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Crop and Soil Environmental Sciences Dep., Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061
* Corresponding author (ttcf{at}vt.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ACOE, Army Corps of Engineers • WDM, Wetland Delineation Manual
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The regulatory growing season definition in the 1987 Wetland Delineation Manual (WDM) is based on soil temperature (Environmental Laboratory, 1987). Soil moisture, vegetation type, and land use were not taken into account in the WDM, even though they all influence soil temperature (Day and Megonigal, 1993; Cole and Brooks, 2000). The growing season defined in Soil Taxonomy as "the portion of the year when soil temperatures are above biological zero in the upper part" (Soil Survey Staff, 1975) was later adopted by the ACOE. "Biological zero in the upper part" is assumed to occur when soil temperatures stay at 5°C or warmer at the 50-cm depth (Environmental Laboratory, 1987). Because of the impracticality of measuring soil temperatures at 50 cm for every potential wetland site, the ACOE allowed approximation of the growing season by the number of frost-free days (Williams, 1992). The ACOE uses the 30-yr average dates between the first and last 0, –2.2, or –4.4°C air temperature occurrence to calculate the number of days above that temperature at a frequency of 5 in 10 yr, as published in NRCS soil survey report tables or downloaded from the Water and Climate Data Center (NOAA-NCDC, 2003). However, using air temperature data as an indicator for soil temperature has been questioned (Day and Megonigal, 1993; Cole and Brooks, 2000), and the lengths of alternative measured or published growing seasons have not been compared in thermic wetlands.
Soil temperature change has been related to soil properties (Mount and Paetzold, 2002; Soil Survey Staff, 1975), specifically land use, which affects soil surface characteristics that mediate reradiation of heat and air temperatures near the surface (Hillel, 1982; Scott, 2000). Two studies have shown that average annual soil temperature is warmer when less protective vegetation or vegetative litter covers the ground surface (Wagai et al., 1998; Aust and Lea, 1991).
The dates between single freezing air temperature events do not appropriately define periods of appreciable biological activity for microbes, non-agronomic perennial plants, or wetland ecosystems (Day and Megonigal, 1993; Tiner, 1999). Microbes have adaptations to cold temperatures and perennial plants have underground root systems that reach into warm soil and carry on physiological activity even when air temperatures dip below freezing (Bernard and Gorham, 1978; DeWald and Feret, 1987). Wet soils are more buffered against changes in temperature than dry soils.
Several studies have found that soil temperatures at 50 cm were above 5°C longer than predicted by the published days above –2.2°C (frost-free days) growing season data. Megonigal et al. (1996) reported a 12-mo microbial growing season in South Carolina, Mississippi, and Louisiana for thermic bottomland hardwood forests based on soil temperature and O2 consumption. A study in a mesic tidal flat in Virginia indicated that soil temperatures were never below 5°C at the 50-cm depth and were below 5°C for only about 2% of the year at the 20-cm depth (Seybold et al., 2002). However, no studies have reported the effect of land use on growing season length in thermic areas or on soil temperatures in wet flats wetlands: wetlands that occur on mineral soils on broad, flat interstream divides with water-tolerant and nutrient demanding pines, oaks, and/or mixed hardwoods (Brinson, 1993; Harms et al., 1998).
We hypothesize that the growing seasons of microbes and higher plants are dependent on soil temperatures rather than air temperatures and that the soil temperatures vary depending on the soil moisture state, vegetation type, and leaf litter cover (Oi, Oe, and Oa horizons) of differing land uses. The objectives of this study were to (i) compare the affect of land use on the length of various measured growing seasons, (ii) compare the measured growing season with alternate growing seasons, (iii) compare the timing and duration of high water table periods with the measured growing season length, and (iv) evaluate and suggest improvements in current regulatory growing season definitions used for wetland identification and delineation at two wet flats wetlands in Southeast Virginia.
