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Range Expansion (Plants - North America: United States, South-Central) -- 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. Consequently, in response to increases in the air's CO2 content, earth's bushes, shrubs and trees would be expected to grow better and expand their ranges more than non-woody species would be expected to do. Simultaneously, increases in atmospheric CO2 concentration often make plants of all types actually prefer warmer temperatures (Idso and Idso, 1994), causing both woody and non-woody plants to expand their ranges and grow more vigorously. In this summary, we review some of the evidence for these phenomena, focusing on what has been learned in North America, while concentrating on the south-central region of the United States.

In an attempt to understand the dynamics of the most recent portion of the 200-year historical range expansion of honey mesquite in southern Texas, Brown and Archer (1999) conducted a field experiment to determine the woody plant's emergence and growth response to different amounts of water and grass density, hoping to determine if the species' recent expansion could be attributed to livestock grazing, which has long been thought to be a driver of the phenomenon. In doing so, they found that the density and level of defoliation (due to grazing) of herbaceous vegetation, did not affect either the emergence or survival of honey mesquite seedlings, which suggested to them that the emergence and establishment of honey mesquite is independent "of resource availability and livestock grazing pressure on grasses." And in examining the question of whether or not the recent expansion of honey mesquite in southern Texas might have been due to a run of years of anomalously high precipitation, the researchers determined, from the size and age-class distribution of honey mesquite in this area, that there was "no indication of episodic establishment or mortality." Consequently, they concluded that "woody plant encroachment into grasslands can be high, regardless of grazing pressure or herbaceous composition and biomass."

So just what has been responsible for woody-plant range expansions in the South-Central United States?

An impressive clue was provided by Dugas et al. (2001), who studied the response of whole-plant transpiration to atmospheric CO2 enrichment in the woody legume Acacia farnesiana, which occurs throughout the South-Central United States and is one of the more aggressive woody-plant invaders of grasslands worldwide. Plants of this species were grown for one year in greenhouse bays maintained at atmospheric CO2 concentrations of either 385 or 980 ppm, after which whole-plant transpiration was assessed via sap flow measurements. This work revealed that the mean transpiration rate of the plants that had been grown at a CO2 concentration of 980 ppm was only about a fourth of the transpiration rate of the plants that had been grown at 385 ppm; and this huge and increasing reduction in whole-plant transpiration rate of A. farnesiana could well account for its ever-increasing ascendancy over grasses as the air's CO2 content has risen ever higher since the dawn of the Industrial Revolution.

What are some of the consequences of this persistent phenomenon?

Working in the La Copita Research Area southwest of Alice, Texas, Hibbard et al. (2001) analyzed several chemical and physical properties of the top ten centimeters of soils in remnant herbaceous areas and patches of woody vegetation in various stages of invasive development. Compared to soils beneath herbaceous vegetation, they found that the soils beneath the tree/shrub areas had much greater concentrations of both carbon (C) and nitrogen (N); and a companion study of soil C and N across woody patches ranging in age from 10 to 110 years indicated that these variables had experienced a linear increase through time.

What was the source of these C and N increases? In a word, roots. The researchers say they "were surprised by the magnitude of root biomass in surficial soils of woody patches, which greatly exceeded that of herbaceous patches and which greatly exceeded that of foliar litter inputs." Citing a number of studies of rates of root turnover in herbaceous and woody-plant ecosystems, they concluded that "the role of belowground inputs in fueling changes in surficial soil C and N stocks ... accompanying shifts from grass to woody plant domination may therefore be more substantial than previously appreciated."

How much more substantial? In broaching this question, Hibbard et al. began by noting that "the contrasts between woody and herbaceous patches reported here are conservative in that they do not include an assessment of whole plant C and N stocks," i.e., root biomass below ten centimeters depth and woody biomass aboveground. With respect to the first of these factors, they cite several studies that have detected greater soil C concentrations beneath woody vs. herbaceous vegetation to depths of 100 to 400 centimeters. With respect to the second factor, they likewise cite evidence suggesting that "plant C mass has increased tenfold with the conversion of grassland to savanna woodland over the past 100 years."

So what do these findings imply about the world as a whole? The research team notes that since "woody plant expansion into drylands has been geographically widespread over the past century," and since "40% of the terrestrial biosphere consists of arid and semiarid savanna, shrubland, and grassland ecosystems, this type of vegetation change may be of significance to the global C and N cycle." To fully quantify the significance of this phenomenon, however, they say we must obtain better information on "the historic or modern rate, areal extent, and pattern of woody plant expansion in the world's drylands."

