In a paper entitled "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999," Zhou et al. (2001) determined that the magnitude of the satellite-derived normalized difference vegetation index (NDVI) rose by 8.44% in North America over this period. Noting that the NDVI "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," they 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, although temperatures may have actually declined throughout the eastern part of the United States over the period of their study.

After lying dormant for about a year, Zhou et al.'s attribution of this "greening" of the continent to concurrent increases in near-surface air temperature was challenged by Ahlbeck (2002), who suggested that the observed upward trend in NDVI was primarily driven by the concurrent increase in the air's CO2 concentration, and that fluctuations in temperature were primarily responsible for variations about the more steady upward trend defined by the increase in CO2. In replying to this challenge, Kaufmann et al. (2002) claimed Ahlbeck was wrong and reaffirmed their initial take on the issue. In our analysis of the question in our editorial of 18 Sep 2002, however, we concluded that it was Ahlbeck who was "clearly the 'more correct' of the two camps." In fact, we came to the conclusion he was actually totally correct.

About the same time, Hicke et al. (2002) computed 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 (see our plant growth databases), 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 that 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 overwhelming positive.

In a vastly different type of study based on a 48-year record derived from an average of 17 measurements per year Raymond and Cole (2003) demonstrated that the export of alkalinity, in the form of bicarbonate ions, from the USA's Mississippi River to the Gulf of Mexico had increased by approximately 60% since 1953. "This increased export," as they described it, was "in part the result of increased flow resulting from higher rainfall in the Mississippi basin," which had led to a 40% increase in annual Mississippi River discharge to the Gulf of Mexico over the same time period. The remainder, however, had to have been due to increased rates of chemical weathering of soil minerals.

What factors might have been responsible for this phenomenon? The two researchers noted that potential mechanisms included "an increase in atmospheric CO2, an increase [in] rainwater throughput, or an increase in plant and microbial production of CO2 and organic acids in soils due to biological responses to increased rainfall and temperature." Unfortunately, they forgot to mention the increase in terrestrial plant productivity that is produced by the increase in the aerial fertilization effect provided by the historical rise in the air's CO2 content, which also leads to "an increase in plant and microbial production of CO2 and organic acids in soils." And as we note in our review of their paper, this phenomenon should have led to an increase in Mississippi River alkalinity equivalent to that which they had observed since 1953.

In a still-different type of study, using data obtained from dominant stands of loblolly pine plantations growing at 94 locations spread across the southeastern United States, Westfall and Amateis (2003) employed mean height measurements made at three-year intervals over a period of 15 years to calculate a site index related to the mean growth rate for each of the five three-year periods, which index would be expected to increase monotonically if growth rates were being enhanced above normal by some monotonically-increasing factor that promotes growth. This work revealed, in their words, that "mean site index over the 94 plots consistently increased at each remeasurement period," which would suggest, as they further state, that "loblolly pine plantations are realizing greater than expected growth rates," and, we would add, that the growth rate increases are growing larger and larger with each succeeding three-year period.

As to what could be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern United States, Westfall and Amateis named increases in temperature and precipitation in addition to rising atmospheric CO2 concentrations. However, they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study. They also suggested that if increased nitrogen deposition were the cause, "such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots." Hence, they tended to favor the ever-increasing aerial fertilization effect of atmospheric CO2 enrichment as being responsible for the accelerating pine tree growth rates.

Returning to satellite studies, 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, 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 their hypothesis, they are right on the mark quantitatively as well.

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 nonforested ecoregions in the conterminous US, with associated implications for carbon budgets and woody proliferation," which once again spells good news for the biosphere.

