moderate-severity effects where the majority of the trees survive the fire, while they also include patches of high-severity. There is a growing body of scientific research showing that the high-severity portions of these fires are creating great habitat.

Again, this may come as a surprise. Right after a fire, the images in the news tend to focus on blackened trees with no sign of life. Timber companies and the Forest Service usually rush in to cut down these trees soon after the fire, so there is no chance to see what comes next. Yet where scientists have managed to find high-severity patches that did not get cut, they have discovered that a remarkable transformation takes place. 

Dead trees, called “snags”, provide homes for many nesting birds and the insects they feed on, so places with a lot of dead trees—i.e. snag forests—are filled with birdsong. Snags also allow more sunlight to reach the forest floor, spurring a surge new growth. In the years following a fire, snag forests are filled with carpets of colorful flowers that attract butterflies and other pollinators, and shrubs with leaves and berries that provide food for deer, bears, and other forest wildlife. Snag forests abound with life.

Yet all these ecological benefits from fire are lost when snag forests are cut down. And because post-fire logging is so common, snag forests have become one of rarest types of forest habitat in the western US, even rarer than old-growth forests. 

One of the justifications used by the Forest Service to log snag forests is the claim that trees will not grow back on their own in these places. Therefore, the Forest Service contends that it needs to clear out the snag forest in order to artificially replant trees. In fact, western forests have evolved with mixed-severity fires and new trees naturally grow back on their own, even in large high-severity fire patches. However, this process was little known because the logging that typically occurs after fires brings in heavy machinery that crushes the new tree seedlings. Yet if you visit snag forest patches that have not been logged, you will find plenty of new tree growth.

Forest ecologist Dr. Chad Hanson of the John Muir Project has spent a lot of time in snag patches for his field research, so he was perplexed when Forest Service-affiliated scientists published a paper claiming to show conifer trees were not growing back after intense fires. Dr. Hanson investigated the sites surveyed for this study and discovered that many of those places had conditions where something other than fire would have precluded new conifer tree growth— such as areas that had been clearcut before the fire, or they were oak tree forests where there were no conifer trees growing before the fire.

Dr. Hanson therefore decided to conduct a study that would examine post-fire tree regeneration at a larger scale. The results of his research were recently published, and they show that trees are readily growing back in all of the high-severity fire patches he surveyed, including deep inside very large patches. These trees soon grow over the shrubs and other vegetation on the forest floor, eventually shading them out of existence as the snag forest conditions are supplanted by the more familiar type of green tree forest. 

Because of this shift, the 4.6% of high-severity fire areas where Dr. Hanson found conifer regeneration is occurring more slowly have an ecologically desirable role as well. Since snag forest habitat is a temporary phenomenon as new green trees displace the snag forest features, having a few spots were trees regenerate more slowly allows the many plants and animals that benefit from snag forests to have a bit more time with this rare but important habitat. 

Unfortunately, the Forest Service recently announced plans to cut much of the best remaining snag forest habitat in California that was created by the Rim Fire of 2013 near Yosemite National Park. Once again, the Forest Service is claiming that there are no new trees growing in many of the places where they want to log, even though these areas were part of where Dr. Hanson surveyed and found wide-spread tree regrowth. Dr. Hanson and the John Muir Project are now working with other environmental organizations such as the Center for Biological Diversity and Sierra Club of California to challenge the logging and save this vibrant snag forest habitat that is home to a variety of imperiled species, including spotted owls, great grey owls, and black-backed woodpeckers.

Last updated May 29, 2018

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