- Variability of plant species distributions, plant connunities,
soil and hydrological factors in an area of the Netarts Spit tidal
marsh are examined. The purpose is to advance general understanding
of Oregon's tidal marsh ecosystems in terms of plant coninunities and
functio'nal habitat conditions and make a contribution to the conceptual
and technical framework used by regulatory agencies to manage
Vegetation, soil and hydrological attributes were measured periodically
from May, 1978 to March, 1979 along three transects extending
parallel to the elevation gradient. Each transect began at point bayward
of the lower limit of marsh vegetation and continued to a point
shoreward of the perceived upper limit of the marsh on the narrow,
stabilized dune ridge separating Netarts Spit from the ocean. Plant
species distribution and abundance by cover class was recorded in microplots
located at increments of five meters or less along each transect.
Soil and hydrological observations at intervals of approximately six weeks were made at 29 primary sampling sites located at increments of
15 meters or less along the transects. Water table fluctuations and
soil moisture tension were examined in situ. Soil cores were extracted
from the marsh for analysis of interstitial soil moisture content and
salinity, soil texture, pH, organic matter content, cation exchange
capacity, magnesium, calcium, phosphorus, potassium and sodium content.
Data on tidal inundation frequency and duration were provided by the
National Ocean Survey from a tide gauge located at Netarts public boat
dock. Weather data were obtained from continuously recording hygrothermographs
and an accumulating rain gauge located in the ecotone
between marsh and upland. Comparative climatic data were obtained
from published National Oceanic and Atmospheric Administration Environmental
Data Center Bulletins.
Analysis of individual environmental gradients was initiated by
arranging the data matrix into a composite transect ordered by elevation.
Each attribute was then plotted against elevation using graphically
comparable scales. Data synthesis was accomplished using a
combination of reciprocal averaging ordination (to identify coimlunity
gradients) and cluster analysis (for sampling site classification)
applied to hierarchical data sets composed of combinations of physical,
chemical, and vegetative sampling site attributes.
Habitat at lower elevations of the study area (0.70 m below mean
high water) is dominated by tidal factors which decrease steadily as
one approaches extreme higher high water (0.52 m above mean high water).
Fine textured sediments are saturated near the middle elevations of
the study area (-0.25 to 1.00 m above mean high water), with the lower and higher elevations dominated by sand. Hydrologic factors (water
table depth and soil moisture tension) dominate the highest elevations
of the study area but decrease steadily as one approaches mean high
water. The remaining environmental factors (interstitial soil moisture
content and salinity, cation exchange capacity, specific ion concentrations,
organic matter, and pH) may each be interpreted as variants
of one or more of the three primary environmental gradients: tides,
sediment texture, and hydrology. Interstitial soil moisture salinity,
soil moisture tension, and water table depth all show maxima during
months with high daily temperatures and low precipitation (Aug.-Sep.,)
and minima during months with low daily temperatures and high precipitation
(Dec.-Mar.). Interstitial soil moisture content shows the reverse
relationship with minima during Aug.-Sep. and maxima during Dec.-
Plant community structure of the tidal marsh may be best understood
as a species continuum beginning at about 0.60 m below mean high water
characterized by snecies dominance at low elevation (_0.51) to 0.40 m
above mean hiah water) givinG way to soecies diversity at higher elevations.
This continuum may be locally qraduated into three or more
subdivisions or plant communities by the dynamic reltionshi of intertidal
deposition and erosion processes. Community structure of the
associated dune upland is distinguishable environmentally and floristically
as distinct from the tidal marsh at about 0.90 m above mean hich
water although no attempt is made to resolve finer details of urland
community structure beyond the marsh interface.