Unsaturated hydraulic properties of xerophilous mosses: towards implementation of moss covered soils in hydrological models

Abstract

Evaporation from mosses and lichens can form a major component of the water balance, especially in ecosystems where mosses and lichens often grow abundantly, such as tundra, deserts and bogs. To facilitate moss representation in hydrological models, we parameterized the unsaturated hydraulic properties of mosses and lichens such that the capillary water flow through moss and lichen material during evaporation could be assessed. We derived the Mualem‐van Genuchten parameters of the drying retention and the hydraulic conductivity functions of four xerophilous moss species and one lichen species. The shape parameters of the retention functions (2.17 < n < 2.35 and 0.08 < α < 0.13 cm−1) ranged between values that are typical for sandy loam and loamy sand. The shapes of the hydraulic conductivity functions of moss and lichen species diverged from those of mineral soils, because of strong negative pore‐connectivity parameters (−2.840 < l < −2.175) and low hydraulic conductivities at slightly negative pressure heads (0.016 < K0 < 0.280 cm/d). These K0 values are surprisingly low, considering that mosses are very porous. However, during evaporation, large pores and voids were air filled and did not participate in capillary water flow. Small K0 values cause mosses and lichens to be conservative with water during wet conditions, thus tempering evaporation compared to mineral soils. On the other hand, under dry conditions, mosses and lichens are able to maintain a moisture supply from the soil, leading to a higher evaporation rate than mineral soils. Hence, the modulating effect of mosses on evaporation possibly differs between wet and dry climates. Copyright © 2013 John Wiley & Sons, Ltd.

Quantifying energy and water fluxes in dry dune ecosystems of the Netherlands

“…Although organic layers, e.g., dry peat, have a larger specific heat (1600 J kg −1 K −1 ) than dry sand (693 J kg −1 K −1 ) (Gavriliev, 2004), the energy required to heat up the moss material is much smaller than for sand, because of the small dry bulk density of ca. 26.8 g L −1 (derived for Campylopus introflexus from Voortman et al, 2014). Therefore, the surface temperature and the emitted longwave radiation are largest for the moss surface.…”

“…This passive evaporation process is similar to the process of soil evaporation, i.e., evaporation becomes limited if the surface becomes too dry to deliver the potential rate. The unsaturated hydraulic properties of the dense Campylopus introflexes moss mat covering the lysimeter soil were based on the hydraulic properties derived by Voortman et al (2014) and used in the first 2 cm of the model domain. Macro pores in the moss mat were neglected by Voortman et al (2014), which implies that direct implementation of these hydraulic properties would result in large amounts of surface runoff generation or ponding, since the unsaturated hydraulic conductivity (K 0 ) of the moss mat is lower than 0.28 cm d −1 .…”

“…The unsaturated hydraulic properties of the dense Campylopus introflexes moss mat covering the lysimeter soil were based on the hydraulic properties derived by Voortman et al (2014) and used in the first 2 cm of the model domain. Macro pores in the moss mat were neglected by Voortman et al (2014), which implies that direct implementation of these hydraulic properties would result in large amounts of surface runoff generation or ponding, since the unsaturated hydraulic conductivity (K 0 ) of the moss mat is lower than 0.28 cm d −1 . Therefore, the dual porosity model of Durner (1994) was used to add 1000 cm d −1 to the hydraulic conductivity curve of Voortman et al (2014) between −1 and 0 cm pressure head (Appendix A).…”

“…Macro pores in the moss mat were neglected by Voortman et al (2014), which implies that direct implementation of these hydraulic properties would result in large amounts of surface runoff generation or ponding, since the unsaturated hydraulic conductivity (K 0 ) of the moss mat is lower than 0.28 cm d −1 . Therefore, the dual porosity model of Durner (1994) was used to add 1000 cm d −1 to the hydraulic conductivity curve of Voortman et al (2014) between −1 and 0 cm pressure head (Appendix A). This permits the infiltration of rainwater at high intensity rain showers without affecting the unsaturated hydraulic behavior at negative pressure heads.…”

“…Mosses and lichens are present in most successional stages in dry dune ecosystems, either as pioneer species or as understory vegetation. Voortman et al (2014) hypothesized that moss-covered soils could evaporate less than a bare soil, since the unsaturated hydraulic properties of moss layers reduce evaporation under relatively moist conditions. Such hydraulic behavior could have large implications on the ecological interactions between vascular and nonvascular plants in water-limited ecosystems, as the presence of a moss cover could facilitate the water availability for rooting plants.…”