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Web-FACE: a new canopy free-air CO2 enrichment system for tall trees in mature forests

Summary

Published in Oecologia 133: 1-9

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 CO2 gas 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 mmol 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 2 tons per day over the whole growing season. The seasonal daytime mean CO2 concentration was 520 mmol mol-1. One-minute 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 gradiant of [CO2], 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 mmol 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.

Sector(s): 

Forests

Categorie(s): 

Scientific Article

Theme(s): 

Forestry Research, Forests, Silviculture

Author(s)

PEPIN, Steeve and Christian KÔRNER

Year of publication :

2002

Keywords :

Evaluated CO2, exposure, forest ecology, global change, stable carbon isotopes, Web-Face, silviculture and yield of natural forests - softwood stands, sylviculture des forêts naturelles résineuses, sylviculture et rendement des forêts naturelles - peuplements résineux

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