10 - Seeing the forest for the trees: forest health monitoring

Summary

Introduction

A recent definition of forest health states that it is dependent on sustainability, productivity, and pest management (Raffa et al. 2009), which is similar to the central premise of this text (see Chapter 1). We suggest that one way to assess sustainability, as the first component of a healthy forest, is to determine if observed landscape-level tree mortality corresponds to baseline mortality (i.e., a stable size structure is maintained so that the number of trees dying within a size class does not exceed the number necessary to replace those in the next larger size class). Meanwhile, productivity, the second component of a healthy forest, involves meeting the management objectives of the landowner.

An understanding of the evolutionary history of the forest and all associated forest processes and components; e.g., fire, climate, insects, disease, etc., is critical when considering the spatial scale at which forest health is being assessed. For example, in the western USA and in Canadian lodgepole pine forests, the baseline mortality concept would need to be applied at the level of tens of thousands or hundreds of thousands of square kilometers. These forests experience repeated long-term cycles whereby forests become susceptible to the mountain pine beetle, die as a cohort, and burn so that seeds may germinate and the forest grow again (Peterman 1978; Berryman 1986). The conflagration that follows a mortality event occurs at large spatial scales and though forests can experience up to 100% mortality of all vegetation layers, they would still be considered “healthy” as this would be an essential renewal stage.