Introduction

A variety of risk assessment tools have been designed to help managers quantify expected losses from bark beetles, losses which can be quite severe. A framework to help managers choose the tools that are most appropriate to their needs is presented in this section.

Several bark beetle species, mostly in the family Curculionidae, subfamily Scolytinae, have the potential for dramatic population increases under favorable forest and climate conditions, which can result in landscape-scale mortality to the host tree species, (e.g., Wood and Unger 1996). For example, the mountain pine beetle (MPB, Dendroctonus ponderosae Hopk.) has killed much of the mature lodgepole pine (Pinus contorta Dougl. ex. Loud.) over an area of approximately 9 million hectares (ha) in British Columbia in recent years (Westfall 2005), and the spruce beetle (Dendroctonus rufipennis) has infested several hundred thousand ha of spruce forest in southwestern Yukon, Canada (R.Garbutt, pers. communication). These events have widespread implications for current and future forest management, ranging from effects on timber supply and operations, wildfire-urban interface, wildlife habitat, and aesthetics.

Landscape-scale risk assessment of bark beetle infestation aims to quantify the spatial and temporal likelihood of attack extent and severity. Methods to assess risk can range from structural risk, (i.e., strictly assessing patterns) to functional risk; (i.e., assessing interactions and feedbacks between pattern and process). Susceptibility and risk rating systems (Bentz and others 1993) generally classify stands without a temporal dimension in a relatively simply spatial context and are useful for a quick overview of landscape state and general patterns. Landscape connectivity methods (O’Brien and others 2006) can join stands of higher susceptibility into a network that can help give an integrated landscape perspective but are still temporally static. Connectivity assessments have been useful in areas with limited current attack to provide an assessment of the spatial pattern of hosts and likely pathways of attack. Empirical projection models explicitly model outbreak dynamics based on historical temporal patterns and have been useful to assess very broad-scale dynamics and potential interactions with management (Eng and others 2005). Population models capture system dynamics and feedback between host patterns and beetle demographics in detail (Dunning and others 1995) and can help to gain insight into likely trends of outbreak development and to explore management alternatives in relatively fine detail.

These methods, and others in the literature, share a common theme: they aim to quantify expected levels of attack and loss due to beetles. Although the way in which risk is quantified may differ among methods, risk can always be cast in terms of a probability distribution that represents likelihood of losses or impacts. This provides a common frame of reference with which methods can be compared on a gradient from simpler, structure-based methods to more complex, process-based methods, allowing methods to be assessed in terms of trade-offs between data required, difficulty of application, and precision of results. To demonstrate this framework, a diverse suite of tools useful to assess risk of bark beetle attack at broad spatial scales is examined. For each, there is an overview of the method as applied to bark beetle infestation risk, with a focus on data requirements and outputs. The pros and cons are highlighted and the types of management questions that can be addressed are outlined and an example of management application is presented.