Plant hydraulic architecture influences transpiration through its effect on gs (Meinzer et al. 1997). Yet the interaction between gs and plant hydraulic architecture within an elevated Ca context has been largely neglected (Phillips et al. 2011). The dependency of transpiration on plant hydraulic architecture is formulated via a simple one-dimensional flow model (Tyree and Zimmerman 2002):
where E is leaf-level transpiration rate (mmol m-2(leaf) s-1), kL is leaf-specific hydraulic conductance (mmol m-2 s-1 MPa-1), is soil water potential (MPa), and is leaf water potential (MPa). The leaf-specific hydraulic conductance can be further divided as:
where HV is the Huber value (sapwood area divided by leaf area, m2 m-2), KS sapwood specific hydraulic conductivity (mmol s-1m-1MPa-1)and h tree height (m) (representing the path length for water transport). If this framework is correct, a decrease in EL can be expected to be accompanied by an increase in, or a decrease in kL. If kL changes with elevated Ca, it can be the result of a decrease in HV or KS, or opposite changes in both these components might compensate (assuming h is constant). Reports from the literature on each of these components are mixed. For example, it is commonly observed that is unaffected by elevated Ca (Phillips et al. 2011), or even decreases (Bunce 1995, 1996). Some studies have reported a decrease in kL with elevated Ca, while did not change (Bunce and Ziska 2002; Eamus et al. 1995). Other studies found that and kL did not change but the HV increased in response to elevated Ca (Phillips et al. 2011), re-calculated from their Table 1). These contrasting results, and lack of critical data, emphasise the scarcity of studies relating the decrease in transpiration at elevated Ca with plant hydraulic architecture.