Fire and fish dynamics in a changing climate
Wildland fire affects native fishes in the Rocky Mountain West by removing riparian vegetation which increases solar radiation to the stream and leads to warmer summer water temperatures. Many native fish, such as bull trout and cutthroat trout, evolved with fire, and their populations are resilient to fire's effects if habitat is otherwise unaffected or movement of fish from nearby, robust populations is ongoing. This resiliency, however, has been reduced in many watersheds because of stream habitat fragmentation and degradation, and the invasion of nonnative fishes such as brook trout and brown trout that tolerate warmer water temperatures. Climate change may further weaken this resiliency in two ways: by directly increasing water temperatures and by changing the influence of fire. Thus, forecasting the persistence of native trout undergoing climate change requires an understanding of fire dynamics - including size, distribution, frequency, severity, and the influence of fuels management - and the responses of nonnative fish species.
To evaluate fire and fish population dynamics, researchers at the Rocky Mountain Research Station, in collaboration with the University of Montana, are linking the landscape fire succession simulation model Fire-BGCv2 with a stream temperature model and trout growth models to predict changes in trout species distributions and productivity in the East Fork Bitterroot River basin in Montana. This basin has a history of large, stand-replacing fires, yet still supports native fish populations in many tributaries. Using model predictions, they will evaluate potential thresholds in fire risk, such as those related to the persistence of federally listed bull trout, and the scales at which to expect recovery under various climate and fire regimes.
To initiate their study, researchers first precisely modeled stream temperature based on a suite of climatic and geomorphological variables. They then linked the stream temperature model to a version of the Fire-BGCv2 model calibrated to the East Fork Bitterroot River basin and are presently running model simulations for the following scenarios: 1. Historical climate - with historical fire regime, and with fire exclusion2. Future warm/wet climate - with fire exclusion, and with fuels management3. Future hot/dry climate - with fire exclusion, and with fuels management
Each scenario will produce a time series on stream temperature and fire disturbance characteristics that can be used to estimate the amount and suitability of habitat for native and nonnative trout. The researchers' goals are to develop an understanding about those fire and landscape characteristics that pose higher risks for native trout species. For example, the researchers expect the probability of bull trout persistence to vary under each scenario as a function of increasing fire frequency, magnitude, and severity. If true, this will help identify those tributaries where it is best to focus bull trout conservation efforts. In addition, the researchers will evaluate which factors, such as fire severity, fire size, vegetation, or fuels, result in large-scale, long-term changes in fish communities. Finally, they will evaluate the effectiveness of fuels management, through its influence on fire dynamics, in promoting the persistence of native trout populations. For more information, please contact Lisa Holsinger at (406) 329-4810.
Forest Service Partners