Monday, March 16, 2009
Praise for the Dead (wood)
By George Wuerthner
Forest Magazine, Spring 2009
Dead. Most of us have negative associations with the word. After all how did Death Valley get its name? Not because it was a favorite vacation spot for prospectors. Is anyone interested in fishing the Dead Sea? And when we say someone looks like “death warmed over,” it’s not usually taken as a compliment. So it’s not surprising that most of us tend to view dead things as undesirable, unless we are talking about mosquitoes and rattlesnakes.
We impose this cultural bias about dead things to our forests as well. Public land management agencies spend billions annually trying to contain wildfire and insect outbreaks based upon the presumption that these natural processes are destroying the forest by killing trees. Even though there is now some grudging acceptance by land managers that wildfires and insect attacks may be potentially beneficial if they do not kill too many trees, stand-replacement fires, ice storms and large beetle outbreaks are still viewed as unnatural and abnormal—something to suppress, slow and control.
When these natural processes kill trees, managers propose logging to “salvage” the economic value of the downed trees. They operate on the tacit assumption that surplus wood can be removed without hurting the forest’s ecosystem, and until now that has formed the basis of scientific and/or sustainable forestry.
But a new perspective is slowly taking root among forest managers, based on growing evidence that forest ecosystems have no waste or harvestable surplus. Rather, it seems that forests reinvest their biological capital back into the ecosystem, and removal of wood—whether dead or alive—can lead to biological impoverishment. Large stand-replacement blazes and major insect outbreaks may be the ecological analogue to the forest ecosystem as the hundred-year flood is to a river. Such natural events are critical to shaping ecosystem function and processes. Scientists are discovering that dead trees and downed wood play an important role in ecosystems by providing wildlife habitat, cycling nutrients, aiding plant regeneration, decreasing erosion and influencing drainage, soil moisture and carbon storage.
“When you start to look at western forests outside of wildernesses and parks, you notice right away that they lack large quantities of downed wood—dead trees,” says Jon Rhodes, an independent consulting hydrologist in Oregon. “Ecologically speaking, there is a big difference between areas that have been logged compared to areas that are left alone.”
Chad Hanson, a University of California, Davis, researcher, agrees. “We are trapped by an outdated cultural idea that a healthy forest is one with nothing but green trees. An ecologically healthy forest has dead trees, broken tops and downed logs.” Such forests may not look tidy from the perception of a forester, he says, but it’s an indication that the forest is healthy and biologically diverse. “Pound for pound, ton for ton, there is probably no more important habitat element in western conifer forests than large snags and large downed logs,” Hanson says.
Studies have consistently concluded that most western forests have a deficit of large snags and downed dead wood. “Large standing trees are important,” Rhodes says, “but they shouldn’t be museum pieces. They should be part of functioning ecosystems.” When old-growth trees burn in wildfires, they aren’t completely lost, he says, but provide the ecosystem with large quantities of snags and downed wood. “While some say we can’t afford to have old growth burned by fire, it’s apparent that we can’t afford for old growth not to burn in fires, due to the importance of large snags and downed wood and its current lack in western forests,” he says.
Writing in a 2004 article in Conservation Biology, University of Montana ecologist Richard Hutto sums up the new thinking about the ecological value of dead trees. “I am hard-pressed to find any other example in wildlife biology where the effect of a particular land-use activity is as close to 100 percent negative as the typical post-fire salvage-logging operation tends to be,” he wrote. “Everything from the system of fire-regime classification, to a preoccupation with the destructive aspects of fire, to the misapplication of snag-management guidelines have led us to ignore the obvious: we need to retain the very elements that give rise to much of the biological uniqueness of a burned forest—the standing dead trees.”
Dead trees are important to wildlife. Think woodpeckers. But many other species depend on dead trees and downed wood for food and shelter.
Hutto reports that upwards of 60 percent of species that nest in severely burned forests use only snags for nest sites. In addition, about 45 percent of all North American native bird species rely on snags for at least a portion of their life cycle.
Hutto has found fifteen species that are most abundant in forests with high numbers of snags resulting from high-intensity stand-replacement crown fire—the kind of fires that foresters pejoratively call catastrophic. Hutto notes it is doubtful that these species would have evolved such dependency on snag abundance if large stand-replacement fires and widespread insect outbreaks were uncommon or unnatural, as some suggest.
