Ecology of water-level manipulations on a northern marsh

Published source details Harris S.W. & Marshall W.H. (1963) Ecology of water-level manipulations on a northern marsh. Ecology, 44, 331-343.
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This study is summarised as evidence for the following.

	Action	Category
	Raise water level to restore degraded freshwater marshes Action Link
	Lower water level to restore/create freshwater marshes from other land uses Action Link

Raise water level to restore degraded freshwater marshes

A replicated study in 1949–1957 in a freshwater wetland in Minnesota, USA (Harris & Marshall 1963) reported that the effects of reflooding on emergent plant abundance depended on the water level and species. Statistical significance was not assessed. In areas with deep water (>15 inches in summer, after reflooding), the density of all emergent plant species declined (e.g. softstem bulrush Scirpus validus: 7.1 stems/ft² after 1 year of reflooding then 0 stems/ft² after four years of reflooding; cattails Typha spp.: 0.8 stems/ft² vs 0.4 stems/ft²). In areas with shallow water (0–10 inches in summer, after reflooding), the density of softstem bulrush and spikesedge Eleocharis palustris declined (9.6–10.3 stems/ft² after one year vs 0.1–0.3 stems/ft² after four years) whilst the density of cattails and sedges Carex spp. increased (1.0–1.5 stems/ft² vs 2.2–2.5 stems/ft²). Methods: At some point between 1949 and 1957, water levels were raised in four separate wetland pools that had been drawn down for the previous 1–5 years. Vegetation was surveyed between one and four years after reflooding, in stands initially dominated by each plant species but with different post-reflooding water depths.

(Summarised by: Nigel Taylor)
Lower water level to restore/create freshwater marshes from other land uses

A replicated, before-and-after study in 1949–1957 in a freshwater wetland in Minnesota, USA (Harris & Marshall 1963) reported that following drawdown of water levels, emergent wetland vegetation colonized the site. Over five years of drawdown, stands of tall emergent plants like softstem bulrush Scirpus validus, cattails Typha spp. and sedges Carex spp. developed on approximately 5,000 acres of 12,000 acres that were previously open water. Elsewhere, exposed mudflats were colonized by species such as marsh fleabane Senecio congestus and red goosefoot Chenopodium rubrum (area not quantified). The study suggested several related factors that affected the type of vegetation that developed, e.g. month of drawdown, soil type (mineral or peat), speed of drying, seed availability, and presence of algal mats. Herbaceous wetland communities present in the first year of drawdown were largely replaced by upland weeds, then woody species, over the following four years. Methods: At some point between 1949 and 1957, water levels were lowered in seven separate wetland pools to stimulate growth of emergent and moist-soil wetland vegetation. Two pools supported islands of emergent vegetation before drawdown. Observations were made after 1–5 years of drawdown in each pool (further details not reported).

(Summarised by: Nigel Taylor)