Direct Effects of Increasing Carbon Dioxide on Vegetation
Carbon dioxide (CO2) enhancement of plant growth is one important direct effect of rising atmospheric CO2. Through photosynthesis, plants produce food and fiber from light and carbon assimilated as CO2 and form the foundation of the Earth's life support system. Rising atmospheric CO2 is thus an essential input to the food-producing process, and effects of CO2 enrichment described in this document have far-reaching implications for agricultural and ecological productivity. Establishing an empirical foundation of plant response to more CO2 and developing the capability to predict crop and ecosystem responses to CO2 enrichment are research goals that have been pursued in the DOE-led program. This State-of-the-Art report on the direct effects of CO2 on vegetation summarizes the current state of knowledge; it identifies uncertainties and suggests where more research is needed to assure understanding and delineates data and modeling requirements for future predictions. Important progress has been made over the past 5 years since the plant science community urged that more attention be given to direct effects of CO2 enrichment. Enhanced plant growth and yield from more CO2 is now widely recognized in different scientific and public sectors as an important element of the CO2 problem. While there have been gains in knowledge about a few crop plants, considerable ignorance remains about responses of native species and ecosystems to CO2 enrichment. Only a few native species or ecosystem types have been investigated. Competition among species in relation to CO2-induced growth differences is essentially unknown. Many interactions involving CO2 enrichment and other physical and biological variables affecting plant growth have not been studied. Among the physiological processes directly affected by CO2 that are not well understood are those involving net transpiration and whole-plant water use, where CO2 reduces stomatal conductance but increases leaf size. Nutrient requirements to sustain increased photosynthesis and growth at enriched levels of CO2 are not understood. This State-of-the-Art volume examines some key responses and identifies research needed to resolve these and other key unknowns.
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