Full length articlePrescribed fire and mechanical thinning effects on bark beetle caused tree mortality in a mid-elevation Sierran mixed-conifer forest
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).
Section snippets
Study location
The study was conducted in a mixed conifer forest in the north-central Sierra Nevada at the University of California Blodgett Forest Research Station (BFRS), approximately 20 km east of Georgetown, California. Blodgett Forest is located at latitude 38°54′45″N, longitude 120°39′27″W, between 1100 and 1410 m above sea level, and encompasses an area of 1780 ha.
Tree species in the study include sugar pine (Pinus lambertiana Dougl.), ponderosa pine (Pinus ponderosa Laws), white fir (A. concolor Gord.
Bark beetle-caused mortality within sample periods
In the pretreatment forest (Table 2), bark beetles appeared to be at endemic levels since bark beetle-caused mortality was uniformly low or zero across all tree species and size categories, with no significant differences among the experimental units. In the POST1 sample period (Table 2), differences among treatments were significant for small sized white fir (F3,8 = 73.1, P < 0.0001), and for medium sized white fir (F3,8 = 15.8, P = 0.0010). For both small and medium white firs, each treatment that
Discussion
Our results indicate that both fire and mechanical treatments resulted in minimal bark beetle-caused mortality in the residual stand when bark beetle populations are at endemic levels (Table 2), at least in the short term (three years post-treatment, 2002–2005). Overall bark beetle-caused mortality across all treatments for all trees species was below 7%. These findings are comparable to other FFS studies in western forests carried out in the Southern Cascades (Fettig et al., 2010a), and in
Acknowledgements
This is publication No. 221 of the National Fire and Fire Surrogate Study funded by the US Joint Fire Science Program. We would like to especially thank the staff at the University of California Blodgett Forest Station for their dedicated cooperation and support of the Fire Surrogate Study. We thank Andrew J. Amacher for his statistical advice. Thanks also to the U.C. Berkeley FFS Research Team and all 2001–2005 summer field assistants.
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