Positive feedback between increasing atmospheric CO2 and ecosystem productivity

I. Gelfand, S. K. Hamilton, G. P. Robertson

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


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.
Original languageEnglish GB
Title of host publicationAGU Fall Meeting Abstracts 2009
StatePublished - 1 Dec 2009
Externally publishedYes


  • 0426 BIOGEOSCIENCES / Biosphere/atmosphere interactions
  • 0439 BIOGEOSCIENCES / Ecosystems
  • structure and dynamics
  • 1615 GLOBAL CHANGE / Biogeochemical cycles
  • processes
  • and modeling
  • 1630 GLOBAL CHANGE / Impacts of global change


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