First-year responses to managed flooding of lower Columbia River bottomland vegetation dominated by Phalaris arundinacea

Published source details Jenkins N.J., Yeakley J.A. & Stewart E.M. (2008) First-year responses to managed flooding of lower Columbia River bottomland vegetation dominated by Phalaris arundinacea. Wetlands, 28, 1018-1027.
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This study is summarised as evidence for the following.

	Action	Category
	Actively manage water level: freshwater swamps Action Link
	Actively manage water level: freshwater marshes Action Link

Actively manage water level: freshwater swamps

A before-and-after study in 2003–2004 of a freshwater wetland with marsh and swamp vegetation in Oregon, USA (Jenkins et al. 2008) found that following a managed flood/drawdown, plant diversity increased and there were changes in cover of individual plant taxa. Plant diversity was higher in the autumn after the flood/drawdown than in the autumn before (data reported as a diversity index). Of 21 plant taxa for which cover data were reported, 12 became more abundant, including knotweeds Polygonum spp. (before: 21%; after: 35%) and Pacific willow Salix lucida (before: 11%; after: 15%). The cover of seven taxa declined, including invasive reed canarygrass Phalaris arundinacea (before: 44%; after: 41%). The largest canarygrass declines occurred under regenerating tree canopies and in areas more deeply flooded during spring 2004 (see original paper for data). Methods: In 2004, a water control structure was used to restore a more natural water regime to a floodplain wetland: high winter and spring water levels (flooding some surveyed areas) followed by summer drawdown (exposure to natural tides). Over the previous 20 years, the water level had been artificially stabilized and reed canarygrass had invaded. Vegetation was surveyed around the edge of the wetland, in the autumn before (2003) and after (2004) the managed flood/drawdown. Plant species were recorded at 10 cm intervals along 27 transects (approximately 25,000 total points sampled), spanning marshy areas (open, herbaceous) and swampy areas (with a tree canopy). The study does not generally separate results from the two habitat types.

(Summarised by: Nigel Taylor)
Actively manage water level: freshwater marshes

A before-and-after study in 2003–2004 of a freshwater wetland with marsh and swamp vegetation in Oregon, USA (Jenkins et al. 2008) found that following a managed flood/drawdown, plant diversity increased and there were changes in cover of individual plant taxa. Plant diversity was higher in the autumn after the flood/drawdown than in the autumn before (data reported as a diversity index). Of 21 plant taxa for which cover data were reported, 12 became more abundant, including knotweeds Polygonum spp. (before: 21%; after: 35%) and Pacific willow Salix lucida (before: 11%; after: 15%). The cover of seven taxa declined, including invasive reed canarygrass Phalaris arundinacea (before: 44%; after: 41%). The largest canarygrass declines occurred under regenerating tree canopies and in areas more deeply flooded during spring 2004 (see original paper for data). Methods: In 2004, a water control structure was used to restore a more natural water regime to a floodplain wetland: high winter and spring water levels (flooding some surveyed areas) followed by summer drawdown (exposure to natural tides). Over the previous 20 years, the water level had been artificially stabilized and reed canarygrass had invaded. Vegetation was surveyed around the edge of the wetland, in the autumn before (2003) and after (2004) the managed flood/drawdown. Plant species were recorded at 10 cm intervals along 27 transects (approximately 25,000 total points sampled), spanning areas with a tree canopy and areas of open marshland. The study does not generally separate results from the two habitat types.

(Summarised by: Nigel Taylor)