Initial improvement of photosynthetic activity and desiccation tolerance in bryophytes is quickly lost in long-term elevated CO2 in vitro conditions.

Exposure of bryophytes to elevated CO2 initially stimulated photosynthetic activity, but this benefit was rapidly lost in time. Desiccation tolerance did not improve in any of the four bryophyte species studied. The majority of studies have focused on the influence of rising CO2 levels on vascular plants, while bryophytes have received less attention, despite being major contributors to biodiversity in high latitudes, to facilitate water regulation in ecosystems, and support carbon and nutrient cycling. Elevated CO2 typically results in greater carbon availability for cell processes (reparation, osmotic regulation) and allows plants to achieve a greater internal CO2 concentration at any level of plant water content. Thus, we hypothesized that growth under elevated CO2 increases moss desiccation recovery. We conducted a one-year growth chamber experiment with four bryophyte species (Conocephalum salebrosum, Dicranum scoparium, Pleurozium schreberi, and Rhytidiadelphus squarrosus) to assess the effect of elevated CO2 (1000 μmol CO2 mol-1) on bryophyte desiccation tolerance based on CO2 assimilation, carbon balance and chlorophyll fluorescence measurements. Despite the initial CO2 assimilation and carbon gain improvements, such benefits were generally rapidly lost. Enhancement of desiccation tolerance through improvement in assimilation recovery was observed for different species at different time points, while generally, these benefits did not preserve either. Especially sensitive to elevated CO2 was the photosynthetic recovery at 24 h, where significant reduction of desiccation tolerance in D. scoparium and P. schreberi below the control levels was observed, indicating a potential decrease of the long-term performance. In summary, our results suggest that there is no clear long-term positive effect of increased CO2 on bryophyte desiccation-rehydration stress tolerance for the species studied, adding a new layer of complexity to the effect of global change on bryophyte flora.