Quantifying the hydrologic impacts of mountain pine beetle and salvage harvest in the Fraser River drainage, British Columbia, Canada

Due to an abundance of mature pine and a series of warmer than normal winters, the province of British Columbia (BC), Canada is experiencing a mountain pine beetle (MPB) outbreak of unprecedented severity, possibly becoming the largest outbreak ever recorded in North America. In just over a decade, widespread mortality of pine trees has occurred in over 14.5 million hectares of forest (an area roughly twice the size of Ireland) and the outbreak continues to kill mature pine in the province. Although the beetle attacks all pine species native to BC forests, lodgepole pine is the most abundant species by area and the most predominant commercial species. Consequently, an aggressive program of salvage harvesting has been initiated and has resulted in elevated cut rates in many affected areas, with many salvage operations creating large (>1000 ha) forest openings. The epicenter of the current outbreak is in the Fraser River Basin (230 000 km²), where over 8 million hectares of forest, approximately 35% of the drainage area, have been affected. Due to the massive size of the affected area, potential exists for widespread hydrologic changes throughout the basin.

In the snow-dominated areas of the BC interior the loss of forest cover due to pine death and salvage harvesting has been observed at the stand scale to result in higher snow accumulation over the winter and higher melt rates during the spring. This effect, combined with a loss of transpiration (i.e., dead trees do not extract moisture from the soil), generally results in more water available for local runoff. At larger scales these changes can potentially lead to increases in freshet (defined as April-May-June period) runoff and changes in the annual maximum peak-discharge regime. Consequently, our investigation seeks to answer the following questions:

•          What is the impact of beetle-kill and salvage harvesting upon streamflow (freshet and annual peak flow) in the Fraser River basin?

•          What, if any, is difference in hydrologic impacts between beetle-kill and salvage harvest?

•          How does hydrologic response to forest disturbance in the Fraser River basin vary by location (e.g. physical properties) and scale?

The size and physiographic heterogeneity of the Fraser River basin precludes both direct observation and extrapolation of hydrologic impacts observed from a limited number of stand-level and small-basin experiments. As a result, the Variable Infiltration Capacity (VIC) hydrology model has been used to assess hydrologic sensitivity of the Fraser River basin to the effects of MPB and projected effects of varying levels of salvage harvest. The VIC model is a spatially-distributed macro-scale model that has been applied at a resolution of 1/16-degree (approximately 27-32 km², depending upon latitude) and used to quantify streamflow impacts for 60 nested sub-basins in the Fraser ranging in area from 330 to 217,000 km². The local and regional sensitivity of streamflow to MPB and harvest disturbance has been assessed using a set of seven scenarios. These include a pre-infestation baseline (forest cover and MPB disturbance c. 1995), a current (forest cover and MPB disturbance c. 2007) forest cover and five hypothetical scenarios of increasing disturbance severity (from baseline) ranging progressively from 100% kill of mature pine plus 0%, 25%, 50%, 75%, and 100% harvest (by area) of killed pine. As the hypothetical scenarios only consider harvesting of beetle-killed pine, the 100% harvest scenario do not necessarily imply the complete removal of forest cover in any region, particularly in areas of mixed forest cover or absent pine forest cover. All scenarios are forced with meteorological boundary conditions interpolated from station data collected during 1915 to 2006.

The simulation results indicate that for a given sub-basin: (a) forest disturbance tends to increase freshet and peak-flow magnitudes, with relative change in magnitude increasing with disturbance severity, and (b) peak flow shows more sensitivity to cumulative effects of beetle-kill and salvage harvesting than to beetle-kill alone. Also, the sub-basins exhibit high spatial variability in their sensitivity to forest disturbance. For instance, relative changes (from baseline) in the magnitude of the 20-year peak flow event range over the sub-basins from no change to a maximum change of (a) 8% for current forest cover, (b) 8% for a scenario of 100% beetle-kill (0% harvesting), (c) 46% for 25% harvesting, (d) 91% for 50% harvesting, (e) 130% for 75% harvesting, and (f) 172% for 100% harvesting of dead pine. Results indicate that basin sensitivity to MPB-related disturbance is affected by local hydro-climatology, the extent and type of original forest cover and the relative area of disturbance. At locations where streamflow is predominantly derived from snowmelt runoff in the high snowfields of the Coast, Columbia and Rocky Mountains, upstream forest disturbance has little impact. The greatest sensitivity to infestation-induced forest disturbance is exhibited by sub-basins located in areas that are characterized by pine-dominated forest cover (i.e., potentially high-disturbance areas) and low topographic relief (i.e., no significant regions of sub-alpine or alpine runoff). In these areas, where sensitivity is high, peak-flow changes can be substantial. The physical and topological properties of the Fraser River basin are such that the level of sensitivity to forest disturbance tends to decrease with increasing drainage area.