All life requires moisture, energy, air, and nutrients. On any given area, those are finite and the life that can be supported (its carrying capacity) is also finite. This applies to ranchers, wildlife biologists, farmers, and backyard gardeners – e.g., farmers don’t plant corn two inches apart with an abundance of weeds and gardeners don’t plant tomatoes two inches apart under the apple tree.
Carrying capacity varies greatly. Coast Range rainfall is plentiful and forests may support several hundred trees per acre as well as other vegetation. Eastern Oregon forests are drier and may support less than a hundred trees and with far less other vegetation.
Post-settlement, we’ve put out fires and greatly increased fuels and numbers of trees. In eastern and southern Oregon, rainfall limits the land’s carrying capacity. Thinning will reduce drought-related stress and the amount of fuels.
Fire needs heat (matches and lightning), oxygen, and fuel. Take one away (dirt smothers flames, water cools fire, or remove fuel) and the fire goes out. Thinning removes some of the fuel and lessens the likelihood of catastrophic fire.
There are many examples where crown-fires came to a thinned area, dropped down, and crept across the thinned forest’s floor. Once through the thinned area, it went back up into the canopy and resumed its catastrophic fire behavior.
A recent writer to The Oregonian opined that thinning will not prevent wildfires. I agree with him on that. Preventing wildfires requires there be no lightning or people who start fires. What thinning will do is reduce the amount of available fuel should a fire start.
A dense canopy lets little sunlight pass through to the understory and has less diversity of flora and fauna. Besides reducing available fuel, thinning opens the canopy, gets more sunlight into the understory, and greatly increases the forest’s diversity.
Until tree crowns grow together, a young forest has lots of sunlight reaching the ground and has the greatest diversity of both flora and fauna. After crown closure, that diversity begins to diminish. Later, as it matures and trees die, fall over, or tops die, sunlight again begins to pass through the canopy and into the understory and diversity increases. Thinning that maturing forest brings diversity back more quickly than if left alone.
Removing fuels and maintaining a green understory is less likely to burn than an accumulation of dry, woody fuels. Similarly, a layer of dry leaves around a house is far more hazardous than if green grass surrounded it.
Oregon Forest Resources Institute data shows that 73 percent of the annual growth on Oregon’s federal forests is kept as live, green, growing stock. (19 percent is lost through mortality!) The land’s carrying capacity simply cannot sustain that added growth year after year after year. Exceeding its carrying capacity leads to an over-crowded and drought-stressed forest and to fires, insects, and disease – problems found all across the West.
The judicious use of harvesting (and thinning) can bring the land back within its carrying capacity.
Dick Powell is the chairman of the Oregon Society of American Foresters. A similar version of this column was published in The Oregonian.