Mechanisms behind species-specific water economy responses to water level drawdown in peat mosses

Abstract

Background and Aims 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. We aimed to compare species-specific responses to water level drawdown within Sphagnum, and investigate which traits affect water economy in this genus. Methods In a mesocosm experiment, we 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. Fv/Fm) of 13 Sphagnum species. Structural traits were measured, and eight anatomical traits were quantified from scanning electron microscopy micrographs. Key Results Mixed-effects models indicated that at high water level, large leaves were the most influential predictor of high WC, and at low water level WC was higher in species growing drier in the field, with larger hyaline cell pore sizes and total pore areas associated with higher WC. Higher stem and peat bulk density increased WC, while capitulum mass per area and numerical shoot density did not. We observed a clear positive relationship between Fv/Fm and WC in wet-growing species. Conclusions While we found that most hummock species had a relatively high water loss resistance, we propose that some species are able to maintain a high WC at drawdown by storing large amounts of water at a high water level. Our result showing that leaf traits are important warrants further research using advanced morphometric methods. 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.

Environmental drivers of Sphagnum growth in peatlands across the Holarctic region

“…The two, S. medium and S. divinum , have been described to occur in contrasting habitats (Yousefi et al, 2017). However, we have observed both species in open as well as shaded habitats (Bengtsson, Granath, Cronberg, & Rydin, 2020). The ecological boundaries between these species need to be further studied, since it seems that both could be plastic enough to occur in either environment.…”

“…Undoubtedly, the growth of Sphagnum and hence peatland function will be affected by environmental change. Sphagnum magellanicum generally has a wider niche in relation to water table level and nutrients, and showed a stronger direct response in LI to precipitation and temperature, which can give it a competitive advantage in warmer and wetter environments, than S. fuscum, which engineers an environment that is more resistant to desiccation (Bengtsson, Granath, et al, 2020). However, due to indirect effects and differences in bulk density the responses of the two species in productivity (NPP) were more similar, suggesting that the response of peatland moss NPP to climate change may be robust to species turnover (cf.…”

Development of an Image Analysis Pipeline to Estimate Sphagnum Colony Density in the Field

“…Plant density varies between Sphagnum cushions and it is thought to affect evaporation rates [ 9 , 18 , 44 ]. However, tools for the quantification of density have not been developed and therefore this still requires either destructive sampling or visual counting, both of which are time consuming.…”

“…Key to conservation efforts is understanding the interactions between the plant communities in the peatland ecosystem and their responses to environmental changes [ 5 , 6 , 7 , 8 ]. A main component in many peatland ecosystems is the peatmoss, Sphagnum , which is often given the status of ‘ecosystem engineer’ [ 9 , 10 , 11 , 12 ]. Many of the species in this genus are bog specialists that waterlog and acidify the environment, excluding many potential competitors and creating optimal peat-forming conditions.…”

“…Some of the water is stored in specialised cells, but the majority is retained within extracellular spaces [ 14 ]. The ability to store water in extracellular spaces is due to morphological adaptations of the Sphagnum plant, including densely packed leaves and branches, and the production of long pendant branches that act as wicks [ 9 , 16 ]. In other bryophytes, colony density has been shown to have a significant impact on water retention by reducing the amount of free space between individual shoots and trapping moisture below the canopy [ 9 , 17 , 18 ].…”