Influences of Thinning and Underburning Restoration Practices on Red Turpentine Beetle Demography in Jeffrey Pine

Abstract

Forest thinnings implemented with cut-to-length and whole-tree harvesting systems followed by prescribed underburning were evaluated for their effects on red turpentine beetle (Dendroctonus valens LeConte) colonization in pure, uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.). Based on pitch tube abundance expressed as bole surface area-based counts, this beetle was significantly more prevalent where thinnings were implemented regardless of harvesting approach than in the unthinned treatment, but the population increases were confined to the burned stand portions. Substantial evidence indicated that fire-induced crown and bole damage culminating in compromised defense mechanisms elevated turpentine beetle colonization, and some evidence suggested a proclivity of this beetle to attack stems exhibiting general characteristics of low vigor, specifically smaller trees with weak crowns. Tree mortality was likely induced by the turpentine beetle infestation but was probably not its sole cause. The responses presented herein to these thinning and burning practices, which are being increasingly relied upon in forest restoration efforts in the western USA, provide natural resource managers insight into potentially compromised forest health outcomes over the intermediate term when implemented in Jeffrey pine and similar dry site forest types.

KEYWORDS:

• bark beetles
• stand density management
• cut-to-length harvesting
• whole-tree harvesting
• prescription fire
• Pinus jeffreyi
• Dendroctonus valens