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Study Areas |
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TOPABSTRACTINTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The 8.7-ha Bruff study area (Bruff) is at an elevation of 15 m, centered at 36°37'02'' N, 76°33'28'' W. Bruff was drained by ditching to 0.5 m and managed as agronomic fields and loblolly pine forest until 1999. Drainage ditches were plugged in March 2000, and the fields have reached an early successional stage of wetland reforestation. Fifty percent of the herbaceous cover in the field consists of invasive perennials and grass species such as Chinese lespedeza (Lespedeza cuneata, Dum. Cours.), panicled ticktrefoil (Desmodium paniculatum Lam.), trumpet creeper (Campsis radicans L.), and tall fescue (Festuca arundinacea Schreb.). The forest was planted to loblolly pine (Pinus taeda L., medium density) in the early 1970's and 90% of the overstory cover consists of loblolly pine and red maple (Acer rubrum L.). Fibric, hemic, and sapric horizons were about 4 to 7 cm thick in the forest and fibric horizons were 1 cm thick in the field plots.
The 22.5 ha Hall study area (Hall) is at an elevation of 10 m, centered at 36°37'57'' N, 76°18'50'' W. Hall has a similar land-use history as Bruff, except that the forest was unmanaged hardwood that had been selectively cut several times. Eighty percent of the dense overstory cover in the forested area consists of sweetgum (Liquidambar styraciflua L.), red maple, swamp chestnut oak (Quercus michauxii Nutt.), and sourwood (Oxydendrum arboretum L.). Forty percent of the herbaceous cover in the early successional field consists of ragweed (Solidago canadensis L.) and broomsedge bluestem (Andropogon virginicus L.). Fibric and hemic horizons were 1 to 2 cm thick in the forest and fibric horizons were 1 cm thick in the field plots. Sapric horizons were discontinuous in the forest.
Both study areas had an intermittently saturated hydroperiod typical of wet flats (Rheinhardt et al., 2002; Burdt, 2003). The surface horizons were saturated with water predominantly between February and April when evapotranspiration was lowest (Lichtler and Walker, 1979). Water occasionally ponded in the bare ground treatments because infiltration was impeded by compacted plowed layers and transpiration was nonexistent. The water levels were lower at Bruff than at Hall in 2001 (Burdt, 2003) because Bruff received 20 cm less precipitation and had high evapotranspiration in the winter from the loblolly pine and tall fescue (Martin, 2000; Pangle and Seiler, 2002).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The study began on 16 Jan. 2001 and ended on 14 June 2002. At each plot, water table depths were monitored twice daily by the automated wells. The wells were installed in auger holes backfilled with medium sand sieved between 0.3 and 0.84 mm (The Quikrete Companies, Atlanta, GA) and capped with bentonite at the soil surface. Within four different, defined growing seasons, days when water tables were at depths of 
30 cm were marked and grouped into clusters of 
7 consecutive days. The clusters were terminated after 2 d of water depths >30 cm, to allow no more than a 24-h interruption when water was below 30 cm.
Air temperatures were measured hourly using Stowaway Tidbit thermistors (Onset Computer Corp., Pocasset, MA). Thermistors were mounted to a PVC pole 1 m above the soil surface placed 1 m south of each well and shaded with styrofoam. Soil temperatures were measured at 4-h intervals by Tidbit thermistors installed at a depth of 50 cm in 3-cm diam. holes placed 1 m north of each well. The soil cores were saved and replaced into their original location and the surface soil and leaf litter were knit across the opening to prevent surface air flow along the edge of the hole. Seedlings in the bare ground plots were pulled from the 3 m2 area around the soil temperature sensors.
Daily air temperature data for the 30-yr period from 1971 to 2000 for Lake Kirby near Suffolk, Virginia were downloaded from the National Oceanic and Atmospheric Administration (NOAA) (NOAA-NCDC, 2003). The average monthly low and high air temperatures and the 30th and 70th percentile air temperatures were calculated using the same methodology used to calculate averages and normal ranges for precipitation data (USDA-SCS, 1985b).
