top chamber and free air CO 2 enrichment-approaches to investigate tree responses to elevated CO 2

Introduction Carbon dioxide (CO2) is the second most abundant greenhouse gas after water vapour. The concentration of greenhouse gases was almost constant in the pre-industrial era (Spahni et al. 2005, Denman et al. 2007). Due to anthropogenic activities such as bur­ ning fossil fuels and deforestation, the con­ centration of atmospheric CO2 has risen dra­ matically from 280 to 380 ppm since the be­ ginning of the industrial revolution (Denman et al. 2007). Along with oceans and the atmosphere, ter­ restrial ecosystems play an important role as a carbon reservoir in the natural carbon cycle. Growing forests act as a carbon sink by means of photosynthetic conversion of carbon dioxide to plant biomass. Mature forests are believed to show a neutral ba­ lance of photosynthetic carbon fixation and CO2 release by respiration (e.g., Graf Pan­ natier 2006). According to the latest studies, the world’s remaining old-growth forests are usually carbon sinks with positive net eco­ system productivity whereas only very few studies show old forests with a negative net carbon balance of the forest including soils (Luyssaert et al. 2008). Most physiological studies have shown that elevated CO2 induces changes in tree growth patterns, tissue structure and developmental processes. It influences the rate of physiolo­ gical gas exchange resulting in enhanced net carbon assimilation (e.g., Teskey 1997). Fur­ thermore, stomatal conductance is reduced (Hättenschwiler et al. 2002) and Rubisco properties are altered (Bowes 1993). Moreover, elevated CO2 may affect cell divi­ sion, expansion and patterning. Trees under elevated CO2 produce taller and thicker stems, increase total leave area and foliar starch concentration. In addition, shifts in timing of developmental phases may be ob­ served (Bowes 1993, Pritchard et al. 1999, Jach & Ceulemans 1999, Taylor et al. 2001). Elevated CO2 can also inhibit various deve­ lopmental effects of the hormone ethylene (Sisler & Wood 1988) and has a marked im­ pact on soil nitrogen availability for plants by increasing soil net nitrification (Carnol et al. 2002). Over the years, a variety of techniques has been applied to study tree responses to ele­ vated atmospheric CO2 at the plant organ, entire trees, or ecosystems. Early studies on the effects of elevated CO2 were typically done under controlled conditions in closed chambers or greenhouses, such as: a) branch and leaf chambers (Teskey 1997); b) phyto­ trons (Liu et al. 2004); c) controlled environ­ ment chambers (Kellomäki et al. 2000); or variations thereof. A shortcoming of these difficult methods is that they create an artifi­ cial environment compared to natural eco­ system conditions. Attempts to bring experi­ mental set-ups in a more natural context have yielded more elaborate techniques that tend to allow, to varying degrees, for an ex­ change with the natural environment. These include: d) open top chambers (OTC e.g., Vanaja et al. 2006); e) free air CO2 enrich­ ment systems (FACE e.g., Karnosky et al. 2001); and f) screen-aided CO2 control (SACC Leadley et al. 1997). OTCand FACE-derived systems are nowadays the most frequently used methods to study tree responses to elevated CO2 under close to natural conditions (Leadley et al. 1997). In this review, the principles of these two methods and the specific experimental approaches of OTC and FACE will be ex­ plained. Their advantages and disadvantages to individual research goals will be dis­ cussed. Screen-aided CO2 control (SACC) will be proposed as a possible compromise eliminating disadvantages and combining advantages of both OTC and FACE for many research applications. The second part of this review compares and validates the ef­ fects of elevated CO2 on plant biomass, leaf expansion, and photosynthesis obtained from