As the air's CO2 content continues to rise, most trees will likely lose less water to the atmosphere via transpiration and produce increasingly greater amounts of biomass, the latter of which phenomena is much more strongly expressed in woody perennial species than in annual herbaceous plants. Consequently, bushes, shrubs and trees will likely expand their ranges more than will non-woody species with future increases in the air's CO2 concentration. In this summary, we thus review the recent scientific evidence for this phenomenon, i.e., woody-plant range expansions in response to past and predicted increases in atmospheric CO2 concentration.
To begin, Campbell et al. (2000) reviewed the work of 83 scientists associated with the Global Change and Terrestrial Ecosystems Pastures and Rangelands Core Research Project 1, which has led to the publication of over 165 peer-reviewed scientific journal articles. After synthesizing this vast amount of information, they report that "elevated CO2 and reductions in water availability are predicted to increase woodland thickening."
Although some individuals have claimed that woody plant range expansions have been favored by the overgrazing of cattle, Brown and Archer (1999) determined that historical honey mesquite encroachment in the southwestern United States occurred at a rapid rate that was unaffected by grazing pressure. In addition, Polley et al. (2002a) demonstrated that seedling survivorship of honey mesquite was correlated with soil moisture content, which increased from 1.5% at 270 ppm CO2 to 28% at 550 ppm CO2 in their experiment. Moreover, Polley et al. (2002b) grew five woody leguminous species, including honey mesquite, at twice-ambient atmospheric CO2 concentrations and reported that CO2-enriched seedlings survived maximum drought conditions 11 days longer than ambiently-grown seedlings. Thus, the historical increase in the air's CO2 content may well have been responsible for the observed expansion of honey mesquite trees in this region. That this is likely to have been the case is also born out by the observation of Dugas et al. (2001) that atmospheric CO2 enrichment significantly reduces whole-plant transpiration in Acacia farnesiana seedlings, which grow in many of the same locations as honey mesquite. Finally, and in response to these various phenomena, Lloyd et al. (1998) found that total bird abundance and species richness both increased with increasing mesquite tree density. Thus, the modest increase in the air's CO2 content over the past two centuries has likely had a pronounced positive effect on local bird biodiversity in the American southwest.
In an unrelated study, Weltzin and McPherson (2000) studied the effect of summer precipitation variability on the germination and survivorship of oak trees in the semi-desert grasslands of the southwestern United States and northern Mexico, since climate models predict this region will become warmer and wetter in the coming decades. The results of their study indicate that seedling emergence and establishment will likely increase from 1.5- to 3-fold if the climate of the area does indeed change as predicted. Likewise, Usami et al. (2001) grew two-year-old oak saplings at 350 and 700 ppm CO2 in combination with ambient and elevated air temperatures (3 to 5°C above ambient) and noted that increased air temperature alone boosted final seedling biomass by approximately 50%, while the synergistic effects of elevated CO2 and temperature boosted biomass by about 125%. Consequently, the authors concluded that any global warming that might occur with increasing atmospheric CO2 concentrations will "enhance the growth of Quercus myrsinaefolia saplings in natural forests, and accelerate [their] succession and poleward migration."
Such woody-plant expansions into America's grasslands have not been restricted to the southwest; they have also been documented on the Great Plains within the state of Kansas. Using aerial photographs taken over a 46-year period (1939-1985), Knight et al. (1994) determined that a small forest on the Konza Prairie Research Natural Area had expanded its range from 157 to 241 hectares, an increase of over 50%. Moreover, after examining historical land survey documents for the area dating back to 1859, the authors calculated that a 97% increase in forest size had already occurred by the time the baseline aerial photographs were taken in 1939.
In the Pacific Northwest region of the United States, similar increases in woody-plant abundance have been observed by comparing present-day surveys with existing historical baseline data. Such was the case in the study of Knapp and Soule (1998), who documented a 59% increase in total tree cover within the Horse Ridge Research Natural Area located in central Oregon over a 23-year period. Of particular interest was the observation that the number of clumps of western juniper increased by 37%, while the number of juniper stems rose by 53%. In an attempt to account for the incredible expansion of this species, Soule and Knapp (1999) evaluated several mechanisms that might possibly be responsible for the observed proliferation. They determined that domestic grazing and altered fire regimes had little to do with the observed increase, speculating that the historical rise in the air's CO2 content may have been the primary factor that caused the observed increases in juniper growth and abundance. In another related study, Knapp et al. (2001) used tree ring chronologies developed for this area and determined that juniper trees recovered better from the effects of drought during the 1994-1998 period, when the CO2 content of the air was 343 ppm, than they did during the period 1896-1930, when the atmospheric CO2 concentration averaged approximately 300 ppm. Thus, they concluded that "greater recovery following drought in the late period is consistent with the expected ameliorating influences of atmospheric CO2 under stressful conditions" and that this phenomenon may be the driving force that has allowed "western juniper to expand its range during the 20th century."
