Title Web-FACE: a new canopy free air CO₂ enrichment system for tall forest trees in mature forests
Author/s Steven Pepin, Christian Körner
Year 2002
Journal Oecologia 133:1-9
Abstract The long-term responses of forests to atmospheric CO₂ enrichment have been difficult to determine experimentally given the large scale and complex structure of their canopy. We have developed a CO₂ exposure system that uses the free-air CO₂ enrichment (FACE) approach but was designed for tall canopy trees. The system consists of a CO₂-release system installed within the crown of adult trees using a 45-m tower crane, a CO₂ monitoring system and an automated regulation system. Pure CO₂gas is released from a network of small tubes woven into the forest canopy (web-FACE), and CO₂ is emitted from small laser-punched holes. The set point CO₂ concentration ([CO₂]) of 500 µmol mol⁻¹ is controlled by a pulse-width modulation routine that adjusts the rate of CO₂ injection as a function of measured [CO₂] in the canopy. CO₂ consumption for the enrichment of 14 tall canopy trees was about 2tons per day over the whole growing season. The seasonal daytime mean CO₂ concentration was 520 µmol mol⁻¹. Oneminute averages of CO₂ measurements conducted at canopy height in the center of the CO₂-enriched zone were within ±20% and ±10% of the target concentration for 76% and 47% of the exposure time, respectively. Despite the size of the canopy and the windy site conditions, performance values correspond to about 75% of that reported for conventional forest FACE with the added advantage of a much simpler and less intrusive infrastructure. Stable carbon isotope signals captured by 80 Bermuda grass (Cynodon dactylon) seedlings distributed within the canopy of treated and control tree districts showed a clearly delineated area, with some nearby individuals having been exposed to a gradient of [CO], which is seen as added value. Time-integrated values of [CO₂] derived from the C isotope composition of C. dactylon leaves indicated a mean (±SD) concentration of 513±63 µmol mol⁻¹ in the web-FACE canopy area. In view of the size of the forest and the rough natural canopy, web-FACE is a most promising avenue towards natural forest experiments, which are greatly needed.
Keywords Elevated CO₂ · Exposure · Forest ecology · Global change · Stable carbon isotopes
Research project The Swiss Canopy Crane Project (SCC)
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