Historical changes in mid-water stands of common reed in the Winnebago pool lakes, Wisconsin

摘要

Despite the tolerance of common reed grass to environmental extremes, mid-water stands in the Winnebago pool lakes of central Wisconsin appear to be diminishing. Formerly occupying shoreline locations, water level manipulations subsequent to dam construction beginning in the 1850's have isolated reed stands off shore. These stands have persisted but casual observations indicate that stand size has been declining. To address this perception we obtained an approximately decadal series of aerial photographs dating back to 1937 for four stands in Lake Poygan. Annual records were available for 1986-94. Using image analysis software, we determined shape and size metrics. Changes in stands varied widely, with areal losses ranging from 2% to 94%. The perimeter showed corresponding losses. Perimeter to area ratio and number of patches indicate that stand loss is characterized by increased fragmentation and shrinking patch size. The pattern of loss appears to proceed through an increase in irregularities along the perimeter, especially along edges facing the summer prevailing winds. This process exposes more of a patch to wind and wave action and eventually results in a division of the patch, culminating in periodic losses of the smallest patches. Despite an overall decrease in stand size, changes in size varied among the years of record with limited recovery occurring during some years.