We compared four measured and four published or defined growing seasons. The measured –2.2°C growing season was the cluster of dates when the minimum daily air temperature remained above –2.2°C. The measured –4.4°C growing season was defined in the same manner but for the –4.4°C threshold. The measured soil temperature growing season was the block of dates when the maximum daily soil temperature at 50 cm reached 5°C. Measured values were rounded to the level of precision used in the regulations. The vegetative growing season was delimited by the observed bud break and leaf abscission of red maple in 2001. Bud break was recorded again in 2002 in both forest and field plots. Published –2.2 and –4.4°C growing season dates were found in soil surveys of adjacent Suffolk and Chesapeake counties (Reber et al., 1981; NOAA-NCDC, 2003). The published thermic soil temperature regime (published thermic) growing season (Soil Survey Staff, 1999) was found on the Hydric Soils—Criteria web page (http://soils.usda.gov/use/hydric/criteria.html, verified 28 Mar. 2004). A continuous growing season was defined as being 365 d long.
Statistical Analysis
The study was an incomplete block design because of equipment failure and loss. Treatments at each study area were compared within each growing season by one-way, unstacked ANOVA (Minitab, Inc., State College, PA). The nonparametric Mann–Whitney t test in Minitab was used to compare pairs of treatments whenever the ANOVA showed treatment differences. All statistical analyses in this study were investigated at the 90% confidence level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The lowest daily soil temperature recorded in the forest plots at Bruff was 8.1°C on 8 Feb. 2001, 5.5°C in the field plots on 9 Feb. 2001, and 2.6°C in the bare ground plots on 21 Jan., 2002. The lowest soil temperature recorded in the forest plots at Hall was 7.5°C on 5 Feb. 2001, 4.6°C in the field plots on 7 Jan. 2002, and 2.5°C in the bare ground plots on 6 Jan. 2002. The leaf litter buffered heat exchange between the air and the soil in the forest and reduced reradiation of heat at night, leading to warmer soil temperatures in the winter. In the spring, the forest air was warmed from heat reradiated from the soil and trapped beneath the dense canopy. The vegetation and leaf litter in the field was thin and discontinuous and had little effect on reradiation of heat. There was no vegetation or leaf litter in the bare ground plots after April 2001.
There was no significant difference in the length of measured soil temperature growing seasons by land use, probably because temperatures were taken at the 50-cm depth. The measured soil temperature growing season was continuous in forest plots but was interrupted in January for 1 to 7 d in some field and bare ground plots. These results are supported by Burdt (2003) who reported that measuring soil CO2 efflux rates produced a similar growing season length, with some variation by land use. The measured –2.2 and –4.4°C growing seasons were longer (P = 0.027 and P < 0.001) in the forest than in the field or bare ground plots at Bruff in 2001 because the forest soil was the warmest in the winter and the vegetation may have trapped some warm air near the surface and prevented some reradiation of heat (Burdt, 2003). The same was true but only for the –2.2°C data at Hall in 2001. There were no differences in measured –2.2 and –4.4°C growing seasons between treatments in 2002, possibly because only the first part of the year was measured.
Growing Season Comparisons
The measured soil temperature growing season was continuous in the forest and field plots and was interrupted for <1 wk in early January in the bare ground plots (Table 1, Fig. 1). Monthly average air temperatures were warmer than normal in Aug. 2001 and again from Nov. 2001 through Apr. 2002, but normal for the rest of the study. The concept that plants and microbes respire year-round in thermic wetlands is supported by the continuously elevated soil surface CO2 efflux rates measured in forested and field plots in an earlier part of this study. The rates were only elevated from Feb. 1 to Dec. 31 in the colder, noninsulated bare ground plots (Burdt, 2003), primarily from soil surface microbial activity. Megonigal et al. (1996) also reported continuous growing seasons in thermic forested wetlands in the southeastern USA. In mesic soil temperature regimes, Pickering and Veneman (1984) reported that poorly drained mesic Massachusetts soils had continued biological activity and caused significant Fe reduction even during the winter, after microbial activity in the adjacent better drained soils had ceased. Seybold et al. (2002) also reported that soil temperatures at 50 cm did not drop below 5°C in a mesic Virginia tidal flat, which implies that wetland ecosystems with dense vegetation and surface organic layers have internal climate properties similar to soils in warmer regions.
