Thinning Jeffrey pine stands to reduce susceptibility to bark beetle infestations in California, U.S.A.

Abstract

Bark beetles (Coleoptera: Curculionidae, Scolytinae) are commonly recognized as important tree mortality agents in coniferous forests of the western U.S.A. High stand density is consistently associated with bark beetle infestations in western coniferous forests, and therefore thinning has long been advocated as a preventive measure to alleviate or reduce the amount of bark beetle‐caused tree mortality. The present study aimed to determine the effectiveness of thinning to reduce stand susceptibility to bark beetle infestations over a 10‐year period in Pinus jeffreyi forests on the Tahoe National Forest, California, U.S.A. Four treatments were replicated three times within 1‐ha square experimental plots. Treatments included thinning from below (i.e. initiating in the smallest diameter classes) to a residual target basal area (cross‐sectional area of trees at 1.37 m in height) of: (i) 18.4 m 2 /ha (low density thin); (ii) 27.6 m 2 /ha (medium density thin); (iii) 41.3 m 2 /ha (high density thin); and (iv) no stand manipulation (untreated control). Throughout the present study, 107 trees died as a result of bark beetle attacks. Of these, 71% (75 trees) were Abies concolor killed by Scolytus ventralis ; 20.6% (22 trees) were Pinus ponderosa killed by Dendroctonus ponderosae ; 4.7% (five trees) were P . jeffreyi killed by Dendroctonus jeffreyi ; 1.8% (two trees) were P . jeffreyi killed by Ips pini ; 0.9% (one tree) were P . jeffreyi killed by Orthotomicus (= Ips ) latidens ; 0.9% (one tree) were P . ponderosa killed by both Dendroctonus brevicomis and D . ponderosae ; and 0.9% (one tree) were P . jeffreyi killed by unknown causes. In the low density thin, no pines were killed by bark beetles during the 10‐year period. Significantly fewer trees (per ha/year) were killed in the low density thin than the high density thin or untreated control. No significant treatment effect was observed for the percentage of trees (per year) killed by bark beetles.