The Loma Prietan - May/June 2010

Forest Thinning Myths Persist

by Dr. Will Russell

The hollowed trunk of this first-growth redwood illustrates its fire resistance, its mature branch structure provides wildlife habitat, and its crown keeps the forest floor cool and shaded. Photo: Anneliese Agren
The hollowed trunk of this first-growth redwood illustrates its fire resistance, its mature branch structure provides wildlife habitat, and its crown keeps the forest floor cool and shaded. Photo: Anneliese Agren

Industrial timber management tools, such as commercial and pre-commercial thinning, are becoming the preferred tools for forest restoration, carbon sequestration and fire safety. The prescription of these tools, however, is based on certain myths regarding forest management. Many of these myths have roots in science but have been applied inappropriately to the coast redwood forests.

Myth #1: Forest Restoration: Regenerating forests need to be thinned in order to produce old-growth forest characteristics.

The theory behind forest thinning is that the removal of a portion of the trees within a regenerating stand will increase the growth rate of the remaining trees. In addition, theory suggests that, due to extreme competition for light, high density "over-stocked" stands will not be able to develop the multi-layered, multi-aged, characteristics of old-growth forests.

While it is true that stand thinning increases the growth rate of remaining trees, and the economic value of large trees from a timber management perspective is obvious, the ecological value of large trees is not as widely accepted. Structural elements of old trees, such as large lateral branches, snags, and hollows, are much more important as habitat features than the overall size of a tree.

Coast redwood trees regenerate primarily through clonal sprouting rather than from seed, so competition between sprouts is not a true competition between individual trees. Rather, competition takes place between sprouts of the same tree to determine which sprout will become the new trunk. As a result, coast redwood forests self-thin with ease, resulting in density levels, within 60 years, similar to old-growth stands. Complexity of structure and composition also develop naturally in the coast redwood forest type, without the need for additional management input.

Myth #2: Carbon Sequestration: Second-growth stands are better for carbon storage than old-growth stands, and stand thinning is beneficial for carbon storage.

The notion that second-growth forests are superior to old-growth for carbon storage is based first on the misconception that old-growth forests are carbon neutral and second on evidence that second-growth forests grow faster than old-growth forests.

Early ecologists suggested that old-growth forests operated in a steady state where the carbon taken in through photosynthesis was equal to the carbon released through respiration. This theory is no longer regarded as true, due to recent studies that demonstrate that old-growth forests continue to grow and sequester carbon at a significant rate.

While it is generally true that second-growth forests sequester carbon at a faster rate than old-growth forests, the total amount of carbon stored is much lower. Using the metaphor of a bank account, a second-growth forest has a high interest rate on a low balance, while old-growth forests have a low interest rate on a high balance. Therefore, converting old-growth to second-growth is counterproductive.

The harvesting of any living tree results in an immediate carbon deficit. Every stage in the extraction and production of timber products requires significant amounts of fossil fuels. Timber products and byproducts begin to decompose fairly quickly after production, particularly in the case of redwood, which is commonly used for fencing and decking. Finally, soil carbon (approximately 50% of the total carbon) begins to be released following harvest, resulting in an even greater loss of carbon.

Myth #3: Fire Safety: Forest stands need to be thinned for fire safety.

Forest thinning for fire safety is based on the theory that reducing ladder fuels (small trees and shrubs that can carry fire from the forest floor to the crown) and interrupting forest canopy will reduce the spread and intensity of fire. While this may be true for some forests types such as pine or fir forests, federal and local policies have promoted intensive forest thinning in all forest types, including coast redwood.

Coast redwood forests are remarkably resistant to fire. Features of the trees such as thick fire-resistant bark and high crown, low density of ladder fuels, combined with moist fuel conditions under the forest canopy, commonly result in a low potential for catastrophic wildfire, particularly in older forests. Fire hazard can increase in a coast redwood forest through forest thinning, particularly when large trees are removed. Opening the canopy increases sunlight to the forest floor, reducing fuel moisture and allowing for the development of ladder fuels such as shrubs and suppressed trees.

Coast redwood forests are remarkably resilient to disturbance, regenerate naturally, and store enormous amounts of carbon. Active management through timber harvest and stand thinning tends to simplify ecosystems, reduces regenerative potential, and produces rather than sequesters carbon.

Will Russell, Ph.D., is an Assistant Professor of Environmental Studies at San Jose State University where he conducts research on the restoration and conservation of coast redwood forests. Reach him at russell_sjsu@yahoo.com