In analyzing this annual atmospheric CO2 oscillation, several scientists have noted that the spring drawdown of the air's CO2 content is currently beginning a few days earlier than it did several decades ago.  Some have attributed this phenomenon to CO2-induced global warming, while others have suggested that the increasingly earlier occurrence of what we may call biological spring may be due to an 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.

One of the first papers to anticipate this phenomenon was that of Lovelock et al. (1999), who enclosed branchlets of 30-m tall Luehea seemannii trees in open-top chambers suspended within the trees' upper canopies and fumigated them with air of either 360 or 750 ppm CO2 for nearly 40 weeks in a study of the effects of elevated CO2 on photosynthesis, growth and reproduction in this deciduous tropical tree species.  By these means they determined that leaves of branchlets grown in CO2-enriched air displayed net photosynthetic rates that were approximately 30% greater than those of leaves of branchlets grown in ambient air.  However, the additional carbohydrates produced by this phenomenon were not used by CO2-enriched branchlets to increase leaf growth or reproductive efforts.  Instead, they were stored in terminal woody tissues, which observation led the authors to suggest that enhanced carbohydrate storage in terminal branchlets may facilitate greater first-flush leaf growth the following year.

Actual documentation of this phenomenon was first provided by Idso et al. (2000), who periodically measured the lengths, dry weights and leaf chlorophyll concentrations of new branches that each spring 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 or 700 ppm CO2.  Although new spring branch growth always 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, new branches on the CO2-enriched trees were typically more than four times more massive than their counterparts on the ambient-treatment trees, while on a per-tree basis, there was over six times more new-branch biomass on the trees growing in the CO2-enriched chambers than in the ambient-air chambers.  Just as rapidly as this ultra-enhancement of new branch growth 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 seasonal standing biomass and fruit production ratios, i.e., just under a doubling.

Based on their experimentally-observed results, Idso et al. calculated the length of time 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, determining that the 300 ppm increase in the air's CO2 concentration caused the trees to begin the significant portion of their spring drawdown of atmospheric CO2 fully two weeks earlier than what would otherwise have been normal.  They then calculated that for the 43-ppm increase in the air's CO2 concentration experienced between 1960 and 1994, biological spring for sour orange trees should have occurred two days earlier in 1994 than it did in 1960.

By way of comparison, there was an approximate seven-day advancement in the time of occurrence of the declining phase of the atmosphere's seasonal CO2 cycle over this time period; and surface reflectance measurements made by satellites revealed a similar advancement in the springtime "greening of the earth" in high northern latitudes.  Idso et al.'s data thus suggest that a significant portion of this advancement may have been due to the dramatic stimulation of initial new-branch growth provided by the increase in the air's CO2 concentration experienced over that time interval.  But where did the CO2-enriched sour orange trees get the extra nitrogen that was needed to support the hugely-enhanced production of new branch and leaf tissue that occurred at the beginning of the growing season?

In an experiment that may hold the answer to this question, Idso et al. (2001) measured the concentrations of three soluble leaf proteins having molecular masses of 33, 31 and 21 kDa at weekly intervals for a period of one full year in the foliage of the CO2-enriched and ambient-treatment trees.  During the central portion of the year, the abundances of the three proteins were generally lower in the leaves of the CO2-enriched trees than they were in the leaves of the ambient-treatment trees; but in the latter part of the year and continuing for a short while into the next year, this relationship was reversed, as the leaf concentrations of the three proteins in the CO2-enriched trees surpassed those of the proteins in the trees growing in ambient air.  Then in the spring, when new growth began to appear, the concentrations of the three proteins in the foliage of the CO2-enriched trees plummeted, possibly as a result of giving up the nitrogen they had stored over winter to supply the needs of the newly developing branches and leaves.

The idea sounds intriguing; but is there any other evidence to support it?  Actually, there is.  In fact, there are a number of observations that argue for its validity.

First of all, Idso et al. found that the N-terminal amino acid sequence of the 21-kDa protein has homology with sporamin B, an implicated vegetative storage protein (VSP) that can comprise 60-80% of the total soluble protein found in sweet potato tubers.  They also determined that it shares low sequence homology with trifoliatin, a soluble leaf protein from trifoliate orange that shares 62% amino acid similarity with sporamin B and can comprise up to 65% of total leaf protein.  In addition, they say that "immunoelectron microscopy demonstrated the presence of the proteins within amorphous material in the vacuoles of mesophyll cells, where VSPs are commonly located."

Perhaps the most telling evidence of all was the fact that starting at about day-of-the-year 225, the CO2-enriched trees experienced a period of leaf senescence and abscission that was not observed in the ambient-treatment trees.  "This phenomenon," Idso et al. report, "peaked at about day 300, when the CO2-enriched trees shed leaves at a rate 2.5-2.7 times greater than the normal background rate of the ambient-treatment trees, whereupon the enriched-tree leaf-fall rate diminished, returning to normal by the end of the year."

According to the hypothesis formulated by Idso et al., nitrogen reabsorbed from these second-year leaves during the process of senescence became available for storage in first-year leaves of the CO2-enriched trees, going initially into the 21-kDa protein starting at about day 225, into the 31-kDa protein starting at about day 265, and into the 33-kDa protein starting at about day 335, according to the trends defined by their weekly measurements of leaf protein concentrations.  Then, when new branches and leaves began appearing in the spring, the stored nitrogen was remobilized from the prior first-year leaves (which now became second-year leaves) to supply the needs of the developing branch and leaf tissues, whereupon the concentrations of the three putative storage proteins in the now-second-year CO2-enriched leaves rapidly dropped to levels that were once again less than those observed in similar-age ambient-treatment leaves, and they remained at that reduced level until the second-year leaves gave up even more nitrogen prior to their senescence in the fall.

