Reduce frequency of vegetation harvest: freshwater marshes

Overall effectiveness category Likely to be beneficial
Number of studies: 3
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How is the evidence assessed?

Effectiveness

50%
Certainty

43%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

Harvesting vegetation less often (e.g. every two years instead of every year) will allow more time for it to recover from the disturbance, potentially growing taller and more densely. It might give plants long enough to mature and reproduce. Caution: In some habitats, regular disturbance such as harvesting may be necessary to maintain the composition and diversity of plant and animal communities.

For this action, “reduction” includes stopping harvest altogether. Note that studies comparing areas that remain unharvested to areas that become harvested, at any frequency, are not summarized as evidence for this action.

Related actions: Reduce intensity of vegetation harvest; Reduce frequency of cutting/mowing, including studies where cut vegetation is not removed.

Study locations

Key messages

Three studies evaluated the effects, on vegetation, of reducing the frequency of harvest in freshwater marshes (or harvesting at different frequencies). There was one study in each of the USA, Belgium and Italy.

VEGETATION COMMUNITY

Overall richness/diversity (1 study): One replicated, paired, controlled, before-and-after study in wet grasslands in Belgium reported that overall plant species richness was similar in plots harvested once or twice/year.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, paired, controlled, before-and-after study in wet grasslands in Belgium reported that the effect of harvesting twice/year (in July and October) on total above-ground biomass was intermediate between the effects of harvesting once/year in July or October.
Individual species abundance (3 studies): All three studies quantified the effect of this action on the abundance of individual plant species. For example, one replicated, paired, controlled study in freshwater marshes in the USA reported that cattail Typha biomass was greater, nine months after the last harvest, in plots harvested every six weeks than in plots harvested every three weeks. One paired, controlled, before-and-after study in reedbeds in Italy found that the common reed Phragmites australis biomass was similar in plots harvested once or twice/year, when measured at least five months after the last harvest.

VEGETATION STRUCTURE

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, paired, controlled study in 1980–1981 in two artificial water treatment marshes dominated by cattails Typha spp. in Michigan, USA (Ulrich & Burton 1984) reported that harvesting cattail less frequently during one summer increased its biomass the following summer. Statistical significance was not assessed. Nine months after the last harvest, cattail biomass was 390 g/m²in plots harvested every six weeks and 190 g/m² in plots harvested every three weeks. There was a similar but less extreme pattern one year after the last harvest: cattail biomass was 760 g/m²in plots harvested every six weeks and 600 g/m² in plots harvested every three weeks. At both times, cattail biomass in unharvested plots was 620 g/m². Methods: In June 1980, nine plots were established in each of two cattail-dominated marshes. Over 12 weeks, six plots (three plots/marsh) were cut every six weeks and six plots (three plots/marsh) were cut every three weeks. Cuttings were removed. The remaining six plots remained unharvested. In June and August 1981, above-ground cattail biomass was collected from each plot, then dried and weighed.
Study and other actions tested
Referenced paper
Ulrich K.E. & Burton T.M. (1984) The establishment and management of emergent vegetation in sewage-fed artificial marshes and the effects of these marshes on water quality. Wetlands, 4, 205-220.
A replicated, paired, controlled, before-and-after study in 1986–1988 in five wet grasslands in Belgium (Dumortier et al. 1996) reported that harvesting plots once per year increased plant species richness and sometimes increased plant biomass, whilst harvesting twice per year increased plant species richness and reduced plant biomass. Statistical significance was not assessed. Over two years, plant species richness increased whether plots were harvested once per year (July: from 15 to 18 species/6 m²; October: from 19 to 20 species/6 m²) or twice per year (July and October: from 17 to 19 species/6 m²). Total above-ground biomass (including litter) increased in plots harvested in July (from 460 to 490 g/m²), but declined in plots harvested in October (from 730 to 480 g/m²) or July and October (from 660 to 630 g/m²). The study also included some data on the abundance of individual plant species under each harvesting regime (see original paper). Methods: In spring 1986, three 7 x 7 m plots were established in each of five adjacent wet grasslands (mown annually for the previous 10 years). From 1986, five plots (one plot/grassland) were mown in July, five were mown in October, and five were mown in July and October. Cuttings were removed. Plant species were recorded each summer between 1986 and 1988. Biomass was cut and collected from five 30 x 30 cm quadrats/plot/year, immediately before the first harvest (so not at the same time in all plots), then dried and weighed.
Study and other actions tested
Referenced paper
Dumortier M., Verlinden A., Beeckman H. & Van D.M.K. (1996) Effects of harvesting dates and frequencies on above and below-ground dynamics in Belgian wet grasslands. Écoscience, 3, 190-198.
A paired, controlled, before-and-after study in 2000–2002 in two lakeshore reedbeds in northern Italy (Fogli et al. 2014) found that plots harvested once or twice each year supported similar common reed Phragmites australis biomass after two years. In both reedbeds, above-ground reed biomass was statistically similar in plots harvested once each year (in winter; 625–1,751 g/m²) and plots harvested twice each year (in summer and winter; 370–1,153 g/m²). Before harvesting, reed biomass was statistically similar in plots destined for each treatment (477–668 g/m²). Methods: In July 2000, a pair of 10 x 10 m plots was established in each of two reedbeds on the shore of Lago di Aslerio. From summer 2000, one plot/reedbed was mown once each year (August 2000 and 2001), one plot/reedbed was mown twice each year (February 2001 and 2002, plus August mowing). Cuttings were removed. The reedbeds had been historically harvested in winter (and sometimes in summer), but not for >30 years. Above-ground biomass was calculated from counts and measurements of reed shoots from three 1-m² quadrats/plot, before intervention (July 2000) and two years later (July 2002).
Study and other actions tested
Referenced paper
Fogli S., Brancaleoni L., Lambertini C. & Gerdol R. (2014) Mowing regime has different effects on reed stands in relation to habitat. Journal of Environmental Management, 134, 56-62.

Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.