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."
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