temperature (Environmental Laboratory, 1987). Soil moisture, vegetation type, and land use were not taken The growing season concepts used by federal agencies in defining into account in the WDM, even though they all influence and regulating wetland hydrology ignore land use and rely on pubsoil temperature (Day and Megonigal, 1993; Cole and lished surrogate indicators. This study compared several growing sea-Brooks, 2000). The growing season defined in Soil Taxson indicators with measured air and soil temperature and hydrology data on three land-use types in the Great Dismal Swamp ecosystem onomy as "the portion of the year when soil temperaof Southeast Virginia to determine how accurate the indicators are tures are above biological zero in the upper part" (Soil on each land use. Water-table depths, 1-m air temperatures, and Survey Staff, 1975) was later adopted by the ACOE. soil temperature at 50-cm depths were measured for 18 mo at plots "Biological zero in the upper part" is assumed to occur representing forest, early successional field (field), and tilled (bare when soil temperatures stay at 5ЊC or warmer at the ground) land-use treatments at two study areas. Land use affected 50-cm depth (Environmental Laboratory, 1987). Because air and soil temperature through vegetation type and soil surface of the impracticality of measuring soil temperatures at properties, both of which are important for wetland restoration. Based 50 cm for every potential wetland site, the ACOE alon soil temperature at 50 cm, the growing season was continuous in lowed approximation of the growing season by the numforests but was interrupted in January for 1 to 7 d in some field and ber of frost-free days (Williams, 1992). The ACOE uses bare ground plots. Soil temperatures at 50 cm rose above biological zero (5؇C) 90 to 128 d before the published Ϫ2.2؇C growing season the 30-yr average dates between the first and last 0, started. The published Ϫ2.2؇C growing season was 28 to 88 d longer Ϫ2.2, or Ϫ4.4ЊC air temperature occurrence to calculate than the measured equivalent, and began after the water tables rose the number of days above that temperature at a freand stayed continuously in the upper 30 cm. A continuous growing quency of 5 in 10 yr, as published in NRCS soil survey season declaration is proposed for federal regulations in thermic wet report tables or downloaded from the Water and Cliflats on all land uses. Lengthening the growing season did not cause mate Data Center (NOAA-NCDC, 2003). However, the studied wetlands to fail the minimum federal wetland hydrology using air temperature data as an indicator for soil temrequirements for identification or delineation. perature has been questioned (Day and Megonigal, 1993; Cole and Brooks, 2000), and the lengths of alternative measured or published growing seasons have not been
CO 2 efflux rates are affected by vegetation type, temperature, and soil surface conditions, and serve as an indicator of the length of the below-ground biological and microbial growing season. This study determined the effect of three land-use treatments on CO 2 efflux and growing season lengths in Southeast Virginia on two forested mineral soil wetlands. CO 2 efflux, soil temperature, and soil moisture were measured 24 times in 18 months at plots representing forest, early successional field, and bare ground landuse treatments. CO 2 efflux differed (p < 0.05) by treatment in the order forest > field > bare ground. CO 2 efflux was higher in hardwood-than conifer-dominated forest and higher in bare ground plots that were not inundated. Appreciable CO 2 efflux took place even once leaves had fallen off deciduous trees, and most of the CO 2 efflux appeared to be from vegetation rather than microbial sources during that period. Variability in CO 2 efflux was best described by the interaction between soil temperature and soil moisture (R 2 = 0.32) (p < 0.05). The below-ground growing season indicated by appreciable CO 2 efflux was similar to that indicated by soil temperatures above 5°C measured at 50 cm, the regulatory reference depth. The CO 2 efflux growing season was 365 days in the forest but was 9-16 days shorter in the field and 21-78 days shorter in the bare ground land-use treatment plots. These data can be used to modify the regulatory growing season definition in forested thermic wetlands and to reflect the environmental variation caused by different land uses.
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