In an oak savanna setting in southeastern Arizona, Weltzin and McPherson (2000) established a number of experimental plots - which were isolated from ambient precipitation and soil moisture - that they hand-watered between July 1994 and October 1996 in accordance with the long-term (30-year) mean characteristics of natural precipitation there, as well as with 50% additions and subtractions from the winter and summer precipitation regimes. In mid-July of 1994 and 1995, they also planted 49 Quercus emoryi acorns in each plot, after which they monitored seedling emergence and survival. Based on what they observed, the two researchers determined that "summer precipitation was most important for Q. emoryi seedling emergence and early establishment," with differences in recruitment ranging from 1.5- to 3-fold greater in the wet summer regime than in the dry summer regime. In addition, they state that this result was in harmony with the results of several other studies, where "establishment of other woody plants within arid and semi-arid grasslands has been attributed to intra to interannual periods of above average precipitation or soil moisture."

Imbedded as we are, or at least have been for some time, in a centennial-scale period of warming that has been concurrent with a similar-scale increase in atmospheric CO2 concentration (which is likely to continue much longer, and which tends to increase plant optimum temperature, i.e., the temperature at which plants grow best, and plant water use efficiency), the results of this study and the others it cites suggest that woody plants, such as oak trees, should gradually expand onto arid and semi-arid grasslands where they presently cannot grow and successfully reproduce. As further support for this conclusion, Weltzin and McPherson cite the work of McClaran and McPherson (1995), "who concluded that the last downslope shift in this ecotone, which occurred 700-1700 years BP [Before Present], coincided with a period of particularly high summer precipitation in the region (i.e., the Medieval Warm Period, 645-1295 years BP)," and perhaps this is why we are already seeing this transformation of the planet (woody-plant range expansions into arid and semi-arid regions) on every continent of the globe on which woody plants grow.

Working in the laboratory, Peterson and Neofotis (2004) grew velvet mesquite seedlings for six weeks from the time of their planting (as seeds) in small pots within environmentally-controlled growth chambers that were maintained at atmospheric CO2 concentrations of 380 and 760 ppm and two levels of water availability (high and low). Although they did not see a significant CO2-induced increase in plant growth in their study, they say that by the end of the six weeks there was a highly significant reduction of approximately 41% in the volume of water transpired by the plants in response to the experimental doubling of the air's CO2 content. "This large reduction in whole-plant water use," as they describe it, "occurred because the reduction in transpiration per unit leaf area at elevated CO2 was not offset by a proportional increase in total leaf area."

In commenting on their findings, the pair of scientists from the Biosphere 2 Center near Oracle, Arizona, say they suggest that under a future high-CO2 scenario, "seedlings may deplete soil moisture at a slower rate than they do currently," and that "this could facilitate seedling survival between intermittent rain events," noting that their work "corroborates the conclusions of Polley et al. (1994, 1999, 2003) that increasing levels of atmospheric CO2 may facilitate the establishment of mesquite seedlings through a reduction in soil water depletion." That such has indeed occurred is suggested by the fact, again quoting Peterson and Neofotis, that "mesquites and other woody species in the semiarid southwestern United States have shown substantial increases in population density and geographic range since Anglo-American settlement of the region approximately 120 years ago," in support of which statement they cite the studies of Van Auken and Bush (1990), Gibbens et al. (1992), Bahre and Shelton (1993), Archer (1995), Boutton et al. (1999), Van Auken (2000), Ansley et al. (2001), Wilson et al. (2001) and Biggs et al. (2002).

Gibbens et al. (2005) utilized notes made by land surveyors in 1858 to estimate cover of grasses and shrubs on the Jornada Experimental Range (JER) and the Chihuahuan Desert Range Research Center (CDRRC) in the northern Chihuahuan Desert in southern New Mexico, after which they analyzed data derived from reconnaissance surveys made in 1915-1916 and 1928-1929 on the JER and in 1938 on the CDRRC, together with vegetation maps of both properties made in 1998, to determine the history of vegetation change on these ranges from 1858 to 1998, over which 140-year time span the atmosphere's CO2 concentration rose by approximately 28%, from a value of 287 ppm to a value of 367 ppm. In doing so, they learned that in 1858, fair to very good grass cover occurred on 98% of the JER, but by 1998, fully 92% of the range was controlled by shrubs, leaving only 8% for grasses. Likewise, fair to very good grass cover occurred on 67% of the CDRRC in 1858, but by 1998, 91% of the range was controlled by shrubs, leaving only 9% for grasses. Nevertheless, and most interestingly, they say "there does not appear to have been any loss of grass species given that all species identified in early 20th-century surveys are still present."

