Positive feedback between increasing atmospheric CO₂ and ecosystem productivity

Increasing atmospheric CO₂ will likely affect both the hydrologic cycleand ecosystem productivity. Current assumptions that increasing CO₂ willlead to increased ecosystem productivity and plant water use efficiency(WUE) are driving optimistic predictions of higher crop yields as wellas greater availability of freshwater resources due to a decrease inevapotranspiration. The plant physiological response that drives these effects is believed to be an increase in carbon uptake either by (a)stronger CO₂ gradient between the stomata and the atmosphere, or by (b)reduced CO₂ limitation of enzymatic carboxylation within the leaf. The(a) scenario will lead to increased water use efficiency (WUE) in plants. However, evidence for increased WUE is mostly based on modeling studies, and experiments producing a short duration or step-wiseincrease in CO₂ concentration (e.g. free-air CO2 enrichment). We hypothesize that the increase in atmospheric CO₂ concentration is having a positive effect on ecosystem productivity and WUE. To investigate this hypothesis, we analyzed meteorological, ANPP, and soil CO₂ flux data sets together with carbon isotopic ratio (13C/12C) of archived plant samples from the long term ecological research (LTER) program at KelloggBiological Station. The datasets were collected between 1989 and 2007(corresponding to an increase in atmospheric CO₂ concentration of ~33ppmv at Mauna Loa). Wheat (Triticum aestivum) samples taken from 1989and 2007 show a significant decrease in the C isotope discriminationfactor (Δ) over time. Stomatal conductance is directly related toΔ, and thus Δ is inversely related to plant intrinsic WUE(iWUE). Historical changes in the 13C/12C ratio (δ13C) in samplesof a perennial forb, Canada goldenrod (Solidago canadensis), taken fromadjacent successional fields, indicate changes in Δ upon uptake of CO₂ as well. These temporal trends in Δ suggest a positivefeedback between the increasing CO2 concentration in the atmosphere, air temperature, and plant iWUE. This positive feedback is expressed by (a)nonparallel changes of δ13C signal of atmospheric CO₂ (δa)and plant samples (δp), (b) negative correlation between theΔ and average temperatures during the growth season, although only for temperatures up to 21°C. The lack of effect at high ertemperatures suggests a negative influence of growing season warming on the iWUE. These results suggest a complex feedback between atmosphericCO₂ increase, plant physiology, ecosystem productivity, and soil CO₂ fluxes. These complex effects support our hypothesis of a CO₂ fertilization effect on plant productivity, and they raise addition alquestions regarding adaptation of plants to changing atmospheric CO₂ and climate.