Plant hydraulic conductance measures how efficiently water flows through a plant, and depends on root, stem, and leaf hydraulic conductance components. A majority of the resistance within this whole-plant pathway is attributed to leaves, which thus may have a strong regulatory effect on whole-plant hydraulic conductance. We investigated whether maximum leaf hydraulic conductance (Kleaf) was correlated with stem hydraulic conductivity (Kstem). A positive correlation would be predicted by optimality theory, in which species with higher flux rates would show higher conductances in all components. By contrast, according to the classical “hydraulic segmentation hypothesis,” leaves may have low hydraulic conductances in part to protect the longer-lived stem xylem from catastrophic loss of function. Species with vulnerable stem xylem, i.e., those which lose 50% of stem xylem conductance due to embolism during mild shoot dehydration, may tend to have low Kleaf. If species with vulnerable stem xylem have high Kstem, due to a trade-off between safety and efficiency, then hydraulic segmentation may result in a negative correlation of Kleaf and Kstem across species.
We made measurements on leafy branches from woody native and non-native species growing in chaparral and coastal sage scrub at the Santa Margarita Ecological Reserve in California. We measured Kleaf using the evaporative flux method, and Kstem on stem segments using low pressure-driven flow. We normalized Kstem by stem segment length, sapwood area, and distal leaf area. We determined stem vulnerability curves using the air injection method with a cavitation chamber, and calculated the pressure at which 50% of the hydraulic conductance was lost (P50). We measured wet and dry season predawn and midday water potentials on transpiring and non-transpiring samples using a pressure chamber.
Across the species tested, observed leaf-stem coordination supported the hydraulic segmentation hypothesis. Kleaf was negatively correlated with Kstem, indicating that species with higher efficiency for water movement in leaves have lower efficiency for water transport in stems. Conversely, Kleaf was positively correlated with stem P50 values, demonstrating that species with greater water movement in leaves had stems more resistant to xylem cavitation, and thus in less need of protection from high tensions. These results indicate that leaves may act as major hydraulic control points for species that are more vulnerable to xylem cavitation. This relationship suggests that leaf-stem coordination is an important component of ecological trait variation with respect to plant water use in Mediterranean-type ecosystems and potentially in other systems prone to seasonal drought.