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Xylem vulnerability and hydraulic conductivity are correlated with rooting depth in co-occurring chaparral shrubs.
Preston, Katherine*,1, Bhaskar, Radika1, Schwilk, Dylan1, Ackerly, David1, 1 Stanford University, Stanford, CA
ABSTRACT- The ability of plants to tolerate water stress is strongly influenced by the vulnerability of their xylem to embolism and hydraulic failure. The degree of water stress experienced by a plant depends on its access to water, which is partly a function of rooting depth. Thus, within a community, variation in rooting depth may be correlated with variation in xylem vulnerability. We measured xylem vulnerability and hydraulic conductivity in five species of co-occurring evergreen chaparral shrubs that varied in rooting depth (predawn water potential). Vulnerability was quantified as the degree of water stress (leaf water potential) required to induce a 50% loss in water transport due to embolism (PLC50). Species with highly resistant xylem (more negative PLC50) showed lower rates of conductivity, indicating a trade-off between safety and hydraulic efficiency. Both traits varied with rooting depth: PLC50 was more negative and conductivity was lower in species with shallower roots. Lower hydraulic efficiency and greater resistance to embolism were also associated with denser wood and lower photosynthetic rate, but greater photosynthetic water use efficiency. Although reduced specific leaf area (SLA, area/mass) is thought to enhance stress tolerance, in our study it was not correlated with resistance to drought-induced embolism. Our results suggest that the balance between the risk of embolism and the cost of reduced conductivity varies within a community as a function rooting depth.
KEY WORDS: embolism, drought tolerance, stress, chaparral