FOCUS ON FIRE
IN THE FOREST
FUNCTION OF FIRE
A GRASP ON SMOKE
BEYOND THE BLAZES
Function of Fire
By Chris Bryant
When a fire moves through a forested area, it’s the destruction that’s easiest to see — blackened landscapes, ruined habitats for plants and animals and, in many cases, a loss of valuable timber resources.
However, work done by UM forest researchers Anna Sala and Tom DeLuca has led them to see that something more is left behind in the wake of fire. They’ve been able to show how dramatic the short- and long-term benefits of fire can be to a forest, and how fire suppression, however well-intentioned, actually can contribute to less productive forest conditions.
Their focus has been on the region’s ponderosa pine and Douglas fir ecosystems. Over time, the once spacious stands of ponderosa pine have become dense stands dominated by Douglas fir. Fire suppression is largely responsible for causing this shift, but little work actually has been done to learn what role natural, periodic fire plays in nutrient cycles, water use by trees and species composition of forests.
DeLuca, a professor in the Department of Ecosystem and Conservation Science, has studied the role of fire in soil nutrient cycling since he arrived at UM a decade ago. In recent years he has focused his research on learning about the effects of fire suppression on forest soils.
Nitrogen, says DeLuca, is the No. 1 limiting nutrient in the region’s forest soils. Since it isn’t naturally part of the geology, nearly all nitrogen found in soils is captured from the atmosphere by microbes. And if you notice that the grass is greener after a lightning storm, it’s not your imagination. Lightning strikes provide a pulse of nitrogen in the soil.
Hungry plants rapidly suck up the nitrogen once it’s in soil — so the snapshot of available nitrogen found in a soil sample doesn’t say much about the nutrient level of the soil over time. Instead, DeLuca uses an instrument called an “ionic resin capsule,” which is sunk into the soil for a period of time. As nitrogen is taken out of the atmosphere and released into the soil, the capsule absorbs it before the plants can. This method gives researchers a better idea of the ability of the soil to provide nitrogen over time.
Using these capsules, DeLuca found that forests that burned five, 10, even 30 years ago had a much higher level of available nitrogen compared to a forest that hadn’t burned for 100 years.
“We’re finding that the availability of nitrogen is especially slow in these late succession forests that have not burned,” he says.
Another, perhaps more obvious, limiting factor in the region’s forests is the availability of water. The dense stands of Douglas fir have a major impact on the ponderosa’s ability to get water — especially when they need it most. Sala, an associate professor in the Division of Biological Sciences, has been able to quantify this effect by monitoring the water movement in trees under a variety of circumstances. To measure the water in trees, she inserted two needles, one spaced just below the other. The bottom needle is a heater and the top needle a sensor. As water moves through the tree Sala measures the heat wave transported by the water, giving her a velocity of water movement. By placing these sensors on trees in thinned stands and in dense stands, she can compare how trees use water in different forest conditions.
In treated areas, she says, she sees a remarkable uptake of water in late June — when the heat starts.
“The trees in the control area shut down; they don’t have water,” Sala says, “but when you’ve removed the Douglas fir, you see a big uptake in water use.”
In effect, the unusually dense stands create a condition where none of the trees get the water that would normally be available.
Sala acknowledges that for some old-growth stands, thinning followed by burning could be the best treatment. But she says it is impossible to do that on a large scale, because each old tree requires such care. You have to remove the duff, for example, so you don’t overheat the base of the tree.
How best to restore forest, or even whether to, can be a socially and scientifically charged question. One goal of Sala and DeLuca’s research is to better understand how the forest works so that better management decisions can be made.
Sala says they realized that in order to be more helpful in formulating management options that can restore these highly-altered forests, they needed to get away from complicating factors such as the legacies of grazing and timber harvest. They needed to find a true reference forest.
So, for the last couple years, DeLuca and Sala have been hiking — with some strong students — way back into the Selway-Bitterroot and Gospel Hump wilderness areas in western Montana and northern Idaho. There, far from the effects of intense forest management, they found study plots with a more natural fire history.
“There was lots of bushwhacking,” Sala recalls. “You had to be in shape to get way out there — I was not.”
But, despite the blisters and rattlesnake encounters, the researchers agree that the effort paid off in many ways. Even though she says the long distances on foot kept her from collecting as much hard data as she could have on heavily managed stands near roads, the experience gave her a better perspective of what natural forests look like.
“We can’t restore ecosystems back to what they were because we don’t know what they were,” she says. “That’s the point of going into the wilderness areas — to see how things really function.”
She compared forest structure in stands subjected to natural fire regimes with stands not burned during the last century. Not all old-growth forests fit the classic open stand image that many people think of as ideal ponderosa pine forest, but she didn’t find the dense Douglas-fir thickets seen in managed forests.
Sala suspects the current high density in managed forests is linked not only to fire-suppression but to grazing and logging as well. But she adds that more research is needed to accurately gauge the relative impacts of various management decisions.
The data they collected confirmed some long-held suspicions. DeLuca buried nutrient-absorbing resin capsules in the study plots and found that the soil in the stands that had burned repeatedly over time was significantly higher in available nutrients.
“This is a big deal because we are not measuring it right after a fire,” he says. “It’s not a short-term response.”
The wilderness study suggests that, despite calls to restore forests to pre-management conditions, there is still a lot to learn about how those forests functioned in the first place. But, as the science clearly shows, a force as dominant as fire can’t be overlooked.
“To get the systems functioning the way they used to,” says DeLuca, “at some point we have to allow fire back in the landscape.”