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Two Decades of Global Dryland Vegetation Change
Andela, N., Liu, Y.Y., van Dijk, A.I.J.M., de Jeu, R.A.M. and McVicar, T.R. 2013. Global changes in dryland vegetation dynamics (1988-2008) assessed by satellite remote sensing: comparing a new passive microwave vegetation density record with reflective greenness data. Biogeosciences 10: 6657-6676.

The authors set the stage for their study by noting that drylands cover nearly 30% of the global land surface, and that over the last few decades many of their native ecosystems "have faced increased pressure from human demands and climate change," citing Asner et al. (2004), Dore (2005) and Liu et al., 2013).

What was done
Seeking to learn how dryland ecosystems around the world may have been responding to the concomitant pressures exerted by both man and nature, Andela et al. employed two satellite-observed vegetation products "to study the long-term (1988-2008) vegetation changes of global drylands: the widely used reflective-based Normalized Difference Vegetation Index (NDVI) and the recently developed passive-microwave-based Vegetation Optical Depth (VOD)," the first of which products "is sensitive to the chlorophyll concentrations in the canopy and the canopy cover fraction, while the VOD is sensitive to vegetation water content of both leafy and woody components," which when used together, in their words, "helps to better characterize vegetation dynamics, particularly over regions with mixed herbaceous and woody vegetation."

What was learned
The five researchers report that "NDVI was more sensitive to herbaceous vegetation changes and short-term precipitation variations," while VOD "was more sensitive to changes in woody vegetation and longer-term precipitation variations." And, as a result, they remark that "co-trends between NDVI and VOD provide evidence of widespread woody vegetation encroachment at the expense of the herbaceous vegetation component in arid regions, and arid shrublands in particular." And as their ultimate conclusion about the matter, they concluded that the "spatial distribution of trends suggests that a global driver (e.g., CO2 fertilization) is causing a change in relative performance of woody vegetation compared to herbaceous vegetation," while further noting that evidence for woody thickening and encroachment was also found for some semi-arid drylands.

What it means
In spite of the postulated growing negative impacts of man and climate alike, the greening of the earth continues - and in places where it's toughest of all to be green (arid lands) - with the proposed impetus for the phenomenon being the likely-enabling role of anthropogenic-induced atmospheric CO2 enrichment.

Asner, G.P., Elmore, A.J., Olander, L.P., Martin, R.E. and Harris, A.T. 2004. Grazing systems, ecosystem responses, and global change. Annual Review of Environment and Resources 29: 261-299.

Dore, M.H.I. 2005. Climate change and changes in global precipitation patterns: What do we know? Environment International 31: 1167-1181.

Liu, Y.Y., Dijk, A.I.J.M., McCabe, M.F., Evans, J.P. and de Jeu, R.A.M. 2013. Global vegetation biomass change (1988-2008) and attribution to environmental and human drivers. Global Ecology and Biogeography 22: 692-705.

Reviewed 15 January 2014