Climatic variability, land-cover change, and forest hydrology in the Pacific Northwest

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Climatic variability, land-cover change, and
forest hydrology in the Pacific Northwest

• David W. Peterson
• JISAO Climate Impacts Group
• Forest Hydrology

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Current ResearchClimate Water Forest
Interactions
Climate
Water
Forests
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Water and ecosystem productivity
Annual Outflow
Gross photosynthesis
Numeric Terradynamics Simulation Group University
of Montana
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Forest dependence on water resources in the PNW

• Trees require water for photosynthesis and growth
• Summer water deficits limit forest productivity
• Total precipitation
• Seasonality of precipitation
• At high elevations, excessive snowpack limits
  growing season length and forest productivity
• Water availability influences forest species
  composition and density

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Forest influences on water resources

• Forests influence water resources by
• Intercepting precipitation
• Influencing infiltration and runoff rates
• Modifying soil water-holding capacity
• Extracting soil water to meet transpiration
  requirements
• Influencing snowpack accumulation and melt rates

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Water and ecosystem productivity
Annual Outflow
Gross photosynthesis
Numeric Terradynamics Simulation Group University
of Montana
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Land-cover changes

• Disturbances (variable intensity)
• Forest harvesting
• Wildfires
• Insects and diseases
• Land conversions
• Forest growth and development
• Forest structure
• Leaf area
• Species composition
• Climate change

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Climatic variability and change

• Annual climatic variability and ENSO
• Affect forest stress levels
• Extreme events can disturb forests
• Decadal climatic variability (e.g., PDO)
• Forests may acclimate to new conditions
• Problems switching from wet to dry?
• Atmospheric carbon dioxide concentrations
• Will higher CO2 levels reduce forest water use?
• Climate change temperature/precipitation

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Simulation model approach

• Identify likely hydrological impacts of
• Vegetation changes
• Climatic variability and change
• Soils and topography
• Integrate results across the region

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Hydrological responses

• Snowpack water storage and snowmelt
• Soil water storage
• Infiltration versus runoff
• Summer water deficits severity and duration of
  plant stress
• Transpiration and evaporation
• Runoff and baseflow

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Forest cover changes due to forest harvest and
regrowth
Brush cover (post-harvest)
Mature forest (50-60 yr)
Mica Potlach Watershed, Idaho, 1933-1990
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Forest cover and snowpack
As forest cover increases
spring snowpack declines by up to 20
March 1
May 1
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Forest density affects water use and soil water
storage
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Forest density affects surface runoff and baseflow
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Questions to be addressed

• How will future climate and vegetation changes
  affect surface water fluxes and soil water
  storage in PNW forests?
• Which has a bigger impact on forest hydrology
  climatic variability or vegetation change?
• What are the hydrological implications of changes
  in forest management practices?
• How might future changes in water resources
  affect forest productivity and species
  composition?

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Presentation Outline

• Forest water interactions
• Forest dependence on water resources
• Forest influences on water resources
• Effects of land-cover change and climate on water
  cycling in PNW forest ecosystems
• Types and effects of land-cover changes
• Effects of climatic variability
• Interactions between climate and forest cover