Fire plays important role in forest ecology

Photo provided by Aaron Cherry
Wild fires have an important function in maintaining the healthy and diversity of a forest ecosystem.

Fire plays important role in forest ecology

Wildfires play a complex role in forest ecosystems. While often perceived by the public as universally destructive, the real picture of the effects of wildfire on forests is more diverse. Fire often performs critical ecosystem functions within the forest ecosystem, but when forests are managed without taking into account the importance of fire within a forest the result is often severe, destructive, and uncontrollable wildfires. Forest management practices play a key role in preventing the deleterious effects so often associated with wildfires. By properly managing forests in relation to fire, the communities surrounding a forest can continue to benefit from the diverse recreational opportunities, abundant wildlife and economic wealth that Oregon’s forests can provide.

Good management of forests must begin with an understanding of the processes at work in a forest ecosystem. One of the key processes is the recycling of nutrients, in which the flow of energy cycles nutrients through the biomass of an ecosystem, into the environment, and then back into the biomass. The most familiar nutrient cycle is the hydrological cycle, in which water moves between the atmosphere, lithosphere, and biosphere. Many other nutrients, such as carbon, flow through similar cycles. Within a forest ecosystem, fire often facilitates this flow of nutrients.

“In certain ecosystems, when trees die the presence of heavy moisture allows organisms such as fungi to break down the woody biomass,” says Jarred Saralecos, forestry instructor at Umpqua Community College. “This releases the nutrients tied up in the biomass and recycles them to the soil, where they are then available for trees and other primary producers to utilize.”

However, in drier environments the low moisture availability limits the ability of organisms to breakdown the woody biomass of a forest, explains Saralecos. The biomass will eventually break down, but the time intervals involved make the decomposition an inconsequential process in the cycle of nutrients. In these ecosystems, the forests will eventually burn the woody material that has accumulated over time. The fire provides an alternative path to decomposition for recycling the nutrients. Fire takes advantage of drier ecosystems where low moisture levels limit the decay of woody biomass by saprobes such as fungi. In a similar process as decomposition, the nutrients tied up in the woody biomass are release by fire and made available to living plants within the forest ecosystem.

In addition to freeing nutrients from dead biomass, fire also serves as a powerful force for natural selection within a forest. “Healthy forests are a lot sparser than people think,” says Saralacos. “Often what we think of as a healthy forest structure is too thick, and eventually these thick forests will burn. Burning provides a stressor that weeds out diseased and nonviable trees.”

Elimination of weak and diseased trees also frees up water in the soil for new saplings, explains Saralecos. When a forest burns, the weaker trees are killed and removed from the community. By removing these trees, the diversity in tree maturity is increased and consequently a forest’s over all resiliency and resistance to diseases and pests is increased as well.

“It is important to realize that not all forest fires are stand replacement fires. In different ecosystems fire burns with differing severity and frequency,” says Saralecos. “For example, Oregon’s Coast Range does not burn often because of its higher moisture levels. However, when fires do occur in the Coast Range they are very intense, and the burned over area must restart from scratch ecologically, beginning with pioneer species.”

In contrast, in drier areas such as Oregon’s intermountain region, fires burn with a much higher frequency, usually every three to 15 years. Because of this high frequency, the fires there are much less intense. These fires usually don’t remove the mature trees. Instead, they just burn the grasses and brush that form the forest understory. Because of the lower intensity of these fires, the burn pattern through a forest is patchy, with a smaller area burning and other adjacent areas of the forest left unburned.

As part of wildfires function in recycling nutrients they can also change the successional stage of a forest, depending on the fire’s severity. After a major ecological disturbance, such as a fire that removes the majority of aboveground biomass, bare ground is first colonized by ruderal species such as mosses or weedy plants. The next successional stage is composed of shrubs and small hardwoods, and eventually mature trees will become established if nothing disturbs developmental progress of the ecosystem.

“Fire functions as a disturbance within a forest ecosystem, moving the forest back in successional stages,” says Benz. “For example, take a recently logged area of the forest. The dominant organisms in the clear cut are weedy plants, grasses, and other early successional species. Soon after harvest, however, new trees are planted. If in 20 years that area burns with enough severity, the growing forest composed of 20 year old trees would be moved back to the initial successional stage, dominated by grass and weedy plants.”

This ecological disturbance can help create a greater diversity of species with a forest community. Not only does this maintain a stand of trees with diverse ages and maturity, but having a variety of successional stages also creates more habitat for wildlife such as birds and mammals. Ecosystems that are diverse are more resilient to further ecological disturbances, such as future timber harvest or fire. Different conditions within each specific forest determine the overall severity of a fire and whether the forest’s successional stage is significantly affected.

“Fire functions differently in each specific forest ecosystem. Moisture is the main factor that determines the frequency with which a forest naturally burns,” says Saralecos. “However, human influence now plays an important role in determining the frequency of wildfires.”

Human influence usually serves to decrease the burn frequency of a forest. Fire prevention and suppression limit the role fire has on a landscape. Unfortunately, fire suppression removes the ecosystem function that fires performs, which to thin the forest canopy, remove diseased trees and free up nutrients from dead woody biomass. This can result in the accumulation of fuels within the forest ecosystem.

“A good example of this is the southern Sierra Nevada Mountains. This forest naturally burns fairly frequently, usually about 15-30 years. A study a few years back looked at the levels of duff accumulation on the forest floor in these areas that have experienced long term fire suppression,” says Saralecos. “They found that between six to 26 inches of duff is now present in these forests, which is unusually deep.”

