Simulations of Hydroecological Responses to Elevated CO₂ at the Catchment Scale

Abstract

A spatially explicit hydroecological landscape model of water, carbon and energy balances (Topog- IRM) is described. The landscape is envisaged as a catchment forested with a single stratum comprising Eucalyptus maculata trees. The model was used to simulate the direct effects of a 2× elevation in atmospheric carbon dioxide at two levels of nitrogen on catchment water yield, soil moisture status and tree growth. Experimental results used to parameterise the model are detailed. Key features of the model are (1) an ability to scale hydrological processes at the catchment scale in three dimensions, and (2) a means to integrate multiple factors/ stresses on plant growth.

The effects of CO₂ on catchment hydrology (water yield or soil moisture content) and forest growth (expressed as leaf area index, LAI) were modelled for a 2.-year period, and contrasted with the effects of added nitrogen. Results were expressed as totals for the catchment or spatially distributed across the catchment. For the total catchment, water yield increased in the order: high CO₂ with low N, high CO₂ with high N, ambient CO₂ with low N, ambient Co₂ with high N. LAI increased from 3.3 to 5.7 in the order: ambient CO₂ with low N, ambient CO₂ with high N, high CO₂ with low N, high Co₂ with high N. These results agree with previous data. New findings are:

(1) with elevated CO₂ a new equilibrium in transpiration is established in which leaf area increases offset decreases in stomatal conductance;

(2) the addition of nitrogen increases transpiration without any indication of a new equilibrium being reached during the simulated period;

(3) the spatial distribution of soil moisture changes, presenting a new resource base for spatial changes to species composition and growth rates.

The major hydroecological responses to elevated CO₂ are seen as increased maximum upper canopy leaf area, increased litter inputs, especially at times of drought (hence changed fire regimes), changes in the composition of the understorey (hence litter composition, soil microfauna, and the spatial expression of biological diversity) and a slight increase in water yield.