In a paper entitled "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999," which describes a study that covered essentially the entire North American continent, Zhou et al. (2001) determined that the satellite-derived normalized difference vegetation index (NDVI) rose by 8.44% over this period. Noting that the NDVI parameter "can be used to proxy the vegetation's responses to climate changes because it is well correlated with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf area, leaf biomass, and potential photosynthesis," Zhou et al. went on to suggest that the increases in plant growth and vitality implied by their NDVI data were primarily driven by concurrent increases in near-surface air temperature. Since this warming was rather muted in North America, however, and in the United States in particular, where temperatures may have actually declined throughout the eastern part of the country over the period of the study, Ahlbeck (2002) suggested that the observed upward trend in NDVI was primarily driven by the concurrent rise in the air's CO2 content. Nevertheless, it is likely that both parameters probably played a role in the observed productivity increase, although the CO2 increase was more likely the predominant one given the lack of temperature increase over the period of time under study.

About the same time, Hicke et al. (2002) conducted an independent study, computing net primary productivity (NPP) over North America for the years 1982-1998 using the Carnegie-Ames-Stanford Approach (CASA) carbon cycle model, which was driven by a satellite NDVI record at 8-km spatial resolution. This effort revealed that NPP increases of 30% or more occurred across the continent from 1982 to 1998. During this period, the air's CO2 concentration rose by 25.74 ppm, as calculated from the Mauna Loa data of Keeling and Whorf (1998), which amount is 8.58% of the 300 ppm increase that is often used as a reference for expressing plant growth responses to atmospheric CO2 enrichment. Consequently, for herbaceous plants that display NPP increases of 30-40% in response to a 300-ppm increase in atmospheric CO2 concentration, the CO2-induced NPP increase experienced between 1982 and 1998 would be expected to have been 2.6-3.4%. Similarly, for woody plants that display NPP increases of 60-80% in response to a 300-ppm increase in atmospheric CO2 (Saxe et al., 1998; Idso and Kimball, 2001), the expected increase in productivity between 1982 and 1998 would have been 5.1-6.9%. Since both of these NPP increases are considerably less than the 30% or more observed by Hicke et al., additional factors must have helped to stimulate NPP over this period, some of which may have been concomitant increases in precipitation and air temperature, the tendency for warming to lengthen growing seasons and enhance the aerial fertilization effect of rising CO2 concentrations, increasingly intensive crop and forest management, increasing use of genetically improved plants, the regrowth of forests on abandoned cropland, and improvements in agricultural practices such as irrigation and fertilization. Whatever the mix might have been, one thing is clear: its ultimate effect was overwhelmingly positive.

Two years later in another satellite-based study, Lim et al. (2004) correlated the monthly rate of relative change in NDVI, which they derived from advanced very high resolution radiometer data, with the rate of change in atmospheric CO2 concentration during the natural vegetation growing season within three different eco-region zones of North America (Arctic and Sub-Arctic Zone, Humid Temperate Zone, and 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. The result of these operations was that in all of the regions but one, according to the researchers, "δCO2 was positively correlated with the rate of change in vegetation greenness in the following month, and most correlations were high," which they say is "consistent with a CO2 fertilization effect" of the type observed in "experimental manipulations of atmospheric CO2 that report a stimulation of photosynthesis and above-ground productivity at high CO2." In addition, 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 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), who discovered that although new spring branch growth of sour orange trees began on exactly the same day of the year in both ambient (400 ppm) and CO2-enriched (700 ppm) open-top chambers, the rate of new-branch growth was initially vastly greater in the CO2-enriched trees. Three weeks after branch growth began in the spring, 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, over six times more new-branch biomass was produced on the CO2-enriched trees, before declining to an approximate 80% stimulation typical of the bulk of the growing season.

Consequently, 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, it can be calculated 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 the mean 19-year increase in NDVI for the seven regions for which Lim et al. present data is calculated, the result is an increase of something just over 40%, indicative of the fact that Lim et al.'s data are not only qualitatively consistent with their hypothesis, they are right on the mark quantitatively as well.

Lastly, in a somewhat similar study, but one that focused more intensely on climate change, Xiao and Moody (2004) examined the responses of the normalized difference vegetation index integrated over the growing season (gNDVI) to annual and seasonal precipitation, maximum temperature (Tmax) and minimum temperature (Tmin) over an 11-year period (1990-2000) for six biomes in the conterminous United States (Evergreen Needleleaf Forest, Deciduous Broadleaf Forest, Mixed Forest, Open Shrubland, Woody Savanna and Grassland), focusing on within- and across-biome variance in long-term average gNDVI and emphasizing the degree to which this variance is explained by spatial gradients in long-term average seasonal climate. The results of these protocols indicated that the greatest positive climate-change impacts on biome productivity were caused by increases in spring, winter and fall precipitation, as well as increases in fall and spring temperature, especially Tmin, which has historically increased at roughly twice the rate of Tmax in the United States. Hence, "if historical climatic trends and the biotic responses suggested in this analysis continue to hold true," in the words of Xiao and Moody, "we can anticipate further increases in productivity for both forested and non-forested ecoregions in the conterminous US, with associated implications for carbon budgets and woody proliferation," which once again spells good news for the biosphere.

In light of each of the findings presented above, and in spite of all the real and perceived assaults on Earth's vegetation (rising temperatures, drought, development and deforestation), plant productivity and growth in the mean has increased across all of North America over the past three decades, demonstrating the great capacity of the biosphere to respond - and in a positive manner - to the many and varied challenges it faces.

References
Ahlbeck, J.R. 2002. Comment on "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999" by L. Zhou et al. Journal of Geophysical Research 107: 10.1029/2001389.

Hicke, J.A., Asner, G.P., Randerson, J.T., Tucker, C., Los, S., Birdsey, R., Jenkins, J.C. and Field, C. 2002. Trends in North American net primary productivity derived from satellite observations, 1982-1998. Global Biogeochemical Cycles 16: 10.1029/2001GB001550.

Idso, C.D., Idso, S.B., Kimball, B.A., Park, H., 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, S.B. and Kimball, B.A. 2001. CO2 enrichment of sour orange trees: 13 years and counting. Environmental and Experimental Botany 46: 147-153.

Keeling, C.D. and Whorf, T.P. 1998. Atmospheric CO2 Concentrations - Mauna Loa Observatory, Hawaii, 1958-1997 (revised August 2000). NDP-001. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.

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.

Saxe, H., Ellsworth, D.S. and Heath, J. 1998. Tree and forest functioning in an enriched CO2 atmosphere. New Phytologist 139: 395-436.

Xiao, J. and Moody, A. 2004. Photosynthetic activity of US biomes: responses to the spatial variability and seasonality of precipitation and temperature. Global Change Biology 10: 437-451.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research 106: 20,069-20,083.