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Response of Mature Spruce Trees to Elevated CO2
Barton, C.V.M. and Jarvis, P.G.  1999.  Growth response of branches of Picea sitchensis to four years exposure to elevated atmospheric carbon dioxide concentration.  New Phytologist 144: 233-243.

What was done
Branch bags were placed around several branches of mature 16-year old Sitka spruce (Picea sitchensis) trees growing near Edinburgh, United Kingdom. Bags were continuously flushed with ambient or twice-ambient concentrations of atmospheric CO2 for four growing seasons to study the effects of elevated CO2 on branch photosynthesis and growth.

What was learned
After four years of treatment exposure, CO2-enriched branches containing current-year needles exhibited net photosynthetic rates that were 100% greater than rates observed in current-year needles on branches receiving ambient CO2 concentrations. In second-year needles, however, atmospheric CO2 enrichment induced photosynthetic acclimation. Despite the occurrence of this phenomenon, CO2-enriched second-year needles still displayed photosynthetic rates that were 43% greater than those of their ambiently-grown counterparts.

In contrast to the CO2-induced increases in net photosynthesis, there were no measurable increases in growth for branches flushed with elevated CO2 concentrations. The lack of an observable growth response in CO2-enriched branches most likely resulted from the export of photosynthetically-derived carbohydrates from such branches to other sinks within the tree, especially since elevated CO2 did not affect respiration rates.

What it means
As the CO2 content of the air rises, it is likely that Sitka spruce trees will respond by increasing their rates of net photosynthesis. As a result of this phenomenon, these trees should be able to utilize additional carbon gains to increase their overall biomass, thus increasing their carbon sequestering abilities.

Reviewed 15 May 2000