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Range Expansion (Woody Plants - Miscellaneous) -- Summary
When the air's CO2 content is experimentally increased, the vast majority of earth's plants lose less water to the atmosphere via transpiration while producing more biomass, the latter of which phenomena is generally more strongly expressed in woody perennial species than in annual herbaceous plants. Consequently, in concert with future increases in the air's CO2 concentration, earth's bushes, shrubs and trees will likely grow better than non-woody species will. Simultaneously, the increase in atmospheric CO2 content will make many plants of all types actually prefer warmer temperatures (Idso and Idso, 1994), which suggests that if air temperatures continue to rise, global warming will cause both woody and non-woody plants to grow more vigorously and expand their ranges, but with woody plants doing so more dramatically. In this summary we review some of the evidence for this phenomenon as it has operated in the past among a wide variety of trees.

Levis et al. (1999) explored the potential for vegetation feedbacks on climate via a fully-coupled climate-vegetation model with boundary conditions appropriate for 21,000 years ago. The results derived from this exercise suggested that under the colder and drier conditions of the Last Glacial Maximum, grasslands and tundra largely replace present-day forests in temperate and boreal latitudes. In addition, the physiological effects of the atmosphere's lower CO2 concentration at the time of the Last Glacial Maximum -- a concentration of about 200 ppm -- "cause a reduction in tropical and subtropical forest cover (compared to present) in favor of C4 grasslands."

These vegetation changes produce two opposing climate feedbacks in different parts of the world. In middle and high latitudes, the reduced tree cover leads to further cooling, as reduced tree cover exposes more snow and raises surface albedo. In the tropics and subtropics, however, the sparser vegetation cover weakens the hydrological cycle and leads to warmer and drier conditions.

Now, if we reverse the direction of change explored in this study and move from the colder and lower atmospheric CO2 concentrations of the Last Glacial Maximum to the warmer and higher CO2 concentrations of the present and beyond, what do we see? In all parts of the planet, including high, middle and low latitudes, there is a proliferation of trees. In the high and middle latitudes, this phenomenon has a warming effect on climate, while in the lower latitudes it has a cooling effect. Together, these two trends reduce the latitudinal temperature gradient, which phenomenon tends to weaken the potential for stormy weather and the many weather extremes that climate alarmists claim will increase as a result of CO2-induced global warming.

Campbell et al. (2000) reviewed research work done between 1994 and 1999 by a worldwide network of 83 scientists associated with the Global Change and Terrestrial Ecosystems (GCTE) Pastures and Rangelands Core Research Project 1 (CRP1), which resulted in the publication of over 165 peer-reviewed scientific journal articles. Overall, they found that the stimulatory effect of a doubling of the ambient CO2 concentration on grassland production averages about 17% in ecosystem-based experiments, although they note that the increase would likely be higher in moisture-limited and warm-season conditions. In addition, they say that these same conditions of "elevated CO2 and reductions in water availability are predicted to increase woodland thickening."

Usami et al. (2001) grew two-year-old saplings of Quercus myrsinaefolia -- an evergreen broad-leaved oak -- in controlled environment chambers having various atmospheric CO2 concentrations and air temperatures for approximately one year, in order to study the interactive effects of elevated CO2 and temperature on the development and growth of this important tree, which is widely distributed throughout Laos, Vietnam, China, Taiwan, South Korea and southwestern Japan. In doing so, they found that 3 and 5C increases in air temperature boosted final sapling biomass by 53 and 47%, respectively, in ambient air, but that at elevated CO2 concentrations that were either 1.5 or 2 times greater-than-ambient, the same 3 and 5C increases in air temperature enhanced final biomass by 110 and 140%, respectively. As a result, the researchers concluded that any global warming that might occur in the future will "enhance the growth of Q. myrsinaefolia saplings in natural forests, and accelerate [their] succession and poleward migration." However, because the trees' positive response to simultaneous increases in air temperature and atmospheric CO2 concentration negates the need for any poleward migration, it seems likely that although the trees' cold-limited range boundaries will likely extend themselves, the tree's heat-limited boundaries may be little changed, leading to a significant expansion of their geographical range.

