PLANT MATERIAL AND CHAMBERS:
Bare-rooted seedlings of hemlock (T. canadensis) and sugar maple (A. saccharum) were 3–5 years old and 45–60 cm tall when planted into the experiment. In May 2004, two seedlings of each species were randomly assigned to each rootbox and two rootboxes were placed under each chamber. In September 2004, the seedlings from one rootbox of each chamber were harvested. In May 2005 new seedlings were planted.

Root boxes were prepared by drilling ~30–2.5 cm holes into the bottoms of plastic storage tubs (82 × 52 × 42 cm). The boxes were buried level with the surrounding soil surface. The boxes were filled with sand and covered with 10 cm of topsoil (from local sources) to establish mycorrhizal associations and a microbial community similar to that of the local forests. This sand/soil layering mimicked the soil structure in the surrounding forests.

One open-top chamber (0.8 m × 0.8 m × 1 m) was placed over each pair of rootboxes. The chamber frames were made of 1/2 inch PVC pipe and were wrapped with transparent 0.8 mm PVC film. Fans encased in metal blower boxes were connected to a perforated ring of PVC that was placed at the bottom of the chamber. A total of 80 chambers were used. Each chamber was randomly assigned to one of eight possible treatment combinations: control (ambient CO2, ambient NO2, no soil NO3- addition), elevated CO2, elevated NO2, high NO3-, elevated CO2 + elevated NO2, elevated CO2 + high NO3-, elevated NO2 + high NO3- and elevated CO2 + elevated NO2 + high NO3-.

TREATMENTS:
The elevated CO2 treatment began on 20 June 2004 and 13 June 2005 and ended on 15 September 2004 and 22 August 2005 when the plants were harvested. The fumigation treatments began after leaf-out. Elevated CO2 chambers were set to 760 ppm CO2, elevated NO2 chambers received 40 ppb NO2. The NO concentration in elevated NO2 chambers was typically 3–7 ppb. The ambient CO2 concentration was 365 ppm and ambient NO2 and NO concentrations were both <1 ppb. Half the chambers were given additional soil N in the form of NaNO3 at a rate of 30 kg N ha−1 year−1. The initial NO3- addition was after bud-break, and the repeated applications were designed to maintain high NO3- in the soil throughout the growing season.

PHOTOSYNTHESIS MEASUREMENTS:
The Li-Cor LI-6400 portable gas exchange system (Li-Cor) was used for all gas exchange measurements. Photosynthesis was measured at two CO2 concentrations for each seedling. First, each seedling was measured at the CO2 concentration under which it was growing (i.e., seedlings grown under ambient CO2 were measured at 380 ppm CO2 and those grown under elevated CO2 were measured at 760 ppm CO2) and then each seedling was measured at 760 ppm.

GROWTH AND ALLOCATION MEASUREMENTS:
At the time of planting, 10 seedlings of each species were harvested and the height, stem diameter and total dry biomass were measured. In September 2005, all seedlings were harvested by removing the rootboxes from the ground and placing the soil and seedlings onto a 2 mm mesh screen to rinse soil from the roots. Once clean, the roots, stems and leaves were separated, the roots frozen, and the leaves pressed. The plant tissues were dried for 3 days at 50 °C and weighed to determine dry mass .

The total leaf area of maple seedlings was determined using a leaf area meter (LI-3100, Li-Cor). Leaf area was divided by total leaf biomass to determine specific leaf area (SLA). To determine the SLA of hemlock seedlings, only needles that were produced in 2005 were used. The area of hemlock needles was determined by photographing several needles on a background of known area. Using Adobe Photoshop software (Adobe Systems, Inc., San Jose, CA, USA), we calculated the percentage of background that was covered with needles. The photographed leaves were then weighed to determine SLA.

Dried leaf tissue was ground and %C and %N were determined using an Elemental Analyzer (FlashFA 1112; ThermoFinnegan, Pittsburg, PA, USA).