Bark Beetle Responses to Stand Structure and Prescribed Fire at Blacks Mountain Experimental Forest, California, USA: 5-Year Data

Abstract

Highly effective fire suppression and selective harvesting of large-diameter, fire-tolerant tree species, such as ponderosa pine (Pinus ponderosa C. Lawson) and Jeffrey pine (P. jeffreyi Balf.), have resulted in substantial changes to the structure and composition of interior ponderosa pine forests. Mechanical thinning and the application of prescribed fire are now commonly used to reduce fuel loads and restore late-seral conditions in interior ponderosa pine forests, but the propensity for some bark beetles (Coleoptera: Curculionidae, Scolytinae) to attack fire-injured trees has led to questions regarding how management objectives may be impacted by levels of delayed tree mortality attributed to bark beetle attack. In this study, we examined bark beetle responses to creation of mid-seral (low structural diversity; LoD) and late-seral (high structural diversity; HiD) forest structures at Blacks Mountain Experimental Forest, California, USA, during a five-year period following the application of prescribed fire (B). Twelve experimental plots (LoD and HiD), ranging in size from 77 ha to 144 ha, were split with one half randomly assigned prescribed fire, resulting in four treatments (HiD, HiD + B, LoD, and LoD + B). A total of 6877 pine and fir (Abies) trees (4.0 % of all trees) were killed by bark beetles (all bark beetle species combined). Most of the tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) and mountain pine beetle (D. ponderosae Hopkins) in ponderosa pine, and fir engraver (Scolytus ventralis LeConte) in white fir (Abies concolor [Gord. & Glend.] Lindl. ex Hildebr.). The majority of bark beetle-caused tree mortality was recorded on HiD + B (41.5 % of trees that were killed by bark beetles) with 30.0 %, 23.5 %, and 5.0 % occurring on HiD, LoD + B, and LoD, respectively. Overall, a significantly higher percentage of bark beetle-caused tree mortality (of available trees) occurred on LoD + B (5.3 %) and HiD + B (4.8 %) compared to LoD (1.1 %). We observed no significant differences in levels of bark beetle-caused tree mortality (all bark beetle species combined) among diameter classes, but the level of western pine beetle-caused tree mortality was concentrated in the large tree component on burned split plots, which differs from results obtained two years after the application of prescribed fire. The amount of bark beetle-caused tree mortality increased substantially on unburned split plots compared to burned split plots following our initial survey (i.e., three years to five years after the application of prescribed fire). The implications of these and other results for the management and maintenance of late-seral conditions in interior ponderosa pine forests are discussed.