How does habitat restoration influence resilience of salmon populations to climate change?

A pressing question for managing recovery of depressed or declining species is: Can habitat restoration increase resilience to climate change? We addressed this question for salmon populations with varying life histories, where resilience is defined as maintaining or increasing population size despite climate change effects. Previous studies indicate that several interrelated mechanisms may influence salmon resilience to climate change, including improving either habitat capacity or productivity, and ameliorating climate change effects on flood flow, low flow, or stream temperature. Using the Habitat Assessment and Restoration Planning (HARP) model, we first examined the relative importance of each mechanism for increasing salmon population resilience by comparing projected salmon spawner abundance for seven individual restoration action types under current and projected mid‐ and late‐century climates. We found that restoring habitats with the greatest restoration potential most increased resilience for all species, but the most beneficial restoration actions varied among species. Increasing habitat capacity and productivity both contributed to resilience, and ameliorating climate change effects was important in a few subbasins where the restoration opportunity was widespread. Cool‐water climate refuges contributed to resilience of some subpopulations by reducing late‐century declines in spawner abundance even without restoration. We also modeled more complex habitat restoration strategies comprised of several restoration action types at varying restoration intensities and found that the restoration action types and level of restoration effort needed to increase resilience varied among species. Less vulnerable species such as coho salmon responded well to four restoration actions (floodplain reconnection, wood augmentation, increased shade, and increased beaver ponds) applied at low restoration intensity and over a large area. More vulnerable species such as spring Chinook responded to fewer action types (floodplain reconnection, wood augmentation, and increased shade), but at much higher intensity and over a much smaller area. The analysis also identified important locations for each restoration action type for each species, which helps focus habitat restoration effort on areas that are likely to provide the largest increases in resilience.