KEYWORDS
A
S : AL • sapwood-to-leaf area ratio •
D
• vapour pressure deficit outside of leaf boundary layer •
D
s
• vapour pressure deficit at the leaf surface •
E
• transpiration per unit of leaf area •
g
bl
• boundary layer conductance for water vapour •
g
s
• stomatal conductance for water vapour •
G
Si
• mean conductance for water vapour of individual trees • mostly based on single sensors •
G
S
• mean canopy stomatal conductance for water vapour •
g
sref
•
G
Siref
• and GSref are the maximum of the corresponding conductance at D = 1 kPa
•
g
sm
• maximum stomatal conductance for water vapour •
J
S
• sap flux density •
k/AL • leaf-specific hydraulic conductance • LAI • leaf area index • ΔΨS-L • water potential difference between soil and leaf
ABSTRACT
Responses of stomatal conductance (gs) to increasing vapour pressure deficit (D) generally follow an exponential decrease described equally well by several empirical functions. However, the magnitude of the decrease – the stomatal sensitivity – varies considerably both within and between species. Here we analysed data from a variety of sources employing both porometric and sap flux estimates of gs to evaluate the hypothesis that stomatal sensitivity is proportional to the magnitude of gs at low D (≤ 1 kPa). To test this relationship we used the function gs=gsref–m· lnD where m is the stomatal sensitivity and gsref=gs at D= 1 kPa. Regardless of species or methodology, m was highly correlated with gsref (average r2= 0·75) with a slope of approximately 0·6. We demonstrate that this empirical slope is consistent with the theoretical slope derived from a simple hydraulic model that assumes stomatal regulation of leaf water potential. The theoretical slope is robust to deviations from underlying assumptions and variation in model parameters. The relationships within and among species are close to theoretical predictions, regardless of whether the analysis is based on porometric measurements of gs in relation to leaf-surface D (Ds), or on sap flux-based stomatal conductance of whole trees (GSi), or stand-level stomatal conductance (GS) in relation to D. Thus, individuals, species, and stands with high stomatal conductance at low D show a greater sensitivity to D, as required by the role of stomata in regulating leaf water potential.
