Cut large trees/shrubs to maintain or restore disturbance: freshwater marshes

Overall effectiveness category Likely to be beneficial
Number of studies: 4
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How is the evidence assessed?

Effectiveness

60%
Certainty

50%
Harms

5%

Calculating overall effectiveness category

Background information and definitions

Disturbance can clear dominant plants (including trees and shrubs), maintain light availability and control nutrient levels – and may maintain vegetation in a desirable and/or species-rich state (Hall et al. 2008; Middleton 2013). Therefore, conservationists sometimes want to actively restore disturbance where it has ceased, or maintain disturbance at a site where it would otherwise be lost.

Large trees and shrubs may need to be managed by cutting individual plants, stems or branches with loppers, saws or chainsaws. These actions are the focus of this section. Afterwards, regrowth of trees and shrubs may be managed by grazing, mowing or herbicide (effects covered elsewhere in synopsis).

Related actions: Use cutting to control problematic large trees/shrubs, whose success is not linked to a change in disturbance regime; Cut/remove/thin forest plantations; Cut/mow herbaceous plants (or small woody plants) to maintain or restore disturbance.

^{Hall S.J., Lindig-Cisneros R. & Zedler J.B. (2008) Does harvesting sustain plant diversity in Central Mexican wetlands? Wetlands, 28, 776–792.}

^{Middleton B.A. (2013) Rediscovering traditional vegetation management in preserves: trading experiences between cultures and continents. Biological Conservation, 158, 750–760.}

Study locations

Key messages

Four studies evaluated the effects, on vegetation, of cutting large trees/shrubs to maintain or restore disturbance in freshwater marshes. Three studies were in the USA. One was in Germany.

VEGETATION COMMUNITY

Community types (1 study): One study of a riparian wet meadow in Germany reported changes in the area of plant community types over four years after cutting trees/shrubs (along with grazing).
Community composition (1 study): One replicated, randomized, controlled, before-and-after study aiming to restore freshwater marshes in the USA found that cutting trees (along with other interventions) significantly affected the overall plant community composition over the following five years.
Overall richness/diversity (1 study): One study of a riparian wet meadow in Germany reported that plant species richness increased over four years after cutting trees/shrubs (along with grazing).

VEGETATION ABUNDANCE

Overall abundance (2 studies): Of two replicated, randomized, paired, controlled, before-and-after studies in the USA, one found that cutting and removing woody plants from a degraded wet prairie had no significant effect on overall vegetation cover three years later. The other study was in wet patches of a pine forest and found that understory vegetation cover increased more, over one year, where trees were thinned than where they were not thinned.
Characteristic plant abundance (1 study): One replicated, randomized, controlled, before-and-after study of overgrown freshwater marshes in the USA reported that of 26 plant taxa that became more frequent after cutting trees (along with other interventions), 16 were obligate wetland taxa.
Herb abundance (1 study): One replicated, randomized, paired, controlled, before-and-after study in wet patches of a pine forest in the USA found that cover of sedges Carex increased more, over one year, where trees were thinned than where they were not thinned.
Tree/shrub abundance (2 studies): One replicated, randomized, paired, controlled, before-and-after study of a wet prairie in the USA found that woody plant cover declined, over three years, in plots where trees/shrubs were cut – but increased in plots where trees/shrubs were not cut. One study of a riparian wet meadow in Germany simply reported that some trees/shrubs regrew over four years after cutting trees/shrubs (along with grazing).
Individual species abundance (1 study): One study quantified the effect of this action on the abundance of individual plant species. The replicated, randomized, paired, controlled, before-and-after study of a wet prairie in the USA found, for example, that cutting trees and shrubs had no significant effect on cover of the dominant herbaceous plant, tussock grass Deschampsia cespitosa, three years later.

VEGETATION STRUCTURE

Height (1 study): One site comparison study of a riparian wet meadow in Germany reported that an area in which trees/shrubs were cut back (along with reinstating cattle grazing) contained shorter vegetation than an adjacent unmanaged area.

OTHER

Survival (2 studies): One replicated, randomized, paired, controlled study in a wet prairie in the USA found that cutting woody plants did not significantly affect their survival in the following year. One study of a riparian wet meadow in Germany simply reported that 20% of black alder Alder glutinosa trees were still alive after being cut back and grazed for four years.

