Hydrologic sensitivity of watersheds to MPB infestation in the B.C. Interior

Hydrology, Maps, and Geographic Data for Watersheds Affected By Mountain Pine Beetle in the Interior of British Columbia

As the mountain pine beetle (MPB) spreads through the interior of British Columbia, the extent of lodgepole pine mortality and salvage logging will be significant in many watersheds.  Stand mortality, salvage logging, and road construction each have the potential to affect hydrologic processes.  The following text provides an introduction to the hydrologic consequences of both the MPB infestation and salvage operations, sources of information, and links to a series of maps and tables indicating the extent of lodgepole pine dominated forest cover types in medium to large watersheds throughout the interior of British Columbia.

Hydrologic Change Expected with Stand Mortality and Salvage Logging

A reduction in forest canopy can result in:

• increased water reaching, stored in, and flowing from hillslopes,
• earlier onset of spring snowmelt,
• increased spring and total annual streamflow volumes,
• changes in summer and fall flows, and
• more rapid streamflow response to storms.

The magnitude of hydrologic change will depend on:

• the severity and time since attack,
• presence, density and extent of understory vegetation,
• the extent of salvage logging within stands and across a watershed,
• the occurrence of fire,
• the physical characteristics of the watershed, and
• the weather.

If attacked stands are left to deteriorate naturally, hydrologic change will be gradual as trees turn from green to red, drop their needles, lose fine branches, and eventually fall to the ground.  At the same time, understory vegetation will be releasing due to increased light levels and reduced competition for nutrients.  In contrast, salvage logging can result in an immediate change to a hydrologic system.  For example, logging will physically remove the dead standing pine, and therefore eliminate any role they might provide through shade or canopy interception of precipitation.  In addition, logging can affect the hydrologic system through:

• removal of non-pine species and understory vegetation including coniferous regeneration, shrubs, and herbs,
• changing the amount and distribution of fine and coarse woody debris,
• site preparation, and
• the conversion of subsurface to surface runoff along roads and skid trails.

The duration of hydrologic change associated with MPB will depend on:

• whether the entire overstory has died or been logged,
• the amount of ground disturbance,
• the presence, age, and density of advance regeneration, and
• the regeneration delay.

At the stand scale, the initial hydrologic effects of logging are expected to be greater than the initial effects of beetle kill alone.  However, with prompt reforestation it is possible that the duration of hydrologic change will be reduced in regenerating clearcuts relative to that in un-salvaged pine stands.  These differences have not been well-quantified and are the subject of current research, as are the effects of MPB at the larger watershed scale.

Water Management Issues

Hydrologic changes that can result from the MPB and salvage harvesting include:

• increased peak flows and water yield,
• increased surface erosion,
• damage to forest road surfaces, cuts and fills, and drainage structures,
• channel destabilization,
• loss of fish habitat
• increased landslide activity,
• elevated water tables,
• loss of soil and site productivity, and
• loss of water quality.

Watershed Planning Recommendations

To mitigate the effects of the MPB infestation on water and watersheds, the following recommendations should be considered by forest resource planners where practicable:

• salvage log in stages by using a variety of cutting intensities and retention strategies, distributed over the landscape, to desynchronize runoff,
• avoid sensitive terrain and soil types and develop erosion control plans where necessary,
• minimize harvesting within riparian areas,
• maintain a diversity of cover types and minimize post-salvage reforestation delays through single tree or patch retention to protect advanced regeneration as well as non-coniferous forest vegetation,
• leave fine and coarse woody debris in openings where possible to delay surface runoff,
• develop watershed management strategies that will maintain other forest resource values,
• construct, inspect, and maintain roads to ensure natural surface and shallow subsurface drainage remain in tact,
• upgrade drainage networks on permanent roads prior to salvage logging as necessary to accommodate expected increases in peak flows, and
• increase inter-agency and public communication regarding the issues, hazards, and risks in watersheds affected by MPB beginning with the identification of watersheds where downstream risks to public safety and infrastructure could be increased.

Research initiatives

Research projects, both field and modeling, address questions about the effects of MPB-related stand mortality, salvage logging, partial retention, and regeneration effects on snow accumulation and melt, stand water balances, streamflow, channel stability, water quality, and aquatic habitat.

• MPB and Hydrology Research Projects List

Sources of information

Summaries of post-MPB hydrology:

• Forest Stewardship for Mountain Pine Beetle Salvage
• Review and Synthesis of Potential Hydrologic Impacts of Mountain Pine Beetle and Related Harvesting Activities in British Columbia
• Streamline Watershed Management Bulletin
• BC Journal of Ecosystems and Management
• The Kamloops Forest Hydrology Abstracts Library abstracts of hydrology papers describing the effects of forest disturbance on water quantity and quality providing insight into the potential changes expected with MPB, at:

Maps

A series of overview maps showing the extent of pine dominated forest cover in the Northern and Southern Interior Forest Regions were produced as part of an inter-agency flood hazard mitigation initiative led by the Provincial Emergency Program.  The objectives of this initiative are:

• “to coordinate government response to flood consequences resulting from beetle infestation, and
• to reduce risk to communities and provincial infrastructure from infestation related hazards.”

One of the key tasks set out by the Assistant Deputy Ministers of the Ministry of Environment, Transportation, and Forests and Range was to develop a risk assessment framework including hazard identification.

The maps show all 3^rd order and higher watershed boundaries, communities, and public infrastructure, the extent of forest consisting of more than 40% lodgepole pine, and areas logged over the past 25 years within a Forest District.  Data are not currently available for the following areas:

• areas logged and affected by recent wildfires from 2003 on TFLs and some parks
• Peace
• Fort Nelson
• Skeena-Stikine
• Nadina - east of Tweedsmuir Park
• Quesnel - west and east tips
• Central Cariboo – south and central areas
• Chilcotin - large part of southern half as well as northern tip
• 100 Mile house - west
• Headwaters – north-east
• Kamloops - south
• Rocky Mountain – north-west tip
• Arrow Boundary - centre and north

Summary tables listing all third and higher order watersheds, their area, the area of pine dominated forest, and the area logged are also provided for each District.  The maps and tables provide a useful indication of the extent of both lodgepole pine leading forest types and past disturbance in most watersheds throughout the interior of B.C.  They will help to focus attention on areas where significant stand mortality is expected.  These maps should be used in conjunction with flood risk maps to identify areas that have a higher level of flood risk in the post-MPB period.

Local Contacts

Contacts for additional forest hydrology information:

Contacts for map related questions: