Duke Forest FACE - Site Characteristics
The Blackwood Division of the Duke Forest is near Chapel Hill, in Orange County, North Carolina (35° 58' 41.430"N, 79° 05' 39.087" W, 163 m asl). Mean annual precipitation is 1140 mm, and mean annual temperature is 15.5 °C. Soils are low-fertility acidic Hapludalf in the Enon Series typical of uplands in southeastern USA, with a clayey loam in the upper 0.3 m, and clay below down to the bedrock at 0.7 m. Maximum soil moisture is 0.54 m3 m-3. The local topographic variations are small (< 5% slopes). The site contains three vegetation types typical of succession after abandonment of agricultural fields in the region: A field covered with herbaceous vegetation (maintained at this stage by annual mowing); a pine forest established in 1983 as a plantation that currently contains more than 40 woody species, some in the canopy but mostly in sub-canopy positions, and an Oak-Hickory type forest with remnant pine component.
The 90-ha block of the stand is even-aged (planted in 1983) and was established from seedlings following clear-cut and burn at the site. Loblolly pine trees from a Piedmont provenance were planted at 2m × 2.4m (6' × 8') spacing, and natural regeneration added a large number of additional species. Density of co-dominant pines in the study portion of the stand is approximately 1600 trees ha-1 and total tree density of dominant and subcanopy hardwood trees is 3700 trees ha-1. The subcanopy and understory are diverse with more than 50 species (mostly woody) but dominated by sweetgum (Liquidambar styraciflua), elm (Ulmus alata), red maple (Acer rubrum), dogwood (Cornus florida) and oak-hickory species. Soil pH is around 6.0, and nitrogen (~1.1% N) and phosphorus (~0.3% P) in the pine foliage tend to be at the middle range for mid-rotation loblolly pine in the region. The site index is between 21 and 22 m at age 25.
Forest Cover Types
Over 100 species of trees have been identified in the Duke Forest. The major types in descending order of prevalence are pine, pine-hardwood, upland hardwood, and bottomland hardwood. Pine stands grow on a variety of sites, which are usually the result of old-field succession or planting. The most common species of pine found within the Forest are loblolly, shortleaf and Virginia pine. Typically these pines grow on drier sites.
Little is known on the history of the site prior to the purchase by Duke University, mostly completed in 1944. Records show that the site was occupied by the "Big Meadows" in 1921, with the herbaceous and grassy cover maintained through repeated burning by European settlers since the early 19th century, perhaps preceded by similar practice of the Native Americans. Local farmers harvested the meadow's vegetaion for hay. By 1964, the Big Meadows were occupied with forest in which junipers were large enough to harvest for posts, indicating that tree species colonized the meadow before or shortly after its purchase by Duke. In the late 1970's/early 1980's a large portion of the area now occupied by two of three AmeriFlux sites (Old Field and Pine Plantation) and the FACE site was cleared, and following site preparation a portion of it was planted into loblolly pine.
The Duke Forest lies near the eastern edge of the North Carolina piedmont plateau and supports a cross section of the woodlands found in the upper coastal plain and lower piedmont of the Southeast. A variety of timber types, plant species, soils, topography, and past land use conditions are represented.
Centrally located between the mountains to the west and the ocean to the south and east, the Forest has a moderate climate. Temperatures range from an average daily maximum of 10.6 degrees and minimum of 0 degrees in January to an average daily maximum of 31.1 degrees and minimum of 20 degrees in July. Precipitation averages about 1117.6 millimeters annually and is well distributed throughout the year. July and August are normally the wettest months with an average of 129.5 mm of rainfall; October and November are normally the driest with an average of 68.9 mm.
Topography and Soils
Much of the Forest exhibits rolling terrain. Elevations range from 250 feet above sea level in the Haw River Division to 760 feet on Bald Mountain in the Blackwood Division.
The soils of the southern Piedmont are derived from diverse and ancient parent materials, including granite, granitic gneiss, metamorphic rock of the Carolina slate formation (from which Duke West Campus buildings are built), Triassic sedimentary rocks, and basic ignaceous intrusives, all of which are well represented on the Duke Forest. Soils within the forest belong to some of the most common soil series within the Piedmont: Georgeville, Herndon, Tatum, Goldston, Whitestore, Creedmoor, Chewacla, Enon, Appling, and Cecil. Most of these series have been substantially impacted by an agricultural past and are characterized by relatively coarse-textured surface soils (sandy loams to loams) and an underlying fine-textured clayey layers. Many of the residual soils that are derived directly from underlying bedrock are many meters deep. A Georgeville series soil on the Durham Division, intensively studied in the 1990s, was determined to be nearly 30 m in total depth over unweathered, solid bedrock. Such deep soils result from extreme weathering conditions that have affected the southern Piedmont for 10's of millions of years.
15N was applied to FACE plots in pairs starting with rings 6&4 on 5/7/03, and ending with 1&2 on 5/9/03. The application used FACE site water at a rate of 250 ml solution per m2 ground area. A total of 0.015g N per m2 ground area was applied as 15NH4Cl (75% by mass) and K15NO3 (25%). This raised d15N of the surface litter horizon (Oi Horizon) to 1000 per mil, a level expected to dilute rapidly as the 15N tracer travels through the system and litterfall ensues. The first ecosystem-scale measurements of 15N redistribution is scheduled for September 2003, with measurements planned through September 2006.