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

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.