1. Climatic variability, land-cover change, and forest hydrology in the Pacific Northwest David W. Peterson JISAO Climate Impacts Group Forest Hydrology

2. Current Research: Climate – Water – Forest Interactions Climate WaterForests

3. Water and ecosystem productivity Annual Outflow Gross photosynthesis Numeric Terradynamics Simulation Group University of Montana

4. 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

5. 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

6. Water and ecosystem productivity Annual Outflow Gross photosynthesis Numeric Terradynamics Simulation Group University of Montana

7. 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

8. 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 CO 2 levels reduce forest water use? Climate change – temperature/precipitation

9. Simulation model approach Identify likely hydrological impacts of: Vegetation changes Climatic variability and change Soils and topography Integrate results across the region

10. 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

11. Forest cover changes due to forest harvest and regrowth Brush cover (post-harvest) Mature forest (50-60 yr) Mica – Potlach Watershed, Idaho, 1933-1990

12. Forest cover and snowpack As forest cover increases… … spring snowpack declines by up to 20% March 1 May 1

13. Forest density affects water use and soil water storage

14. Forest density affects surface runoff and baseflow

15. 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?

16. 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