Fire Effects on Forest Health in Loblolly Pine Forests

In most cases, few data are available to assess the relationship between fire and insect outbreaks in loblolly pine forests. However, there is some evidence to suggest that low-severity fires may increase resistance to some insects. The southern pine beetle (Dendroctonus frontalis) is a major insect pest of southern pines. The oleoresin-producing systems of loblolly and other pine species are the major line of defense against beetle attacks. The formation of radial resin ducts in loblolly pines is positively correlated with growth rates (Deangelis et al. 1986). High-density patches of pine are most susceptible to beetle attack, and prescribed fire can be used to eliminate smaller, high risk trees in these areas (McNab 1977). To the extent that thinning improves vigor of the remaining trees, it will likely increase the resistance to beetle attack. However, fire may increase susceptibility to pine beetles if it reduces the vigor of surviving trees through crown scorch or root damage (Conner et al. 1991). Beetle outbreaks can also influence the intensity of subsequent fires by producing heavy fuel loads (White et al. 1998). Pales weevils and pitch-eating weevils consume the cambium of young loblolly pines, and are mostly a problem in plantations. Burning clear-cut areas eliminates the residual slash in which weevils breed, and may reduce weevil buildup (Fox and Hill 1973).

Fire also influences the susceptibility of loblolly pine to some fungal pathogens. For example, Annosum root rot is a major fungal disease of southern pines. Fire may reduce susceptibility to this disease by removing the litter layer, which harbors most of the Annosum spores (Gooding et al. 1966, Froelich et al. 1978). However, burning has been shown to be less effective than other control and prevention methods, including the application of granular borax over fresh stumps and thinning during the summer month when few spores are released. Also, slow-moving backfires may actually increase susceptibility to root pathogens if they consume most of the duff layer and damage tree roots (Sullivan et al. 2003).