Vascular Plant Succession on an Exposed Fluvial Delta Plain After a Permanent Lake Water Level Drawdown. A 33-Year Time Course

ABSTRACT A permanent lake water level drawdown initiates a vegetation succession on exposed sediments. This study aims to quantify long-term vascular plant changes on the sediments in time (number of years after drawdown) and space (elevation above the new water level). Shortly after a 1.4 m permanent drawdown of Myrkdalen lake, W Norway, a 40 m long transect running from the original lowermost vegetation toward the new water level was established. Species cover percentages in the 2 m2 study plots were determined by visual estimates and data sampling was performed from 1988 until 2020; in total, 19 times. During the study period both mean richness and plant percentage cover varied significantly in relation to space and time. Carex vesicaria and Phalaris arundinacea increased gradually especially in the lower parts. The successional changes as quantified by a DCA ordination showed rapid changes during the first 15-20 years, but later the vegetation appeared to be more stabilized. Elevation explained the mean floristic variation in a linear fashion while a quadratic model better explained the variation in relation to time. Variation in summer and June water level explained a small part of the succession change during the study period.