Three studies were conducted to characterize and present early-seral
competition between Douglas-fir seedlings and the surrounding vegetation
communities during Pacific Northwest forest establishment. The first experiment
served as the foundation for this dissertation and was designed to quantify tradeoffs
associated with delaying forest establishment activities by introducing a fallow year
in order to provide longer-term management of competing vegetation. A range of six
operationally relevant treatments were applied over two growing seasons that
included in the first (1) a no-action control, (2) a spring release only, (3) a fall site
preparation without sulfometuron methyl followed by a spring release, as well as (4) a
fall site preparation with sulfometuron methyl and a spring release. In the second
year, there was (5) a fall site preparation without sulfometuron methyl followed by a
spring release and also in the second year (6) a fall site preparation with sulfometuron
methyl and a spring release. Treatments 5 and 6 were left fallow without planting
during the first year. These treatments were applied in two replicated experiments
within the Oregon Coast Range.
After adjusting for initial seedling size, year-3 results indicated that plantation
establishment and competition control immediately after harvest (i.e. no fallow
period) enabled seedlings to be physically larger than those planted after a one year
delay. At the Boot study site, limiting vegetation below 20% for the first growing
season improved year-3 Douglas-fir seedling stem volume over 273 cm³. Delaying
establishment activities one year and reducing competing vegetation below 11%
enabled seedling volume after two years to be statistically the same as three year old
seedlings in the no-action control, a volume range of between 148 to 166 cm³.
Delaying forest establishment at Jackson Mast improved seedling survivorship over
88% when a spring heat event reduced survivorship of trees planted a year earlier to
less than 69%. The combined effect of applying a fall site preparation and spring
release was necessary to reduce competitive cover below 10% in the year following
treatment and provided longer-lasting control of woody/semi-woody plants. Less
intense control measures (i.e. no-action control and treatment 2) were not able to
restrain woody/semi-woody plant cover which grew to nearly 40% at Boot and over
24% at Jackson Mast in three years. No treatment regime provided multi-year control
of herbaceous species. Including sulfometuron methyl in the fall site preparation
tank-mix did not have a negative effect on seedling growth or provide significant
reductions in plant community abundance in the year following application when
compared to similar regimes that did not include the chemical. Delaying
establishment lengthened the amount of time associated with forest regeneration
except on a site that accentuated a spring heat event.
In the second study, horizontal distance and azimuth readings provided by a
ground-based laser were used to stem map seedling locations and experimental unit
features at Boot. These data were used to create a relative Cartesian coordinate
system that defined spatially explicit polygons enabling, for the first time, the ability
to collect positional data on competing forest vegetation within an entire experimental
unit. Deemed "vixels" or vegetation pixels, these polygons were assessed for
measures of total cover and cover of the top three most abundance species during the
initial three years of establishment. An alternate validity check of research protocols
was provided when total cover resulting from this vixel technique was compared to a
more traditional survey of four randomly located subplots. The resulting linear
regression equation had an adjusted R² of 0.90 between these two techniques of
assessing total cover. When compared within a treatment and year, total cover
differed by less than 12 percentage points between the two techniques. Analysis of
year-3 woody/semi-woody plant cover produced by the techniques led to identical
treatment differences. Two treatments resulted in woody/semi-woody cover of
approximately 1500 ft² by the vixel method and nearly 40% cover by the subplot
method while the remaining four treatments were grouped below 600 ft² or 20%
cover, respectively. With continued refinement, these techniques could visually
present forest development through all phases and provide long-term information
used to bolster growth and yield models, measures of site productivity, as well as
community ecology research.
The third study evaluated the season-long gas exchange and biomass
partitioning of four weedy plant species capable of rapidly colonizing Pacific
Northwest regenerating forests. Cirsium arvense, Cirsium vulgare, Rubus ursinus
and Senecio sylvaticus were studied at two sites. A greenhouse was used to introduce
two levels of irrigation (well-watered and droughty). These species were also studied
while growing among a larger vegetation community at a field site. Irrigation
treatments had little impact on gas exchange rates. Species achieved maximum
photosynthetic rates of 30, 20, 15 and 25 μmol CO₂ m⁻² s⁻¹ (respectively) prior to
mid-July coinciding with an active phase of vegetative growth. As the season
progressed, photosynthetic rates declined in spite of well-watered conditions while
transpiration rates remained relatively consistent even when soil water decreased
below 0.25 m³ H₂O/m³ soil. Water use efficiency was high until late-July for all study
species, after which time it decreased below 5 μmol CO₂ · mmol H₂O⁻¹. Multi-leaf
gas exchange measurements as well as biomass data provided a holistic view of plantlevel
mechanisms used to shunt activity toward developing tissues. Herbaceous
species had assimilation rates that differed vertically (within each species) by as much
as 10 to 20 μmol CO₂ m⁻² s⁻¹ from July to September as lower leaves senesced in
favor of those higher on study plants. Specific leaf area was greatest in June for all
species then declined indicating species placed little effort into sacrificial early season
leaves when compared to those higher on the plant that could continue to support
flowering or vegetative growth. The study of seasonal gas exchange in the presence
of declining water availability has helped to describe competitive mechanisms at
work during forest regeneration as well as provide physiologic support for the
application of vegetation management regimes. / Graduation date: 2013
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29479 |
| Date | 17 May 2012 |
| Creators | Dinger, Eric J. |
| Contributors | Rose, Robin |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Relation | Forest Explorer |
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