 Normal burning frequency would have limited the accumulation of fine forest debris, such as needles and twigs (known as duff), but fire suppression has let the duff accumulate to the present levels. Now, any fire that burns there will be a severe stand replacement fire, not a cooler fire that would have occurred naturally.

  “I think people forget that trees and forests are living things, and they will all eventually die. We often want to preserve large, unique or special trees. So, we suppress fires in these areas,” says Saralecos. “But these forests will burn eventually. Now, in the Sierra Nevadas, even a prescribed burn would kill the very giants that we want to protect.”

Because fire plays such a key role in many forest ecosystems, fire suppression without taking measures to replace the function of fire will be ineffective in the long term. In fact, unless forest management takes steps to replace the ecosystem functions performed by fire, long term fire suppression will allow fuels to accumulate and eventually the forest will burn in extremely severe, destructive, and uncontrollable fires.

“An interesting example of this is comparing the relationship of fire starts and uncontrolled fires on public and private land in the west. As one would expect, the number of fire starts is roughly proportional to the amount of land held privately and publicly,” says Saralecos. “But uncontrolled fires resulting for these starts occur disproportionately on public land, even when accounting for differences in access between public land and private.”

This is because the private sector has harvested and thinned, creating a high enough turnover in the forest to have replaced fire. The removal of timber and opening of the forest canopy with cutting performs similar ecosystem functions to that of fire.

Thinning is the timber harvest practice of removing only a portion of the trees in an area, leaving a majority of the trees standing. This form of timber management mimics the effects of lower severity fires by opening up the forest canopy and removing underperforming trees. It also can create the patchy, diverse forest ecosystem in which different areas vary in successional stage.

“The major challenge to thinning as a method of timber is that it is restricted to areas that can be accessed by machinery,” says Luke Harris, a forester that has worked extensively in the Pacific Northwest and the Southeast United States. “This is because leaving trees standing within a thinning site makes the use of a yarder unsafe and inefficient. Consequently, logging equipment must be able to access the trees where they are growing.”

This logistical challenge limits thinning to areas that are relatively flat and navigable by machinery. “In the loblolly pine plantations of the south thinning works well because the plots are in flat areas or rolling hills,” explains Harris. “However, in the Pacific Northwest the majority of forest areas are rugged terrain that make thinning on a large scale unfeasible.”

Clear-cutting does not have the obstacles that restrict the use of thinning. The process of clear cutting removes the majority of trees in a specific area of forest, making the use of a yarder logging system safe and efficient. Consequently, this allows effective timber harvest in areas that would be inaccessible to thinning.

In contrast to thinning, clear-cutting mimics the effects of a more severe fire on forest ecosystems. By removing all the trees in a specific area, the biomass of accumulating fuel is reduced. This emulates the effect of a major fire in reducing the fuel load in a forest.

In contrast to private forest land, the policy for managing public lands has often been to restrict harvest of timber. This is because while private forest lands often have profitable timber production as the primary management goal, public lands such as the National Forests have many different and often competing management objectives including preservation of wildlife and plant diversity and providing a wide span of recreational opportunities in addition to promoting profitable timber production.

“Certain areas of the National Forests are dedicated to management for late successional characteristics,” says Benz. “This means preserving old growth timber along with the diversity of tree maturity characteristic of a mature forest.”

Some forest managers view the old growth forest as the most stable forest structure with respect to fire. “The argument is that a forest of mature tree is more resistant to fire than a tree plantation. This is because these old growth trees have survived fires in the past,” says Benz. “Part of the reason mature trees are able to survive most forest fires is that they have very thick bark which insulates them from the heat that would kill younger, thinner-barked trees.”

In the past, suppression has been the main management goal in relation to wild fires. The National Forest’s Smokey Bear ad campaign, which is the longest running and most successful public announcement campaign in American history, has seared a negative view of fire into the public conscience. However, management attitudes toward fire on public lands have been shifting in recent years.

“I have seen a change in the way National Forest management views fires in the last decade. Instead of extinguishing every fire possible, risk assessments are now performed on fire starts,” says Benz. “Some fires are allowed to burn if they are determined to be low risk for becoming uncontrollable.”

Prescribed burning is another method that can be used where timber harvest is restricted. However, controlled burning requires extensive site preparation, permitting, and the right set of weather conditions before it can be performed safely and effectively. Depending on the requirements of the area to be burned the need for large fire crews or helicopters to start the fires can drive up the cost of prescribed burning.

Despite careful planning, prescribed burns are not guaranteed to remain controllable. “Forests with similar characteristics such as climate, fuel loads, species composition, and terrain scale and slope usually respond similarly to fire, and thus prescribed burning can usually be executed well,” says Benz. “However, weather adds uncertainty to any prescribed burn. The prevailing wind direction can change and cause a planned fire to go awry. Ultimately, no prescribed burn is risk free – there are just too many different variables at play.”

A variety of different practices are needed to meet the management objectives of diverse forests. The goal that unifies both private and public forest management is the desire to prevent large, severe, destructive fires in our forests. In certain areas practices such as thinning and carefully planned clear cutting fill the gap created when the role of fire is limited in a forest ecosystem. On public lands where timber harvest is limited, controlling natural fires and utilizing prescribed burning can help reduce the accumulation of fuels and promote the diverse structure of a healthy forest.

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