Reed bed vegetation structure and plant species diversity depend on management type and the time period since last management

Prepared by Line Holm Andersen, Petri Nummi, Simon Bahrndorff, Cino Pertoldi, Kristian Trøjelsgaard, Torben Linding Lauridsen, Jeppe Rafn, Cecilie M. Skak Frederiksen, Mads P. Kristjansen & Dan Bruhn

Line Holm Andersen using the pinpoint analysis in the recently cut down reed bed. Photo credit: Cecilie M. S. Frederiksen.

The reed bed is a succession driven wetland habitat dominated by the tall grass species common reed Phragmites australis. If unmanaged, the reed bed will eventually dry out and turn into scrubland.

Reed management, including harvest, burning and grazing, can halter the natural succession, and multiple studies compare the plant communities of managed compared to unmanaged reed beds. However, some claim that reed management increases the plant species richness, others that management does not change the species richness, and yet others that the plant species richness decrease following management. We wondered if these disagreements might be explained by including the age of the unmanaged reed bed, here defined as the time since last management. We also wanted to explore the effect of the two management methods reed winter harvest and reed winter cutting.

We conducted fieldwork during May and August of 2018 across four reed bed treatments; first, a reed bed harvested with reed stems removed in 2018 (aged 0-year-old); second, a reed bed cut down with reed stems left behind (0-year-old); third, a reed bed last harvested in 2015 (3-year-old); and fourth, a reed bed last harvested in 1993 (25-year-old). Here, we determined how the plant community, species richness and structural qualities of the differed between reed bed treatments.

Jeppe Rafn and Cecilie M. S. Frederiksen doing vegetation analyses in the 3-year-old reed bed. Photo credit: Line Holm Andersen.

The age of the reed bed had a large effect on both the plant community and the structural qualities of the reed bed. Each of the 0-, 3- and 25-year-old harvested reed beds contained unique species. Interestingly, we found that the effect of reed management on the plant species richness depended on multiple factors. The effect depended on when the field was conducted, as the species richness was only significantly different between reed bed ages in May, but not in August. In May, we saw that the age of the unmanaged control was important, as species richness was significantly higher in the 3-year-old reed bed compared to the 0-year-old harvested, but that the 0-year-old harvested reed bed had significantly higher species richness than the 25-year-old reed bed. This shows that using reed beds of different ages as an unmanaged control can result in different conclusions on the effect of management. Lastly, we saw that reed harvest rejuvenated the reed bed due to an increase in green reed shoots and that this effect was still apparent three years following harvest.

We found that the management method had a significant impact on the reed bed. The plant community was significantly different between the 0-year-old harvested and cut reed beds, that each contained a number of unique plant species. Also, the plant community of the harvested reed bed was adapted to a significantly higher level of light availability than the cut reed bed, likely due to the left-behind reed stems in the cut reed bed.

In conclusion, we recommend that reed management should strive towards securing reed bed patches of different ages. This will result in the highest species richness and the most structurally diverse reed bed at the landscape level.

A canal running along the edge of the 25-year-old reed bed. Photo credit: Line Holm Andersen.

This is a plain language summary for the paper of Andersen et al. published in Applied Vegetation Science (