Volume 8, Number 43: 26 October 2005
Many scientists have observed that the spring "green-up" of vegetation in the Northern Hemisphere is currently beginning a few days earlier than it did several decades ago, and that certain activities of various forms of animal life that are correlated with vegetative development are likewise occurring earlier. Some have attributed this phenomenon to global warming, while others have suggested that the increasingly earlier occurrence of what we may call biological spring may be due to an ultra-amplification of early spring branch growth that is provided by the ever-increasing aerial fertilization effect of the ongoing rise in the air's CO2 content (see Trees (Early Spring Growth) in our Subject Index). We here report on the findings of two new studies that contribute to our understanding of this phenomenon.
In the first study, Jasoni et al. (2005) report on Net Ecosystem CO2 Exchange (NEE) in an ongoing long-term FACE experiment being conducted in a Mojave Desert (USA) shrubland, where half of the FACE arrays have been continuously enriched with an extra 180 ppm of CO2 during all non-freezing hours of the year since April 1997. This desert ecosystem is dominated by the evergreen shrub Larrea tridentata. It also contains the subdominant drought-deciduous shrubs Lycium andersonii and Ambrosia dumosa, as well as the perennial grasses Achnatherum hymenoides and Pleuraphis rigidia. In addition, there is a large contingency of annual and perennial forbs that at times may be comprised of as many as 75 different species.
Perhaps Jasoni et al.'s most important finding, in our view, was that at the eight-year point of their experiment, as they describe it, "maximum NEE in ecosystems exposed to elevated CO2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO2." In addition, their data show that the initial spring ramp-up of NEE also occurred one month earlier in the CO2-enriched FACE rings. Hence, for the 30-day advancement in the initiation of "biological spring" that was driven by the 180-ppm increase in the air's CO2 concentration employed in this study, we calculate a CO2-induced rate-of-advancement of biological spring of 0.167 day per ppm increase in CO2.
Most interestingly, but likely somewhat fortuitously - since other studies have yielded significantly different rates-of-advancement of biological spring (see, for example, Idso et al. (2000), who observed a rate-of-advancement of biological spring of 0.047 day per ppm of CO2 in their study of sour orange trees) - this number is essentially identical to the observed rate of temporal advancement of the declining phase of the annual cycle of the air's CO2 content (7 days / 43 ppm = 0.163 day per ppm increase in CO2, based on data obtained between 1960 and 1994), which is driven by the ever-earlier-occurring spring-induced photosynthetic drawdown of the air's CO2 content that takes place when much of the Northern Hemisphere's vegetation awakens from its long winter's nap.
The second new experiment that touches on this topic is that of Handa et al. (2005), who conducted a FACE study of European larch (Larix deciduas L.) and mountain pine (Pinus uncinata L.) trees at high elevation in the Swiss Central Alps. In their experiment, the trees were exposed to an extra 213 ppm of CO2, causing larch needles to unfold an average of 6 days earlier in two consecutive years and pine needles to unfold an average of 2 days earlier, yielding CO2-induced rates-of-advancement of biological spring for these two species of 0.028 and 0.009 day per ppm CO2, respectively.
The range of rate-of-advancement values derived from these several experiments is large, most likely as a consequence of the different means employed to measure them in the different studies. Nevertheless, they all point in the same direction, i.e., to a CO2-driven tendency for biological spring to occur at an ever earlier date as the air's CO2 content continues to climb. It would be helpful for a common convention to be used to better quantify this phenomenon in all of the CO2-enrichment experiments being conducted around the world where the vegetation being studied produces new foliage each spring that can be carefully monitored from the time of its first appearance. Such a cooperative effort would do much to quantitatively evaluate this phenomenon, a better understanding of which is essential for determining what part of the historically-observed advancement of biological spring is due to direct physiological effects of atmospheric CO2 enrichment and what part may be due to global warming. As things stand now, it is difficult to even know which of the two potential causes predominates.
Sherwood, Keith and Craig Idso
References
Handa, I.T., Korner, C. and Hattenschwiler, S. 2005. A test of the treeline carbon limitation hypothesis by in situ CO2 enrichment and defoliation. Ecology 86: 1288-1300.
Idso, C.D., Idso, S.B., Kimball, B.A., Park, H.-S., Hoober, J.K. and Balling Jr., R.C. 2000. Ultra-enhanced spring branch growth in CO2-enriched trees: Can it alter the phase of the atmosphere's seasonal CO2 cycle? Environmental and Experimental Botany 43: 91-100.
Jasoni, R.L., Smith, S.D. and Arnone III, J.A. 2005. Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2. Global Change Biology 11: 749-756.