Between November of 1987 and January of 2005, the longest
CO2-enrichment study of a long-lived woody plant species ever to be
conducted was carried out at Phoenix, Arizona (USA), by an international
team of researchers. The results show elevated levels of CO2 boost both the
total amount of biomass produced by the trees as well as the harvestable
yield, that is, fruits. The effects were long lasting.
The findings have been reported in the October issue of Global Change
Biology. They confirm that an increase in atmospheric CO2 could be
beneficial to many types of vegetation. However, scientists have warned that
these effects could be largely offset by an increase in atmospheric ozone.

The tests where organised by Bruce Kimball of the US Agricultural
Research Service's Arid Land Agricultural Research Center, Sherwood Isdo
from the Center for the study of Carbon Dioxide and Global Change, Stephanie
Johnson of the University of Montana and by Matthias Rillig of the Institute
of Biology at the Free University of Berlin. They were initiated with the
out-of-doors planting of eight sour orange (Citrus aurantium L.) seedlings,
and with the initial project researchers surrounding pairs of the seedlings
with clear-plastic-wall open-top chambers, within which four of the trees
were exposed to a continual bottom-to-top flow of ambient air, while the
other four trees were exposed to a similar 24-hour 7-day-per-week upward
flow of air enriched with an extra 300 ppm of CO2. The scientists state that
"the trees were fertilized and flood irrigated similar to practice in
commercial orchards so as to maintain ample nutrients and soil moisture."

So what was learned from the historic 17-year experiment? In terms of
total biomass production, which was the primary focus of the summary report,
Kimball and collegues state that the CO2-enriched to ambient ratio of annual
wood plus fruit production "peaked in years 2-4 of the experiment at about
2.4," but that "following the peak, there was a decline through year 8."
Thereafter, they found that the annually-produced-biomass ratios "were more
or less at a plateau that corresponded with the value of the ratio at final
harvest of 1.69."

In terms of harvestable yield, that is, fruit production, lead author
Kimball writes that "the cumulative amount of biomass due to fruit
production over the duration of the experiment was increased 85% due to
elevated CO2," which increase "was entirely from an increase in fruit
number".In addition, the scienistst report that "the vitamin C content of
the fruit was increased 7% based on samples taken from the fourth through
the 12th years of the experiment." Consequently, not only were there a whole
lot more oranges produced by the trees in the CO2-enriched chambers, a whole
lot more better-quality oranges were produced.

In their concluding discussion of one of the major implications of the
study, the researchers write that "rather than a continual acclimation" -
i.e., rather than a gradual long-term decline in the aerial fertilization
effect of the extra 300 ppm of CO2 supplied to the CO2-enriched trees
"instead there was a sustained enhancement of about 70% in annual fruit and
incremental wood production over the last several years of the experiment."

This observation thus led them to conclude that "the effects of
elevated CO2 on trees can be large and sustained for many years," as they
indeed demonstrated to be the case with sour orange trees, there having been
a 70% sustained increase in biomass production over the entire last decade
of the study in response to the 75% increase in the air's CO2 content
employed throughout the experiment.
[biopact.com]