These observations depict an amazing transformation of the landscapes of these two rangelands, from a condition of close to complete dominance by grasses to one of almost complete dominance by shrubs; but these dramatic changes are not unique. They report, for example, that "long-term records have revealed similar changes in the Sonoran Desert (Hastings and Turner, 1965; Martin and Turner, 1977; McClaran, 2003)," and that "even in areas of the southwestern United States receiving more precipitation than the Chihuahuan and Sonoran deserts there has been an encroachment of shrubs into former grasslands and savannas (Archer, 1988; Archer, 1994)."

In California, Zavaleta and Kettley (2006) examined patterns of production, standing biomass, carbon (C) and nitrogen (N) storage, community composition, and soil moisture along a 25-year chronosequence of sites within an annual, exotic-dominated grassland at Stanford University's Jasper Ridge Biological Preserve in the interior foothills of the central coast range south of San Francisco, various parts of which have been invaded at a number of different times over the past quarter-century by Baccharis pilularis shrubs. They report finding that increasing above- and below-ground biomass along the chronosequence "drove increases in ecosystem N sequestration of ~700% and in C storage of over 125%," including a 32% increase in total soil C over the 25-year period. What is more, they found that "increases in carbon storage also did not appear to be saturating at 25 years after shrub establishment in any pool, suggesting the potential for additional carbon gains beyond 25 years." In addition, they found that Baccharis shrubs began to decline in prominence after about 20 years, as native oaks "with life spans of centuries" and the potential to drive even larger ecosystem changes began to grow in the shrub-dominated areas.

Of further interest, the two researchers say they "initially hypothesized that Baccharis-invaded sites would experience increasing N limitation as N was immobilized in biomass and litter." However, as they continue, they found that "total soil N increased rapidly with shrub age" and that "the magnitude of increase in total soil nitrogen was much larger than the increase in nitrogen immobilization in biomass and litter over time." Consequently, they say that their findings "illustrate the potential for important vegetation-mediated ecosystem responses and feedbacks to atmospheric CO2 and climate change," as indeed they truly do.

In a somewhat different type of study, Dole et al. (2003) modeled potential future changes in the geographic distribution of the Joshua Tree (Yucca brevifolia), the current distribution of which is roughly coincident with the Mojave Desert of the southwestern United States (southern California, southern Nevada, and western Arizona), based on (1) climate changes predicted to accompany a doubling of the air's CO2 content and (2) the experimental observation that a doubling of the atmosphere's CO2 concentration, in the words of the researchers, "enhances the low-temperature tolerance of Y. brevifolia seedlings, such that the lethal low temperature is lowered by 1.6°C," as reported by Loik et al. (2000). This exercise revealed that "the increase in freezing tolerance caused by doubled CO2 would increase the potential habitat of this species by 14%, independent of any climate change." On the other hand, if only the predicted warming effect of the doubled CO2 is considered, the total area occupied by Y. brevifolia declines by 25%. When both effects are considered together, however, Dole et al. report that the model predicts a "slightly larger future distribution."

In a final study that looked at one of the important ecological ramifications of the expanding ranges of woody plants, Lloyd et al. (1998) evaluated the relationship between bird abundance and a number of large-scale vegetation features, including the density and distribution of mesquite trees at the Buenos Aires National Wildlife Refuge in southeastern Arizona in an effort to understand changes that occur within bird communities as a result of changes in ecosystem composition, specifically, changes that have arisen due to the documented expansion of woody tree species into this region. The results of this undertaking indicated that among the following variables - overall grass, herb and shrub cover, percent cover of native grasses, percent cover of an introduced grass (Lehmann lovegrass), average size of mesquite trees, and the density of mesquite trees - only the density and distribution of mesquite trees were found to influence bird populations. Specifically, total bird abundance was found to increase with increasing mesquite density, and greater bird species richness was found on plots with higher mesquite densities.

In conclusion, as near-surface air temperatures and atmospheric CO2 concentrations climbed ever higher over the past hundred or more years, various types of woody-plants concomitantly expanded their ranges into arid and semiarid regions of the American Southwest that had been unable to support their growth for many prior centuries, in harmony with what has been revealed by numerous CO2-enrichment and soil-water-manipulation experiments. This phenomenon has been demonstrated to significantly increase ecosystem productivity and carbon and nitrogen storage in both plants and soils, as well as ecosystem species richness, demonstrating that the "twin evils" of the radical environmental movement - global warming and atmospheric CO2 enrichment - have to date been "twin goods," suggesting that more of the same, i.e., continued increases in both parameters, may yield still further biospheric benefits.

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