Cover thresholds for impacts of an exotic grass on the structure and assembly of a wet prairie community

Published source details Toth L.A. (2016) Cover thresholds for impacts of an exotic grass on the structure and assembly of a wet prairie community. Wetlands Ecology and Management, 24, 61-72.
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This study is summarised as evidence for the following.

	Action	Category
	Use herbicide to control problematic plants: freshwater marshes Action Link
	Use herbicide to control problematic plants: freshwater marshes Action Link
	Raise water level to restore/create freshwater marshes from other land uses Action Link

Use herbicide to control problematic plants: freshwater marshes

A before-and-after study in 2006–2010 of a floodplain wetland invaded by limpo grass Hemarthria altissima in Florida, USA (Toth 2016) reported that over the four years after applying herbicide, cover of wet-prairie indicator species and total plant species richness were typically higher than before intervention. Statistical significance was not assessed. In the spring before applying herbicide, the wetland had 96% limpo grass cover and <1% cover of native wet-prairie indicator species. There were 3 plant species/100 m². Between one and four years after applying herbicide, limpo grass cover ranged from 2% to 22%. Indicator species cover ranged from 1% to 13%. There were between 13 and 30 plant species/100 m². Methods: In May 2006, glyphosate herbicide was applied to 6 ha of a recently rewetted, limpo-grass-invaded floodplain. Plant species and their cover were surveyed in twelve 100-m² plots, before intervention (spring 2006) and for approximately four years after (spring 2007–summer 2010). This study was in the same area as (17), but used a different plot.

(Summarised by: Nigel Taylor)
Use herbicide to control problematic plants: freshwater marshes

A replicated, before-and-after, site comparison study in 2007–2010 of floodplain wet prairies invaded by limpo grass Hemarthria altissima in Florida, USA (Toth 2016) reported that applying herbicide reduced limpo grass cover and maintained cover of other wetland-characteristic herbs in some plots 2–3 years later, but failed to do so in others. Statistical significance was not assessed. In three of seven treated plots, applying herbicide reduced limpo grass cover (before: 47%; 2–3 years after: 4%). Cover of other wetland-characteristic herbs was similar before (21%) and after (18%) applying herbicide. In the other four of seven treated plots, applying herbicide failed to prevent an increase in limpo grass cover 2–3 years later (before: 47%; 2–3 years after: 59%). Meanwhile, cover of other wetland-characteristic herbs declined (before: 21%; after: 10%). Nearby untreated plots were always dominated by wetland-characteristic herbs (before: 54%; after: 43%) with little limpo grass cover (before: 5%; after: 1%). Across all seven treated plots, three key metrics increased over time after herbicide application, reaching similar or higher levels three years after intervention than before: cover of native wet-prairie indicator species (before: 2–5%; after: 9%), total vegetation cover (before: >80%; after: >80%), and total plant species richness (before: 13–17; after: 23 species/100 m²). Methods: Eighteen 100-m² plots were established in wet prairies on a recently rewetted floodplain, with varying limpo grass cover. In autumn 2007, glyphosate herbicide was applied to seven plots with the greatest limpo grass cover. Plant species and their cover were surveyed in all 18 plots, before intervention (spring 2006–summer 2007) and for approximately three years after (spring 2008–summer 2010). This study was in the same area as (16), but used different plots.

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

A before-and-after study in 1998–2007 in Florida, USA (Toth 2016) reported that after dechannelizing the river to rewet the floodplain, both non-native limpo grass Hemarthria altissima and native wetland-characteristic vegetation increased in abundance in historic wetland zones, although individual plots became dominated by one or the other. Statistical significance was not assessed. Before rewetting began, limpo grass was present in only 3 of 27 marsh and wet prairie plots with only 0–4% cover. Roughly six years after rewetting was complete, limpo grass was present in 15–17 of 27 plots, with an average cover of 22–26%. Further data were presented for the 18 wet prairie plots. Here, there were 14–17 plant species/100 m² before rewetting, then 17–19 plant species/100 m² six years after. Cover of three wet-prairie indicator species was <1–3% before rewetting, then 16–25% six years after. However, these averages mask a dichotomy. After rewetting, nine wet prairie plots were dominated by the indicator species (38% cover; limpo grass cover: <12%) whilst nine were dominated by limpo grass (43% cover; indicator species cover: 3%). Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly six years after (until 2007), in twenty-seven 100-m² plots (18 in historic wet prairies and nine in historic marshes). This study used the same rewetted floodplain section as (21) and shared plots with (20), (22) and (26).

(Summarised by: Nigel Taylor)