The influence of soil waterlogging in determining plant species distributions among interdunal swales and between dunes and swales on Cape Cod was investigated employing field and greenhouse experiments. Most species exhibited significant responses to experimental treatments that closely matched their distributions in the field. In general dune species such as Ammophila breviligulata and Solidago sempervirens grew poorly under waterlogged conditions. When transplanted to the dry sand dune all species grew poorly or died. Swale species exhibited a range of responses to waterlogging conditions. Growth of Juncus canadensis, Scirpus pungens and Vaccinium macrocarpon increased with waterlogging. Cyperus dentatus, Juncus pelocarpus and Solidago tenuifolia grew equally well across all waterlogging conditions and are present in most swales. Juncus greenei and Myrica pensylvanica grew best in drier swales and their distributions appear to be restricted to the perimeters of swales.

Soil waterlogging is an important determinant of species distributions among swales and between swales and dunes. However, the effect of biotic factors such as competition, seed dispersal, soil seed banks and seedling recruitment needs to be examined as well to show how swale communities change over time.