Background
Drylands -- arid and semiarid ecosystems consisting of mosaics of herbaceous and woody vegetation -- cover about 45% of the planet's land area (Burgess, 1995; Bailey, 1998).  During the past century, however, Asner et al. note that "the balance between plant lifeforms has shifted to favor trees and shrubs in many drylands (Archer, 1994, 2002)."  Factors contributing to this transition are vigorously debated, but they are generally acknowledged to include fire frequency, grazing intensity, atmospheric CO2 concentration and nitrogen deposition.

What was done
To learn more about this phenomenon, the authors "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 C [carbon] stocks between 1937 and 1999 in a 400-km² region of northern Texas, USA."  They say "this undertaking represents the largest scale, highest spatial resolution analysis of its kind to date."

What was learned
It was 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%)."  The net result for the entire area was a 30% increase in woody plant cover and a 32% increase in above-ground carbon stocks.

What it means
Over the past century of global environmental change (increasing air temperature, CO2 concentration and nitrogen deposition), shrubs and trees have gradually been extending their ranges to cover more and more of the earth's arid and semiarid land areas, even when man has tried to turn the tide of woody plant encroachment via various means of intervention.  This phenomenon, in turn, has led to an ever-increasing storage of carbon in woody biomass, both above- and below-ground, exerting a powerful and growing negative influence on the CO2-induced warming potential of the atmosphere that keeps earth's climate within bounds that promote the continued existence of life in all its variety.

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

Archer, S.  2002.  Proliferation of Woody Plants in Grasslands and Savannas: A Bibliography.  Texas A&M University, College Station, TX.

Bailey, R.G.  1998.  Ecoregions: The Ecosystem Geography of the Oceans and Continents.  Springer, New York, NY.

Burgess, T.L.  1995.  Desert grassland, mixed-shrub savanna, shrub-steppe, or semi-desert scrub?  The dilemma of coexisting growth forms.  In: McClaran, M.P. and van Devender, T.R. (Eds.).  Desert Grasslands.  University of Arizona Press, Tucson, AZ.