Do we need soil moisture measurements in the vegetation–environment studies in wetlands?

Abstract

Questions Water level parameters are normally used as explanatory variables in ecological studies, but soil moisture may be more causally connected with species composition. Can measurements of volumetric soil moisture in vertical profiles using electromagnetic sensors improve vegetation–environment analyses in fens and bogs? Are there inter‐habitat differences in water level–moisture relationships that could explain different sensitivities of particular vegetation types to water level changes? Location Calcareous fens, poor fen grasslands, poor fens and ombrotrophic bogs in the Czech Republic. Methods Monitoring of soil moisture (Profile Probe PR2), water level, pH and electrical conductivity of the water; measurements of soil pH and organic carbon; vascular plant and bryophyte data sampling; Pearson's correlations; ANOVA with post‐hoc tests; PCA of environmental factors; CCA and partial CCA that controlled for the effects of pH, conductivity, and year and region of sampling; Monte Carlo permutation tests. Results The median volumetric soil moisture correlated with the log‐transformed organic carbon concentration. This correlation was particularly strong when all vegetation types were merged or when poor fens and poor fen grasslands were analysed separately. The latter vegetation types further displayed the strongest correlations between water level and uppermost soil moisture. Calcareous fens had the highest water level and the lowest correlation between water level and soil moisture. In bog profiles the moisture continuously increased downwards and its variability over time correlated with water level fluctuation less strongly than in the case of Sphagnum fens. The median water level explained most variation in species data in all cases, except for separately analysed poor fen grasslands, whose variation was best explained by soil organic carbon. Moisture characteristics became insignificant whenever water level was included into the forward selection model. Conclusions In fens and bogs, water level still describes hydrological conditions better than directly measured moisture, and serves as a useful proxy of complex environmental conditions related to the hydrological regime, especially in Sphagnum fens with a shallow peat layer. Our results validate existing knowledge of species–environment relationships based on water table measurements.

Soil moisture's underestimated role in climate change impact modelling in low‐energy systems

“…Soil moisture has historically been difficult to measure cheaply, accurately and quickly under field conditions (Hájek et al ., ). Moreover, as topography and derived indices can be strongly correlated with soil moisture, proxy variables have often been used as surrogates instead (e.g.…”

“…Parviainen et al ., ; Piedallu et al ., ). However, with the development of new technologies, a shift to explicit measures of soil wetness is possible (Hájek et al ., ). Our results suggest that studies examining fine‐scale vegetation patterns should be based on field‐quantified soil moisture measurements where possible, because TWI was weakly related to observed soil moisture (as observed by, e.g.…”

Topographically determined water availability shapes functional patterns of plant communities within and across habitat types

“…However, the predictive power of TWI differed among habitat types. In a study of the contribution of water table level versus soil moisture in explaining species composition, Hájek et al (2013) found that deep peat layers buffer the effect of water level decline, an effect which was detected by direct measurements of soil moisture, but not water table depth. This may be why we see weak predictive power of TWI on mean Ellenberg F values in habitats like active raised bogs, transition mires and quaking bogs, and depression on peat substrates (habitat code 7110, 7140, and 7150).…”

“…This may be why we see weak predictive power of TWI on mean Ellenberg F values in habitats like active raised bogs, transition mires and quaking bogs, and depression on peat substrates (habitat code 7110, 7140, and 7150). Hájek et al (2013) conclude that soil organic matter must be included to account for water retention abilities of different soil types. Furthermore, in groundwater-dependent habitats such as fens and bogs, community-weighted Ellenberg F was not controlled by topographically determined water availability.…”

Modelling soil moisture in a high‐latitude landscape using LiDAR and soil data

“…The role of soil moisture is often underestimated in studies regarding high-latitude and high-alpine landscapes, due to the lack of data (Kammer et al, 2013;le Roux et al, 2013;Myers-Smith et al, 2015). Spatially extensive soil moisture measurements are challenging, since they are time-consuming, expensive, and hard to obtain (Famiglietti et al, 2008;Hajek et al, 2013). Thus, terrain-based surrogates, such as wetness indices, are commonly used in the absence of field-obtained soil moisture data.…”