In an extensive review of the scientific literature, Idso (2001) described a number of proven biological consequences of elevated atmospheric CO2 concentrations, one of the most important of them being an increase in plant water use efficiency that enables plants to grow and reproduce in areas that were previously too dry for them. Due to consequent increases in ground cover in these regions, the adverse effects of wind- and water-induced soil erosion are also reduced, and there is a tendency for vast tracts of previously unproductive land to become supportive of more abundant plant and animal life in what could appropriately be called a "greening of the earth." Idso's review also indicates that elevated levels of atmospheric CO2 help plants to better cope with a number of other environmental stresses, such as low soil fertility, low light intensity, high soil and water salinity, high oxidative stress, and the stress of herbivory.

On top of all of these benefits, many experiments have indicated that enrichment of the air with CO2 increases the temperature at which plants function at their optimum, making them even better suited to life in a progressively warming world than they were to the cooler state that preceded the Little Ice Age-to-Current Warm Period transition. And since trees are generally more responsive to atmospheric CO2 enrichment than are other plants, it is only natural to expect we would already be seeing evidence of their benefiting from the 100-ppm increase in the air's CO2 content experienced since the inception of the Industrial Revolution. Idso confirms this expectation as well, with information gleaned from 33 publications originating from all parts of the world.

Menyailo and Hungate (2003) assessed the influence of six boreal forest species (spruce, birch, Scots pine, aspen, larch and Arolla pine) on soil methane (CH4) consumption in a Siberian artificial afforestation experiment, where the six common boreal tree species had been grown under common garden conditions for the prior 30 years. This work revealed, in the words of the two researchers, that "soils under hardwood species (aspen and birch) consumed CH4 at higher rates than soils under coniferous species and grassland." Under low soil moisture conditions, for example, the soils under the two hardwood species consumed 35% more CH4 than the soils under the four conifers; while under high soil moisture conditions they consumed 65% more. Consequently, since Pastor and Post (1988) have suggested, in the words of Menyailo and Hungate, that "changes in temperature and precipitation resulting from increasing atmospheric CO2 concentrations will cause a northward migration of the hardwood-conifer forest border in North America," this range expansion will likely lead to an increase in methane consumption by soils and a reduction in methane-induced global warming potential.

Citing climate-alarmist predictions that global warming may occur so rapidly that trees "may not be capable of dispersing into newly available habitats quickly enough to match the rate of environmental change," Hamrick (2004) reviewed the findings of a number of studies that focus on the responses of trees to conditions analogous to those of computer-generated simulations of rapid global warming. This work revealed, as Hamrick describes it, that "trees combine life-history traits and levels of genetic diversity that will allow them to adapt relatively quickly to environmental changes [our italics]," and that they have high genetic mobility, especially via pollen. The plant biology and genetics professor also notes that "tree species have faced large-scale global environmental changes many times during their evolutionary histories," and that even though these changes "have occurred quite quickly, most tree species have survived." Consequently, Hamrick concludes that "characteristics of tree species that are often overlooked in discussions of the effects of global climatic changes" may indeed "allow many tree species to survive predicted global climatic changes while preserving much of their genetic diversity."

Working at a site some 40 km northwest of Longreach, Queensland, Australia, Krull et al. (2005) measured vertical profiles of δ13C and 14C of soil organic matter to infer the time course of changes in these parameters along a transect spanning the dynamic transition zone from C4-dominated grassland to C3-dominated woodland. This work revealed, in their words, that "much of the vegetation change at this site occurred over the last 50 years," and in discussing this finding they note that the colonization of grasslands or savannas by trees over the last 50-100 years has been attributed by many researchers "to the increase in atmospheric CO2, causing CO2 fertilization and resulting in increased water-use efficiency in C3 plants (Berry and Roderick, 2002; Grunzweig et al., 2003)."

In another study conducted in Australia, Banfai and Bowman (2006) report that "a number of processes are thought to be threatening the ecological integrity of monsoon rainforests in Northern Australia," including "the combined effects of an increase in late dry season fires, feral animal damage and weed invasion." In addition, climate alarmists everywhere contend that rainforests the world over are in danger of succumbing to the supposedly deleterious effects of a continuation of what they call the unprecedented global warming of the late 20th century, which they claim was driven by unprecedented increases in the air's CO2 content, which together comprise the "twin evils" of what we call the radical environmentalist movement.

