Multi-year water level drawdown and wildlife grazing drive wetland vegetation succession

Abstract

In wetlands, multi-annual water level drawdowns and herbivory can induce cyclic vegetation succession. While water level drawdowns can be used in wetland management to increase the area of reed vegetation, an important habitat for wetland birds, herbivory may interfere with this process. Here, we studied the combined effects of a human-induced water level drawdown, i.e. the intentional temporarily and large scale lowering of the water level, and herbivores on wetland vegetation development.In the Oostvaardersplassen wetland, we used satellite imagery to assess vegetation development with and without water level drawdown and with and without red deer presence (introduced in 1992). An herbivore exclosure experiment (2022–2024) across an elevational gradient tested the effect of grazing on vegetation development during a drawdown.Satellite imagery showed an expansion of reed cover by 560 ha in the period without red deer (1987–1991) and by 420 ha with red deer (2020–2024), only in the area with drawdowns. The exclosure experiment highlighted an interaction between herbivory and water depth: The presence of red deer at drier locations had minor effects on reed expansion, whereas reed expansion was strongly inhibited at wet locations with presence of geese.Our findings provide large-scale quantitative evidence of the interaction between a water level drawdown and herbivory on the restoration of reed-dominated wetlands. We show the effectiveness of a water level drawdown, when dry conditions can be maintained for several consecutive years, as a restoration tool to promote reed development and the potential to steer the impact of herbivores during restoration.