Jumping a couple years closer to the present, Goetz et al. (2005) transformed satellite-derived NDVI data obtained across boreal North America (Canada and Alaska) for the period 1982-2003 into photosynthetically-active radiation absorbed by green vegetation and treated the result as a proxy for relative June-August gross photosynthesis (Pg), stratifying the results by vegetation type and comparing them with spatially-matched concomitant trends in surface air temperature data. Over the course of the study, this work revealed that area-wide tundra experienced a significant increase in Pg in response to a similar increase in air temperature; and Goetz et al. say "this observation is supported by a wide and increasing range of local field measurements characterizing elevated net CO2 uptake (Oechel et al., 2000), greater depths of seasonal thaw (Goulden et al., 1998), changes in the composition and density of herbaceous vegetation (Chapin et al., 2000; Epstein et al., 2004), and increased woody encroachment in the tundra areas of North America (Sturm et al., 2001)." In the case of interior forest, on the other hand, there was no significant increase in air temperature and essentially no change in Pg, with the last data point of the series being essentially indistinguishable from the first. This latter seemingly aberrant observation is in harmony with the fact that at low temperatures the growth-promoting effects of increasing atmospheric CO2 levels are often very small or even non-existent (Idso and Idso, 1994), which is what appears to have been the case with North American boreal forests over the same time period. As a result, Canada's and Alaska's tundra ecosystems exhibited increasing productivity over the past couple of decades, while their boreal forests did not.

Also working in Alaska, Tape et al. (2006) analyzed repeat photography data from a photo study of the Colville River conducted between 1945 and 1953, as well as 202 new photos of the same sites that were obtained between 1999 and 2002, to determine the nature of shrub expansion in that region over the past half-century. This approach revealed, in their words, that "large shrubs have increased in size and abundance over the past 50 years, colonizing areas where previously there were no large shrubs." In addition, they say their review of plot and remote sensing studies confirms that "shrubs in Alaska have expanded their range and grown in size" and that "a population of smaller, intertussock shrubs not generally sampled by the repeat photography, is also expanding and growing." Taken together, they conclude that "these three lines of evidence allow us to infer a general increase in tundra shrubs across northern Alaska."

So what is the cause of the shrub expansion? ... and when did it begin?

Tape et al. are inclined to attribute it to large-scale pan-Arctic warming; and from analyses of logistic growth curves, they estimate that the expansion began about 1900, "well before the current warming in Alaska (which started about 1970)." Hence, they conclude that "the expansion predates the most recent warming trend and is perhaps associated with the general warming since the Little Ice Age." These inferences appear reasonable, although we would add that the 80-ppm increase in the atmosphere's CO2 concentration since 1900 likely played a role in the shrub expansion as well. If continued, the researchers say the transition "will alter the fundamental architecture and function of this ecosystem with important ramifications," the great bulk of which, in our opinion, will be positive, as the greening of the earth continues.

Working at eight different sites within the Pacific Northwest of the United States, Soule and Knapp (2006) studied ponderosa pine trees to see how they may have responded to the increase in the atmosphere's CO2 concentration that occurred after 1950. The two geographers say the sites they chose "fit several criteria designed to limit potential confounding influences associated with anthropogenic disturbance." In addition, they selected locations with "a variety of climatic and topo-edaphic conditions, ranging from extremely water-limiting environments ... to areas where soil moisture should be a limiting factor for growth only during extreme drought years." They also say that all sites were located in areas "where ozone concentrations and nitrogen deposition are typically low."

At all eight of the sites that met all of these criteria, Soule and Knapp obtained core samples from about 40 mature trees that included "the potentially oldest trees on each site," so that their results would indicate, as they put it, "the response of mature, naturally occurring ponderosa pine trees that germinated before anthropogenically elevated CO2 levels, but where growth, particularly post-1950, has occurred under increasing and substantially higher atmospheric CO2 concentrations." Utilizing meteorological evaluations of the Palmer Drought Severity Index, they thus compared ponderosa pine radial growth rates during matched wet and dry years pre- and post-1950.

So what did they find? Overall, the two researchers discovered a post-1950 radial growth enhancement that was "more pronounced during drought years compared with wet years, and the greatest response occurred at the most stressed site." As for the magnitude of the response, they determined that "the relative change in growth [was] upward at seven of our [eight] sites, ranging from 11 to 133%."

