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
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/30049 |
| Date | 23 May 2003 |
| Creators | Lindh, Briana C. |
| Contributors | Muir, Patricia S. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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