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

Terrestrial higher plants exchange large amounts of CO2 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 CO2 in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO2 content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO2 concentration might influence the rate of atmospheric CO2 increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO2 concentration may modulate plant and ecosystem responses to CO2 concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO2 concentration may be as significant to plant and ecosystem C balance as CO2 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 CO2 concentration and climatic change. In short-term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO2 concentration affects terrestrial higher plants in several ways. Elevated CO2 can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO2 concentration. Acclimation and adaptation of photosynthesis to increasing CO2 concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO2 concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO2 concentration. Plant respiration may be inhibited by elevated CO2 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 CO2 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 CO2 concentration range from negligible to fantastic. Well-reasoned analyses point to moderate global plant responses to CO2 concentration. Transfer of C from plants to soils is likely to increase with elevated CO2 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 CO2 concentration will have, by themselves, significant long-term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO2 concentration is a matter for speculation, not firm conclusion. Long-term field studies of plant responses to elevated atmospheric CO2 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 CO2 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 CO2 concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO2 concentrations near vents is limited. On the whole, terrestrial higher-plant responses to increasing atmospheric CO2 concentration probably act as negative feedbacks on atmospheric CO2 concentration increases, but they cannot by themselves stop the fossil-fuel-oxidation-driven increase in atmospheric CO2 concentration. And, in the very long-term, atmospheric CO2 concentration is controlled by atmosphere-ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO2 concentration.