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Range Expansion (Plants - North America: United States, Scattered Locations) -- Summary
When the atmosphere's CO2 concentration is experimentally increased, the vast majority of earth's plants lose less water to the atmosphere via transpiration, but they produce more biomass, the latter of which phenomena is generally more strongly expressed in woody perennial species than it is in annual herbaceous plants. As a result of increases in the air's CO2 content, therefore, earth's bushes, shrubs and trees would be expected to grow better and expand their ranges into drier terrain more readily than would be expected of non-woody species. Simultaneously, increases in atmospheric CO2 often make plants of all types actually prefer warmer temperatures (Idso and Idso, 1994), causing both woody and non-woody plants to grow more vigorously and expand their ranges during periods of global warming. In this summary, we review some of the evidence for, and the consequences of, these phenomena, focusing on what has been learned in studies conducted at a number of sites scattered throughout the United States.

In a study designed to evaluate historic climate change more than biological responses to it, Klasner and Fagre (2002) studied summer temperatures and spring snowpack over the period 1927-1991 in the McDonald Creek drainage basin of Montana's Glacier National Park, as well as altitudinal and areal changes in subalpine fir forests between 1945 and 1991 at six 40-hectare sites located between elevations of 1900 and 2200 meters. This work revealed no net change in spring snowpack from the beginning to the end of the record. Likewise, there was no net change in summer minimum temperature; but in the case of summer maximum temperature, there was a change: a net drop of about 0.7C from the beginning to the end of the study period. Thus, it should come as no surprise that altitudinal changes in the location of the alpine treeline were not observed. However, and in spite of the drop in summer maximum temperature, there was a 3.4% increase in the area of tree coverage from 1945 to 1991, as well as an increase in the density of trees.

Dropping down to Colorado, Perfors et al. (2003) employed a set of overhead infrared radiative heaters to continuously warm five 3- x 10-meter plots of ungrazed montane meadow at the Rocky Mountain Biological Laboratory in Gunnison County, while five similar plots served as controls. The extra downward flux of infrared radiation warmed the top 15 cm of soil by about 1.5C and dried it by about 15% on a gravimetric basis during the growing season, prolonging the snow-free season at each end by a total of 20 days. They then developed and applied a method for extracting the age-detrended growth rate of common sagebrush in an effort to determine the effect of a modest warming on this widespread woody plant.

Annual sagebrush growth rates in the heated plots were found to be approximately 50% greater than those in the control plots, due primarily to the warming-induced increase in the length of the snow-free season, suggesting, in the words of the researchers, that "global climate change, which is expected to result in a contracted period of snow accumulation in the montane west, will result in increased growth and range expansion of sagebrush near high-elevation range boundaries in the western US." Although Saleska et al. (2002) had earlier demonstrated that the experimental warming decreased soil organic carbon content, Perfors et al. additionally suggest that "because sagebrush litter is more recalcitrant to decomposition than is the litter from the forb species that are in decline in the heated plots of our climate manipulation experiment, enhanced sagebrush growth could also contribute to a negative feedback [to CO2-induced warming] by increasing the turnover time of soil carbon."

Slipping over into Kansas, Knight et al. (1994) used aerial photographs to analyze the dynamics of gallery forest on the Konza Prairie Research Natural Area (KPRNA) between 1939 and 1985. This effort indicated that over the period of study, total gallery forest area increased from 157 hectares to 241 hectares. Going back further in time, additional historical information obtained from the original Land Office Surveys of KPRNA revealed that total forest area in the region had increased 97% between 1859 and 1939, leading the researchers to conclude there is "no question that the absolute amount of forested areas has increased," and, we would add, substantially.

In discussing the significance of their findings for the Great Plains of America, Knight et al. remark that Coronado in 1541 stated "there is not any kind of wood in all these plains, away from the gullies and rivers, which are very few." A dramatic increase in forest growth has certainly occurred in this region since that time, however, and in particular over the last century and a half. One of the reasons for this increase is most certainly the historical increase in earth's atmospheric CO2 concentration. Rising from a value of approximately 265 ppm at the time of Coronado to a value in excess of 380 ppm today, the 43% or more increase in atmospheric CO2 has had a pronounced positive impact on the photosynthesis and growth of woody species on every continent where they are found (Idso, 1995), as well as on their water use efficiencies; and the findings of Knight et al. are just one more testament to the beneficent consequences of that "fact of life."

In another study conducted on the Konza Prairie Research Natural Area, McCarron et al. (2003) measured the effects of mesic grassland-to-shrubland conversion on soil CO2 flux, extractable inorganic N, and N mineralization beneath isolated C3 shrub "islands" and surrounding undisturbed native tallgrass prairie. This work revealed that "a shift in plant community composition from grassland to shrubland resulted in a 16% decrease in annual soil CO2 flux with no differences in total soil C or N or inorganic N."

