Ultra-enhanced spring branch growth in CO₂-enriched trees: Can it alter the phase of the atmosphere's seasonal CO₂ cycle?

Since the early 1960s, the declining phase of the atmosphere's seasonal CO₂ cycle has advanced by approximately 7 days in northern temperate latitudes, possibly as a result of increasing temperatures that may be advancing the time of occurrence of what may be called 'climatological spring.' However, just as several different phenomena are thought to have been responsible for the concomitant increase in the amplitude of the atmosphere's seasonal CO₂ oscillation, so too may other factors have played a role in bringing about the increasingly earlier spring drawdown of CO₂ that has resulted in the advancement of the declining phase of the air's CO₂ cycle. One of these factors may be the ongoing rise in the CO₂ content of the air itself: for the aerial fertilization effect of this phenomenon may be significantly enhancing the growth of each new season's initial flush of vegetation, which would tend to stimulate the early drawdown of atmospheric CO₂ and thereby advance the time of occurrence of what could be called 'biological spring.' Working with sour orange (Citrus aurantium L.) trees that have been growing out-of-doors in open-top chambers for over 10 years in air of either 400 or 700 ppm CO₂, this hypothesis was investigated by periodically measuring the lengths, dry weights and leaf chlorophyll concentrations of new branches that emerged from the trees at the start of the 1998 growing season. The data demonstrate that the hypothesis is viable, and that it might possibly account for 2 of the 7 days by which the spring drawdown of the air's CO₂ concentration has advanced over the past few decades. (C) 2000 Published by Elsevier Science B.V.