Against this backdrop of despair, the two Australian researchers decided to test the contracting rainforest claim with a comprehensive repeat aerial photography study of the Northern Territory's Kakadu National Park, where monsoon rainforest exists as an archipelago of hundreds of small patches scattered within a larger eucalypt savanna matrix. In this undertaking, in the words of the two scientists, "changes to the boundaries of 50 monsoon rainforest patches were assessed using temporal sequences of digitized aerial photography [taken in 1964, 1984, 1991 and 2004], with a view to understanding the relative importance of the drivers of change."

So what did they find?

Banfai and Bowman report that rainforest patches increased in size between 1964 and 2004 by an average of 28.8%; and after lengthy analyses of several phenomena that might possibly have been responsible for the range increases, they concluded that "the expansion is likely to have been primarily driven by increases in variables such as rainfall and atmospheric CO2." In this regard, they note that the average area change for dry rainforests from 1964 to 2004 was an increase of 42.1%, whereas for wet rainforests the increase was 13.1%. In addition, in the case of dry rainforests, they report there was "an almost linear increase in rainforest area over the study period," in harmony with the concomitant upward trends of both atmospheric CO2 and rainfall.

In further support of the validity of their findings, and "contrary to the view that monsoon rainforests are contracting," which is one of the chief pessimistic mantras of the world's climate alarmists, the two researchers inform us that other repeat aerial photography studies conducted in Northern Australia have also revealed rainforest "expansion at the expense of more open vegetation." These studies include those of monsoon rainforests in Litchfield National Park near Darwin (Bowman et al., 2001) -- where forest patches nearly doubled in size between 1941 and 1994 -- and in the Gulf of Carpentaria (Bowman et al., 2006). In addition, they write that "these changes parallel the observed expansion of tropical rainforest on the east coast of Australia (Harrington and Sanderson, 1994; Russell-Smith et al., 2004)."

Added to these Australian findings, we note that in a recent review of the scientific literature, Lewis (2006) reports that most other tropical forests around the world also experienced significant increases in productivity over the last several decades; and he too concludes that the ongoing rise in the air's CO2 concentration is likely the key factor responsible for their increased robustness. Hence, it would appear that wherever one looks around this amazing planet of ours, the CO2-induced greening of the earth continues, especially in the case of the globe's forests.

Last of all, in a pair of companion papers, Berg et al. (2007) analyzed annual growth rings of 167 black cottonwood trees growing along two creeks in the Rocky Mountain region of Alberta, Canada, while Rood et al. (2007) measured the growth responses to the temperature differences experienced by saplings of fourteen cottonwood genotypes generated from stem cuttings of trees growing in three distinct ecoregions along the elevational gradient of the two streams: an upper montane region, an intermediate aspen parkland, and a lower fescue prairie region. The work of Berg et al. revealed that despite a 3.8C June-December mean temperature difference between the fescue prairie and montane zones -- which led to a 42% increase in growing degree days when going from the montane zone to the fescue prairie zone -- the growth rate of most trees was fairly consistent across the ecoregions, such that there was only about an 11% difference in the typical trunk diameter of century-old black cottonwoods across the three mountain zone ecoregions. In addition, the work of Rood et al. revealed the existence of what they call "localized temperature adaptation" and "elevational ecotypes of black cottonwood," which findings are, in their words, "consistent with results from studies of other deciduous and coniferous trees along mountain profiles (Sparks and Ehleringer, 1997; Oleksyn et al., 1998; Weih and Karlsson, 1999)."

The findings of Berg et al. and Rood et al., augmented by the similar observations of the other researchers they cite, suggest that the black cottonwood trees of Alberta, Canada, are indeed prepared for whatever nature might throw at them in the way of continued global warming, having the capacity to migrate upward in altitude without having to necessarily give up ground at lower elevations, as their existent genotypes appear to exhibit a wide range of adaptability to temperature.

In conclusion, it would appear that not only are earth's woody plants capable of doing whatever they need to do to survive global warming, they are doing it with the help of what the leaders of the U.S. Environmental Protection Agency so desperately desire to designate a dangerous air pollutant. What is more, the planet's trees, shrubs and bushes are actually expanding their ranges in the process. So how could "the powers that be" at the EPA have possibly gotten things so wrong? Not by the application of science, that's for sure.

