The effects of thinning treatments on growth and survivorship of white spruce (Picea glauca [Moench] Voss) plantations affected by recent eastern spruce budworm (SBW) outbreaks were examined over a 5-year period in northern Minnesota. Thinning treatments increased individual tree growth, live crown ratios (LCRs), and survival relative to unthinned stands. Overall, stands affected by SBW had lower rates of volume production than unaffected stands. In addition, individual tree volume growth was greater in thinned SBW-affected stands relative to unthinned SBW-affected stands. Across stand conditions, individual tree postthinning volume growth response was best predicted by the interaction of prethinning LCR and postthinning relative density (RD). In particular, at low stocking levels (RD = 0.20) higher live crown values resulted in the highest volume growth ratios. On the other hand, at higher stocking levels (RD 0.40–0.55) volume growth was fairly consistent, regardless of LCRs. Across all stocking levels, a minimum LCR of 40% appears to ensure high tree and stand growth rates and is also an indicator of a tree's ability to respond positively to thinning. This plasticity of white spruce suggests that stands maintained at these crown target levels can achieve high levels of stand and individual tree productivity as long as appropriate LCRs are maintained.
Silvicultural strategies such as thinning may minimize productivity losses from a variety of forest disturbances, including forest insects. This study analyzed the 10-year postthinning response of stands and individual trees in thinned white spruce (Picea glauca [Moench] Voss) plantations in northern Minnesota, USA, with light to moderate defoliation from eastern spruce budworm (Choristoneura fumiferana Clemens). Using the Forest Vegetation Simulator, model results suggested overprediction of stand basal area growth and tree diameter increment in these stands. Growth modifiers indicated that trees growing in unthinned stands and with greater defoliation levels (i.e., 20-32%) would need the largest adjustment for diameter increment. Modifiers for height were similarly specified to compensate for the underprediction of height increment in these stands. Thinned stands continued to maintain target live crown ratios in excess of 0.40, suggesting long-term productivity. Results highlight the need for simulation models that represent appropriate responses to stands and trees affected by forest insects and diseases. Ultimately, accurate representations of growth and development in these models that account for influences of biotic disturbance agents are essential under future global change scenarios, particularly as silvicultural strategies are implemented to reduce the impacts of forest health threats and other stressors.
1 The jack pine budworm Choristoneura pinus pinus is a defoliator distributed throughout the range of jack pine Pinus banksiana in North America. Jack pine budworm feeds primarily on jack pine. More than 2400 hectares of defoliation of red pine Pinus resinosa were noted during an outbreak of C. pinus pinus from 2004-2008 in Minnesota and Wisconsin, U.S.A. 2 Defoliation of red pine across thousands of hectares, detectable during aerial surveys, is a significant deviation from historical records. 3 Fifty-six study plots were established across 15 counties of Minnesota and Wisconsin to quantify densities and impacts of jack pine budworm in locations showing defoliation of both red pine and jack pine. 4 Populations of jack pine budworm on red pine behaved similarly to those on jack pine, consuming pollen cones and vegetative shoots. The mean number of egg masses per branch and per site was greater on red pine than on jack pine at study sites in Minnesota and Wisconsin, respectively. 5 It is not known what led to the outbreak of jack pine budworm on red pine, nor is it clear whether landscape-scale defoliation of this magnitude represents an isolated incident or a shift in relative host use. Hypotheses regarding the mechanisms behind this alternative host use by jack pine budworm are discussed.
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