With respect to the significance of their observations, Soule and Knapp say their results show that "radial growth has increased in the post-1950s period ... while climatic conditions have generally been unchanged," which further suggests that "nonclimatic driving forces are operative." In addition, they say the "radial growth responses are generally consistent with what has been shown in long-term open-top chamber (Idso and Kimball, 2001) and FACE studies (Ainsworth and Long, 2005)." Hence, they conclude their findings "suggest that elevated levels of atmospheric CO2 are acting as a driving force for increased radial growth of ponderosa pine, but that the overall influence of this effect may be enhanced, reduced or obviated by site-specific conditions."

Summarizing their findings, Soule and Knapp recount how they had "hypothesized that ponderosa pine ... would respond to gradual increases in atmospheric CO2 over the past 50 years, and that these effects would be most apparent during drought stress and on environmentally harsh sites," and in the following sentence they say their results "support these hypotheses." Hence, they conclude their paper by stating that "an atmospheric CO2-driven growth-enhancement effect exists for ponderosa pine growing under specific natural conditions within the interior Pacific Northwest," providing yet another important example of the ongoing CO2-induced greening of the earth.

Last of all, and most recently, we come to the study of Wang et al. (2006), who examined ring-width development in cohorts of young and old white spruce trees in a mixed grass-prairie ecosystem in southwestern Manitoba, Canada, where a 1997 wildfire killed most of the older trees growing in high-density spruce islands, but where younger trees slightly removed from the islands escaped the ravages of the flames. There, "within each of a total of 24 burned islands," in the words of the three researchers, "the largest dominant tree (dead) was cut down and a disc was then sampled from the stump height," while "adjacent to each sampled island, a smaller, younger tree (live) was also cut down, and a disc was sampled from the stump height."

After removing size-, age- and climate-related trends in radial growth from the ring-width histories of the trees, Wang et al. plotted the residuals as functions of time for the 30-year periods for which both the old and young trees would have been approximately the same age: 1900-1929 for the old trees and 1970-1999 for the young trees. During the first of these periods, the atmosphere's CO2 concentration averaged 299 ppm; during the second it averaged 346 ppm. Also, the mean rate-of-rise of the atmosphere's CO2 concentration was 0.37 ppm/year for first period and 1.43 ppm/year for the second.

The results of this exercise revealed that the slope of the linear regression describing the rate-of-growth of the ring-width residuals for the later period (when the air's CO2 concentration was 15% greater and its rate-of-rise 285% greater) was more than twice that of the linear regression describing the rate-of-growth of the ring-width residuals during the earlier period. As the researchers describe it, these results show that "at the same developmental stage, a greater growth response occurred in the late period when atmospheric CO2 concentration and the rate of atmospheric CO2 increase were both relatively high," and they say that "these results are consistent with expectations for CO2-fertilization effects." In fact, they say "the response of the studied young trees can be taken as strong circumstantial evidence for the atmospheric CO2-fertilization effect."

Another thing Wang et al. learned was that "postdrought growth response was much stronger for young trees (1970-1999) compared with old trees at the same development stage (1900-1929)," and they add that "higher atmospheric CO2 concentration in the period from 1970-1999 may have helped white spruce recover from severe drought." In a similar vein, they also determined that young trees showed a weaker relationship to precipitation than did old trees, noting that "more CO2 would lead to greater water-use efficiency, which may be dampening the precipitation signal in young trees."

In summary, Wang et al.'s unique study provides an exciting real-world example of the tremendous benefits the historical increase in the air's CO2 content has likely conferred on nearly all of earth's plants, and especially its long-lived woody species. Together with the results of the other North American studies we have reviewed, this body of research thus paints a picture of the planet's terrestrial vegetation that is just the opposite of what is promulgated by the world's climate alarmists. Land-based plants are not "headed to hell in a hand basket." They are thriving, thanks in large part to the ongoing rise in the atmosphere's CO2 concentration - as well as global warming! - as ever more real-world observations continue to demonstrate.

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