Compensatory responses of CO2exchange and biomass allocation and their effects on the relative growth rate of ponderosa pine in different CO2and temperature regimes.
Increases in the concentration of atmospheric carbon dioxide may have a fertilizing effect on plant growth by increasing photosynthetic rates and therefore may offset potential growth decreases caused by the stress associated with higher temperatures and lower precipitation. However, plant growth is determined both by rates of net photosynthesis and by proportional allocation of fixed carbon to autotrophic tissue and heterotrophic tissue. Although CO2fertilization may enhance growth by increasing leaf-level assimilation rates, reallocation of biomass from leaves to stems and roots in response to higher concentrations of CO2and higher temperatures may reduce whole-plant assimilation and offset photosynthetic gains. We measured growth parameters, photosynthesis, respiration, and biomass allocation of Pinus ponderosa seedlings grown for 2 months in 2×2 factorial treatments of 350 or 650μ bar CO2and 10/25° C or 15/30° C night/day temperatures. After 1 month in treatment conditions, total seedling biomass was higher in elevated CO2, and temperature significantly enhanced the positive CO2effect. However, after 2 months the effect of CO2on total biomass decreased and relative growth rates did not differ among CO2and temperature treatments over the 2-month growth period even though photosynthetic rates increased ≈7% in high CO2treatments and decreased ≈10% in high temperature treatments. Additionally, CO2enhancement decreased root respiration and high temperatures increased shoot respiration. Based on CO2exchange rates, CO2fertilization should have increased relative growth rates (RGR) and high temperatures should have decreased RGR. Higher photosynthetic rates caused by CO2fertilization appear to have been mitigated during the second month of exposure to treatment conditions by a ≈3% decrease in allocation of biomass to leaves and a ≈9% increase in root:shoot ratio. It was not clear why diminished photosynthetic rates and increased respiration rates at high temperatures did not result in lower RGR. Significant diametrical and potentially compensatory responses of CO2exchange and biomass allocation and the lack of differences in RGR of ponderosa pine after 2 months of exposure of high CO2indicate that the effects of CO2fertilization and temperature on whole-plant growth are determined by complex shifts in biomass allocation and gas exchange that may, for some species, maintain constant growth rates as climate and atmospheric CO2concentrations change. These complex responses must be considered together to predict plant growth reactions to global atmospheric change, and the potential of forest ecosystems to sequester larger amounts of carbon in the future.
Plant Biology, University of Illinois, 61801, Urbana, IL, USA.
R M
RM
Callaway
Plant Biology, University of Illinois, 61801, Urbana, IL, USA.
E H
EH
DeLucia
Department of Botany and Geology, Duke University, 27708, Durham, NC, USA.
E M
EM
Thomas
Department of Botany and Geology, Duke University, 27708, Durham, NC, USA.
W H
WH
Schlesinger
Biomass allocation; Climate change CO2 enhancement; Photosynthesis; Relative growth rate