ASPEN FACE Site Characteristics
The Aspen FACE experiment, formerly referred to as Forest – Atmosphere Carbon Transfer and Storage – II (FACTS-II), was conducted on a 32 ha USDA Forest Service experimental farm near Rhinelander, Wisconsin, USA (45.6°N, 89.5°W). The legal description of the site is SW80, Sect. 21, T37N, R7E, Cassian Township, Oneida County, Wisconsin. The site was cultivated for crops from the 1920s until 1972. From 1972 until 1996, it served as a short-rotation tree crop and forest genetics research facility for the U.S. Forest Service. In 1996-1997 the site was cleared of most of the existing plantations, and the FACE experiment was constructed. The site was surrounded by a 3.7 m deer-proof fence with an automatic security gate.
The soil is a Pandus series, mixed, frigid, coarse loamy Alfic Haplorthod. It consists of a 30 cm clay loam plow layer that grades into sandy loam above stratified sand and gravel. Average soil texture across the site is 55.6% sand, 36.6% silt, and 7.8% clay. The topography is level to gently rolling. When the FACE experiment was initiated, total soil N was 0.12%, with 13.53 mg g-1 of NO3- N, and 0.84 mg g-1 of NH4+ N, and the C:N was 12.9. Much more detailed information on site and soil characteristics is presented in Dickson et al. (2000).
The Aspen FACE experiment was constructed in 1996 and 1997, and the first full year of fumigation occurred in 1998. The experiment consisted of 12 treatment “rings”: three replicates each of control, elevated CO2, elevated O3, and elevated CO2 + O3 atmospheres. Target elevated CO2 and O3 concentrations ([CO2] and [O3], respectively) were 560 µll-1 CO2, and 1.5 times ambient [O3].
The treatment rings were dispersed around the 32 ha site at spacing of about 100 m between rings. The three replicates were blocked roughly into north (rep. 1), central (rep. 2), and south (rep. 3) locations of the site. Each treatment ring was planted with three model communities of trembling aspen (Populus tremuloides Michx.): trembling aspen monoculture, trembling aspen and paper birch (Betula papyrifera Marsh.), and trembling aspen and sugar maple (Acer saccharum Marsh.). Thus the experiment was a fixed effects, split-plot, two-factor, complete block design (n=3) with community type as the split-plot factor. Meteorological monitoring occurred at a 20 m instrumented tower located on the north end of the site, and in each treatment ring (see details below).
In March 1997, five clones of trembling aspen were propagated as rooted greenwood cuttings (Karnosky et al. 1996) from selected stock plants, and birch and maple seedlings were propagated from open-pollinated seed. Birch and maple seeds were collected below parent trees in Houghton Co., Michigan in late summer 1996, and in Baraga Co., Michigan is autumn 1996, respectively. Refer to Dickson et al. (2000) for details on the plant propagation methods. In summer 1997, the treatment rings were planted at 1×1 m spacing with the three species. In each ring, the east half was aspen monoculture, the northwest quarter was aspen-maple mixed (1:1) community, and the southwest quarter was aspen-birch (1:1) mixed community.
The aspen monoculture consisted of the five clones that had been studied previously for responses to elevated CO2 and O3 (Karnosky et al., 1996; Kubiske et al., 1998). In general, it was reported that aspen clones 271, 216, and 259 were relatively tolerant, intermediate, and sensitive in terms of O3 sensitivity (Karnosky et al., 1996), and clones 42E and 8L were relatively more and less responsive, respectively, to elevated CO2 (Kubiske et al, 1998). The clones were planted as pairs of individuals of each clone, with the pairs randomly distributed throughout the 1×1 m grid. Some individuals of clones 216 and 271 (labeled E216 and E271) were raised in a growth chamber with CO2-enriched atmosphere (ambient + 350 µll-1 CO2) prior to out-planting to test the effect of exposure during early growth upon subsequent responses. The aspen-birch and aspen-maple mixed communities were planted in an alternating pattern throughout the 1×1 m grid. Aspen in the mixed communities was entirely clone 216.
