- I used greenhouse and field studies to investigate how intraspecific and
interspecific competition, soil disturbance, and fertilization affect the biomass and
reproductive output of Senecio sylvaticus and Epilobium paniculatum, common winter
annuals which invade and dominate western Cascades forests during the first two seasons
after clearcutting and slashburning.
Field work experiments were conducted separately for each species on two
clearcuts in their third and fourth growing seasons post-disturbance. The clearcuts were
located in the Willamette National Forest near Blue River, Oregon. Experiments were
conducted fall 1992 through summer 1993. Research goals were to determine the effects
of nutrient availability, interspecific competition, and ground disturbance on the
establishment, growth, and reproductive output of Senecio sylvaticus and Epilobium
paniculatum and to determine if the typical decline of Senecio sylvaticus in the third and
fourth growing seasons is prevented by these treatments. One treatment was fertilization
of plots; treated plots were fertilized with 300 kg of N, 11.9 kg P, and 22.6 kg K per
hectare. The other treatment was above-ground removal of interspecific competition
(community removal) or soil disturbance.
Community removal and soil disturbance had significant effects on many
production and reproduction variables for one or both species, whereas fertilizer had
minimal impact. For the site in its fourth growing season post-burning, which was
accidentally broadcast fertilized after project establishment, community removal and soil
disturbance significantly increased the above-ground biomass per square meter of Senecio on the site, but plant density was not significantly affected. In addition, community
removal and soil disturbance increased the estimated number of Senecio seeds per square
meter by over thirty times and increased the total number of pods per square meter by
over ten times on the same site. While community removal and soil disturbance response
variables were significantly different from the control, they were not significantly different
from each other.
On the site in its third growing season post-disturbance, the community removal
increased the density of fall Senecio seedlings, but densities plummeted by spring and in
the summer densities were at or near zero for all treatment combinations. The small
number of plants made it impossible to analyze the data. The very low density was typical
for Senecio growing on clearcuts in their third and subsequent seasons post-disturbance,
and experimental treatments were unable to reverse this trend.
On the site in its fourth growing season post-disturbance, Epilobium paniculatum
final density was unaffected by community removal and soil disturbance, but above-ground
biomass per square meter increased by at least five times. The estimated number
of seeds per square meter had five to ten times the mean for the control; however, a
significant treatment interaction prevented mean separation.
The site in its third growing season post-disturbance exhibited decreased fall,
spring, and summer Epilobium densities for the soil disturbance treatment, likely because
the treatment buried seeds on this site which had abundant on-site seed sources in the
previous season. The estimated number of seeds and total number of pods per square
meter increased several-fold for community removal and soil disturbance treatments.
Fertilization significantly increased above-ground biomass, but not by a great magnitude.
The goal of the greenhouse studies was to determine how height, biomass, and
reproductive output of Senecio and Epilobium vary with increasing intraspecific
competition. Separate pot experiments were conducted for each species at Forest
Research Laboratory's greenhouse at Oregon State University, Corvallis, Oregon from
March through September of 1993.
Epilobium above-ground biomass and below-ground biomass per plant were
significantly affected by density by large magnitudes. In addition, pots with one plant had more above-ground biomass and below-ground biomass than all remaining treatments.
Reproductive output per pot was largely plastic for Epilobium; it remained similar across
all densities for total number of pods per pot and seeds per pod. Pots with one plant had
more estimated number of seeds than remaining treatments, but the magnitude was small.
On a per-plant basis, there were significant differences with large magnitudes for seeds per
plant and mean number of pods per plant.
Senecio growth and reproduction were significantly affected by plant density. All
treatments were significantly different from each other for both above-ground biomass per
plant and below-ground biomass per plant, with a ten-fold increase as density dropped
from 10 to 1 plant per pot. However, above-ground and below-ground biomass per pot
were not significantly different, indicating that biomass exhibited a plastic response to
All reproductive variables were significant for Senecio. The magnitude of
difference between plants in the lowest density pots and plants in the highest density pots
was 100 times for biomass of reproductive parts per plant and 137 times for seeds per
plant. Unlike Epilobium, Senecio reproductive output was also highly sensitive to density
for estimated seeds per pot, total fruits per pot, and seeds per fruit. The estimated number
of seeds per pot exhibited a 10-fold magnitude of difference between pots with one plant
when compared with pots with ten plants.
Thus, increasing intraspecific competition had a much greater effect on the total
reproductive output of Senecio than on Epilobium. Whereas Epilobium reproductive
output is independent of density (within the range of densities tested), Senecio does not
exhibit plasticity in reproductive output and output declines with increasing density. If
these results are mirrored in the field, then Senecio may produce the greatest number of
seeds when not densely colonizing a clearcut.