Background
The authors write that "woody plant expansion within grassland ecosystems is a worldwide phenomenon, and dramatic vegetation shifts from grassland to savanna/woodlands have occurred over the past 50-100 years in North America," while noting that one of the chief factors that has contributed to this phenomenon is believed by many to have been the concomitant historical increase in the air's carbon dioxide concentration, as suggested in the studies of Archer et al. (1995), Polley (1997), Bond and Midgley (2000) and Bond et al. (2003). They also indicate that once shrublands are established, they tend to persist for a number of possible reasons, one of which is a type of feedback phenomenon referred to as islands of fertility, which "occurs when resources accumulate in soils beneath woody plants due to litterfall, interception of wet and dry deposition, nitrogen fixation, and animal droppings," as described by Schlesinger et al. (1990), Archer et al. (1995), Reynolds et al. (1999) and Lopez-Pintor et al. (2006). And they report in this regard that "changes in soil attributes under woody vegetation have been documented in the arid grasslands of the southern Great Plains, including increases in soil carbon and nitrogen," citing the work of Reynolds et al. (1999), Hibbard et al. (2001, 2003), McCulley et al. (2004), Schade and Hobbie (2005) and Liao et al. (2006).

What was done
Working at the USDA-ARS Northern Great Plains Research Laboratory near Mandan, North Dakota (USA), Springsteen et al. examined near-surface (upper 15 cm) soil biogeochemistry along a 42-year (1963-2005) chronosequence, which encompassed grassland, woodland, and grassland-woodland transition zones in a northern Great Plains grassland, in order to determine the influence of woody plant expansion on soil carbon and nitrogen contents.

What was learned
The four researchers report that total soil carbon content rose by 26% across the chronosequence from grassland to woodland within the 0-15 cm soil depth, while total soil nitrogen content rose by 31%. And they add that the rate of woody shrub expansion from 1963 to 1988 (25 years) was ~1,800 m² per year at their study site, while from 1988 to 2005 (17 years) it was ~3,800 m² per year, or just a little more than doubled.

What it means
As ever more experiments of this type are conducted at ever more sites around the world, it is becoming increasingly evident that soil carbon sequestration driven by woody-plant invasions of grasslands (which are driven to a significant degree by the ongoing rise in the air's CO2 content), as well as the increases in soil nitrogen content required to sustain them, are growing ever greater with the passage of time, as the greening of the earth continues.

References
Archer, S., Schimel, D.S. and Holland, E.A. 1995. Mechanisms of shrubland expansion: land use, climate or CO2? Climatic Change 29: 91-99.

Bond, W.J. and Midgley, G.F. 2000. A proposed CO2-controlled mechanism of woody plant invasion in grasslands and savannas. Global Change Biology 6: 865-869.

Bond, W.J., Midgley, G.F. and Woodward, F.I. 2003. The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas. Global Change Biology 9: 973-982.

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.

Hibbard, K.A., Schimel, D.S., Archer, S., Ojima, D.S. and Parton, W. 2003. Grassland to woodland transitions: integrating changes in landscape structure and biogeochemistry. Ecological Applications 13: 911-926.

Liao, J.D., Boutton, T.W. and Jastrow, J.D. 2006. Storage and dynamics of carbon and nitrogen in soil physical fractions following woody plant invasion of grassland. Soil Biology and Biochemistry 38: 3184-3196.

Lopez-Pintor, A., Sal, A.G., Benayas, J.M. R. 2006. Shrubs as a source of spatial heterogeneity -- the case of Retama sphaerocarpa in Mediterranean pastures of central Spain. Acta Oecologia 29: 247-255.

McCulley, R.L., Archer, S.R., Boutton, T.W., Hons, F.M. and Zuberer, D.A. 2004. Soil respiration and nutrient cycling in wooded communities developing in grassland. Ecology 85: 2804-2817.

Polley, H.W. 1997. Implications of rising atmospheric carbon dioxide concentration for rangelands. Journal of Range Management 50: 561-577.

Reynolds, J.F., Virginia, R.A., Kemp, P.R., de Soyza, A.G. and Tremmel, D.C. 1999. Impact of drought on desert shrubs: effects of seasonality and degree of resource island development. Ecological Monographs 69: 69-106.

Schade, J.D. and Hobbie, S.E. 2005. Spatial and temporal variation in islands of fertility in the Sonoran Desert. Biogeochemistry 73: 541-553.

Schlesinger, W.H., Reynolds, J.F., Cunningham, G.L., Huenneke, L.F., Jarrell, W.M., Ross, V.A. and Whitford, W.G. 1990. Biological feedbacks in global desertification. Science 247: 1043-1048.