Vigorous pursuit of this information via remote sensing techniques that show promise of quantifying grass vs. woody plant biomass in grasslands and savannas, coupled with ever-evolving ecosystem modeling techniques, may soon provide the answers we seek. From what we already know, however, it's a good bet that Hibbard, Archer, Schimel and Valentine have laid the necessary groundwork for resolving the dilemma of the world's so-called missing carbon: it could well be found in the soils and standing biomass of woody plants that have invaded earth's grasslands and savannas over the period of rising atmospheric CO2 concentration that began with the advent of the Industrial Revolution.

Searching for more evidence of the role played by the ongoing rise in the air's CO2 content in the ongoing expansion of the ranges of earth's woody plants, Polley et al. (2002b) grew seedlings of five woody leguminous species for close to one month in greenhouses maintained at atmospheric CO2 concentrations of 390 and 700 ppm under well-watered and water-stressed conditions, in order to determine the effects of elevated CO2 and soil moisture on plant survivorship in the face of drought. This work revealed that the seedlings grown in the CO2-enriched air exhibited greater rates of photosynthesis and more favorable leaf water potentials than the seedlings grown in ambient air. In addition, the elevated CO2 increased seedling biomass from 11 to 43%, except in one of the species whose biomass was indifferent to atmospheric CO2 concentration. Also, the CO2-enriched seedlings did not reach the point where 50% of their population had succumbed to drought-induced death until four days after the seedlings grown in ambient air had reached that point. What is more, the CO2-enriched seedlings survived eleven days longer than the ambient-air seedlings when subjected to maximum drought conditions. Consequently, there would appear to be a number of soil-moisture-related ways by which rising atmospheric CO2 concentrations foster the transformation of grasslands into woodlands.

Further exploring these phenomena, Polley et al. (2002a) grew representative grassland plants in plastic tunnels having a continuous atmospheric CO2 gradient that ranged from 200 to 550 ppm, in order to determine how the air's CO2 concentration impacts grassland water use and subsequent soil moisture contents. Concurrently, in a parallel study, honey mesquite seeds were germinated and seedling survivorship was followed in grasslands where grass roots were excluded from the first 0.15 meters of soil or where supplemental irrigation was provided to study the effects of soil water content on these parameters. Among the many things learned from these studies, the researchers found that the presence of grasses (and their roots in the first 0.15 meters of soil) significantly reduced average soil water content over the three months of the study. Second, honey mesquite seedling survivorship was correlated with soil moisture content and increased from 1.5 % at 270 ppm CO2 to 15% at 360 ppm and 28% at 550 ppm. Consequently, Polley et al. concluded that "by reducing water limitation to establishing seedlings, the continuing rise in the atmospheric CO2 concentration could facilitate mesquite establishment on dry grasslands."

In one final study conducted in the South-Central United States, Asner et al. (2003) "used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatial specific changes in woody vegetation cover and aboveground carbon stocks between 1937 and 1999 in a 400-km2 region of northern Texas," noting that "this undertaking represents the largest scale, highest spatial resolution analysis of its kind to date." As a result, they determined that "rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years," while "areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (-75% to +280%)." Lumped together, the net result for the entire area was a 30% increase in woody plant cover and a 32% increase in above-ground carbon stocks.

In conclusion, it is evident that over the past century of significant global environmental change - including increasing air temperature, CO2 concentration and anthropogenic nitrogen deposition - woody plants throughout the South-Central United States have been gradually extending their ranges to cover more and more arid and semiarid land areas, even when man has tried to limit their expansion; and this phenomenon has led to an ever-increasing storage of carbon in woody-plant biomass, both above- and below-ground, which exerts a powerful negative influence on the potential for CO2-induced global warming. This latter phenomenon, in turn, helps to keep earth's climate within bounds conducive to the continued existence of life, while the former phenomenon enhances both the abundance and diversity of life in arid and semi-arid areas.

Asner, G.P., Archer, S., Hughes, R.F., Ansley, R.J. and Wessman, C.A. 2003. Net changes in regional woody vegetation cover and carbon storage in Texas drylands, 1937-1999. Global Change Biology 9: 316-335.

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.

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.

Hibbard, K.A., Archer, S., Schimel, D.S. and Valentine, D.W. 2001. Biogeochemical changes accompanying woody plant encroachment in a subtropical savanna. Ecology 82: 1999-2011.

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.

Polley, H.W., Johnson, H.B. and Tischler, C.R. 2002. 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.

Last updated 2 May 2007