But it’s not just the use of snags for nesting, or even feeding as with woodpeckers, that attracts birds and other wildlife to recently killed forests. Burned forests also are used extensively by seed-eating species that are attracted by the abundance of new seeds shed by cones and colonizing plants.
Even the presumption that large blazes are a threat to spotted owls is being challenged. “There are several studies which indicate that spotted owls actually benefit from substantial patches of high-severity fire within their home ranges,” says researcher Hanson. “They selectively forage in unlogged, high-severity burn patches.” However, he adds, if these burned areas are salvage logged, spotted owls avoid them.
In a paper presented at a conference on the ecology and management of dead wood in western forests, researcher Timothy Kent Brown estimated that two-thirds of all wildlife species use dead trees or downed wood during some portion of their life cycle. Among Pacific Northwest vertebrates, sixty-nine species depend upon cavities for shelter or nesting, while forty-seven other species are strongly associated with downed wood. And it’s not just the obvious species like woodpeckers that demonstrate this dependence. Many bat species, for instance, hide in cavities in dead trees or under the loose bark of dead and/or dying trees.
Jim Andrews, a professor at Middlebury College in Vermont, studies amphibians and reptiles in northeastern forests. “Foresters tend to look at the forest from the floor up,” he says. “I have occasionally gone on field trips with them, and they were rather proud of how they had managed their forests, but the forest has nothing in it. There’s no cover. No places to find live critters.”
Andrews notes that dead and dying trees are important for many cold-blooded species, from gray frogs to arboreal rat snakes. “Standing snags, once they get big enough so that they have hollow centers—what foresters call ‘overmature’…are the places where wildlife reside,” Andrews says. “To a biologist you don’t have overmature trees—you have wildlife habitat.”
Andrews notes that the greatest biomass of terrestrial vertebrate species found in eastern forests are salamanders, not the more charismatic large mammals like deer and moose. Salamanders provide food to many other species, from wild turkeys to shrews.
But salamanders are also significant predators in their own right, Andrews says. They eat beetle larvae, fly larvae, ground beetles, spiders, sow bugs, round worms and other invertebrates that feed on forest debris. In doing so, they shape the forest ecosystem much as wolves do on another scale. “Salamanders, by preying upon these species that consume leaf litter, help to maintain a deeper layer of leaves and other organic debris that holds moisture, reduces floods and that kind of stuff,” Andrews says.
SMALL BUT CRUCIAL
It’s easy to identify an ecosystem for its most photogenic species, but there are dozens of small cogs that are of equal importance. One of those is ants, and downed logs are their preferred home. Ants are among the most common invertebrate in forest ecosystems and, not surprisingly given their abundance, are critical elements in forest ecosystems.
The most obvious value of ants is as food—from birds such as flickers to much larger animals like bears. In fact, research suggests that ants are among the most important food for bears in Oregon during June and July, as well as later in the summer if the berry crop is small. A British Columbia study found that grizzly bears rely on ants for food late in the fall when berries are unavailable. Reducing the number of dead trees, and thus ants, has a direct consequence for bear survival.
But ants also prey on insects that attack trees. For example, studies in Washington and Oregon discovered that ants accounted for an 85 percent reduction of pupae from two tree-defoliating moths.
Dead logs and snags are also home to pollinating insects. Solitary and colonial bees, of which there are hundreds of species that reside in downed logs and/or snags, are among the major pollinators of flowers and berry-producing shrubs.
Dead trees are even important for other plant species. Bureau of Land Management botanist and lichen expert, Roger Rosentreter, says that dead snags, by creating suitable habitat for lichen growth, carry the legacy of lichen species to the next generation of live trees in the forest. Research by Oregon State University professor Bruce McCune found that some common lichens were more abundant on barkless branches of dead trees than on live ones.
Healthy forest soils also require decomposing material. Below the litter layer in the soil is yet another layer of life that depends on dead wood. “There’s a whole complex food web in the soil that is a combination of bacteria, fungi, protozoa, micro-fauna like arthropods, springtails, mites—all those organisms thrive and are important to the composition of the forest,” says soils specialist Tom Deluca, a forest scientist at the Wilderness Society’s Northern Rockies office.
Deluca notes that while forest litter, such as fallen needles and branches, is important to forest soils, forest soil development is also “very dependent upon the influx of carbon from [whole] trees that have a life cycle of hundreds of years.”
