- Wetland degradation has negative influences on wetland functions and services. The understanding of wetland functions requires the intersection of two science disciplines, ecohydrology and plant physiology for a correct approach on wetland restoration and management. Although studies assessing ecohydrological relationships to protect wetlands are relatively common, studies evaluating both ecohydrological and plant physiological aspects are not common. This study characterizes the relationship between herbaceous vegetation species and soil properties, including surface (0–20 cm) soil volumetric water content, soil texture, and soil bulk density. Non-metric multidimensional scaling (NMDS) and generalized linear models were used for this analysis. This study also evaluates the interaction between soil volumetric water content and water potential of representative wetland plants of the area to understand plant stress responses during the dry period of the year. This study was conducted in a conservation wetland near the Oregon State University Sheep Farm in the Willamette Valley, OR.
Our findings indicate fluctuations in surface soil water content during the period of study (February 2019–2020) followed the variations of precipitation in the study area. In general, the wettest period of the year was the spring while the driest period was from late June to mid-September. Soil water content ranged from 53.5 % to 83.3 % in April; 27.9 % to 72.7 % in August; 48.4 % to 77.2 % in October; and 51.6 % to 88.9 % in January. In total, 27 plant species were characterized on the six transects by using the line-point intercept and grouped in four distinct plant community types: rushes, forbs, grasses and sedges. The representative eight plant species correlated with a different range of soil volumetric water content percentage.
Typha latifolia, Scirpus microcarpus, and Schoenoplectus acutus had a range of higher soil
water content (measured about 45.0–88.9 % during a year) whereas Alopecurus pratensis,
Phalaris arundinacea, Juncus patents, and Juncus effusus had a range of lower soil water content (measured about 20.5–65.0% during a year). Distributed throughout the study area,
Dipsacus fullonum (an invasive exotic species) seemed to be the most adapted to the high variation in soil water content. Plant community analysis indicated that dry season (August and
October) soil moisture, as well as soil bulk density, were the major abiotic drivers of plant community structure. According to step-wise generalized linear modelling, rushes were positively correlated with soil moisture in August, while sedges were negatively correlated with soil moisture in October. Soil bulk density was positively correlated with grasses and negatively correlated with rushes, and litter cover.
The target plant species for the association between soil moisture and water potential during the dry season were J. patens, J. effusus, T. latifolia, and S. microcarpus. Midday water potential of the four species changed significantly in response to changes in soil moisture.
Midday water potential values correlated more strongly with volumetric water content in the two Juncus species than in S. microcarpus and T. latifolia. In addition, the two Juncus species experienced lower midday water potentials (up to about -2.5MPa) and were found on areas with lower soil water content (as low as 27 %) than S. microcarpus and T. latifolia. The water potential results of individual species corroborated the findings of the community analysis.
Juncus (rush) species are more tolerant to drought than other typical wetland species such as S.
microcarpus and T. latifolia. So, potential changes due to climate change or water diversion from the wetland, which may cause dryer conditions, may favour rushes persistence over other species. However, the observation that invasive D. fullonum is dominating the area brings more questions on the future survival of the native species. Results of this study contribute to foundational knowledge concerning the influence of soil conditions and moisture availability on the physiological response and distribution of wetland plant species that should be considered for wetland management practices, conservation, and restoration.