Quantitative and qualitative approaches to assess tree vigor and stand health in dry pine forests

Abstract

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.