Positive interactions between moss cushions and vascular plant cover improve water economy on Öland’s alvar, Sweden

Dense moss cushions colonize bare limestone pavements on Öland’s alvar, southeastern Sweden. As these cushions grow larger and thicker and can store more water, they should physically protect and facilitate their own performance as well as the colonization by vascular plants. We tested these predictions by measuring the airflow and water economy of moss cushions. We found that cushions are imbedded in boundary layers formed by ground and moss surfaces. Near-surface flow was reduced immediately upwind and negligible downwind of the moss cushions, which should facilitate their centrifugal expansion. The calculated diffusion boundary layer was thin (<0.7 mm) above moss cushions exposed to free airflow of 1–6 m·s−1 in accordance with substantial turbulence measured by small sensors at 0.5 cm distance from moss surfaces. Evaporation from the wetted cushions increased linearly with wind speed (0 and 8 m·s−1) in wind-tunnel experiments, and neither evaporation nor airflow followed standard formulas for objects in free flow. Higher wind speed reduced the diffusion boundary layer and simultaneously cooled the moss surface, thereby reducing the drop in water vapour concentration from moss surfaces to air. As desiccation of cushions progressed during three dry summer days, the profound decline of evaporation rate with cushion size gradually became positive when only large cushions still contained water. Water economy in relation to cushion size predicted that rehydrated large cushions retained photosynthesis for 80% and small cushions for only 50% of the time. A cover of the succulent plant Sedum album L. reduced sun and wind exposure and water loss from the moss surface. Sedum remained hydrated after mosses had dried out. Moss cushions and Sedum can, therefore, mutually benefit from each other’s water economy.