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

Abstract

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.

Physiological Adaptation and Plant Distribution along a Steep Hydrological Gradient

“…Mosses on the solid limestone surface form dense cushions reducing wind exposure and evaporation [21]. Moss cushions effectively suck up rainwater, retain it for several hours or days and tolerate subsequent desiccation.…”

“…On land, photosynthesis of mosses is constrained by the loss of water in dry air. However, they tolerate water loss and rapidly become hydrated and resume photosynthesis when moisture or water becomes available [21]. Thus, we foresee that terrestrial plants and mosses photosynthesize under water at a rate depending on the species type and the CO 2 concentration.…”

Positive interactions between an exotic invader and moss biocrusts vary across life stage and correspond with the effect of water pulses on soil nitrogen

“…First, Centaurea produces large canopies that may provide more shade for biocrusts than the scattered bunchgrasses and forbs it replaces. Shade benefits biocrusts by prolonging hydration and decreasing oxidative damage incurred by biocrusts in full sun (Beckett et al, 2004;Cole et al, 2010;Martínez et al, 2006;Sand-Jensen et al, 2015). These benefits are similar to those provided by dryland shrubs for understorey herbaceous species (Lucero et al, 2020;Pugnaire et al, 1996).…”