There are two ways to test how plants will react to elevated CO2 levels.  One way is to use a chamber (fig 1.1), the other is to do free air experiments (Fig 1.2). Although plants do well with elevated CO2 levels in chambers they are problematic.  Even though plants in chambers receive ~25% less sunlight there is greater amounts of diffuse light which plants respond positively to.  Other environmental changes that result from chambers are altered air flow, intercepted rainfall, restricted access by disease and pests, a much higher water vapor pressure deficit*, and a full 4.3 degrees C increase in temperature.  Even the best of chambers that are designed to minimize environmental differences will often have an effect on the plant that is "greater than" that of elevated CO2.  Because of these problems large scale free air or outdoor experiments were developed.  These experiments are typically referred to as FACE or Free Air CO2 Enrichment.  These experiments show a much smaller boost to production than chamber experiments.  Woody plants such as trees and cotton still showed a significant increase in biomass production.  However, crop plants such as grains showed a much lower boost in production.  While wheat and rice showed some increases in yield, the increases were so small that they "were not statistically significant".  Sorghum yield was not affect by growth at elevated CO2.  Rising temperatures combined with decreasing soil moisture, which are side effects of increased greenhouse gases, will work to retard plant growth.  Recent review papers state that there is "serious doubt .... that rising [CO2] will fully offset losses due to climate change."  A list of review papers can be found here.  Real time data as well as archive data can be found here.

Chamber Experiment	FACE at Duke
Fig 1.1 Source: Finnish Forest Research Institute	The white rings are giant CO2 emitters which can be seen in Fig 2 and 3 Fig 1.2 Source: Duke FACE

CO2 Emitter Rings at Duke Forest	Closer look at a CO2 Emitter Ring
Fig 2.1 Source: Duke FACE	Fig 2.2 Source: Duke FACE

Individual CO2 Gas Port	Air Testing Station
These release CO2 into the air Fig 3.1 Source: Duke FACE	3D sonic & open path analyzer for for testing CO2 content Fig 3.1 Source: Duke FACE

Aspen FACE Project	FACE with Temperature
Source: Aspen FACE	Source: Duke FACE

FACE Experiments Around the World	Temperature of Plants
Source: Brookhaven National Labs	Plants that receive extra CO2 move transpire less. The lack of water evaporation causes them to heat up. Source: Long et. all 2006

Soybean FACE		Swiss Grass FACE
Source: Soy FACE		Source: Swiss FACE

Desert FACE		Desert FACE
Source: Desert FACE		Source: Desert FACE

Sweetgum FACE

Source: Oak Ridge National Lab

*Vapor pressure deficit (VPD) is the difference (deficit) between the amount of moisture in the air and how much moisture the air could hold at 100% humidity.  A vapor pressure deficit (VPD) of 0.2 kPa promotes damage from fungal and other disease infections.  In chamber experiments the vapor pressure deficit (VPD) was a full 0.8 kPa higher.  

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Sources:

• Ainsworth EA, Rogers A (2007) The response of photosynthesis and stomatal conductance to rising [CO₂]: mechanisms and environmental interactions. Plant, Cell & Environment 30, 258-270. (pdf)
• Long SP, Ainsworth EA, Leakey ADB, Nösberger J, Ort DR (2006) Food for Thought: Lower-than-expected crop yield stimulation with rising CO₂ concentrations. Science 312, 1918-1921. (pdf).
• Rogers A, Ainsworth EA, Kammann C (2006) FACE value, perspectives on the future of Free Air CO₂ Enrichment Studies. In: Managed Ecosystems and CO₂ (eds J. Nösberger, S.P. Long, R.J. Norby, M. Stitt, G.R. Hendrey, H. Blum), pp.431-449. Springer, Berlin Heidelberg New York. (pdf)
• Ainsworth EA & Long SP (2005) What have we learned from 15 years of free-air CO₂ enrichment (FACE)? A meta-analytic review of responses to rising CO₂ in photosynthesis, canopy properties and plant production. New Phytologist. 165, 351-372. (pdf)
• Norby RJ et al. (2005) Forest response to elevated CO₂ is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences USA 102, 18052-18056. (pdf)
• Long SP et al. (2004) Rising atmospheric carbon dioxide: Plants FACE the future. Annual Review of Plant Biology. 55, 591-628. (pdf)
• Nowak RS et al. (2004) Functional responses of plants to elevated atmospheric CO₂ – do photosynthetic and productivity data from FACE experiments support early predictions? New Phytologist. 162, 253-280. (pdf)
• Zak DR, et al. (2003) Soil nitrogen cycling under elevated CO₂: A synthesis of forest FACE experiments. Ecological Applications. 13, 1508-1514. (pdf)
• Kimball BA et al. (2002) Responses of agricultural crops to free-air CO₂ enrichment. Advances in Agronomy. 77, 293-368.

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