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 f or the concomitant increase in the amplitude of the atmosphere's seasonal C O2 oscillation, so too may other factors have played a role in bringing abo ut 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 fa ctors may be the ongoing rise in the CO2 content of the air itself; for the aerial fertilization effect of this phenomenon may be significantly enhanc ing the growth of each new season's initial flush of vegetation, which woul d tend to stimulate the early drawdown of atmospheric CO2 and thereby advan ce the time of occurrence of what could be called 'biological spring.' Work ing 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 70 0 ppm CO2, this hypothesis was investigated by periodically measuring the l engths, dry weights and leaf chlorophyll concentrations of new branches tha t 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 accou nt for 2 of the 7 days by which the spring drawdown of the air's CO2 concen tration has advanced over the past few decades. (C) 2000 Published by Elsev ier Science B.V.