References
Banfai, D.S. and Bowman, D.M.J.S. 2006. Forty years of lowland monsoon rainforest expansion in Kakadu national Park, Northern Australia. Biological Conservation 131: 553-565.

Berg, K.J., Samuelson, G.M., Willms, C.R., Pearce, D.W. and Rood, S.B. 2007. Consistent growth of black cottonwoods despite temperature variation across elevational ecoregions in the Rocky Mountains. Trees 21: 161-169.

Berry, S.L. and Roderick, M.L. 2002. CO2 and land-use effects on Australian vegetation over the last two centuries. Australian Journal of Botany 50: 511-531.

Bowman, D.M.J.S., McIntyre, D. and Brook, B.W. 2006. Is the Carpentarian Rock-rat (Zyzomys palatalis) critically endangered? Pacific Conservation Biology 12: 134-139.

Bowman, D.M.J.S., Walsh, A. and Milne, D.J. 2001. Forest expansion and grassland contraction within a Eucalyptus savanna matrix between 1941 and 1994 at Litchfield National Park in the Australian monsoon tropics. Global Ecology and Biogeography 10: 535-548.

Campbell, B.D., Stafford Smith, D.M., Ash, A.J., Fuhrer, J., Gifford, R.M., Hiernaux, P., Howden, S.M., Jones, M.B., Ludwig, J.A., Manderscheid, R., Morgan, J.A., Newton, P.C.D., Nosberger, J., Owensby, C.E., Soussana, J.F., Tuba, Z. and ZuoZhong, C. 2000. A synthesis of recent global change research on pasture and rangeland production: reduced uncertainties and their management implications. Agriculture, Ecosystems and Environment 82: 39-55.

Grunzweig, J.M., Lin, T., Rotenberg, E., Schwartz, A. and Yakir, D. 2003. Carbon sequestration in arid-land forest. Global Change Biology 9: 791-799.

Hamrick, J.L. 2004. Response of forest trees to global environmental changes. Forest Ecology and Management 197: 323-335.

Harrington, G.N. and Sanderson, K.D. 1994. Recent contraction of wet sclerophyll forest in the wet tropics of Queensland due to invasion by rainforest. Pacific Conservation Biology 1: 319-327.

Idso, C.D. 2001. Earth's rising atmospheric CO2 concentration: Impacts on the biosphere. Energy & Environment 12: 287-310.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.

Levis, S., Foley, J.A. and Pollard, D. 1999. CO2, climate, and vegetation feedbacks at the Last Glacial Maximum. Journal of Geophysical Research 104: 31,191-31,198.

Lewis, S.L. 2006. Tropical forests and the changing earth system. Philosophical Transactions of the Royal Society B 361: 195-210.

Menyailo, O.V. and Hungate, B.A. 2003. Interactive effects of tree species and soil moisture on methane consumption. Soil Biology & Biochemistry 35: 625-628.

Oleksyn, J., Modrzynski, J., Tjoelker, M.G., Zytkowaik, R., Reich, P.B. and Karolewski, P. 1998. Growth and physiology of Picea abies populations from elevational transects: Common garden evidence for altitudinal ecotypes and cold adaptation. Functional Ecology 12: 573-590.

Pastor, J. and Post, W.M. 1988. Response of northern forests to CO2-induced climate change. Nature 334: 55-58.

Rood, S.B., Berg, K.J. and Pearce, D.W. 2007. Localized temperature adaptation of cottonwoods from elevational ecoregions in the Rocky Mountains. Trees 21: 171-180.

Russell-Smith, J., Stanton, P.J., Edwards, A.C. and Whitehead, P.J. 2004. Rain forest invasion of eucalypt-dominated woodland savanna, Iron Range, north-eastern Australia: II. Rates of landscape change. Journal of Biogeography 31: 1305-1316.

Sparks, J.P. and Ehleringer, J.H. 1997. Leaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects. Oecologia 109: 362-367.

Usami, T., Lee, J. and Oikawa, T. 2001. Interactive effects of increased temperature and CO2 on the growth of Quercus myrsinaefolia saplings. Plant, Cell and Environment 24: 1007-1019.

Weih, M. and Karlsson, P.S. 1999. Growth response of altitudinal ecotypes of mountain birch to temperature and fertilization. Oecologia 119: 16-23.

Last updated 26 August 2009