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Influences of riparian canopy on aquatic communities in high desert streams of eastern OregonTait, Cynthia K. 12 September 1997 (has links)
Because riparian canopy controls most energy inputs to stream
ecosystems, it directly affects the structure of aquatic food webs and the
ecological processes that govern interactions among trophic levels. This
study addresses the interdependence among riparian canopy, benthic
community structure, and the carrying capacity of high desert streams for
salmonid fishes. In streams in the lower John Day River Basin in eastern
Oregon, algal, invertebrate, and fish communities were compared in reaches
with varying densities of riparian canopy. Water temperatures varied with
the density and upstream extent of canopy. Densely canopied sites were
cool, while sites with high irradiances had temperatures exceeding the upper
lethal limit for salmonids. Periphyton and grazer biomasses were greater in
well-lighted sites, but 90% of grazer biomass consisted of Dicosmoecus
gilvipes, a large caddisfly inedible by juvenile trout. Warmer water
increased metabolic demands for salmonids, while the overwhelming
dominance of Dicosmoecus in open sites shifted energy flow away from
trout and shrunk their food base. High water temperatures, however,
provided suitable habitat for many warmwater fishes which would otherwise
not enter tributaries of this size. At higher elevation study sites in Camp
Creek, light levels were higher and less variable than at the lower sites.
Periphyton and invertebrate abundances were not correlated with irradiance.
Rather, periphyton was maintained at low levels by grazers, particularly
Dicosmoecus and snails. Manipulations of fish densities in enclosures
showed that trout and dace had no negative impacts on numbers of
invertebrate prey, and that grazers played a larger role in regulating lower
trophic levels than did fish. Dicosmoecus acted as a keystone species in
the benthic food web of Camp Creek by simultaneously influencing the
trophic level both below and above its own. When irradiance was
experimentally reduced under artificial canopies, periphyton standing crops
were not different from those in open control pools after 4 wks. However,
grazers were more abundant in open pools. The cropping of periphyton to
uniform levels in both sunlight and shade indicated that mobile grazers
targeted sites of varying productivities. Comparisons between benthic
communities in Camp Creek and in a densely canopied reference stream
suggested that benthic community structure shifted to accommodate
changes in energy resources that occur when canopy density is altered. / Graduation date: 1998
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Understory herb and shrub responses to root trenching, pre-commercial thinning, and canopy closure in Douglas-fir forest of the western Cascades, OregonLindh, Briana C. 23 May 2003 (has links)
This thesis examines factors limiting understory herb presence and flowering
in young second-growth Douglas-fir (Pseudotsuga menziesii) forests on the west side
of the Cascade Mountains, Oregon, USA. I studied the belowground effects of canopy
trees on understory herbs and shrubs in old-growth forests using trenched plots from
which tree roots were excluded. Effects of tree density and stand age were tested by
comparing the understory community composition of old-growth stands and pre-commercially
thinned and unthinned young second-growth stands. I also examined
the effect of conifer basal area on understory herb presence and flowering within one
young second-growth watershed. In young stands, I focused on three groups of
understory herb species: disturbance-responsive (release), forest generalist and old-growth
associated.
The effects of root trenching on vegetation and soil moisture were tested in
closed-canopy and gap locations in two old-growth Douglas-fir (Pseudotsuga
menziesii) forests. Ten years after installation, trenched plots averaged 92% total
understory cover while untrenched plots averaged 47% cover. Trenched plots under
closed canopies were moister than control plots throughout the growing season; the
trenching effect on soil moisture became apparent in the generally wetter gaps only at
the end of the growing season. Vegetation responses to trenching were concomitantly
larger under closed canopies than in gaps.
Stands that had been pre-commercially thinned 20 years earlier exhibited
understory composition more similar to old growth than did unthinned stands.
Thinned stands exhibited higher frequencies, abundances and density of flowering of
old-growth associated herbs than did unthinned stands, but lower than did old-growth
stands. Forest generalist and release species showed mixed responses to thinning.
I used both general linear models and classification and regression tree models
to explore the association of herb species presence and flowering with conifer basal
area and abiotic variables. Both modeling approaches yielded similar biological
insights. Flowering was more sensitive than presence to current stand basal area.
Flowering of old-growth associated and release species was negatively correlated with
conifer basal area. Linear models allowed clearer hypothesis tests, while tree-based
models had greater explanatory power and provided information about interactions
between variables. / Graduation date: 2004
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