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The measured soil temperatures serve as the control data set because they are the parameter that the regulations are based on and that the alternative indicators attempt to duplicate. Figure 1 shows an earlier average start time and duration of measured growing seasons in the order of soil temperature > vegetative > –2.2°C > –4.4°C. This agrees with Pickering and Veneman (1984), who reported that soil temperatures at 50 cm were above 5°C in all studied soils longer than the published –2.2°C growing season. Huddleston and Austin (1996) also reported that the soil microbe growing season could not be predicted from published air temperatures as currently done for federally regulated wetlands.
Bud break of red maple and eastern redbud (Cersis canadensis L.) that served as the vegetative indicators of growing season startup were observed in the first week of February 2001 at Bruff (Fig. 1). Sweetgum initiated bud break in early March 2001 and 90% of the hardwood species were leafing out by March 20. Redbud was producing floral material in early February 2001 at Hall, bud break of red maple and sweetgum occurred in mid-February, and by the middle of March about 80% of the overstory was budding.
The deciduous tree species began to senesce in mid-Nov. 2001 at both study areas and by December there was widespread loss of chlorophyll, evidenced by browning and leaf loss. The hardwood trees exhibited termination of growth, but the loblolly pines continued to undergo sap flow and stand transpiration even in the winter (Martin, 2000). Buds began to swell earlier at Bruff in 2002 due to the drier soils and unseasonably warm Jan. temperatures. The vegetative growing season defined by bud break of red maple was about 10 to 12 wk shorter than the measured soil temperature growing season in the forest plots in the winter of 2001–2002, and does not appear to be a viable surrogate because of the interaction between winter chilling and photoperiod on dormancy (Garber, 1983; Falusi and Calamassi, 1990) and the lack of standard indicator tree species on some land use types.
The published –2.2°C growing season was 90 to 130 d shorter and started about 74 d after the measured soil temperature growing season that it was intended to replace (Table 1, Fig. 1). The use of any measured or published air temperatures alone to approximate soil temperature at 50 cm appears insufficient. The published –2.2 and –4.4°C growing seasons started 1 to 8 wk before and were 4 to 11 wk longer than the equivalent measured –2.2 and –4.4°C growing seasons. Neither published air temperature growing season was an adequate surrogate for measured air temperatures.
The published thermic growing season had a reasonable starting date but an ended about 8 wk earlier than the measured soil temperature growing season in 2001 (Fig. 1). The published thermic growing season dates should be re-evaluated for use in thermic wetlands, since this study was conducted in the coldest part of that soil temperature region and previous research has reported a continuous growing season in both thermic and mesic wetlands.
Effects of Growing Season Dates on Wetland Hydrology Requirements
The number of days in blocks of 7 continuous days of saturation within 30 cm of the surface was totaled during four growing seasons (Tables 2 and 3). The total days of saturation were compared with each growing season length to determine if they met WDM hydrology requirements for identification and delineation (5 and 12.5% of the growing season). Four of 12 plots at Bruff and 7 of 12 at Hall failed to meet the 5% threshold in 2001. Those failed only for the measured –2.2°C growing season, because it started so late (Fig. 1). In 2002, only the driest plot at Bruff with marginal hydric soil morphology failed the 5% threshold, and none failed at Hall. Even though the published –2.2°C growing season missed some of the springtime high water table, it produced the same results concerning the 5% threshold as the measured soil temperature and continuous growing seasons. Therefore, changing the WDM growing season regulations to use measured air temperatures may not provide any improvement over using the published air temperatures. Declaring a continuous growing season in the WDM regulations would produce the same results as using the (control) measured soil temperature growing season because the two are almost identical.
