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Title: SPRING BRANCH GROWTH IN CO2-ENRICHED SOUR ORANGE TREES AND ITS POSSIBLE RELATIONSHIP TO THE ONGOING PHASE SHIFT OF THE ATMOSPHERE'S SEASONAL CO2 CYCLE

Author
item IDSO, C - DEPT OF GEOG, ASU, TEMPE
item Idso, Sherwood
item Kimball, Bruce
item PARK, H - DEP PLANT BIO, ASU, TEMPE
item HOOBER, J - DEP PLANT BIO, ASU, TEMPE
item BALLING, JR, R - DEPT OF GEOG, ASU, TEMPE

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/1999
Publication Date: 2/15/2000
Citation: Idso, C.D., Idso, S.B., Kimball, B.A., Park, H.S., Hoober, J.K., Balling, Jr, R.C. 2000. Spring branch growth in co2-enriched sour orange trees and its possible relationship to the ongoing phase shift of the atmosphere's seasonal co2 cycle. Global Change Biology. 43:91-100.

Interpretive Summary: The amplitude of the atmosphere's seasonal CO2 cycle has risen by approximately 20% over the past few decades. Simultaneously, the spring draw-down of the air's CO2 concentration has been starting progressively earlier and now begins about seven days earlier than it did in 1960. To better assess the implications of this latter phenomenon for future agricultural activities, we conducted an experiment to provide a rough estimate of the relative contributions of the aerial fertilization effect of CO2 and concurrent global warming to this earlier occurrence of "biological spring." Following spring bud-break in January of 1998, and continuing for a period of several months, we measured weekly biomass increase of new branches and leaves on sour orange trees that had been grown out-of-doors at Phoenix, Arizona, for over ten years in clear- plastic-wall open-top chambers exposed to normal air of 400 parts per million (ppm) CO2 or air enriched to 700 ppm CO2. We discovered an initia growth stimulation of new spring foliage in the CO2-enriched trees that led to their reaching their maximum growth rates about two weeks earlier than the ambient-treatment trees. If this phenomenon is characteristic of the rest of the planet's vegetation - which, of course, is a very big if - it would account for approximately two of the seven days by which the commencement of "biological spring" has advanced over the past four decades. This finding should help us determine how agricultural practices may need to be adjusted in the future, as the air's CO2 content continues to rise and the temperature of the planet fluctuates in response to various climatic perturbations.

Technical Abstract: Since the early 1960's, the declining phase of the atmosphere's seasonal CO2 cycle has advanced by approximately seven days, possibly as a result of global warming, which may be advancing the time of occurrence of "climatological spring." However, just as several different phenomena are thought to have been responsible for the increasing amplitude of the atmosphere's seasonal CO2 cycle over this time period, so too may other factors have been partially responsible for the increasingly earlier spring drawdown of atmospheric CO2 that brought about the concomitant advancement in the phase of the air's seasonal CO2 cycle. One of these factors may be the rising CO2 content of the air itself, the aerial fertilization effect of which could be enhancing the growth and development of each new season's initial flush of vegetation and thereby advancing the time of occurrence of "biological spring." We investigated the viability of this hypothesis by measuring new-branch length, dry weight and leaf chlorophyll contents of sour orange trees that have been growing out-of-doors in open-top chambers for over ten years in air of either 400 or 700 ppm CO2. Our data demonstrate that our hypothesis has merit; but they also suggest that the mechanisms underlying it are complex. In addition, for the single species studied, it could probably account for no more than two of the seven days by which the spring drawdown of the air's CO2 concentration has advanced over the past few decades.