The effect of mountain pine beetle attack and salvage harvesting on stream flows
The Mountain Pine Beetle (MPB) epidemic, and the large-scale salvage logging of beetle infested trees, are resulting in large areas of dead pine forest and clearcuts in watersheds across the central interior. These disturbances have potential effects on water yield (the total amount of water flowing out of a watershed in a year), peak flows (the highest stream flows of the year) and flood timing.
Forests regulate streamflow by four processes. First, the forest canopy intercepts a percentage of the snowfall and returns it to the atmosphere, reducing the amount of snow reaching the ground to become runoff. Secondly, the forest provides shade, reducing snowmelt rates. In addition the reduction in wind speed in a forested stand also reduces snow melt rates. Finally, trees transpire water during growth.
The effects of a MPB- attack on these processes are different from forest harvesting. The insect-killed trees remain in place, and have a residual canopy that can intercept a portion of the snowfall. Also, the mortality is never 100% and individual trees continue to intercept water. The standing dead trees also provide considerable shade, reducing radiation and snowmelt rates. Forest harvesting contributes to increased spring peak flows by removing the intercepting canopy and removing shade.
In order to determine the scale of these hydrological changes, peak streamflow magnitude, and timing and water yield were simulated using a computer model for Baker Creek, a 1570-square kilometre watershed tributary to the Fraser River at Quesnel, British Columbia.
Four scenarios were modeled:
- Baseline, prior to harvesting
- Conventional harvesting
- The MPB attack of 75% of the mature pine and past harvesting
- Salvage harvest of 80% of the watershed with 20% of the watershed retained in reserves
The model of Baker Creek showed MPB attack of 75% of the mature pine stands, plus the past conventional harvesting (scenario 3), resulted in annual peak flood increases of 60% and annual total water yield increases of 30%. Salvage harvesting of the dead pine (scenario 4) results in a further increase in annual peak streamflow compared to leaving the MPB-attacked trees standing. In Baker Creek, salvage harvesting of all pine-leading stands, (but retaining 20% of the watershed in reserves), increases annual peak flows by 92 %. Flood frequency also increases: a former 20-year interval peak flow discharge will now be expected every 3 years. These changes represent a major shift in stream flow regime.
The peak flow changes have implications on flooding, channel stability and fish habitat within watersheds similar to Baker Creek. These results also have salvage harvest management implications. The current MPB infestation has already created a substantial peak flow hazard. Any salvage harvesting of these stands will increase the peak flow hazard even more.
The FRPA legislation and the Cariboo-Chilcotin higher level plan (CCLUP) do not require landscape level watershed assessments or planning for most MPB-affected watersheds. Government needs to develop policy and strategies for protecting drinking water and fish habitat in the MPB- attacked watersheds.