Timber Harvest Influences on Hydrology and Water Quality of

PNW Forested Wetlands

Possibly the world’s shortest talk, if we base it on known facts.

C. Rhett Jackson, Assistant Professor of Hydrology

Warnell School of Forest Resources

University of Georgia, Athens, GA

How can timber harvest affect wetland water quality?

• Increased solar insolation.*

• Decreased ET - higher water tables.*

• Nutrient release from clearcut areas.*

• Fertilizer runoff.*

• Fine sediment from logging roads.*

• Rutting from yarding within the wetland.*

• Slash deposition from harvest within wetland.

* All previously observed, to some degree, in studies from the eastern U.S.

Till

Water TableHydric Soils

A Forested Wetland (or bog),

small, semi-isolated, on till

Characteristics:

Very shady below canopy, low primary productivity in understory and forest floor, seasonal surface saturation.

What critters live here?

Conceptual: Not to Scale.

Increased solar insolation, local decrease in ET, small water table rise

Shrub and herbaceous vegetation response,

Possible rutting of hydric soils depending on yarding methods,

Possible heavy deposition of logging slash (limbs and tops).

From: Batzer, Jackson, and Mosner. 2000. Hydrobiologia 441:123-132.

Study conducted in Georgia Coastal Plain depressional wetlands.

Increased solar radiation, decreased ET in GW contributing area, water table rise, possible nitrogen release. Sediment? Fertilizer?

Higher water tables, solar insolation, and nutrient release from uplands spur vegetation conversion. Shrub and herbaceous vegetation response, possible die-off of wetland trees. Sediment inputs are only likely from road surfaces.

Scary scenario: Habitat becomes suitable for bullfrogs - eat native amphibs.

Hydrologic Setting

• Actual evapotranspiration (AET) from forests in Western WA is about 20 inches/year.

• Rainfall in the lowlands varies from 40 to 90 inches per year - So, 20 to 70 inches of rainfall enters groundwater and streams.

• Overall, hydrologic impacts should be less than observed in eastern United States.

In what types of wetlands would hydroperiod changes be most

pronounced?• Isolated, groundwater-fed wetlands on flat

topography with small groundwater contributing areas and with relatively low annual rainfall (40 - 55 inches/year).

• Isolated wetlands with small contributing areas into which a large area of road runoff is delivered.

What types of wetlands are least sensitive to hydrologic effects of timber

harvest?• Wetlands with contributing areas that are large

relative to clearcut areas (a big clearcuts covers 100 - 200 acres).

• Valley floor wetlands. Examples: flow-through wetlands on third-order streams or larger; floodplain wetlands on third-order streams or larger; beaver ponds.

• Wetlands on small benches in steep topography.

How much hydroperiod change is too much?

The analogous question for streamflows in forested basins has been difficult to answer.

The hydrologic changes associated with urbanization, known to harm wetlands, are much larger.

Need to frame this question in terms of life histories of key species.

Duration of Water Quality Changes

• Hydroperiod effects - about 7 years.

• Nutrient release - a few years (1- 5).

• Solar insolation - fifteen to twenty years.

• Rutting - semi-permanent.

• Compare to rotation length of 35 to 60 years.

A Related Issue - Warming of Shallow Groundwater

From Jenelle S.D. Black, U.W. MS Thesis, 2001

Issue: Will clearcutting an area of shallow groundwater flow to a headwater stream cause unacceptable water temperature increases?

This was a big issue in the Dickey River watershed analysis.

Issue is the subject of a groundwater study for the Bull trout group.

My opinion:

• Changes in solar insolation and nutrient release from clearcut areas are likely to drive more significant wetland changes than is hydrologic change.

• For many wetlands in the PNW, hydrologic effects of harvest on wetlands will be small compared to inter-annual climate variability.

My recommendation:

• Basic, descriptive, inter-disciplinary pre-/post-harvest wetland studies using a block design. This has worked decently in some eastern studies.

• Hope for “normal” climatic conditions during the study.

• Need a botanist, herpetologist, wetland entomologist, and a water chemistry person.

Thanks for the opportunity to speak with you.

Contact Info:

Rhett Jackson

Warnell School of Forest Resources

University of Georgia, Athens, GA 30602-2152

rjackson@forestry.uga.edu

(706) 542-1772