Light responses of mire mosses – a key to survival after water‐level drawdown?

Mosses are important ecosystem engineers in mires. Their existence may be threatened directly or indirectly by anthropogenic drying, which further leads to shading and changed competition conditions via increased arboreal plant cover. Yet, some species are able to acclimate to the changing habitat, while some give way to new colonizers. In the shaded conditions, acclimation or adaptation to low light levels is likely to be a winning strategy to survive. We studied the light responses of photosynthesis and photosynthetic pigment concentrations in mosses from an open mire and its shaded, i.e. drained and forested counterpart. Against our expectations, the Sphagnum species found only in the open habitat had lower photosynthetic capacity and maximum quantum yield than those found to grow in the shade. Chlorophyll fluorescence results suggested that photoinhibitory damage to photosystem II is responsible for the low photosynthetic performance of the Sphagna of the open habitat, which were inefficient to utilize any light level. In the shaded habitat, Sphagnum mosses showed adaptation to lower light conditions only by possessing a higher chlorophyll content. Pleurozium schreberi reached photosynthetic light saturation at half the irradiance level compared to Sphagna. The lack of efficient photoprotection or repair mechanism after photodamage may constrain the success of these species in the open habitat. Thus, the dominant Sphagna in the open pristine conditions seem to be stress tolerant, while the dominants of the shaded drained mire appear to be species capable of maximizing their growth and production to compete in the unstressful conditions in terms of light and desiccation.