Burn Baby Burn: Fire, Forests, and Carbon

I know, I know. I've been a bit absent over the last few weeks. I'm going to call laying on the beach in South Florida at Christmas enough of an excuse. That and I was finishing up my Wildland Firefighter course. Yep, I'm now all certified to start and stop forest fires. Should you be afraid? Perhaps. But it got me to thinking about prescribed burning, an area of ecology that I know about but don't regularly keep up with the literature. So I decided to take a look at what has recently been published on the topic and found a nice paper in the journal Forest Ecology and Management about the balance of carbon sequestration and habitat conservation as they relate to fire.

Most people tend to think Fire = Bad. In many situations that's true, but in others fire is actually a natural, healthy process. In the past, various federal and state agencies have instituted wildfire suppression policies because they thought it was the best way to preserve the natural state of the land. All it really did was create a massive amount of fuel loading - lots and lots of fuel (dead logs, leaf litter, etc.) around to burn - which created even larger, more destructive fires. Now we know better, and the idea of a more natural fire regime is being used to control fuels and keep the balance of natural habitats.

Today's paper takes a look at woodlands, a subset of forests that are composed of low density, short trees  that typically have an open canopy with a ground layer made up of shrubs, grasses, and forbs. They can be economically important for lumber and livestock grazing, but they are also naturally important as significant carbon (C) sinks and as a biologically diverse habitat. Fire is known to be an important process in this ecosystem and is managed such that willdfires are allowed to burn unhindered, landscape level fuel management helps contain fire spread, and prescribed burning reduces fuels. This study investigates the effects of past fires on carbon storage, woody vegetation community composition, and habitat suitability. They do this by utilizing a "chronosequence" approach. This means that they grouped and compared stands of trees at different successional stages following fire events. These stands were located in the woodlands of Balcones Canyonlands National Wildlife Refuge (BCNWR), located northwest of Austin, Texas, USA. The dominant woody vegetation in this area is the evergreen Ashe's juniper (Juniperus ashei), with subdominant species including several oak (Quercus spp.) species and Escarpment Black Cherry (Prunus serotina var. eximia). These woodlands are also inhabited by the golden-cheeked warbler (Dendroica chrysoparia), an endangered bird species they used to assess habitat suitability. The mixed juniper-oak are critical for this warbler as they use the stringy, sloughed bark from the mature Ashe's juniper exclusively for nesting material and requre Lepidopteran (Geometridae) larvae that feed on broadleafed foliage during the spring breeding season.

The researchers established 60 plots next to trees that had been used for a previous dendrochronology (tree rings) study. That means that they had the history of the tree from the tree rings, this history including fire scars that could be pinned to exact years. The fires were low-intensity, surface burns that mostly consumed the ground layer, seedlings, and saplings. From this they were able to pick sites with single fire events and then assign their plots into three groups: recent-fire (less than 40 years), old fire (greater than 40 years), and no fire. Then, for each plot, they identified tree species and measured their diameter and heights, collected data on woody debris and leaf litter depths, and collected soil samples for carbon and nitrogen analysis.

The authors found significantly different total aboveground biomass C averages for the 60 study plots. Recent-fire showed 5.25 kg m-2, old fire averaged 6.86 kg m-2, and no fire had 9.18 kg m-2 of C. Previous studies have shown that low to moderate intensity fires have less effects on live biomass, stabilize carbon fast afterwards, and primarily affect trees in an age-specific and density-related way. This study found that "the impact of low-intensity fire on C storage increases for sites with higher proportions of biomass in small trees" (greater than 20cm in diameter) as these are more greatly impacted by surface fires.

 This study's results support that periodic surface fires maintain many forests by favoring fire-tolerant species, such as oaks, and decreasing the density of fire sensitive species, such as juniper. They found that sites with a single fire disturbance had a higher density of oak seedlings than juniper seedlings (though not significantly so), especially in years that had low precipitation in the summer. The densities of mature trees was found to be mainly influenced by differences in topography, herbivory, and historical climate.

I would like to say that all of this added up to a good conclusion about habitat suitability for the golden-cheeked warblers but, unfortunately, they found significant differences. Fire reduces mature tree density, which has a negative impact on habitat suitability, but high intensity fire is related to higher oak recruitment (food source for their food source). Managing these woodland for the golden-cheeked warbler may require both mechanical treatment and the use of fire. The critical core breeding habitat for this warbler includes old-growth stands composed of large, mature junipers and about a 40 percent oak species mix. These stands should be protected from intense fire through the mechanical thinning of the understory and the removal of heavy fuels.

Overall, the authors conclude that fire management can achieve multiple outcomes on a single site. This means that detailed ecological information about the species composition and population structure of an area are needed in order to assess conditions and projections on restoration.

Yao, J., Murray, D., Adhikari, A., & White, J. (2012). Fire in a sub-humid woodland: The balance of carbon sequestration and habitat conservation Forest Ecology and Management, 280, 40-51 DOI: 10.1016/j.foreco.2012.05.042

More on wildland fires and prescribed burning:
GeoSTAC's Wildland Fires page
University of Florida IFAS Extention's "Benefits of Prescribed Burning"
National Interagency Prescribed Fire Training Center


(image from Texas Department of Agriculture)