Vegetation Patterns in and among Pannes (Calcareous Intradunal Ponds) at the Indiana Dunes National Lakeshore, Indiana

Abstract

ABSTRACT: The relationships between plant species composition and dispersion, water chemistry and water depth/depth to water table were studied in five calcareous intradunal ponds (pannes) bordering the southern tip of Lake Michigan. The panne systems contained eight plant species threatened and endangered in Indiana. The aquatic zone was dominated by Chara, the pond edge by Rhynchospora capillacea, Juncus balticus and Utricularia cornuta, and the area surrounding the pond by Hypericum kal- mianum. The water chemistry was typical of hardwater ponds in the area, probably af- fecting species composition but not species dispersion within the pannes. A significant correlation between the first axis scores from a reciprocal-averaging ordination and water depth/depth to water was demonstrated. Panne species are fitted to a model based on hydrology proposed by van der Laan for dune-slack vegetation in the Neth- erlands. INTRODUCTION There is a series of intradunal ponds (pannes) just inland from the beach dunes at the Indiana Dunes National Lakeshore. These pannes differ in origin, age and plant species composition from numerous interdunal ponds found farther inland in older systems. The pannes now provide habitat for plant species found nowhere else in Indiana. Depressions which support the pannes were scoured out by wind around the turn of the century when Lake Michigan and groundwater levels were relatively low. When lake levels rose in the 1930s, the water table also rose and was exposed in the base of the depressions. These shallow ponds (< Im in depth) are normally perennial and the water is well-buffered with bicarbonates. Pannes are associated with systems along the E coast of Lake Michigan, along the southern shore of Lake Huron, and in one known location on Lake Superior. The number of plant species these have in common with the pannes at Indiana Dunes de- creases as distance from them increases (Chapman, 1982). Similar systems called dune slacks are associated with marine systems in the Netherlands (van der Laan, 1979; Londo, 1974), North Wales (Onyekwelu, 1972), North Devon (Willis