Over the past three years, a set of coordinated field studies has been conducted in the south-eastern United States on the response of loblolly pine (Pinus taeda, L.) to elevated CO2. The studies were located in North Carolina, Georgia and Oklahoma and represent a transect across the natural range of the species. All three studies used branch chambers to impose elevated CO2 treatments on trees in plantations. At these sites different studies were conducted on the effects of nutrient and water availability and elevated air temperature on carbon gain in elevated CO2 concentrations. Treatments were imposed by fertilisation and/or irrigation of the sites and by elevating air temperature within the branch chambers.
The response to elevated CO2 was consistent among the sites. On average, CO2 enrichment of 1.5 times the ambient concentration increased net photosynthesis by about 45%, and at twice ambient it was increased by about 95% over rates at ambient. After 2 and 3 years of measurement there was no indication of photosynthetic acclimation. Elevating air temperature (+2 °C) reduced carbon gain at elevated CO2 concentrations about 10% in the summer when air temperature exceeded 30 °C, but increased carbon gain by about the same amount in spring and autumn when air temperatures remained below 30 °C. Irrigation and fertilisation increased the maximum rates of net photosynthesis, but there was no interaction between the availability of these resources and enhancement of net photosynthesis in elevated CO2 treatments. In other words, the relative increase in net photosynthesis with increase in CO2 concentration was the same regardless of resource availability.
Growth was enhanced by elevated CO2 and leaf area increased by 50% at twice ambient CO2 concentrations. Stomatal conductance to water vapour did not appear to change with CO2 treatment. In elevated CO2 treatments, leaf carbohydrate content increased, specific leaf area decreased, and most macro-nutrient concentrations decreased on a leaf weight basis, but not on a leaf area basis.
These results indicate that CO2 enrichment will enhance carbon gain of this species on sites ranging from fertile to infertile and moist to dry, and that elevated air temperatures will have compensating positive and negative effects on carbon gain in different seasons.
Key words: elevated CO2, photosynthesis, productivity.
Correspondence: Robert O. Teskey, School of Forest Resources, University of Georgia, Athens, GA 30602, USA