Tree Mortality following Thinning and Prescribed Burning in Central Oregon, U.S.

Christopher J Fettig,Leif A Mortenson,Jackson P Audley

doi:10.3390/f12121677

Abstract

We examined causes and levels of tree mortality one year after thinning and prescribed burning was completed in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests at Pringle Falls Experimental Forest, Oregon, U.S. Four blocks of five experimental units (N = 20) were established. One of each of five treatments was assigned to each experimental unit in each block. Treatments included thinning from below to the upper management zone (UMZ) for the dominant plant association based on stand density index values for ponderosa pine followed by mastication and prescribed burning: (1) 50% UMZ (low density stand), (2) 75% UMZ (medium density stand), (3) 75% UMZ Gap, which involved a regeneration cut, (4) 100% UMZ (high density stand), and (5) an untreated control (high density stand). Experimental units were thinned in 2011 (block 4), 2012 (block 2), and 2013 (blocks 1 and 3); masticated within one year; and prescribed burned two years after thinning (2013–2015). A total of 395,053 trees was inventoried, of which 1.1% (4436) died. Significantly higher levels of tree mortality occurred on 100 UMZ (3.1%) than the untreated control (0.05%). Mortality was attributed to prescribed fire (2706), several species of bark beetles (Coleoptera: Curculionidae) (1592), unknown factors (136), windfall (1 tree), and western gall rust (1 tree). Among bark beetles, tree mortality was attributed to western pine beetle (Dendroctonus brevicomis LeConte) (881 trees), pine engraver (Ips pini (Say)) (385 trees), fir engraver (Scolytus ventralis LeConte) (304 trees), mountain pine beetle (D. ponderosae Hopkins) (20 trees), Ips emarginatus (LeConte) (1 tree), and Pityogenes spp. (1 tree).

Highlights

In a recent review, Achim et al [1] argue that the science of silviculture must adapt to the reality that many forest ecosystems have been altered significantly by human interventions and climate change
A total of 395,053 trees was inventoried across all experimental units, including ponderosa pine (300,417), grand fir (53,663), lodgepole pine (35,661), western white pine (3403), Engelmann spruce (Picea engelmannii Parry ex Engelm.) (834), western hemlock (585), Douglas fir (354), sugar pine (84), and white fir (52)
Tree mortality was attributed to prescribed fire (2706), several species of bark beetles (1592), unknown factors (136), windfall (1 tree) and western gall rust caused by the fungus

Summary

Introduction

Achim et al [1] argue that the science of silviculture must adapt to the reality that many forest ecosystems have been altered significantly by human interventions and climate change. As such, they question the utility of older studies to inform contemporary forest management, and highlight the need for innovative studies at appropriate scales where a range of silvicultural treatments are tested [1]. Management actions have focused on reducing fire-sensitive vegetation; diversifying tree age-class structure; and reducing surface, ladder, and canopy fuels [6]

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