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Woody Thickening in Australia
Macinnis-Ng, C., Zeppel, M., Williams, M. and Eamus, D. 2011. Applying a SPA model to examine the impact of climate change on GPP of open woodlands and the potential for woody thickening. Ecohydrology 4: 379-393.

"Woody thickening," in the words of Macinnis-Ng et al. (2011), is typically defined as "the increase in woody standing biomass in a landscape already containing woody biomass." Both it and woody plant invasion, as they continue, "are global phenomena that are commonly observed in arid and semi-arid regions, including Australia (Bowman et al., 2001; Burrows et al., 2002; Asner et al., 2003; Fensham et al., 2005; Scott et al., 2006; Witt et al., 2009)," the "tropical rainforests of Central and South America (Phillips et al., 1998), and temperate forests globally (Birdsey et al., 1993)." And although they note that "the cause of woody thickening remains debated," they say "there is an increasing awareness of potential roles for climate and changes in atmospheric CO2 concentration in causing woody thickening (and woody invasion)," citing the studies of Fensham et al. (2005), Berry and Roderick (2006), Davis et al. (2007) and Sankaran et al. (2008).

What was done
The four researchers examined the responses of gross primary production (GPP) and water use of a typical Australian woodland using the soil-plant atmosphere (SPA) model of Williams et al. (1996), which they successfully applied to the functioning of a temperate open woodland in Australia (Zeppel et al., 2008) that provided a methodology for testing the conceptual model of Eamus and Palmer (2007), which posits that the increasing atmospheric CO2 concentration and the declining evaporative demand "may explain the global phenomenon of woody thickening."

What was learned
As a result of their real-world-data-based analysis, Macinnis-Ng et al. were able to demonstrate that as the air's CO2 content rises, plant stomatal conductance decreases, such that water use per tree decreases and, therefore, soil water content increases, leading to increases in leaf area index that allow more light to be intercepted, enabling existing trees to grow bigger (even in the case of photosynthetic acclimation), which set of phenomena comprises the complex process of woody thickening.

What it means
Noting that their results "provide a valid mechanism for the conclusion of Berry and Roderick (2002) that evergreen vegetation has increased across Australia over the past 200 years as a result of CO2 enrichment," they conclude that "woody thickening in Australia and probably globally can be explained by the changes in landscape GPP and soil moisture balance arising principally from the increased atmospheric CO2 concentration."

Asner, G.P., Archer, S., Hughes, R.F. and Ansleys, R.J. 2003. Net changes in regional woody vegetation cover and carbon storage in Texas Drylands, 1937-1999. Global Change Biology 9: 316-335.

Berry, S.L. and Roderick, M.L. 2002. CO2 and land-use effects on Australian vegetation over the last two centuries. Australian Journal of Botany 50: 511-531.

Berry, S.L. and Roderick, M.L. 2006. Changing Australian vegetation from 1788 to 1988: effects of CO2 and land-use change. Australian Journal of Botany 54: 325-338.

Birdsey, R., Plantinga, A. and Heath, L. 1993. Past and prospective carbon storage in United States forests. Forest Ecology & Management 58: 33-40.

Burrows, W.H., Henry, B.K. and Back, P.V. 2002. Growth and carbon stock change in eucalypt woodlands in northeast Australia: ecological and greenhouse sink implications. Global Change Biology 8: 769-784.

Davis, M.A., Reich, P.B., Knoll, M.J.B., Dooley, L., Hundtoft, M. and Attleson, I. 2007. Elevated atmospheric CO2: a nurse plant substitute for oak seedlings establishing in old fields. Global Change Biology 13: 2308-2316.

Eamus, D. and Palmer, A.R. 2007. Is climate change a possible explanation for woody thickening in arid and semi-arid regions? Research Letters in Ecology 2007: 10.1155/2007/37364.

Fensham, R.J., Fairfax, R.J. and Archer, S.R. 2005. Rainfall, land use and woody vegetation cover change in semi-arid Australian savanna. Journal of Ecology 93: 596-606.

Phillips, O.L., Malhi, Y. and Higuchi, N. 1998. Changes in the carbon balance of tropical forests: evidence from long-term plots. Science 282: 439-442.

Sankaran, M., Ratnam, J. and Hanan, N. 2008. Woody cover in African savannas: the role of resources, fire and herbivory. Global Ecology and Biogeography 17: 236-245.

Scott, R.L., Huxman, T.E., Cable, W.L. and Emmerich, W.E. 2006. Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland. Hydrological Processes 20: 3227-3243.

Williams, M., Rastetter, E.B. and Fernandes, D.N. 1996. Modeling the soil-plant-atmosphere continuum in a Quercus-Acer stand at Harvard forest: the regulation of stomatal conductance by light, nitrogen and soil-plant hydraulic properties. Plant, Cell & Environment 19: 911-927.

Witt, G.B., Harrington, R.A. and Page, M.J. 2009. Is 'vegetation thickening' occurring in Queensland's mulga lands -- a 50-year aerial photographic analysis. Australian Journal of Botany 57: 572-582.

Zeppel, M., Macinnis-Ng, C., Palmer, A., Taylor, A., Whitley, R., Fuentes, S., Yunusa, J., Williams, M. and Eamus, D. 2008. An analysis of the sensitivity of sap flux to soil and plant variables assessed for an Australian woodland using a soil-plant-atmosphere model. Functional Plant Biology 35: 509-520.

Reviewed 17 August 2011