Dynamics of common reed (Phragmites australis Trin.) in Swiss fens with different management.

Abstract

Dynamics of common reed (Phragmites australisTrin.) in Central Europe have so far mostly been investigated in connection with studies on reed ‘die-back’ along lake shores. However, there has recently been increasing concern about reed expansion at terrestrial sites, such as fens and wet grasslands. In this paper we report on the results of four separate studies which monitored reed dynamics in Swiss fens with various mowing regimes over a period of 4 to 15 years. The first study compared unmown plots with plots mown in winter in a triennial rotation; the second one included unmown plots, plots mown in summer, and plots mown in winter; the third one compared plots mown in June and September with plots only mown in September; the fourth study investigated only plots mown in September. Shoot number and shoot size were recorded in permanent quadrats. In all studies the performance of P. australis fluctuated without trend or tended to decrease during the period investigated. The decreasing tendency concerned shoot size rather than shoot number, and within a given study it was stronger for plots with initially taller shoots. The various mowing regimes did hardly influence these changes. Mowing in winter every three years reduced shoot size in the year after mowing, but not on the long term. Mowing every year in late summer reduced the shoot size compared with unmown plots on the short term, but this effect almost disappeared on the long term, after mowing had become biennial. Mowing in June (in addition to in September) caused no noticeable effects. We conclude that other factors (e.g. weather conditions, competition, or population processes) are more important than management in determining the abundance of P. australis in the fen communities investigated here, although long-term effects of mowing in summer still need more investigation. As a practical consequence it is suggested that at sites which are not strongly dominated by P. australis, as most of those investigated here, reducing the performance of this species should not constitute a major target of nature conservation management, nor can its dynamics be used as an indicator for management success before underlying causes are better understood.