The ecological value of dead trees

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. (Actually I appreciate both and don’t advocate early death for either.)

We carry this cultural bias to our view of forests. Like most people I once viewed dead trees as an indicator of some presumed problem in the forest—that a ‘healthy” forest was one with a minimum of dead trees and largely free of wildfire, insects, and disease. Oh yes, I knew that a few snags were good for woodpeckers, and as a fly fisherman I understood that trout tended to be found hiding behind logs in the stream. I suffered from the same cultural bias as most people and thought that large numbers of dead trees meant that the forest was “out of balance” or “sick.” But the more I studied ecology, the more I questioned these assumptions. I now understand that large numbers of dead trees are critical to functioning forest ecosystems and sometimes, at the risk of hyperbole, I occasionally say they are ultimately more important to forest ecosystems than live trees.

There is no disputing the ecological importance of dead trees. Dead trees and down wood play an important role in ecosystems by providing wildlife habitat, cycling nutrients, aiding plant regeneration, decreasing erosion, and influencing drainage and soil moisture and carbon storage, among other values. Richard Hutto, an ecology professor at the University of Montana, sums up this new way of thinking about the long-term ecological value of dead trees when he notes, “Snags are important biological legacies that are passed from one forest generation to the next.”

OUTDATED PARADIGM

Old perceptions about dead trees are rampant in our society. In a January 2009 Christian Science Monitor article about bark beetle and forests, Tim Love, Seeley Lake District Ranger on the Lolo National Forest, was quoted saying there was an “overabundance” of dead trees on his district from both wildfire and beetles. His viewpoint is all too common.

In our traditional paradigm, fires and insects are generally viewed as undesirable, and tolerated only if they are small, limited, and controllable. We spend billions annually trying to contain wildfire and insect outbreaks based upon the presumption that these natural processes are “destroying” the forest. Even though there is now some grudging acceptance of some limited wildfires and even some small insect attacks as potentially “beneficial,” most people still view large stand replacement fires and large beetle outbreaks as “unnatural,” “abnormal,” and something to suppress, slow, and control.

NEW PARADIGM

A new perspective is slowly taking root that questions the idea that an abundance of dead trees are bad and whether we are “losing” anything to wildfire and insects. Indeed, 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.

Jon Rhodes, an independent consulting hydrologist in Oregon says, “When you start to look at managed western forests outside of wildernesses and parks, you notice right away that they lack large quantities of down wood—dead trees. Ecologically speaking, there is a big difference between areas that have been logged/thinned compared to areas that are left alone.” Rhodes turns common perception on its head when he notes that sanitized, managed forests are less healthy than so-called “sick” natural forests. “The one thing you notice in unmanaged, healthy systems is there’s a lot of dead trees and down wood.”

Chad Hanson, a UC Davis researcher and Sierra Club board member, sums it up. “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 down logs. Such forests may not look tidy from the perception of a forester, but it (a forest with lots of dead trees) is the most biologically diverse and healthy, from a forest ecosystem perspective….Pound for pound, ton for ton, there is probably no more important habitat element in western conifer forests than large snags and large down logs,” claims Hanson.

Rhodes states that studies have consistently concluded that there is deficit of large snags and downed dead wood in most western forests. “Large standing trees are important. I have spent much of my life protecting such trees, 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, because that’s how we get large quantities critically important of snags and downed wood in the forest ecosystem.” Rhodes declares, “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.”

Hanson suggests that even the conservation community is often stuck in the old paradigm. “Too many conservationists believe that forests were all open and fires were all low-severity. In the last ten years, the scientific evidence presents a much different picture of historic forests. Not that the low-severity fires didn’t occur, they just weren’t exclusive. There were vast areas of high-severity fires in the same forests.”

As Hutto suggested in a 2004 article in Conservation Biology, “Everything from the system of fire-regime classification, to a preoccupation with the destructive aspects of fire, to the mis-application 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 TREE IMPORTANCE FOR WILDLIFE

One of the few ways that dead trees are appreciated by most people is their wildlife value. Think woodpeckers. But most people have no idea how many species actually depend on dead trees and down wood.

Richard Hutto reports that upwards of 60% of species that nest in severely burned forests use only snags for nest sites. As many as 45% of all North American native bird species rely on snags for at least a portion of their life cycle.

And Hutto has found 15 species that are most abundant in forests with high snag numbers resulting from severe 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 large insect outbreaks were uncommon or unnatural, as some suggest.

But it’s not just the use of snags for nesting, and 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 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. Chad Hanson asserts, “There are several studies which indicate that spotted owls actually benefit from substantial patches of high severity fire within their home ranges. They selectively forage in unlogged high-severity burn patches. However, if these high-severity patches are salvage logged, spotted owls avoid them.”

So important are dead trees to wildlife that researcher Timothy Kent Brown estimates that two thirds of all wildlife species use dead trees or down wood during some portion of their life cycle. Among Pacific Northwest vertebrates, 69 species depend upon cavities for shelter or nesting, while 47 other species are strongly associated with downed wood. And it’s not just the obvious species like woodpeckers. 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. Andrews says, “Foresters tend to look at the forest from the floor up.  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,’ (which I hate since it is so board-foot focused)—are the places where wildlife reside.”  Andrews quips, “To a biologist you don’t have overmature trees—you have wildlife habitat.” 

