Despite a critical need to evaluate effectiveness of forest treatments in improving stand health, practitioners lack quantitative, repeatable metrics to assess tree vigor and stand health. We evaluated canopy and whole tree attributes of ponderosa pine (Pinus ponderosa Dougl. Ex Laws) related to carbon balance, water balance, and susceptibility to insects and pathogens in dry, pine-dominated forest stands during a multi-year drought, an environmental challenge to stand resilience. Metrics of trees in two unmanaged, and seven treated forested stands, in both uplands and lowlands to develop the quantitative approach. Whole tree and crown attributes including needle length and color, branchlet length and diameter, needle retention (needle ages and retention within ages), and frequency of insects, fungi, and abiotic needle damage were statistically selected to assess tree vigor. Cluster analysis of vigor attributes revealed that trees responded or persisted independently within a forest treatment; forest treatments did not necessarily yield similar tree responses within a stand. A rapid, qualitative assessment was developed to rank trees as low, average, and above-average vigor. To demonstrate an application of our approach, trees were ranked annually over six years in most stands, as well as in a stand where the prescription was adjusted due to the evaluation. The proportion of trees in the three tree vigor ranks differed, suggesting differing levels of stand health. Quantitative metrics and qualitative ranking of tree vigor could assist in selecting trees to be retained to meet specific management objectives, to evaluate treatment implementation, and to monitor post-treatment changes in stand health.

Citation: Hrinkevich, K., and K. J. Lewis. 2011. Northern range limit mountain pine beetle outbreak dynamics in mixed sub-boreal pine forests of British Columbia. Ecosphere 2(10):116. doi:10.1890/ES11-00150.1Abstract. Recent widespread mountain pine beetle (Dendroctonus ponderosae) (MPB) outbreaks in western North America have caused unprecedented tree mortality across much of the beetle's known historic range and into novel habitats near the range margins. Our current understanding of outbreak dynamics and their relationship to forest structure is based almost wholly on research from climatically optimal habitats where infestations have historically been most severe. As range expansion continues marginal habitats may become increasingly susceptible to outbreaks, yet little is known about the factors driving outbreak development in these forest types or how the disturbance regime differs. This study uses dendroecology and forest structure analysis to reconstruct historic outbreak dynamics at the northern MPB distribution limit in British Columbia to extend the disturbance record beyond the scope of documented history and to evaluate the long-term relationships between stand structure, host suitability, and regime characteristics at the range margin. We sampled 22 sites with evidence of past infestations, and using mortality dates, survivor growth releases, and tree vigor analyses we reconstructed five probable outbreak periods predating the current epidemic. At least two of these events fit the profile of large epidemics occurring in the 1900s-1910s and again in the 1940s-1950s, with evidence of two 19th century infestations and chronic localized mortality beginning around the 1980s. Stand age appears to be the most important factor limiting outbreaks prior to 1870, indicating a strong relationship with fire history at centennial time scales. Outbreak return intervals were comparable to those reported in other locations, yet beetle preference for larger and older trees suggests that provincial susceptibility models may not accurately represent northern outbreak risk, particularly in older stands. Following stand maturation, host availability does not appear to limit outbreak severity or recurrence. Here density-independent factors associated with additional climate pressures are likely more important. With enhanced climate suitability, outbreak scale and severity should increase dramatically in this landscape which is currently not limited by host distribution. Our reconstruction may inform management strategies as this occurs and outbreaks move into previously uninfested forests that share many structural and ecological similarities with our sites.

No abstract