The researchers additionally note that their results are "consistent with two other recent studies in a nearby tallgrass prairie that assessed the effects of juniper forest invasion on C and N cycling," i.e., those of Norris (2000) and Smith (2001); and their similar findings add to the mounting evidence that refutes the ill-founded conclusion of Jackson et al. (2002) -see our Editorial of 21 August 2002 - that woody-plant invasion-induced losses of soil organic carbon at mesic sites are large enough to offset concomitant increases in plant biomass carbon. Quite to the contrary, McCarron et al.'s results clearly demonstrate that the invasion of mesic grasslands by woody plants leaves soil carbon stores essentially unaltered, while greatly boosting aboveground inventories of sequestered carbon. Hence, the invasion of mesic grasslands by shrubs and trees mostly likely enhances the biological sequestration of carbon in these widespread and globally-dispersed ecosystems that cover vast areas of the earth's surface.

Lastly, in a most unusual serendipitous study conducted in North Carolina, McCarthy et al. (2006) determined that the non-intensively managed pine plantation that is home to the Duke Forest FACE study experienced a huge reduction in living biomass carbon during a severe ice storm that affected much of the southeastern United States between 4 and 5 December of 2002. Drawing on weather and damage survey data from the entire storm cell, they calculated that the amount of carbon that was transferred from the living to the dead biomass pool during that single event was equivalent to about 10% of the annual carbon sequestration of all forests in the conterminous United States.

Taking advantage of this unique situation, McCarthy et al. compared how the ambient-air and CO2-enriched loblolly pine trees of the Duke Forest FACE study stood up to the devastating effects of the storm. After carefully tabulating all of the damage done to the trees in their experimental plots, they found that the loblolly pine trees growing in ambient air experienced an ice-storm-induced live biomass carbon reduction of 254 g C m-2, while those growing in air enriched with an extra 200 ppm of CO2 (about a 50% increase above the ambient concentration) suffered a live biomass carbon reduction of only 80 g C m-2, which loss was about 70% less than the loss experienced by the trees growing in ambient air. What is more, they found that because of the lesser leaf area reduction caused by the storm in the CO2-enriched plots, the trees in those plots "also exhibited a smaller reduction in biomass production the following year."

In discussing the significance of their findings, the seven researchers who conducted the analysis say their results suggest that "forests may suffer less damage during each ice storm event of similar severity in a future with higher atmospheric CO2." And of particular significance to the subject of this Summary, they say that "the lessening of crown breakage by ice storms in a future CO2-enriched atmosphere may allow loblolly pine to expand its range northerly," because "ice damage to upper crowns decreases the reproductive capacity of species that rely on wind for seed dispersal and therefore produce seeds exclusively in the upper crown," which can limit the range expansion capacities of such trees and the creation of new forests.

In concluding, we note that the findings of these scattered United States investigations, which stretch from the northwestern part of the country to its southeastern sector, are pretty much the same as those observed in more intensively studied parts of the land. And in the face of what climate alarmists describe as the most serious threat facing the world today (concomitant unprecedented increases in atmospheric temperature and CO2 concentration), the findings of these many studies clearly demonstrate that nothing but good has come of the "threat" over the entire course of the 20th century.

References
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.

Idso, S.B. 1995. CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution. Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN.

Jackson, R.B., Banner, J.L., Jobbagy, E.G., Pockman, W.T. and Wall, D.H. 2002. Ecosystem carbon loss with woody plant invasion of grasslands. Nature 418: 623-626.

Klasner, F.L. and Fagre, D.B. 2002. A half century of change in alpine treeline patterns at Glacier National Park, Montana, U.S.A. Arctic, Antarctic, and Alpine Research 34: 49-56.

Knight, C.L., Briggs, J.M. and Nellis, M.D. 1994. Expansion of gallery forest on Konza Prairie Research Natural Area, Kansas, USA. Landscape Ecology 9: 117-125.

McCarron, J.K., Knapp, A.K. and Blair, J.M. 2003. Soil C and N responses to woody plant expansion in a mesic grassland. Plant and Soil 257: 183-192.

McCarthy, H.R., Oren, R., Kim, H.-S., Johnsen, K.H., Maier, C., Pritchard, S.G. and Davis, M.A. 2006. Interaction of ice storms and management practices on current carbon sequestration in forests with potential mitigation under future CO2 atmosphere. Journal of Geophysical Research 111: 10.1029/2005JD006428.

Norris, M. 2000. Biogeochemical Consequences of Land Cover Change in Eastern Kansas. In: Division of Biology, Kansas State University, Manhattan, Kansas, USA.

Perfors, T., Harte, J. and Alter, S.E. 2003. Enhanced growth of sagebrush (Artemisia tridentata) in response to manipulated ecosystem warming. Global Change Biology 9: 736-742.

Saleska, S.R., Shaw, M.R., Fischer, M.L., Dunne, J.A., Still, C.J., Holman, M.L. and Harte, J. 2002. Plant community composition mediates both large transient decline and predicted long-term recovery of soil carbon under climate warming. Global Biogeochemical Cycles 16: 10.1029/2001GB001573.

Smith, D. 2001. Changes in Carbon Cycling as Forests Expand into Tallgrass Prairie: Mechanisms Driving Low Soil Respiration Rates in Juniper Forests. In: Division of Biology, Kansas State University, Manhattan, Kansas, USA.

Last updated 30 May 2007