Thirteen years of wetland vegetation succession following a permanent drawdown, Myrkdalen Lake, Norway

Abstract

Myrkdalen Lake in Western Norway was subjected to a 1.4 m drawdown in June 1987. Plant establishment and vegetation succession on the exposed sediments of a fluvial delta plain was monitored through 2000. The investigated area extended from the original Equisetum fluviatile zone to the new lake edge. The substrate was homogeneous and consisted mainly of minerogenous fluvial sediments. Vegetation data were sampled within continuous quadrats along transects perpendicular to the shore, and they dropped 93 cm in elevation. Detrended Correspondence Analysis and Canonical Correspondence Analysis confirmed that “time since drawdown” and “elevation of the quadrats” appeared to be of nearly equal importance in explaining succession. Plant establishment was rapid on the exposed sediments due to a seed bank and to rapid invasion of plants. The succession includes both floristic change as a function of time and a spatial separation in relation to the water level. The species succession was marked by a growth form progression: mosses and annuals → non-clonal perennials → clonal perennials. After one month, the annual Subularia aquatica and small acrocarpous mosses dominated the site. Dominance then shifted to Deschampsia cespitosa, Juncus filiformis, Blasia pusilla and Polytrichum commune. Subsequently there was an increase of Carex vesicaria in the lower zone and Calamagrostis purpurea in the upper zone, while Phalaris arundinacea was common over most of the elevational gradient. Equisetum declined where it had dominated before drawdown, but it expanded gradually towards the new shoreline. The vegetation remained dynamic after 13 years and it is not considered to be in equilibrium with the new environmental gradient yet. However, annual changes measured by DCA scores have slowed and two vegetation zones have developed. The major vegetational differences along the elevational gradient can be explained by the height of the mean June water limit. This example of species turnover in space and time may be a model for other successions that occur along a strong gradient.