NCCSP

As wildland fires increasinly grab national attention, the call to "do something" gains almost inexorable momentum.  To reduce interface home loss, the obvious thing "to do" is increase the prevalence with which homeowners follow defensible space rules - after all, 90% of homes with nonflammable roofs and a defensible space survive even hot burning, high-intensity fires.

But, what about our forests, rivers and streams?  Are they, too, threatened by changes in fire behavior or are they more threatened by the actions taken to address fire threats?  Our two cover features address these ecological questions, one from a terrestrial and the other from an aquatic perspective.

FEATURE ARTICLE

RESTORATION THINNING: WHAT, WHY, WHERE AND HOW

by Dave Perry, NCCSP Board Member and Professor Emeritus, Oregon State University

Thinning is an old silvicultural tool used to reduce competition for resources and produce larger, more vigorous trees.  In recent years thinning also has been promoted as a tool to restore overly dense stands and landscapes to a more fire and insect resistant condition.

Over the past century forests throughout the low- and mid-elevations of the West have been highly altered in two distinct ways.  First, extensive clearcutting has converted landscapes once dominated by old growth to widespread plantations.  In addition to a deficit of large, dead wood, these plantations often are overly dense and highly susceptible to crown fires and other disturbances.  While nature occasionally produced dense young stands following wildfires (especially in lodgepole pine), research throughout western and southwestern Oregon shows old growth stands commonly established at low conifer densities and gradually filled in over time.  During their first 100 years or so these stands probably were diverse mixtures of conifers and hardwoods, a condition that promoted both biological richness and, because common hardwoods don't burn as readily as conifers, fire resistance.  In dry ponderosa pine forests, dense regeneration was confined to small patches within a matrix of old growth trees.  The objectives of plantation thinning include hastening the return of big trees, restoring the rich variety of species, and eventually producing a landscape that is less susceptible to crown fires.

Second, fire suppression has transformed many dry ponderosa pine and dry mixed conifer forests.  Historically, relatively frequent, low intensity fires spared most big trees and killed many smaller ones.  With fire suppression, many smaller trees that historically would have been killed have survived, greatly increasing stocking density and fuel loads, and putting the old growth trees at risk to crown fires and bark beetles.  Because upwards of 80% of the original old growth in these types has been lost to logging, further losses to fire are of considerable conservation concern.

Restoration thinning in both plantations and the relatively dry older forests should be done in a way that moves stands and landscapes back toward dominance by large, fire resistant, vigorous trees.  That means thinning smaller trees and leaving the larger ones (the actual size depends on the stand).  There may be occasions where taking a few larger trees has a sound ecological justification (e.g. to reduce competition with a relatively uncommon, desirable species such as California black oak), but thinnings with a restoration objective should not reduce average stand diameter.

Some forest types don't need restoration thinning, especially those at higher elevations, where neither logging nor fire suppression have significantly altered forests or landscapes. Even in forest types with an overall need for thinning, stocking density often is quite heterogeneous and not every locale is likely to need it.

Restoration thinning is not a panacea, but a form of preventative medicine intended to reduce risk.  All thinning should be guided by the dictum "do no harm," which in this case means careful logging that does not damage residual trees, soils, and streams, that avoids introducing invasive plants or pathogens, and that does not defeat its purpose by increasing fire hazard (e.g. through leaving untreated logging slash or opening stands so widely they become windier and drier).  Thinnings that require new road building should be avoided wherever possible.  To protect habitat for species that require closed forests thinnings should be planned at the landscape level.  One promising approach is "variable density thinning," in which a mosaic of thinned and unthinned patches is created.  The challenge is to deal with one set of problems – the stresses and risks associated with overstocking – without creating new problems or exacerbating the old ones.

Restoration thinning "done right" can be an important tool to move stands and landscapes back toward dominance by large, fire resistant, vigorous trees.  Photo credit: Dave Perry