Other experimental evidence for a CO2-induced stimulation of new spring branch growth has been provided by Olszyk et al. (2001), who grew Ponderosa pine seedlings out-of-doors for two years in controlled-environment chambers maintained at atmospheric CO2 concentrations of 390 and 670 ppm in combination with low (40 ppb) and high (60 ppb) ozone concentrations.  They found that the elevated CO2 enhanced mean annual rates of net photosynthesis by 39%, and that it increased bud lengths at the end of the second growing season in the low ozone treatment by 17%, which led to a transitory stimulation (+38%) of elongation and growth of terminal buds the following spring.

That the super-stimulation of initial perennial plant growth in the early spring may be an ubiquitous phenomenon is suggested by the research of Bushway and Pritts (2002), who grew over-wintering strawberry plants in controlled environment chambers maintained at ambient (375 ppm) and elevated (700 to 1000 ppm) atmospheric CO2 concentrations until new blooms began to form on plants, after which they moved the plants to a common greenhouse maintained at the ambient CO2 concentration.  The extra CO2 stimulated rates of photosynthesis in the leaves of the over-wintering strawberry plants by more than 50%, which led to greater amounts of starch being found in key plant organs when new spring growth began.  In fact, plants grown in elevated CO2 had two-, three- and four-times the amount of starch in their crowns, leaves and roots, respectively, than their ambiently-grown counterparts.  In addition, plants grown in elevated CO2 flowered and fruited an average of four and seven days earlier than plants grown in ambient air, respectively.

In one final study of the ultra-enhancement of early spring branch growth in earth's trees and shrubs in response to the historical and ongoing rise in the air's CO2 concentration, Lim et al. (2004) correlated the monthly rate of relative change in normalized difference vegetation index (NDVI), which they derived from advanced very high resolution radiometer data, with the rate of change in atmospheric CO2 concentration (δCO2) during the natural growing season within three different eco-region zones of North America (the Arctic and Sub-Arctic Zone, the Humid Temperate Zone, and the Dry and Desert Zone, which they further subdivided into 17 regions) over the period 1982-1992, after which they explored the temporal progression of annual minimum NDVI over the period 1982-2001 throughout the eastern humid temperate zone of North America.  In doing so, they found that in all 17 regions but one, "δCO2 was positively correlated with the rate of change in vegetation greenness in the following month, and most correlations were high," which observations they say are consistent with a CO2 fertilization effect of the type observed in experimental manipulations of the air's CO2 content that report a stimulation of photosynthesis and above-ground production at high CO2.  In addition, and most importantly, they determined that the yearly "minimum vegetation greenness increased over the period 1982-2001 for all the regions of the eastern humid temperate zone in North America."

As for the cause of this latter phenomenon, Lim et al. say that rising CO2 could "increase minimum greenness by stimulating photosynthesis at the beginning of the growing season," citing the work of Idso et al. (2000).  Hence, by looking for a manifestation of the CO2 fertilization effect at the time of year it is apt to be most strongly expressed, Lim et al. may well have found it.  Between 1982 and 2001, for example, the air's CO2 concentration rose by approximately 30 ppm.  From Idso et al.'s findings of (1) more than a 300% initial increase in the biomass of new sour orange tree branches for a 300-ppm increase in the air's CO2 concentration and (2) more than a 500% initial increase in per-tree new-branch biomass, we calculate that yearly minimum greenness should have increased by something between something just over 30% and something just over 50%, if other woody plants respond to atmospheric CO2 enrichment as sour orange trees do; and when we calculate the mean 19-year increase in NDVI for the seven regions for which Lim et al. present data, we get an increase of something just over 40%, indicative of the fact that Lim et al.'s data are not only qualitatively consistent with the hypothesis of CO2-induced ultra-stimulation of early spring branch growth, they are right on the money quantitatively as well.

References
Bushway, L.J. and Pritts, M.P.  2002.  Enhancing early spring microclimate to increase carbon resources and productivity in June-bearing strawberry.  Journal of the American Society for Horticultural Science 127: 415-422.

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.

Idso, K.E., Hoober, J.K., Idso, S.B., Wall, G.W. and Kimball, B.A.  2001.  Atmospheric CO2 enrichment influences the synthesis and mobilization of putative vacuolar storage proteins in sour orange tree leaves.  Environmental and Experimental Botany 48: 199-211.

Lim, C., Kafatos, M. and Megonigal, P.  2004.  Correlation between atmospheric CO2 concentration and vegetation greenness in North America: CO2 fertilization effect.  Climate Research 28: 11-22.

Lovelock, C.E., Virgo, A., Popp, M. and Winter, K.  1999.  Effects of elevated CO2 concentrations on photosynthesis, growth and reproduction of branches of the tropical canopy trees species, Luehea seemannii Tr. & Planch.  Plant, Cell and Environment 22: 49-59.

Olszyk, D.M., Johnson, M.G., Phillips, D.L., Seidler, R.J., Tingey, D.T. and Watrud, L.S.  2001.  Interactive effects of CO2 and O3 on a ponderosa pine plant/litter/soil mesocosm.  Environmental Pollution 115: 447-462.