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Whole ecosystem CO flux under ambient (340 μl/l) and elevated (680 μl/l) CO was measured in situ in Eriophorum tussock tundra on the North Slope of Alaska. Elevated CO resulted in greater carbon acquisition than control treatments and there was a net loss of CO under ambient conditions at this upland tundra site. These measurements indicate a current loss of carbon from upland tundra, possibly the result of recent climatic changes. Elevated CO for the duration of one growing season appeared to delay the onset of dormancy and resulted in approximately 10 additional days of positive ecosystem flux. Homeostatic adjustment of ecosystem CO flux (sum of species' response) was apparent by the third week of exposure to elevated CO. Ecosystem dark respiration rates were not significantly higher at elevated CO levels. Rapid homeostatic adjustment to elevated CO may limit carbon uptake in upland tundra. Abiotic factors were evaluated as predictors of ecosystem CO flux. For chambers exposed to ambient and elevated CO levels for the duration of the growing season, seasonality (Julian day) was the best predictor of ecosystem CO flux at both ambient and elevated CO levels. Light (PAR), soil temperature, and air temperature were also predictive of seasonal ecosystem flux, but only at elevated CO levels. At any combination of physical conditions, flux of the elevated CO treatment was greater than that at ambient. In short-term manipulations of CO, tundra exposed to elevated CO had threefold greater carbon gain, and had one half the ecosystem level, light compensation point when compared to ambient CO treatments. Elevated CO-acclimated tundra had twofold greater carbon gain compared to ambient treatments, but there was no difference in ecosystem level, light compensation point between elevated and ambient CO treatments. The predicted future increases in cloudiness could substantially decrease the effect of elevated atmospheric CO on net ecosystem carbon budget. These analyses suggest little if any long-term stimulation of ecosystem carbon acquisition by increases in atmospheric CO.

Growth and development of native species of trees in response to long-term increases in atmospheric CO2 concentration were studied. Seedlings of two competing perennials, Pinustaeda L. and Liquidambarstyraciflua L., were obtained from germinated seeds and grown through one complete growing season at 350, 500, and 650 μL•L−1 CO2. The plants were grown in CO2 controlled greenhouses under natural photoperiods and light regimes, with temperature controlled to simulate mean local climate. Stem length and basal stem diameter increased with increasing CO2 in both species. Liquidambarstyraciflua maintained size dominance in all concentrations. The dry weights of stems, roots, and leaves increased in both species. In P. taeda, however, the seedlings reached maximum size at 500 μL•L−1 while L. styraciflua continued to increase up to 650 μL•L−1. Liquidambarstyraciflua produced significantly more branches and leaves at the higher CO2 concentrations than at 350 μL L−1. Differences in plant shape and responses in growth rate of these two naturally competing tree species suggest that continuing atmospheric CO2 enrichment could affect future interactions between the species and might produce changes in community composition.

SUMMARYSeedlings of ponderosa pine {Pinus ponderosa Laws,) were grown for three years under three atmospheric ozone concentrations -clean air (CF), amhient ozone (NF), and 15 times amhient ozone (NF150)-at a moderatelypolluted site in the Sierra Nevada, under either well-watered or drought-stressed conditions. When the trees were 5 years old, photosynthetic capacities of 2-year-old, 1-year-old, and current-year needles were measured during August and September of the 3rd season of exposure. Current-year needles of NF150 trees had higher photosynthetic capacity than NF and CF trees during late summer, an effect due to greatly enhanced photosynthesis in well-watered plants that had lost older needles as a result of ozone damage. This photosynthetic compensation in well-watered NF150 seedlings was related to higher tissue nitrogen concentration in the currentyear foliage and possibly to increased inorganic phosphate cycling, hoth responses to the loss of older needles. Drought-stressed NFl 50 seedlings were partially protected from ozone damage hy decreased stomatal conductance and did not exhibit the same degree of photosynthetic compensation. No differences in photosynthetic rate were found between CF and NF seedlings or hetween well-watered and drought-stressed seedlings (across ozone treatments) in any needle age class.