What was done
In the spring of 1998, the authors periodically measured the lengths, dry weights and leaf chlorophyll concentrations of new branches that emerged from sour orange trees that had been growing out-of-doors in clear-plastic-wall open-top chambers for over ten years in air of either 400 and 700 ppm CO2.

What was learned
Although new branch growth began on exactly the same day in both the ambient and CO2-enriched chambers, the initial rate of new-branch biomass production was vastly greater in the CO2-enriched trees.  Three weeks after branch growth began, for example, each new branch on the CO2-enriched trees was more than four times more massive than its counterpart on the ambient-treatment trees; and on a per-tree basis, there was over six times more new-branch biomass on the trees growing in the 700 ppm CO2 treatment than in the 400 ppm CO2 treatment.  Just as dramatically as this growth enhancement began, however, it also declined; and ten weeks into the growing season, the CO2-enriched / ambient-treatment new-branch biomass ratio had leveled out at a value commensurate with the seasonal standing biomass and fruit production ratios, in the range of 1.7 to 2.0.

What it means
Based on their experimentally-observed results, the authors calculated the days by which the new-branch dry weight of the CO2-enriched trees led that of the ambient-treatment trees over the first two months of the growing season.  They found that at the time at which the new-branch biomass of the CO2-enriched trees began its rapid ascent phase in the spring, it was approximately two weeks ahead of the new-branch biomass of the ambient-treatment trees.  Hence, they determined that a 300 ppm increase in the air's CO2 content caused sour orange trees to commence the significant portion of their spring drawdown of atmospheric CO2 fully two weeks earlier than similar trees growing in ambient air.  They then calculated that for the 43 ppm increase in the air's CO2 content experienced between 1960 and 1994, this phenomenon would have been pushed ahead by two full days.  That is, if all vegetation responds to atmospheric CO2 enrichment as sour orange trees do - which is by no means demonstrated by this experiment - "biological spring" should have occurred two days earlier in 1994 than it did in 1960.

By way of comparison, it has been observed that there has been a seven-day advancement in the time of occurrence of the declining phase of the atmosphere's seasonal CO2 cycle over this same time period; and surface reflectance measurements made by satellites have observed a similar advancement in the springtime greening of the planet in high northern latitudes.  The authors' data suggest that some of this earlier occurrence of what they call "biological spring" - as opposed to "climatological spring" - may indeed be due to the dramatic new-branch growth enhancement of trees provided by the increase in the air's CO2 concentration experienced over this time interval.  In addition to the impetus provided by global warming, it is thus possible that the ongoing rise in the air's CO2 content may also be causing the terrestrial surfaces of the globe to green-up earlier each succeeding spring.