Increased woody-plant expansions have also been observed in subalpine forests at high altitudes (1900-2200 m). In Glacier National Park, located in Montana, USA, for example, Klasner and Fagre (2002) studied changes in subalpine fir forests between 1945 and 1991 and reported a 3.4% increase in tree coverage over this period, in spite of a net drop in the maximum summer air temperature of 0.7°C. In addition, at latitudes even farther north, Sturm et al. (2001) used repeat photography (1948-50 to 1999-2000) to look for changes in birch, willow, green alder and white spruce near the Brooks Range and Arctic coast of Alaska, USA. Over this 50-year period, the authors reported "increases in the height and diameter of individual shrubs, in-filling of areas that had only had a scattering of shrubs in 1948-50, and expansion of shrubs into previously shrub-free area." Moreover, they noted "a marked increase in the extent and density of the spruce forest." In each of these locations, human and natural disturbances have been minimal, and cannot account for the observed changes. However, the atmospheric CO2 content of the air increased by approximately 60 ppm during the period between the two assessments, and therefore, could have played a significant role in the observed range expansions.
Woody-plant range expansions have also occurred in Europe. In Norway, for example, Olsson et al. (2000) report that grasslands and heathlands that had long dominated the mountain slopes of two study areas are shrinking in response to a "forest invasion" that is characterized by the "spread of subalpine woodlands and a raised treeline." In this case, however, the authors feel that human activities are the cause of the ecological transformations; yet, again, the increasing CO2 content of the air has likely contributed to the phenomenon.
In summary, the historical rise in the atmosphere's CO2 concentration has likely played a major role in the many woody-plant range expansions that have been observed around the globe. Consequently, as the air's CO2 content climbs ever higher, we can expect this trend to continue.
References
Brown, J.R. and Archer, S. 1999. Shrub invasion of grassland: Recruitment is continuous and not regulated by herbaceous biomass or density. Ecology 80: 2385-2396.
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.
Dugas, W.A., Polley, H.W., Mayeux, H.S. and Johnson, H.B. 2001. Acclimation of whole-plant Acacia farnesiana transpiration to carbon dioxide concentration. Tree Physiology 21: 771-773.
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.
Knapp, P.A., Soule, P.T. and Grissino-Mayer, H.D. 2001. Post-drought growth responses of western juniper (Juniperus occidentalis var. occidentalis) in central Oregon. Geophysical Research Letters 28: 2657-2660.
Knapp, P.A. and Soule, P.T. 1998. Recent Juniperus occidentalis (Western Juniper) expansion on a protected site in central Oregon. Global Change Biology 4: 347-357.
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.
Lloyd, J., Mannan, R.W., Destefano, S. and Kirkpatrick, C. 1998. The effects of mesquite invasion on a southeastern Arizona grassland bird community. Wilson Bulletin 110: 403-408.
Olsson, E.G.A., Austrheim, G. and Grenne, S.N. 2000. Landscape change patterns in mountains, land use and environmental diversity, Mid-Norway 1960-1993. Landscape Ecology 15: 155-170.
Polley, H.W., Johnson, H.B. and Tischler, C.R. 2002a. Woody invasion of grasslands: evidence that CO2 enrichment indirectly promotes establishment of Prosopis glandulosa. Plant Ecology 164: 85-94.
Polley, H.W., Tischler, C.R., Johnson, H.B. and Derner, J.D. 2002b. Growth rate and survivorship of drought: CO2 effects on the presumed tradeoff in seedlings of five woody legumes. Tree Physiology 22: 383-391.
Soule, P.T. and Knapp, P.A. 1999. Western juniper expansion on adjacent disturbed and near-relict sites. Journal of Range Management 52: 525-533.
Sturm, M., Racine, C. and Tape, K. 2001. Increasing shrub abundance in the Arctic. Nature 411: 546-547.
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.
Weltzin, J.F. and McPherson, G.R. 2000. Implications of precipitation redistribution for shifts in temperate savanna ecotones. Ecology 81: 1902-1913.