Transcript No Slide Title

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.
A Forested Wetland (or bog),
small, semi-isolated, on till
Hydric Soils
Water Table
Till
Conceptual: Not to Scale.
Characteristics:
Very shady below canopy, low primary productivity in understory
and forest floor, seasonal surface saturation.
What critters live here?
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: flowthrough 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
[email protected]
(706) 542-1772