Water Level Fluctuations and Air Temperatures Affect Common Reed Habitus and Productivity in an Intermittent Wetland Ecosystem

Lake Cerknica is an intermittent wetland ecosystem with extreme water level fluctuations. It hosts extensive reed stands that have colonized different habitat types. Two different stands were compared: a lake stand not directly influenced by the intermittent River Stržen and a riparian stand near River Stržen. Reed productivity (growth and assimilate allocation) was monitored for these reed stand types over 13 years (2007–2019), and this measurement was compared to monthly water levels and air temperatures. Reeds from the lake reed stand were significantly shorter with a lower shoot density, overall biomass production, and ratio of flowering plants. A correlation analysis revealed stronger and more numerous significant correlations between environmental and reed productivity parameters for the lake reed stand compared to the riparian reed stand. The variabilities of the growth and assimilate allocation parameters in the lake reed stand were both mostly explained by the combined water levels for June and July, which explained 47% and 52% of the variability, respectively. The most influential temperatures were in May, which explained 29% and 19% of the variability of growth and assimilate allocation parameters, respectively. For the riparian reed stand, water levels and temperatures out of the vegetation season appeared more important. Therefore, habitats with permanent water are more suitable for reeds than those with fluctuating water. However, fluctuating water conditions are expected to become more common due to climate change.