Open top chamber and free air CO2 enrichment - approaches to investigate tree responses to elevated CO2

Open Top Chamber (OTC) and Free Air CO2 Enrichment (FACE) are currently the prevailing approaches to study plant responses to elevated carbon dioxide. Method-inherent characteristics of either method distinctively influence results. Advantages and disadvantages of both methods are reviewed here, leading to the conclusion that Open Top Chambers seem to be more suitable for investigating the physiological responses of single trees to high levels of carbon dioxide, while Free Air CO2 Enrichment systems are more useful for studying the effects of elevated carbon dioxide on whole forest ecosystems since they have a large diameter, thus allowing to work with larger trees. Free Air CO2 Enrichment systems also provide a natural microclimate, thus leading to ecologically more meaningful results. Methods involving Screen-Aided CO2 Control (SACC) are proposed as a compromise eliminating disadvantages and combining advantages of both the Open Top Chamber and the Free Air CO2 Enrichment methods. Considering the wide variety of experiments under a range of additional environmental factors it is difficult to identify a typical bias that may be inherent in the data generated by the Open Top Chamber and the Free Air CO2 Enrichment. Meta analysis of large number of past studies revealed that Open Top Chamber experiments produce a stronger growth enhancing effect of carbon dioxide than Free Air CO2 Enrichment experiments. Future comparative discussion of Open Top Chamber and Free Air CO2 Enrichment data needs to take into account this potential bias to yield biologically meaningful interpretations.