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Suggested Improvements in the Regulatory Growing Season Definition
Using measured or published –2.2°C growing seasons discounted some high water table periods because they started too late in the year after the soil temperature at 50 cm had warmed up above 5°C, and place wet flats and similar wetlands in jeopardy of failing to meet the 5 and 12.5% hydrology requirements. Using the continuous growing season as a surrogate for the measured soil temperature growing season may seemingly also place wetlands in jeopardy of meeting regulatory hydrology limits, but that would be true only in extended periods of below average precipitation, as occurred in winter and early spring 2001–2002. In fact, during the spring of 2001, the saturation percentage during the continuous growing season was higher than in the published or measured –2.2°C growing season (Tables 2 and 3). The opposite was true in 2002, when the period of continuous saturation with 30 cm started 1.5 to 3 mo later and lasted only 1/5 to 1/2 as long. It seems clear that a more accurate definition of the growing season concept should be added to the regulations or the concept should be removed entirely.
In mesic and colder soil temperature regimes, it may be possible to develop empirical relationships or regression models between published air temperatures and measured soil temperature and hydrologic data in wetlands. The effect of surface litter cover or land use should be investigated as another variable. The approach has been shown to be successful using a large number of soil, vegetation, and climate inputs to predict soil temperature at 50 cm on well-drained uplands by Isard and Schaetzl (1995), and a similar but simpler model using recent digital data sources could be developed and validated with replicated studies. The effect of snowpack > 20 cm thick on soil freezing was found to be important in mesic and colder areas but would not be in warmer areas. Modeling is not needed in thermic areas if a continuous growing season is used.
An alternative measurement depth for soil temperature may also be considered. For example, the National Research Council (1995) recommended that the upper part of the soil be defined at 30 cm rather than 50 cm because the 30-cm depth is the bottom of the rooting zone in most wetlands. Soil temperature measurements at 15- or 30-cm depths should coincide more closely to air temperatures than the 50-cm measurements.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The published –2.2°C growing season is not a good surrogate for the measured soil temperature growing season because it starts later than all other measured or published growing seasons reported in this study. The late starting date produces potential jeopardy of missing up to two-thirds of the period when soil temperature is above 5°C and water table heights are near the surface. Therefore, based on the results of this and several other studies, the declaration of a year-long growing season for forested wetlands in the thermic soil temperature regime is warranted to replace the current regulatory standard. This study took place in a marginally saturated thermic wetland. The soil temperatures in wetter systems should be even warmer because of the buffering capacity of wet soil and any surface O horizons present.
Land use affects soil temperature the most when the soil surface is bare for parts of the year and when leaf litter is absent, and shortens the growing season at 50 cm by about 1 wk (2% of the year). Therefore a continuous growing season for all land uses is more accurate than using any surrogate studied here. Vegetated wetlands in mesic and colder soil temperature areas may also have continuous growing seasons, although additional studies should be conducted to see if the continuous growing season holds.
Long-term studies should be conducted in selected Ecoregions (McMahon et al., 2001) or major land resource areas (mLRAs) (USDA-SCS, 1981) to relate soil temperature data in wetlands with regional soil temperatures. The data set could be used by modelers to validate models that predict soil temperature in wetlands from existing air temperature databases. The ecologically based maps that result would be an improvement from the published data sets that change across county line boundaries. These studies could be accomplished within 5 to 10 yr, provided that precipitation and temperatures would fall within normal ranges during at least half of the study period. While this seems like a long time, it has been more than 15 yr since the WDM was published and more than 10 yr since the published frost-free days were allowed to be used as a surrogate growing season indicator.
In wetland mitigation areas, on-site measurement of air and soil temperatures should be required in addition to hydrologic monitoring currently required, so that the timing of surface saturation and the period of biological activity are properly matched. The additional cost to monitor soil temperature would be minimal. A mandatory submission of the prior land use, current hydrology, and air and soil temperature data set to the appropriate regulatory agency that permits mitigation sites could yield a better understanding of the timing of the growing season and high water tables and could be used to improve current regulatory limits. The requirement would also allow each mitigation site to qualify as a wetland based on complete measured on-site properties.
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ACKNOWLEDGMENTS |
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Received for publication March 26, 2004.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONStudy AreasMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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