- Relationships between the structure and composition of riparian vegetation
with channel morphology were examined in three montane meadow streams in the
headwaters of the Upper Grande Ronde River and North Fork John Day River in the
Blue Mountains, northeast Oregon. Vegetation composition, root biomass, and
channel morphology cross-sections were sampled along the streambanks at Crane
Creek, Little Fly Creek, and Squaw Creek, at each site: a non-incised stream section
and an incised stream section were sampled.
Root biomass was approximately 2 times greater in non-incised sections than
incised sections and decreased with depth more rapidly in incised sections. Total root
biomass in the upper 40 cm of the streambank ranged from 2,150 g m⁻² to 4,760 g m⁻²
in non-incised sections and from 1,110 g m⁻² to 2,220 g m⁻² in incised sections. Shifts
in rooting structure due to incision were observed. Less than 50% of the total root
biomass was found in the top 10 cm of the non-incised sections, with approximately 20% in successive 10-cm depth increments. In contrast, incised sections had >60% of
the total root biomass in the top 10 cm, ~15% in the 10 to 20 cm depth, <15% in the
20 to 30 cm depth, and <10% in the 30 to 40 cm depth.
Bank height was strongly correlated with species composition, explaining
~60% of the variance in community structure at the three streams. Relationships
between species composition and channel width were different at each stream, and
appeared to be a product of the stage of channel incision (Schumm et al. 1984).
Sections dominated by hydrophytic vegetation had narrower channels and lower banks
than sections dominated by mesophytic vegetation, indicating a connection with the
water table elevation (bank height) and overbank flows (channel area).
The distribution of root biomass and different community structure between
the non-incised and incised sections suggest a positive feedback between vegetation
and channel incision. The process is as follows: as incision progresses, there is a loss
of hydrologic connectivity, which causes a shift to a drier vegetation assemblage and
decreased root structure, resulting in a reduced erosive resistance capacity in the lower
streambank, thereby allowing further incision and widening. The greater loss of
hydrologic connectivity causes a shift to a yet drier vegetative assemblage that results
in yet less erosive resistance capacity, thereby allowing further incision and widening.
These findings suggest that a relatively minor channel incision, and the
associated shift in the hydrologic regime, may lead to a loss of wet meadow
communities and a decrease in the diversity of wetland-dominant species.
Understanding these interactions and predicting the response to management activities
are necessary elements of successful process-based management.