Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit

Responses of stomatal conductance (g(s)) to increasing vapour pressure defi cit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease - the s tomatal sensitivity - varies considerably both within and between species, Here we analysed data from a variety of sources employing both porometric a nd sap flux estimates of g(s) to evaluate the hypothesis that stomatal sens itivity is proportional to the magnitude of g(s) at low D (less than or equ al to 1 kPa), To test this relationship we used the function g(s) = g(sref) - m.InD where m is the stomatal sensitivity and g(sref) = g(s) at D = 1 kP a, Regardless of species or methodology, m was highly correlated with gs,ef (average r(2) = 0.75) with a slope of approximately 0.6. We demonstrate th at this empirical slope is consistent with the theoretical slope derived fr om a simple hydraulic model that assumes stomatal regulation of leaf water potential, The theoretical slope is robust to deviations from underlying as sumptions and variation in model parameters. The relationships within and a mong species are close to theoretical predictions, regardless of whether th e analysis is based on porometric measurements of g(s) in relation to leaf- surface D (D-s), or on sap flux-based stomatal conductance of whole trees ( G(si)), or stand-level stomatal conductance (G(s)) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D sh ow a greater sensitivity to D, as required by the role of stomata in regula ting leaf water potential.