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Woody Plant Encroachment on the USA's Southwestern Desert
Brunelle, A., Minckley, T.A., Delgadillo, J. and Blissett, S. 2014. A long-term perspective on woody plant encroachment in the desert southwest, New Mexico, USA. Journal of Vegetation Science 25: 829-838.

The authors write that the "historic encroachment of woody plants into grassland or savanna ecosystems is observed worldwide ," citing Archer et al. (2001), Morgan et al. (2007), Van Auken (2009), Bowman et al. (2010) and Bond and Midgley (2012)." And they state that "woody plant encroachment into grassland ecosystems of the US desert southwest has been discussed in the literature since as early as the 1930s," while noting that "numerous hypotheses have been proposed to explain Madrean [pine-oak] woodland expansion across the semi-arid grasslands."

What was done
In their own effort to determine the driving force behind this phenomenon, Brunelle et al. used fossil pollen and charcoal data obtained from wetland sediments of the Cloverdale Cienega near the upper elevation transition of grassland to woodlands in southwestern New Mexico, in order to develop "a long-term record of variations in vegetation composition and fire activity," which they compared with published data on drought, the El Nino/Southern Oscillation (ENSO), carbon dioxide concentrations and known dates of grazing impacts to evaluate various hypotheses regarding the timing and causes of woody plant encroachment in desert grasslands."

What was learned
The four researchers report that (1) they were "able to reject any suggestion that changes in the natural system have caused the encroachment of Madrean woodlands into the grassland communities," that (2) "the expansion of Madrean woodlands into the desert grasslands is unprecedented compared to the previous 5500 years," that (3) "it does not appear that climate, as represented by ENSO and PDSI [Palmer Drought Severity Index] records, is related to the exponential increases in woody taxa and charcoal production in these systems," but that (4) "the contemporaneous increase in woody taxa and CO2 concentration suggests that the increase in abundance of woody plants at ~1750 CE may be attributable to increased levels of CO2 in the atmosphere associated with the onset of the Industrial Revolution."

What it means
The results of this study provide ever more evidence for earlier thoughts on the subject that were put forth by Idso and Quinn (1983), Idso (1992) and Polley et al. (1997), who described how the historical increase in the air's CO2 content has been fostering a great greening of previously not-so-green portions of the earth.

Archer, S., Boutton, T.W. and Hibbard, K.A. 2001. Trees in grasslands: biogeochemical consequences of woody plant expansion. In: Schulze, E.-D., Heimann, M., Harrison, S.P., Holland, E.A., Lloyd, J., Prentice, I.C. and Schimel, D.S. (Eds.). Global Biogeochemical Cycles in the Climate System. Academic Press, San Diego, California, USA, pp. 115-137.

Bond, W.J. and Midgley, G.F. 2012. CO2 and the uneasy interactions of trees and savannah grasses. Philosophical Transactions of the Royal Society B 367: 601-612.

Bowman, D.M.J.S., Murphy, B.P. and Banfai, D.S. 2010. Has global environmental change caused monsoon rainforests to expand in the Australian monsoon tropics? Landscape Ecology 25: 1247-1260.

Idso, S.B. 1992. Shrubland expansion in the American Southwest. Climatic Change 22: 85-86.

Idso, S.B. and Quinn, J.A. 1983. Vegetational Redistribution in Arizona and New Mexico in Response to a Doubling of the Atmospheric CO2 Concentration. Climatological Publications Scientific Paper No. 17. Laboratory of Climatology, Arizona State University, Tempe, AZ, USA.

Morgan, J.A., Milchunas, D.G., Lecain, D.R., West, M. and Mosier, A.R. 2007. Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe. Proceedings of the National Academy of Sciences USA 104: 724-729.

Polley, H.W., Mayeux, H.S., Johnson, H.B. and Tischler, C.R. 1997. Viewpoint: atmospheric CO2, soil water, and shrub/grass ratios on rangelands. Journal of Range Management 50: 278-284.

Van Auken, O.W. 2009. Causes and consequences of woody plant encroachment into western North American grasslands. Journal of Environmental Management 90: 2931-2942.

Reviewed 6 August 2014