Abstract
(English)
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The Basel research team's Future-VOC part contributed significantly to the development of a new CO2 enrichment technology, called webFACE, which permits CO2 simulations in mature forest trees in situ, without any constraints to atmospheric circulation. The webFACE technology made it possible for the first time to assess in situ VOC emission of mature temperate forest trees in a future CO2-enriched atmosphere (see WP 3 + 6). The technology was developed, installed and preliminary tests performed during the year 2000. In essence, webFACE is a system of thin tubes woven into the natural tree canopy, with pure CO2 jets emitted through 0.5 mm laser-punched holes. The system is virtually invisible in the canopy and requires the release of ca. 1.8 tons of pure CO2 per day to maintain a 550 ppm atmosphere around 12 adult trees of 35 m height. The system depends on a construction crane (the Swiss Canopy Crane, SCC).
During the year 2001 mature forest trees experienced a doubled pre-industrial CO2 concentration for the first time for a full growing season. The system performance was assessed by 25 sampling lines and infrared gas analysis. In addition, we installed a biomonitor system to assess the CO2 mixing ratio in the canopy in 2001. The CO2 delivered to the webFACE site represents recycled industrial CO2 cleaned to food quality. This gas carries with it a stable isotope composition which differs significantly from that in the free atmosphere (?13C -38‰ instead of -8‰). We installed 100 phytometers, i.e. 50 ml containers tied to major branches at various canopy positions, planted with a fast growing C4 grass. The C4 grass does not exert a biochemical fractionation of 13C-CO2, hence its leaf biomass ?13C reflects the actual mixing ratio of atmospheric CO2 and supplemetal CO2 in the canopy (Paul Scherrer Institute Würenlingen, Dr. R. Siegwolf). The CO2 mixing ratio derived from 13C data perfectly matched the infrared gas analysis data perfectly. After this first full season we are confident that this technology is paving the way to future natural forest CO2 enrichment. Serious interest for this technology was expressed by colleagues at Duke University NC, and by the global canopy research board which will consider the adoption of this technology for global comparisons of tree responses to a future CO2-rich atmosphere during their March meeting in Göttingen. Our detailed analysis of frequency distribution and temporal as well as spatial variability of the CO2-mixing ratio in the canopy revealed an impressive degree of control, not self-evident given the rough aerodynamic situation at the top of such a forest, the windy conditions and the inherent irregularity of CO2-release tubing. The performance of the system differs little from conventional FACE technology, which requires towers, large pipes and blowers, neither affordable nor practical in a forest of this stature. The impact such constructions on trees and on soils would be massive, given the enormous foundations required for towers, overtopping such a forest by at least 10 m, to become operative in the conventional way of FACE. Steel cables needed for slender towers of this size would 'skin' trees during storms. In other words, we can now offer the scientific community a new, versatile and cheap technology, which permits simulating elevated CO2 in fully grown forests, which represent 90% of the global biomass carbon pool. We have shown during the Future-VOC research campaign in 2001 that all trees exposed to elevated CO2 accumulate non-structural carbohydrates in their leaves, carry with them a stable isotope signal, export the stable isotope signal rapidly to soil biota, in particular to mycorrhiza fungi. We documented the dynamics of carbon distribution within the forest biomass. By measuring respiratory C emission from 250 CO2 wells under the trees and in control areas we could map the area of canopy CO2 enrichment by ground data. This data set nicely reflects the coupling of crown CO2 exposure and the soil and rhizosphere biota. By using the gondola of the SCC, our partners were able to document substantial VOC-emissions by adult Quercus petraea.
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