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

Abstract

Terrestrial higher plants exchange large amounts of CO 2 with 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 CO 2 in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO 2 content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO 2 concentration might influence the rate of atmospheric CO 2 increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO 2 concentration may modulate plant and ecosystem responses to CO 2 concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO 2 concentration may be as significant to plant and ecosystem C balance as CO 2 concentration 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 CO 2 concentration and climatic change. In short‐term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO 2 concentration affects terrestrial higher plants in several ways. Elevated CO 2 can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO 2 concentration. Acclimation and adaptation of photosynthesis to increasing CO 2 concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO 2 concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO 2 concentration. Plant respiration may be inhibited by elevated CO 2 concentration, 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 CO 2 concentration 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 CO 2 concentration range from negligible to fantastic. Well‐reasoned analyses point to moderate global plant responses to CO 2 concentration. Transfer of C from plants to soils is likely to increase with elevated CO 2 concentrations 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 CO 2 concentration will have, by themselves, significant long‐term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO 2 concentration is a matter for speculation, not firm conclusion. Long‐term field studies of plant responses to elevated atmospheric CO 2 are 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 CO 2 degassing 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 CO 2 concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO 2 concentrations near vents is limited. On the whole, terrestrial higher‐plant responses to increasing atmospheric CO 2 concentration probably act as negative feedbacks on atmospheric CO 2 concentration increases, but they cannot by themselves stop the fossil‐fuel‐oxidation‐driven increase in atmospheric CO 2 concentration. And, in the very long‐term, atmospheric CO 2 concentration is controlled by atmosphere‐ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO 2 concentration.