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Woody Plants Take Possession of Rangelands in Southern New Mexico, USA
Gibbens, R.P., McNeely, R.P., Havstad, K.M., Beck, R.F. and Nolen, B.  2005.  Vegetation changes in the Jornada Basin from 1858 to 1998.  Journal of Arid Environments 61: 651-668.

In a review of all large FACE studies conducted over the prior 15 years, Ainsworth and Long (2005) report that the greatest CO2-induced benefits were accrued by trees, which experienced a mean biomass increase of 42% in response to a 300-ppm increase in the atmosphere's CO2 concentration.  In comparison, they found that C4 sorghum posted a yield increase of only 7%, while the C3 crops rice and wheat exhibited yield increases of 16% and 22%, respectively, although cotton (which is actually a woody perennial and more like a tree than a rice or wheat plant) exhibited a yield increase of 63%.  Hence, it is only natural to presume that as the air's CO2 content continues to climb, woody plants (whose growth is much more stimulated by rising CO2 concentrations than is that of herbaceous plants) will gradually encroach upon areas where herbaceous plants previously ruled the landscape; and, in fact, such is typically observed to be the case the world over (see, for example, Trees (Range Expansions) in our Subject Index).

What was done
The authors 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, USA, 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.

What was learned
Gibbens et al. report 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."

What it means
Gibbens et al.'s results 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)."  Although they do not state a preference for the driving force or forces for these changes, mentioning several that have been put forth over the years, Idso (1995) describes the case for the historical rise in the air's CO2 concentration being the predominant factor among the mix of hypotheses.

Ainsworth, E.A. and Long, S.P.  2005.  What have we learned from 15 years of free-air CO2 enrichment (FACE)?  A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2New Phytologist 165: 351-372.

Archer, S.  1988.  Autogenic succession in a subtropical savanna: conversion of grassland to thorn woodland.  Ecological Monographs 58: 111-127.

Archer, S.  1994.  Woody plant encroachment into Southwestern grasslands and savannas: rates, patterns and proximate causes.  In: Vavra, M., Laycock, W.A. and Pieper, R.D., Eds.  Ecological Implications of Livestock Herbivory in the West.  Society for Range Management, Denver, Colorado, USA, pp. 13-68.

Hastings, J.R. and Turner, R.M.  1965.  The Changing Mile: An Ecological Study of Vegetation Change with Time in the Lower Mile of a Arid and Semiarid Region.  University of Arizona Press, Tucson, Arizona, USA.

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

Martin, S.C. and Turner, R.M.  1977.  Vegetation change in the Sonoran Desert region of Arizona, USA and Sonora, Mexico.  Journal of the Arizona Academy of Science 12: 59-69.

McClaran, M.P.  2003.  A century of vegetation change on the Santa Rita Experimental Range.  In: McClaran, M.P., Ffolliott, P.F. and Edminster, C.B. Technical Coordinators.  Santa Rita Experimental Range: 100 Years (1903-2003) of Accomplishments and Contributions.  Conference Proceedings RMRS-P-30. Ogden, Utah, USA.  United States Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Reviewed 11 May 2005