Response of mid-water common reed stands to water level variations and winter conditions in Lake Poygan, Wisconsin, USA

Because of wetland recession, common reed (Phragmites australis) stands in the Upper Winnebago Pool Lakes, Wisconsin, now mark the former outer margins of destroyed cattail (Typha spp.) marshes, occurring off shore in water up to 1.5 m deep. These mid-water common reed stands have experienced fragmentation and losses in area. This study examined the relation between changes in characteristics of common reed stands and some key environmental factors that may affect their success, especially those related to seasonal patterns in water level management and exposure to wind and severe winter temperatures. Individual patches (28-26,598 m(2)) within each common reed stand (6740-43,048 m(2)) were characterized with regard to stem densities (1-245 m(-2)) and water depths around the perimeter (0.10-1.52 m) and in the interior (0.22-1.30 m). Water-level cumulative frequency duration curves were used to quantify duration and continuity of exposure of common reed stands to various extreme water levels during 1986-1996, and we correlated these with annual percent changes in area derived from aerial photographs. Higher stem densities corresponded to larger patch size, greater historical stability, and less fragmentation. In addition, larger patches tended to be deeper, and covered a greater range of water depths. Conversely, more "ragged" patches tended to have lower maximum depths, both in the interior and on the perimeter, as well as smaller water level ranges. Overall, higher stem densities were associated with shallower water, though intermediate depths have experienced the greatest decline. Although combinations of extreme water levels and winter temperatures did not significantly predict annual changes in area of all common reed stands, these factors explained up to 75% of the variance in stands with the greatest loss. This indicates recent losses were caused by combined effects of extreme water levels and winter temperatures. (C) 2003 Elsevier Science B.V. All rights reserved.