If the carbon influx (dead trees) created after a wildfire or beetle outbreak are removed, he says, the soil is robbed of energy for micro-organisms. “The organic influx is essential to micro-community,” he says.
People commonly assume that wildfire destroys trees and leaves a smoldering pile of ashes. In truth, some live trees and a lot of dead wood physically survive blazes. Beyond the value of dead trees as feeding, hiding and resting habitat for wildlife, downed logs play an important role in forest regeneration.
Snags and downed logs modify micro-sites that can affect seedling establishment. For instance, snags provide some shade and reduction of drying winds, creating more favorable conditions for tree seedling survival. Researching the effects of fires on snags in Wyoming, Dan Tinker, of the University of Wyoming, found that only 8 percent of the downed wood was consumed in fires. He also says that 35 percent of the downed wood in clear-cuts was a biological legacy left by past fires that occurred prior to logging. Tinker and his associates found that these legacy trees intercepted precipitation and funneled it to the ends of the log, creating a moister micro-site that was often more favorable for tree seedling germination and survival.
Other researchers have found that, when it comes to trees, all death is not equal. How a tree dies affects its ultimate role in the forest ecosystem. A tree killed by bark beetles has a different decay trajectory than, say, a tree dying from disease or wildfire. For instance, bark beetles, by breeching the outer bark of a tree, create tiny openings that allow fungi and other insects to enter the tree’s core.
Bark beetles emit pheromones that not only attract other bark beetles but also insects that prey on bark beetles. And the volatiles released from the decomposing trees attract another entirely different group of organisms that feed upon dead wood. That is why one researcher in Europe found that bark beetle outbreaks increased biodiversity in forest ecosystems.
William Laudenslayer, a U.S. Forest Service researcher at the Pacific Southwest Forest and Range Experimental Station, and his colleagues experimentally girdled trees to kill them, a common forestry practice used to produce snags for wildlife. They compared those snags to trees killed by bark beetles. They found that “bark beetle-killed trees provided significantly greater woodpecker feeding activity, cavity building and insect diversity” compared to snags created by girdling.
Trees heated and killed by fire create sapwood that resists rotting and lasts longer in the ecosystem. Trees dead prior to the fire tend to become blackened and charred. Charred trees are also resistant to decay. Thus, a wildfire creates long-lasting biological legacies that can survive for a century or more.
DEAD IN THE WATER
Wayne Minshall, professor of ecology with the Stream Ecology Center in the Department of Biological Sciences at Idaho State University, points out the importance of logs to aquatic ecosystems as well. “Wherever the logs occur, they cause the stream to meander or braid. And whenever you get braiding or meandering, you’re getting a reduction in the power of the stream and delivering the water in a way so as to dissipate that energy so the flow becomes less destructive. That’s important in keeping streams healthy.”
Wildfires and/or insect outbreaks create downed logs that fall into streams and across slopes. Downed logs, by slowing the velocity of the water, allow sediment to settle out and help return sediment flows to pre-burn levels. Minshall points out that while organisms have evolved to deal with episodic sediment flush events, such as those occurring immediately after a wildfire, they are unable to cope with forestry-induced sedimentation. To these organisms, a forest fire is no big deal, he says. “We get a short few years of sediment runoff, but it’s not a major thing that organisms can’t handle.” But aquatic organisms can’t take unexpected events they haven’t evolved with, such as the presence of fine sediment all year round for extended periods of time. “If we clear-cut, salvage log or put roads in, then the sediment flows tend towards chronic, and it’s a major detriment to organisms,” he says.
Rhodes says that scientists have not identified an upper threshold of logs in streams that is too much for fish. “The more wood, the more fish, all things being equal,” he says. “Lots of wood is a big part of the productivity for streams.” The loss of salmonids in many parts of the West, he says, can be attributed to the absence of wood in streams.
The criteria for healthy ecosystems can’t be easily defined or exhaustively listed. But healthy ecosystems have a full array of processes operating unimpaired, including hydrologic function, soil productivity, carbon sequestering, provision of wildlife habitats and keystone disturbances such as fires, floods, storms and insect outbreaks.
One crucial element present in unmanaged, healthy systems is a significant amount of dead trees and downed wood, Rhodes says. “There is seldom too much dead wood in forests and certainly not in unmanaged ones. However, there is almost always a dearth of it in managed forests.”