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, randomized, paired, controlled, before-and-after study in 1994–1997 in an ephemeral wet prairie in Oregon, USA (Clark & Wilson 2001) found that cutting woody plants reduced their cover (but not their short-term survival) and affected cover of forbs – but not the dominant herb species or vegetation overall. Over three years, woody plant cover declined in plots where they were cut (by 79%), but increased in plots where they were not cut (by 20%). This is despite no significant effect on woody plant survival in the first year after cutting (cut: 60%; uncut: 83%). Changes in forb cover also significantly differed between cut and uncut plots, although the precise effect depended on whether forbs were native (cut: 167% increase; uncut: 77% decrease) or non-native (cut: 45% decrease; uncut: 28% increase). Plots under each treatment experienced statistically similar increases in overall vegetation cover (cut: 42%; uncut: 31%) and cover of the dominant herb species, tussock grass Deschampsia cespitosa (cut: 8%; uncut: 31%; see original paper for data on other individual plant species). Methods: In 1994, five pairs of plots (each 56–140 m²) were established in a degraded, seasonally flooded prairie. Woody plants had grown over 200 years of fire suppression. In autumn 1994 and 1996, all woody vegetation was cut with pruners or loppers, then removed, from one plot/pair. Vegetation was surveyed before (summer 1994) and after cutting. Survival of six tagged woody plants/plot was recorded in summer 1995. Cover of selected herb species was recorded in three 0.5-m² quadrats/plot in summer 1997.
Study and other actions tested
Referenced paper
Clark D.L. & Wilson M.V. (2001) Fire, mowing, and hand-removal of woody species in restoring a native wetland prairie in the Willamette Valley of Oregon. Wetlands, 21, 135-144.
A site comparison study in 1996–2000 in a riparian wet meadow in southern Germany (Zahn et al. 2003) reported that in plots where woody vegetation was cut (along with reinstating grazing), there were changes in the area of plant community types, an increase in plant species richness, a reduction in vegetation height and growth of some woody vegetation. Statistical significance was not assessed. Over four years after intervention, there were slight increases in the area of reedbed/marsh vegetation (from 10 to 14%) and herbs typical of disturbed areas (from 45 to 50%) and a slight decrease in the area of meadow and pasture vegetation (from 45 to 36%). Total plant species richness increased in seven of seven plots, from 5–45 species/plot to 11–57 species/plot (increase of 3–22 species/plot). After four years, the cut/grazed area contained shorter vegetation than adjacent unmanaged land, including patches <10 cm tall not present in unmanaged land (data reported graphically). Finally, woody vegetation grew back despite grazing: up to 15 bushes/100 m², reaching a height of >1 m after four years. Around 80% of 400 black alder (Alder glutinosa) trees that had been cut back died over the four years. Methods: The focal wetland had been abandoned for 20 years, becoming overgrown with tall herbs and, in places, woody plants. In 1996, woody vegetation was cut back, near ground level, from a 6-ha study area (details not reported). Annual summer grazing was also reinstated. The study does not distinguish between the effects of these interventions. Vegetation was surveyed each summer 1996–2000, in seven grazed 100-m² plots. In 2000, vegetation height was measured along a 34-m transect spanning the cut/grazed and unmanaged areas.
Study and other actions tested
Referenced paper
Zahn V.A., Meinl M. & Niefermeier U. (2003) Effects of low maintenance grazing on the vegetation of a wetland fallow. Naturschutz und Landschaftsplanung, 35, 171-178.
A replicated, randomized, controlled, before-and-after study in 2000–2005 aiming to restore ephemeral freshwater marshes within pine forest in Georgia, USA (Martin & Kirkman 2009) found that cutting trees (along with applying herbicide and prescribed burning) altered the overall plant community composition, favouring herbaceous and wetland-characteristic species. Over five years, the community composition of managed wetlands diverged significantly from that of unmanaged wetlands (data reported as a graphical analysis). This effect was stronger in the core of the wetlands than on the wetland-upland boundary. Of 26 plant taxa whose frequency increased in managed wetlands (statistical significance not assessed), 25 were herbs and 15 were obligate wetland taxa. Methods: In summer 2000, mature stands of oak Quercus spp. trees (that had developed following fire suppression) were removed from five depressional wetlands by cutting and/or applying herbicide. Then, the wetlands were then burned three times (once every two years). The study does not distinguish between the effects of cutting, applying herbicide and prescribed burning. Five additional wetlands were not managed (trees not removed and no burning). Plant species presence/absence was recorded before (2000) and after (2005) intervention, in three to seven 100-m² plots/wetland.
Study and other actions tested
Referenced paper
Martin K.L. & Katherine K.L. (2009) Management of ecological thresholds to re-establish disturbance-maintained herbaceous wetlands of the south-eastern USA. Journal of Applied Ecology, 46, 906-914.
A replicated, randomized, paired, controlled, before-and-after study in 2011–2012 aiming to restore marsh patches in a pine forest in North Carolina, USA (Aschehoug et al. 2015) found that thinning trees increased understory vegetation cover, including sedges. In plots where trees were thinned, there were increases in total understory vegetation cover (from 34% one month before thinning to 57% one year after) and total cover of sedges Carex spp. (from 7% to 22%). These increases were significantly larger than in plots where trees were not thinned (total understory cover: increase from 44% to 48%; sedge cover: decrease from 10% to 8%). The effect of tree thinning was statistically similar in dammed and undammed plots (reported as a statistical model result). Methods: In May 2011, sixteen 30 x 30 m plots were established (in four blocks of four) in wet patches of a pine forest. Development of sedge marshes in wet patches of the forest had been restricted by fire suppression and the extirpation of beavers Castor canadensis. In eight plots (two/block), 90% of the trees were manually removed. Trees were not thinned in the other plots. Four thinned and four unthinned plots were also dammed. Vegetation cover was visually estimated one month before (April 2011) and one year after (April 2012) intervention.
Study and other actions tested
Referenced paper
Aschehoug E.T., Sivakoff F.S., Cayton H.L., Morris W.F. & Haddad N.M. (2015) Habitat restoration affects immature stages of a wetland butterfly through indirect effects on predation. Ecology, 96, 1761-1767.

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.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Cut large trees/shrubs to maintain or restore disturbance: freshwater marshes Action Link	Likely to be beneficial	4	Synopsis Link
Use herbicide to maintain or restore disturbance: brackish/saline swamps Action Link	No evidence found (no assessment)	0	Synopsis Link
Use grazing to maintain or restore disturbance: freshwater marshes Action Link	Likely to be beneficial	5	Synopsis Link