Terrestrial higher‐plant response to increasing atmospheric [CO2] in relation to the global carbon cycle

Abstract

AbstractTerrestrial higher plants exchange large amounts of CO2with the atmosphere each year; c. 15% of the atmospheric pool of C is assimilated in terrestrial‐plant photosynthesis each year, with an about equal amount returned to the atmosphere as CO2in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO2content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO2concentration might influence the rate of atmospheric CO2increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO2concentration may modulate plant and ecosystem responses to CO2concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO2concentration may be as significant to plant and ecosystem C balance as CO2concentration itself. Moreover, human activities such as deforestation and livestock grazing can have impacts on the C balance and structure of individual terrestrial ecosystems that far outweigh effects of increasing CO2concentration and climatic change.In short‐term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO2concentration affects terrestrial higher plants in several ways. Elevated CO2can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO2concentration. Acclimation and adaptation of photosynthesis to increasing CO2concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO2concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO2concentration. Plant respiration may be inhibited by elevated CO2concentration, and although a naive C balance perspective would count this as a benefit to a plant, because respiration is essential for plant growth and health, an inhibition of respiration can be detrimental. The net effect on terrestrial plants of elevated atmospheric CO2concentration is generally an increase in growth and C accumulation in phytomass. Published estimations, and speculations about, the magnitude of global terrestrial‐plant growth responses to increasing atmospheric CO2concentration range from negligible to fantastic. Well‐reasoned analyses point to moderate global plant responses to CO2concentration. Transfer of C from plants to soils is likely to increase with elevated CO2concentrations because of greater plant growth, but quantitative effects of those increased inputs to soils on soil C pool sizes are unknown.Whether increases in leaf‐level photosynthesis and short‐term plant growth stimulations caused by elevated atmospheric CO2concentration will have, by themselves, significant long‐term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO2concentration is a matter for speculation, not firm conclusion. Long‐term field studies of plant responses to elevated atmospheric CO2are needed. These will be expensive, difficult, and by definition, results will not be forthcoming for at least decades. Analyses of plants and ecosystems surrounding natural geological CO2degassing vents may provide the best surrogates for long‐term controlled experiments, and therefore the most relevant information pertaining to long‐term terrestrial‐plant responses to elevated CO2concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO2concentrations near vents is limited.On the whole, terrestrial higher‐plant responses to increasing atmospheric CO2concentration probably act as negative feedbacks on atmospheric CO2concentration increases, but they cannot by themselves stop the fossil‐fuel‐oxidation‐driven increase in atmospheric CO2concentration. And, in the very long‐term, atmospheric CO2concentration is controlled by atmosphere‐ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO2concentration.