Patterns in the secondary succession of a Carex vesicaria L. wetland following a permanent drawdown

Abstract

The Myrkdalen Lake, western Norway, was subjected to a ca. 1.4 m permanent drawdown in June 1987. This left the original wetland vegetation belts “hanging” over the new water level. One year after the drawdown, a permanent transect with contiguous 1.0m×0.5 m quadrats was established through a Carex vesicaria wetland belt, and was analysed annually through 2001. The transect was 17 m long and included the lower part of a Phalaris arundinacea vegetation and the upper part of an Equisetum fluviatile vegetation. All quadrats were located on the same type of substrate and extended through a 65-cm difference in elevation. During the study period, the transect was completely inundated for only 6 days. A secondary succession was initiated immediately after the drawdown. The main trend was a gradual decreased cover of C. vesicaria and an increase particularly in the cover of P. arundinacea and Calamagrostis purpurea. Regression analyses indicated that variation in the cover of the different species could be expressed as linear, quadratic, or exponential functions of the number of years since drawdown. Parallel with the decreased abundance of C. vesicaria, there was a linear decrease in the number of its fertile shoots. Very low numbers of flowering shoots were found after the mean cover of P. arundinacea exceeded the cover of C. vesicaria. The study indicates that the drawdown resulted in a hydrological regime that was not optimal for C. vesicaria. Despite this, a clonal wetland sedge may obviously survive a major drawdown for more than 14 years.