Water Economy Responses to Water Level Drawdown in Sphagnum Mosses

The ecosystem engineers Sphagnum (peat mosses) are responsible for sequestering a large proportion of carbon in northern peatlands. Species may respond differently to hydrological changes, and water level changes may lead to vegetation shifts in peatlands, causing them to revert from sinks to sources of carbon. Fia Bengtsson and colleagues aimed to compare species-specific responses to water level drawdown within Sphagnum, and investigate which traits affect water economy in this genus.

In a mesocosm experiment, they investigated how water level drawdown affected water content (WC) in the photosynthetically active apex of the moss and maximum quantum yield of photosystem II (i.e. F_v/F_m) of 13 Sphagnum species. Structural traits were measured, and eight anatomical traits were quantified from scanning electron microscopy micrographs.

While the team found that most hummock species had a relatively high water loss resistance, they propose that some species are able to maintain a high WC at drawdown by storing large amounts of water at a high water level.

“We found some evidence that anatomical traits explain the water economy responses. In particular, leaf width was important for maximum water-holding capacity and water retention, which we attributed to leaf convexity and large extracellular spaces,” write Bengtsson and colleagues. “Cell-level traits linked to hyaline pore size and pore area also appeared to influence water-holding capacity and retention. However, leaf and cell trait variation was largely correlated with species identity, which limits mechanistic inference, and further studies are needed to test our results.”

As climate change may lead to more frequent droughts and thereby water level drawdowns in peatlands, a mechanistic understanding of species-specific traits and responses is crucial for predicting future changes in these systems.