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 atmos phere each year; c. 15% of the atmospheric pool of C is assimilated in terrestrial-plant photosynthesis each year, with an about equal amoun t returned to the atmosphere as CO2 in plant respiration and the decom position of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO2 content dur ing 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. Clima tic changes and increases in pollution associated with increasing atmo spheric CO2 concentration may be as significant to plant and ecosystem C balance as CO2 concentration itself. Moreover, human activities suc h as deforestation and livestock grazing can have impacts on the C bal ance and structure of individual terrestrial ecosystems that far outwe igh effects of increasing CO2 concentration and climatic change. In sh ort-term experiments, which in this case means on the order of 10 year s or less, elevated atmospheric CO2 concentration affects terrestrial higher plants in several ways. Elevated CO2 can stimulate photosynthes is, but plants may acclimate and (or) adapt to a change in atmospheric CO2 concentration. Acclimation and adaptation of photosynthesis to in creasing CO2 concentration is unlikely to be complete, however. Plant water-use efficiency is positively related to CO2 concentration, imply ing the potential for more plant growth per unit of precipitation or s oil moisture with increasing atmospheric CO2 concentration. Plant resp iration 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 inhi bition of respiration can be detrimental. The net effect on terrestria l plants of elevated atmospheric CO2 concentration is generally an inc rease in growth and C accumulation in phytomass. Published estimations , and speculations about, the magnitude of global terrestrial-plant gr owth responses to increasing atmospheric CO2 concentration range from negligible to fantastic. Well-reasoned analyses point to moderate glob al plant responses to CO2 concentration. Transfer of C from plants to soils is likely to increase with elevated CO2 concentrations because o f greater plant growth, but quantitative effects of those increased in puts to soils on soil C pool sizes are unknown. Whether increases in l eaf-level photosynthesis and short-term plant growth stimulations caus ed by elevated atmospheric CO2 concentration will have, by themselves, significant long-term (tens to hundreds of years) effects on ecosyste m C storage and atmospheric CO2 concentration is a matter for speculat ion, not firm conclusion. Longterm field studies of plant responses to elevated atmospheric CO2 are needed. These will be expensive, difficu lt, and by definition, results will not be forthcoming for at least de cades. Analyses of plants and ecosystems surrounding natural geologica l CO2 degassing vents may provide the best surrogates for long-term co ntrolled experiments, and therefore the most relevant information pert aining to long-term terrestrial-plant responses to elevated CO2 concen tration, but pollutants associated with the vents are a concern in som e 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 in crease in atmospheric CO2 concentration. And, in the very long-term, a tmospheric CO2 concentration is controlled by atmosphere-ocean C equil ibrium rather than by terrestrial plant and ecosystem responses to atm ospheric CO2 concentration.