Abstract:
The purpose of this study was to: (1) evaluate land
use and vegetation history of the Willamette Floodplain
Research Natural Area; (2) characterize present day
vegetation; (3) evaluate the relation of vegetation to
environment, and (4) establish a baseline dataset for future
trend analysis in order that response of different plant
species to fire frequencies may be assessed.
I used historical records, aerial photographs,
interviews, and literature to ascertain land use and
vegetation history. I assumed that the RNA had been
maintained as prairie by periodic aboriginal burning. Fire
ignited by settlers continued as a management tool into the
late 19th century. Grazing was a major disturbance in the
late 19th and early 20th centuries, and plowing or some
other soil disturbance on the raised elongated mounds may
have occurred. With removal of livestock grazing in the early 1960s and only a sporadic burning program, shrub cover
increased markedly over the past several decades.
I surveyed vegetation in the summer of 1991 in 36
random permanent plots. 1 classified species cover with
two-way indicator species analysis, identifying two major
communities. One is dominated by tall dense Rosa
eg'lanteria, Hypericum perforatum, and many alien weedy
species located on slightly elevated mounds, and the other
is dominated by shorter but also dense Rosa eglanteria,
Deschampsia cespitosa, and many species with wetland
affinity on intermounds. Monotypic patches of Spiraea
douglasii also occur on intermounds.
Species composition strongly correlates with
raicrotopography related to an indirect soil moisture index
based on wetland status of individual species. Composition
correlates less strongly with year since last burn. I
confirmed vegetation-environment relations by null
hypothesis analysis, tested by multi-response permutation
procedure. Vegetation significantly differs on mounds vs.
intermounds and vegetation significantly reflects burning
history. I ordinated floristic data by non-metric scaling.
I also tallied nested frequency data as a baseline for
future trend analysis in order to assess change in
vegetation in response to fire frequency. Frequency change
of Rosa eglanteria will best be detected by change in a 0.01
m2 plot size, Holcus lanatus by a 0.10 m2 plot, and
remaining species by a 1.00 m2 plot.