The importance of down logs to wildlife can’t be overstated. 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. As a result they are important as food to many other species, from wild turkeys to shrews.

But salamanders are also significant predators in their own right. Andrews says salamanders eat beetle larvae, fly larvae, ground beetles, spiders, sow bugs, round worms, and other invertebrates that feed on forest debris.

Salamanders are the top predator that shape ecosystem processes much as wolves are the top predator at another scale in terrestrial ecosystems. “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, “says Andrews.

DEAD WOOD AND SMALL CREATURES

Our focus on the big and flashy species often causes us to ignore the small cogs that often drive the ecosystem. One of the small cogs is ants, and down 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.

Their most obvious value of ants is as food for many species, from birds like flickers to much larger animals like bears. In fact, research suggests that ants are among the most important food for bears in Oregon in June and July, as well as later in the summer if the berry crop fails. Another British Columbia study found that grizzly bears relied upon ants for food late in the fall when berries were unavailable. Reducing the number of dead trees, and thus ants, has a direct consequence for bear survival.

But ants are also major predators on insects that attack trees. For example, studies in Washington and Oregon discovered that ants accounted for an 85 % reduction of pupae of two tree defoliating moths.

Dead logs and snags are also the major home for pollinating insects like wasps and bees. Solitary and colonial bees, of which there are hundreds of species that reside in down logs and/or snags, are among the major pollinators of flowers and berry-producing shrubs.

Dead trees are even important for other plant species. BLM botanist and lichen expert, Roger Rosentreter says that dead snags, by creating suitable habitat for lichen growth, carries the legacy of lichen species to the next generation of live trees in the forest. And research by Oregon State University professor Bruce McCune found that some common lichens were more abundant on bare barkless branches of dead trees than found on live ones.

IMPORTANCE FOR SOIL STRUCTURE

Below the litter layer in the soil there is yet another layer of life that depends on dead wood. Tom Deluca, Forest Scientist at the Wilderness Society’s Northern Rockies office and a former forestry professor at the University of Montana specializing in soils says: “There’s a whole complex food web in the soil that is a combination of bacteria, fungi, and protozoa, micro-fauna like arthropods, springtails, mites—all those organisms thrive and are important to the composition of the forest.”

Rhodes states that research has consistently shown that downed wood is “absolutely essential for watershed and soil processes. One of the most important parts of soil function and productivity is organic matter in soil. The prime source for organic matter is decomposing woody material in forests, much of which comes from dead trees that fall down.”

Deluca notes that while forest litter, such as fallen needles and branches, are 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.” 

Deluca says, “If you remove the carbon influx (dead trees) created after a wildfire or beetle outbreak, you are ultimately robbing the soil of the energy for micro-organisms. The organic influx is essential to micro-community.

STRUCTURAL VALUE OF DEAD WOOD IN FOREST REGENERATION

People commonly assume that wildfire destroys trees, and consumes down trees leaving a smoldering pile of ashes. In truth, most dead wood physically survives blazes. Beyond the obvious value of dead trees as feeding, hiding and resting habitat for wildlife, down logs can even play an important role in forest regeneration.

Snags and down 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 from the University of Wyoming, found that only 8% of the down wood was consumed in fires. Even 35% of the down wood in clearcuts 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.

CREATION OF DEAD SNAGS

Death and taxes may be the two things you can count on, but 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 pheremones 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.

Forest Service researcher, William Laudenslayer 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 wildfire creates long-lasting biological legacies that can survive for a century or more.

DEAD TREES’ IMPORTANCE FOR AQUATIC ECOSYSTEMS

Wayne Minshall, formerly Professor of Ecology with the Stream Ecology Center in the Dept. of Biological Science 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.”

Logs are also important in trapping sediment. Down logs, by slowing the velocity of the water, allow sediment to settle out. 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. “They (aquatic organisms) can’t take unexpected events they haven’t evolved with, events that are chronic like fine sediment all year round for extended periods of time. Fire in a forest is no big deal. We get a short few years of sediment runoff, but it’s not a major thing that organisms can’t handle. If we clearcut, salvage log, or put roads in, then the sediment flows tend towards chronic and it’s a major detriment to organisms,” says Minshall.

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. And the loss of salmonids in many parts of the west is attributed to loss of wood,” says Rhodes.

GRATEFUL FOR THE DEAD

The ecological cost of salvage logging speaks for itself, and the message is powerful, writes Hutto. “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% negative as the typical post-fire salvage-logging operation tends to be.”

Although Hutto’s findings are specific to post-fire logging, many of his conclusions could equally be applied to any forest management that significantly reduces the overall biomass of dead trees.

Chris Maser, biologist and author of Forest Primeval: The Natural History of an Ancient Forest, The Redesigned Forest, as well as other books on forest ecology, remarks, “A forest both invests and re-invests biological capital into the ecosystem when it dies.” Removal of this biological capital jeopardizes the future productivity of the forest. Maser muses, “The question that needs to be asked is:  How much of the forest is necessary to leave intact to protect its functional integrity—not how much can be taken in the name of short-term economic profitability?”