CO2 and O3 Treatment Protocols
Elevated CO2 and O3 treatments operated only during the leaf-on season, and only during daylight hours. The fumigation treatments were initiated each spring with bud burst when aspen leaves were about 1/4 expanded. Fumigation was terminated each autumn when aspen leaves had changed color and were just beginning to abscise. Aspen was typically the first species to begin leaf expansion in spring, and Maple was typically the last species to senesce leaves in autumn. Fumigation seasons averaged 147 days. Each of the 9 treatment rings operated independently but employed the same CO2 and O3 treatment protocols. The target CO2 concentration [CO2] for the 6 elevated [CO2] treatment rings was 560 µll-1, which was approximately ambient daytime [CO2] + 200 µll-1. This treatment concentration was chosen to approximate the predicted atmospheric [CO2] in about the year 2060 ( ref ), and also for consistency with the FACE experiment that was in operation at Duke University (ref).
The CO2 fumigation system operated automatically each day from dawn to dusk, when the solar elevation exceeded 6° from the horizon. Treatment CO2 was released every day of the operating season except during brief interruption for system maintenance, rare equipment failure, or when wind speeds exceeded 5 m s-1. The treatment CO2 used to augment [CO2] of the ambient air was produced as a byproduct of fertilizer manufacture from methane and atmospheric nitrogen; it had a δ13C of about –44‰. The 13C content was verified by analysis of periodic samples from the storage tanks on a Finnigan Delta Plus mass spectrometer (K.S. Pregitzer).
The O3 fumigation protocol in 1998 and 1999 was designed around diurnal profiles measured at Leelenaw, Michigan during summer 1987. The profiles were then modified based on regional 6-year averages (Pinkerton and Lefohn, 1987). In those years, the site operator selected a daily peak [O3] based on the weather forecast. The maximum peak [O3] was 100 µll-1. for hot, sunny days, with a lower maximum, typically 50-60 µll-1, for cooler, cloudier days. Beginning in 2000, the O3 fumigation protocol was modified so that the automated control system would track ambient [O3] and dose the 6 treatment rings with 1.5 × the ambient concentration. Throughout the experiment, no O3 was released when foliage was wet from rain, dew, or fog, when maximum temperatures were projected to be less than 15°C. Also, and in addition to the high-wind speed threshold for system shut down programmed for the CO2 treatments, the O3 fumigation system incorporated a low wind speed cutoff threshold of 0.4 m s-1 for 40 seconds. Because the FACE system relies on wind to mix and deliver the treatment gasses into the plots, this low wind speed threshold helped prevent unusually high O3 concentrations from forming near the plot perimeters. Other than the automatic wind speed cutoff thresholds, the O3 fumigation system was switched on and off each day by the site operator.
- Dickson, R.E.; Lewin, K.F.; Isebrands, J.G.; Coleman, M.D.; Heilman, W.E.; Reimenschneider, D.E; Sober, J.; Host, G.E.; Zak, D.R.; Hendrey, G.R./ Pregitzer, K.S.; Karnosky, D.F. 2000. Forest Atmosphere carbon transfer and storage (FACTS-II) the aspen Free-air CO2 and O3 Enrichment (FACE) project: an overview. Gen Tech. Rep. NC-214. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Research Station. 68p.
- Karnosky, D.F.; Gagnon, jZ.E.; Dickson, R.E.; Coleman, M.D.; Lee, E.H.; Isebrands, J.G. 1996. Changes in growth, leaf abscission, and biomass associated with seasonal tropospheric ozone exposures of Populus tremuloides clones and seedlings. Canadian Journal of Forest Research. 26: 23-37.
- Kubiske, M.E.; Pregitzer, K.S.; Zak, D.R.; Mikan, C.J. 1998. Growth and C allocation of Populus tremuloides genotypes in response to atmospheric CO2 and soil N availability. New Phytology. 140: 251-260.
- Pinkerton, J.E.; Lefohn, A.S. 1987. The characterization of ozone data for sites located in forested areas of the eastern United States. Journal of Air Pollution Control Association. 33: 1005-1010.