Multivariate analysis of a tidal marsh ecosystem at Netarts Spit, Tillamook County, Oregon

Abstract

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 wetlands. 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.- Mar. 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.