Abstract:
Increased woody plant density (woody encroachment or woody thickening) is a globally observed phenomenon. Similarly, increased
atmospheric carbon dioxide concentrations and decreased pan evaporation rates are globally observed phenomena. In this
paper, we propose that the former (increased woody plant density) is a product of the latter. We propose that decreased stomatal
conductance and increased rates of carbon fixation arising from an enriched atmospheric carbon dioxide concentration, in conjunction
with reduced rates of pan evaporation, result in increased woody plant density. We suggest that this is analogous to the
increased woody plant density that is observed along rainfall gradients that span arid to mesic environments. From this conceptual
model, we make three predictions, namely, that (a) long-term trends in tree water-use-efficiency should reveal increased values;
(b) run-off data should show an increase where woody thickening is occurring; (c) enriched CO2 experiments should reveal an
enhanced plant water status. These three predictions are discussed and shown to be supported by experimental data.
