Comparison of hydraulic design of an evergreen and a deciduous chaparral shrub

Background/Question/Methods

The hydraulic design of plant water transport systems determines plant responses to seasonal changes in water availability. We investigated how the hydraulic design of an evergreen and a deciduous chaparral shrub differed. In chaparral shrub communities, evergreen and deciduous shrub species coexist interspersed with each other. Evergreen species maintain some leaves throughout the year. In contrast, deciduous species shed all their leaves during part of the year in response to environmental changes such as low water availability. We characterized shrub hydraulic design by measuring the decline in water transport efficiency through leaves, stem, and roots with decreasing leaf water potential for Ceanothus spinosus (evergreen) and Ceanothus integerrimus (deciduous) chaparral shrubs. Samples were collected from shrubs growing in a common garden at California State University-Bakersfield. Decline in leaf hydraulic conductance with decreasing leaf water potential was measured using the evaporative flux method, whereas the decline in stem and root hydraulic conductivity with decreasing xylem water potential was measured using a centrifuge method.

Results/Conclusions

Ceanothus spinosus had a higher maximum leaf hydraulic conductance than Ceanothus integerrimus. Ceanothus spinosus also exhibited a greater decrease in leaf hydraulic conductance (relative to maximum) than Ceanothus integerrimus in response to decreasing water potential. The leaf hydraulic conductance of both Ceanothus spinosus and Ceanothus integerrimus dropped near zero at similar water potential (-2 MPa). Thus, some aspects of leaf hydraulic conductance differed with leaf habit while others did not differ. Preliminary analyses suggest that maximum stem and root hydraulic conductivity was also higher for Ceanothus spinosus than Ceanothus integerrimus, but that vulnerability to decreasing water potential was lower for Ceanothus spinosus. In combination, these measurements will be used to develop an electronic circuit model of whole water transport using an electronic circuit simulator. Comparing the effect of leaf, stem and root hydraulic conductance on whole water transport with declining plant water potential between an evergreen and a deciduous species will improve our understanding of how water transport systems differ with leaf habit.