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Stream temperature, as an important component of stream
ecosystems, can be affected by forest harvesting through removal of
riparian shade and changes in hydrology. Riparian Management Areas
(RMAs), as implemented through the current Oregon Forest Practice
Rules, are designed, in part, to maintain stream temperature following
forest harvesting. However, effectiveness of RMAs in achieving this
outcome is uncertain. The objective of this research was to examine
effectiveness of RMAs, as outlined by the current Oregon Forest Practices
Act and the Northwest State Forests Management Plan, in maintaining
warm-season temperature patterns of streamwater. Twenty-two
headwater streams, on either private- or state-owned forestlands in the
Oregon Coast Range that encompassed a range of RMA widths and
harvest prescriptions, were evaluated for effectiveness of RMAs on stream
temperature. A Before-After-Control-Impact/Intervention design was used,
and each stream had an upstream control and a downstream treatment
reach. Temperature probes were placed 1) at the top of the control reach,
2) at the boundary between the control and treatment reaches, and 3) at
the bottom of the treatment reach from June to September for four years
starting in 2002. All but one stream have at least two years of pre2
harvest temperature data, and one year of post-harvest temperature data.
Selected stream and riparian characteristics were collected every 60 m
within the control and treatment reaches once prior to and once following
harvest. I hypothesized that RMAs would be effective if pre-harvest warmseason
maximum temperature patterns were maintained following harvest
treatments. Comparisons of temperature patterns between control and
treatment reaches both pre- and post-harvest indicate that my hypothesis
should be rejected because warm-season maximum temperature patterns
were not maintained when mean values in treatment reaches across all
study streams were considered. Difference in temperature gradients
between control and treatment reaches averaged 0.6°C, based on two
years of pre-harvest and one year of post-harvest data. This indicates that
more warming or less cooling occurred in treatment reaches than
occurred in control reaches when pre-harvest and post-harvest periods
were compared, suggesting that current RMAs for small- and medium fishbearing
streams of the Oregon Coast Range are not effective for
maintenance of warm-season maximum temperature patterns. |
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