Prescribed fire and mechanical thinning effects on bark beetle caused tree mortality in a mid-elevation Sierran mixed-conifer forest

Highlights

• We analyzed bark beetle-caused tree mortality after fire and mechanical thinning treatments.

• Treatments were prescribed fire, mechanical, and both fire and mechanical.

• Overall mortality from bark beetles was lower than 7% across all treatments.

• Mortality of small and medium white firs increased in both fire treatments.

• Mortality of medium sugar pines was elevated in the fire only treatment.

Abstract

We assessed tree mortality caused by bark beetles in a mixed-conifer forest in the central Sierra Nevada in response to fire and mechanical treatments. The treatments were: (1) no treatment, (2) prescribed fire, (3) mechanical (crown thinning-from-below followed by rotary mastication), and (4) mechanical followed by prescribed fire. Ponderosa pine (Pinus ponderosa Laws) mortality caused by the western pine beetle (Dendroctonus brevicomis LeConte), sugar pine (Pinus lambertiana Dougl.) mortality caused by mountain pine beetle (D. ponderosae Hopkins), and white fir (Abies concolor Gord. and Glend) mortality caused by the fir engraver beetle (Scolytus ventralis LeConte) was assessed pre-treatments, one-year post-treatments, and three years post-treatments. For the duration of the study, bark beetle caused mortality across all treatments for each tree species was less than 7%. Bark beetle-caused mortality of small and medium white firs increased in treatments that included fire, and bark beetle-caused mortality of medium size sugar pines was elevated in the fire only treatment compared with other treatments. Our results indicate that mechanical treatments cause little risk of mortality to residual trees from bark beetles in the short term. The higher secondary mortality in the small and medium size white firs in both fire treatments can be considered a benefit in overly dense mixed conifer forests where the understory is dominated by shade-tolerant white firs.

Introduction

Since the beginning of the 20th century, the suppression and intended control of fire has been at the forefront of U.S. forest management and forest policy (Biswell, 1989, Agee, 1993, Stephens and Ruth, 2005). Fire suppression has been justified over the years for many reasons other than preventing loss of human lives and structures, including protecting valuable timber resources (Show and Kotok, 1924, Show, 1926), and general aesthetic values (Berry and Hesseln, 2004). It has only been since the mid-1990s that U.S. Forest Service policy recognized fire as an important ecological process that is a critical component of forest dynamics (Stephens and Ruth, 2005). Likewise, bark beetles have been a challenge to forest managers since the importance of forest protection was recognized by federal and private owners in the western U.S. (Craighead et al., 1931), and they often affect larger areas than fire (Raffa et al., 2008). Forest insects are now commonly recognized by forest managers as an integral part of forest ecosystems (Barker, 2003, Wood and Storer, 2002, Wood and Storer, 2009).

American Indians such as the Nisenan community of the western Sierra Nevada (Kroeber, 1925, Kroeber, 1929, Matson, 1972) have been using fire as a management tool for nearly 2000 years prior to the arrival of European-American settlers (Cook, 1976; as in Stephens and Collins, 2004). Historically, these human-ignited, frequent, low- to moderate-intensity burning patterns, together with lightening ignited, varying intensity wildfires, created a mosaic of forest stand structures (Anderson and Moratto, 1996, Husari and McKelvey, 1996, Collins et al., 2011). As land use patterns changed, so did forest stand structure. Beginning in 1905, Euro-American-based management practices attempted to keep fire out of forests at all costs (McCullough et al., 1998). These fire suppression efforts, together with forest harvesting, contributed to dense forest re-growth of smaller diameter, fire intolerant trees (Hessburg and Agee, 2003) such as white firs (Abies concolor Gord. and Glend). These stands were very different from the open, park-like stands that were comprised of few, large diameter, shade intolerant pines (Agee, 1993, Skinner and Chang, 1996, Agee and Skinner, 2005, North et al., 2007) that were typically produced by American Indian management practices. It follows that as the structures of these forests changed, so did the response of bark beetles; so much so that early forest entomologists ranked insect-caused damage to the second-growth forests to be as significant as the threat of fire (Barker 2003). In 1899, motivated by the threat bark beetles posed to standing timber (Barker 2003), A.D. Hopkins traveled throughout California, Oregon, Washington, and Idaho, and recorded the extensive mortality of old growth western conifers caused by bark beetles (Wood and Storer 2002).

Prescribed fire has been used for nearly a century as a tool in forest management to reduce the severity of wildland fires (Biswell 1989), and more recently in the western U.S. to reduce hazardous fuels by restoring stand structure to an idealized previous condition, or by promoting “resiliency” (Covington et al., 1997, Keifer et al., 2000, Skinner, 2005, Baker et al., 2007, Abella et al., 2007, Zhang and Ritchie, 2008, Stephens et al., 2010, van Mantgem et al., 2011). Mechanical treatments (e.g. thinning from below) are often used to produce similar stand structures as fire with many similar objectives, often when prescribed burning is not a feasible option (for example, due to air quality restrictions and other public health concerns, or increased risk of structure fires)(Stephens et al. 2012). Such thinning efforts have also been federally mandated to reduce hazardous fuels (Stephens and Ruth 2005). Regardless of the management option, the overall common concern is to reduce fire hazard and minimize large tree mortality in the residual stand.

Mortality from “first-order” fire effects (i.e. direct fire effects) occurs at the time of fire or immediately afterward (Reinhardt et al., 2001), and can result from crown scorch, cambial damage, and root damage (Reinhardt et al., 2001, Kobziar et al., 2006). Such effects are often manipulated and/or mitigated depending on the desired outcome of the prescribed burn management plan. However, mortality from “second order” fire effects (i.e. indirect fire effects) such as mortality from bark beetles, can be problematic as the manifestations of these effects occur over a longer period of time (Reinhardt et al., 2001). Since bark beetles are attracted to previously stressed and weakened trees (Furniss and Carolin, 1977), any management option, whether it be prescribed fire, mechanical thinning, or fire suppression, has the potential to increase bark beetle activity and subsequent mortality (Wood et al., 1985, Fettig et al., 2007, Jenkins et al., 2008, Youngblood et al., 2009).

The objective of our study was to assess this subsequent bark beetle caused mortality from prescribed fire, mechanical harvesting (crown thinning followed by thinning-from-below), and a combination of these two treatments, as part of the Fire and Fire-Surrogate Study (FFS) (McIver et al., 2009) in the central Sierra Nevada. First order fire effects from this study site are described elsewhere (Kobziar et al., 2006, Stephens and Moghaddas, 2005).