Do we need soil moisture measurements in the vegetation–environment studies in wetlands?
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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?
Calcareous fens, poor fen grasslands, poor fens and ombrotrophic bogs in the Czech Republic.
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.
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.
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.