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Forests 2018, 9(9), 536; https://doi.org/10.3390/f9090536

Severity of Overstory Mortality Influences Conifer Recruitment and Growth in Mountain Pine Beetle-Affected Forests

1
Rocky Mountain Research Station, USDA Forest Service, Fort Collins, CO 80526, USA
2
Department of Forest and Rangeland Stewardship, Colorado State University (Retired), Fort Collins, CO 80523, USA
*
Author to whom correspondence should be addressed.
Received: 14 August 2018 / Revised: 28 August 2018 / Accepted: 30 August 2018 / Published: 1 September 2018
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Abstract

The severity of lodgepole pine mortality from mountain pine beetle outbreaks varies with host tree diameter, density, and other structural characteristics, influencing subcanopy conditions and tree regeneration. We measured density and leader growth of shade-intolerant lodgepole pine, shade-tolerant Engelmann spruce, and very shade-tolerant subalpine fir regeneration beneath stands that experienced moderate and high overstory lodgepole pine mortality (average 40% and 85% of total basal area) a decade earlier. Lodgepole comprised >90% of the overstory basal area and mature spruce and fir were present in both mortality levels, though live basal area and disturbance history differed. Post-beetle outbreak recruitment was high in both mortality levels, but there were more lodgepole in high than moderate mortality plots (1140 stems ha−1 vs. 60 stems ha−1) and more subalpine fir in moderate than high mortality plots (4690 stems ha−1 vs. 2870 stems ha−1). Pine advance regeneration, established prior to outbreak, was more dense in high mortality than moderate mortality sites (930 stems ha−1 vs. 310 stems ha−1), but the trend was generally the opposite for the other conifers. Lodgepole recruitment increased and subalpine fir decreased with greater forest floor light availability. All species grew faster in high mortality areas than their counterparts in moderate mortality areas. However, in high mortality areas pine grew faster than the more shade tolerant species, and in moderate mortality areas spruce and fir grew faster than pine. These species-specific responses to the degree of overstory mortality will influence future stand composition and rate of forest recovery after mountain pine beetle outbreaks. View Full-Text
Keywords: mountain pine beetle; bark beetles; lodgepole pine; Engelmann spruce; subalpine fir; Colorado; Rocky Mountains mountain pine beetle; bark beetles; lodgepole pine; Engelmann spruce; subalpine fir; Colorado; Rocky Mountains
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Pelz, K.A.; Rhoades, C.C.; Hubbard, R.M.; Smith, F.W. Severity of Overstory Mortality Influences Conifer Recruitment and Growth in Mountain Pine Beetle-Affected Forests. Forests 2018, 9, 536.

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