Climate change makes it less likely conifer forests will regenerate after wildfires

Recent research suggests that the iconic conifer forests of the Klamath Region in the U.S. Pacific Northwest are less likely to regenerate as climate change leads to hotter, drier conditions and makes high-severity wildfires larger and more frequent.

Article by Mike Gaworecki.

The Klamath Region of the Western United States, which spans northern California and southwestern Oregon, is considered a richly biodiverse botanical hotspot. It boasts nearly 30 species of conifer, for instance, like the Douglas fir, which the region is closely identified with, as well as a number of other plant species not found anywhere else on Earth.

Recent research suggests, however, that it is precisely the iconic conifer forests of the Klamath Region that are less likely to regenerate as climate change leads to hotter, drier conditions and makes high-severity wildfires larger and more frequent.

The shrublands of northern California could end up replacing forests composed of a mixture of broadleaf trees, evergreen hardwoods, and conifers across large swaths of the Klamath, new research by scientists with the Smithsonian Conservation Biology Institute (SCBI) and Harvard Forest suggests. Their findings are detailed in a study published in the journal Global Change Biology in April.

There are two main impacts of climate change that could drive the conversion of the landscape into shrub-dominated systems, according to study author Jonathan Thompson, a senior ecologist at the Harvard Forest in Petersham, Massachusetts. For one thing, as temperatures rise, the region is becoming more drought-prone and forest fires are becoming more frequent and more severe. And as those fires hit forests after increasingly shorter intervals, they can greatly impede the regeneration of conifers that find it harder to thrive in hotter, more arid climates — what’s referred to as the “interval squeeze” by researchers.

“After a severe fire you’ve got the shrubs and the conifers coming up together, and if the conifers can’t get up above the shrubs pretty quickly, they get trapped in the morass of that, in a shrub trap,” Thompson told Mongabay. “They grow more slowly when they’re in that mix, and they’re more susceptible to the next fire. And so you can get caught in this cycle of repeated fires, unless the conifers can break free and grow up above them.”

Thompson and co-authors write in the study that “by the end of the century just over half of the area currently capable of supporting mixed-conifer/mixed-evergreen forests could be at risk of minimal conifer recruitment following severe fire, even in relatively small high-severity patches.”

In order to study the dynamics of the interval squeeze in the Klamath Region, the researchers took samples from 57 sites that burned at high severity between 1987 and 2008. “We sampled vegetation composition and structure, and we recorded the heights and crown diameters of all the broadleaf trees, shrubs, and conifer seedlings that were regenerating after fire,” Alan Tepley, a forest ecologist with SCBI and the study’s lead author, told Mongabay.

Aftermath of the 1994 Bear Fire in the Klamath region. Photo by Alan Tepley.

The samples were taken as evenly as possible across a gradient of “time since fire” — from seven to 28 years after the last high-severity fire swept the area — as well as a gradient of “climatic water deficit,” or, in other words, a moisture gradient. Tepley and team harvested more than 1,000 trees and collected cross-sections of the trees at regular intervals all the way up their trunks.

“By doing that we could count the rings at each of those heights. So for example if you count 10 tree rings at the base of the tree, and you count five tree rings up at a meter, you would know that tree took five years to reach a meter tall,” Tepley said. “By comparing the number of rings at all those different sections that we collected, we could develop a growth curve for each tree, a height growth curve for each tree, and then we could look at how long it’s taking the trees to grow taller than the competing vegetations, and which trees have a growth advantage compared to the others.”

The team found that 89 percent of all the conifers that they sampled had established themselves within the first four years following a high-severity fire — meaning that there’s a really short window of opportunity for the conifers to lay down their roots and start growing if they hope to out-compete shrubs and broadleaf or hardwood trees, which typically regenerate by sprouting from an existing root system or from seeds that stay viable in the soil even after a high-severity fire. That gives them something of a headstart, because conifers can only regenerate when their seeds blow in from a nearby stand that didn’t burn.

Tepley and team found that hardwoods and shrubs typically dominate the regenerating biomass for at least the first three decades after fire. “In general hardwoods and shrubs accounted for 85 to 90 percent of the live biomass,” Tepley said. “So conifers are a pretty small portion of what’s regenerating, and they have to grow through that competing hardwoods and shrubs. We found that those conifers that established earlier, grew faster. The sooner they established after a fire, the faster their height growth. So it was a big advantage to establish quickly and get a jumpstart on the hardwoods and shrubs. The conifers that established later, after four or five years or so, grew really slowly.”

These late-established conifers have an especially hard time growing in drier areas, whereas the trees proved more resilient and more capable of regeneration in wetter parts of the Klamath Region regardless of how quickly after the fire they established themselves, the team found. Conifers also accounted for a smaller proportion of the total biomass at drier sites, so they faced an even more competitive environment in addition to growing more slowly.

Tepley and team looked at the density of regenerating conifers as a function of available seed sources and moisture availability, as well. “We took a 400-meter radius circle around each plot, and then used recent aerial photos to calculate the proportion of the area of that circle that had seed sources that survived the fire,” Tepley said. A recent study of Douglas fir regeneration after fire found that seedlings were abundant up to about 400 meters from a seed source, he noted. “We found that, on the wetter sites, you tend to have abundant post-fire seedling density even when there are relatively few seed sources available. But as you get to drier and drier sites, you need more seed sources to maintain the same level of regenerating seedlings. So as the climate gets warmer and presumably drier, it might lead to bigger fires, or potentially bigger patches of high-severity fire, which would remove seed sources over bigger areas. And also as the climate gets warmer and drier, our analysis suggests that more seed sources would be needed to maintain the same level of regenerating seedlings.”

These findings have implications beyond the composition of forests in the Klamath Region, of course. There are a lot of fire-prone forests that could feel the interval squeeze, Harvard Forest’s Thompson said, with Mediterranean ecosystems being perhaps the most vulnerable, though the phenomenon of high-severity fires becoming more frequent and leading to permanent forest loss isn’t limited to those ecosystems.

Then there’s all of the other environmental impacts besides forest loss. “From a carbon perspective, if we want to talk about it in terms of ecosystem services, you’re moving from one of the most carbon-dense ecosystems, which is mature conifer forests, to a really carbon-poor ecosystem,” Thompson added. “You’re also losing the capacity to sustain low-intensity, low-severity fires. Every fire in a shrubland system is a high-severity fire, it’s impossible to have a creeping ground fire when all your vegetation is one meter off the ground. It just all goes.”

That not only makes vegetation more vulnerable to fires in that ecosystem, but also the wildlife that relies on the disappearing forests for habitat. Plus, soils are more likely to erode, making it even more likely that the future will hold more and larger fires. “So you get this positive feedback,” Thompson said, “where each time the fire is a little more severe, in a little bigger patches, and the likelihood that conifers will come back is that much lower.”

Biscuit Fire reburning the area of the 1987 Silver Fire in the Klamath region. Photo by Thomas Link.


Tepley, A. J., Thompson, J. R., Epstein, H. E., & Anderson‐Teixeira, K. J. (2017). Vulnerability to forest loss through altered postfire recovery dynamics in a warming climate in the Klamath Mountains. Global Change Biology. doi:10.1111/gcb.13704

Follow Mike Gaworecki on Twitter: @mikeg2001

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