Growth and photosynthesis of loblolly pine (Pinus taeda) after exposure to elevated CO2 for 19 months in the field

To detect seasonal and long-term differences in growth and photosynthesis of loblolly pine (Pinus taeda L.) exposed to elevated CO2 under ambient conditions of precipitation, light, temperature and nutrient availability, seedlings were planted in soil representative of an early, abandoned agricultural field and maintained for 19 months in the field either in open-top chambers providing one of three atmospheric CO2 partial pressures (ambient, ambient +15 Pa, and ambient +30 Pa) or in unchambered control plots. An early and positive response to elevated CO2 substantially increased total plant biomass. Peak differences in relative biomass enhancement occurred after 11 months of CO2 treatment when biomass of plants grown at +15 and +30 Pa CO2 was 111 and 233% greater, respectively, than that of plants grown at ambient CO2. After 19 months, there was no significant difference in biomass between +15 Pa CO2-treated plants and ambient CO2-treated plants, whereas biomass of +30 Pa CO2-treated plants was 111% greater than that of ambient CO2-treated plants. Enhanced rates of leaf-level photosynthesis were maintained in plants in the elevated CO2 treatments throughout the 19-month exposure period despite reductions in both leafN concentration and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity during the first 11 months of CO2 exposure. Reductions in Rubisco activity indicated photosynthetic adjustment to elevated CO2, but Rubisco-mediated control of photosynthesis was small. Seasonal shifts in sink strength affected photosynthetic rates, greatly magnifying the positive effects of elevated CO2 on photosynthesis during periods of rapid plant growth. Greater carbon assimilation by the whole plant accelerated plant development and thereby stimulated new sinks for carbon through increased plant biomass, secondary branching and new leaf production. We conclude that elevated CO2 will enhance photosynthesis and biomass accumulation in loblolly pine seedlings under high nutrient conditions; however, reductions over time in the relative biomass response of plants to elevated CO2 complicate predictions of the eventual magnitude of carbon storage in this species under future CO2 conditions.