Woody thickening: a consequence of changes in fluxes of carbon and water on a warming globe

Abstract

Understanding patterns, rates and controls of water and CO2 exchange between land surfaces and the atmosphere is central to the sciences of meteorology, ecology, hydrology, ecophysiology, forestry and related endeavours. Measurements involving sapflow sensors, eddy covariance and remote sensing have contributed substantially to our understanding of these issues. In this talk, we apply a combination of methods in order to apply a soil-plant-atmosphere model to the question: what is causing the globally observed phenomenon of woody thickening? The density of woody plants in arid and semi-arid regions is increasing regionally and globally (Fensham et al., 2005, Hoffman et al., 1999, Bowman et al. 2001, Burrows et al. 2002). This can be deduced from analyses of tree-ring widths, forest inventory data, aerial photo-interpretation and from long-term monitoring sites (Spiecker et al., 2003). Potential causes of woody thickening have been extensively discussed in the past. Mechanisms that have been proposed include the (a) Walther model, which invokes competition for water and nutrients among the deeper roots of woody plants versus the shallower roots of shrubs and grasses; (b) a role for changes in the timing, intensity and frequency of fire; and (c) changes in herbivory by large herbivores. Such thickening may have a large impact on regional CO2 budgets, atmospheric CO2 concentration and ecosystem function and regional water budgets. We propose an alternative mechanism to explain woody thickening based upon changes in water and carbon fluxes within the soil-plant-atmosphere continuum resulting from a change in global atmospheric conditions. Such a mechanism is global in reach, appears consistent with a number of phenomena and has several testable predictions, which we briefly discuss.