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With population growth, climate change, and increasing forest disturbance, understanding the complex relationships between forests and water is key to sustaining future forest resources, aquatic habitat, and water supplies. Research into forest and water interactions continues to expand our understanding of ecohydrological processes and our ability to assess the hazards associated with natural and human-related forest disturbances.

Outbreaks of mountain pine beetle are evaluated as a generic disturbance agent, and comparisons are made with other forest disturbances such as wildfire, windthrow, and logging. A useful basis for comparison is the degree of disruption to the overstorey, understorey, and forest floor layers. Clear differences are observed in the impacts of bark beetles, fire, and windthrow, but there is overlap with various harvesting systems. Insects are selective in terms of the species or size of tree that is killed; this selectivity varies with stand composition, stand structure, and outbreak stage. The mountain pine beetle functions as part of larger natural disturbance regimes in western North America, which vary with climate and forest type. Outbreaks of many different insects occur throughout western Canada, with the relative role of fire and insects differing among ecoregions and over time. Beetle-killed stands may facilitate extreme fire behaviour and may be more susceptible to future burning. Large expanses of dead or removed trees also result in altered soil water balance and stream flows, disposing some sites to mass movement or flooding. All disturbances generate heterogeneity, with much of the value to biodiversity and ecosystem recovery depending on residual structure and biological legacies. The capacity for unassisted recovery and the value of each stand to timber supply, carbon balance, and habitat needs in a landscape context are relevant when considering salvage logging or forest rehabilitation. The future role of forest pests is expected to fluctuate in response to changes in climate and the altered composition and structure of western forests.