Season length indicators and land-use effects in Southeast Virginia wet flats

The growing season concepts used by federal agencies in defining and regulating wetland hydrology ignore land use and rely on published surrogate indicators. This study compared several growing season indicators with measured air and soil temperature and hydrology data on three land-use types in the Great Dismal Swamp ecosystem of Southeast Virginia to determine how accurate the indicators are on each land use. Water-table depths, 1-m air temperatures, and soil temperature at 50-cm depths were measured for 18 mo at plots representing forest, early successional field (field), and tilled (bare ground) land-use treatments at two study areas. Land use affected air and soil temperature through vegetation type and soil surface properties, both of which are important for wetland restoration. Based on soil temperature at 50 cm, the growing season was continuous in forests but was interrupted in January for 1 to 7 d in some field and bare ground plots. Soil temperatures at 50 cm rose above biological zero (5 degrees C) 90 to 128 d before the published -2.2 degrees C growing season started. The published -2.2 degrees C growing season was 28 to 88 d longer than the measured equivalent, and began after the water tables rose and stayed continuously in the upper 30 cm. A continuous growing season declaration is proposed for federal regulations in thermic wet flats on all land uses. Lengthening the growing season did not cause the studied wetlands to fail the minimum federal wetland hydrology requirements for identification or delineation.