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

Abstract

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 leaf N 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.