Print PageThe xylem of terrestrial woody angiosperms has three main functions: long distance transport of water, mechanical support of stems and leaves, and storage of water and nutrients. Since these functions rely on different cell types comprising the xylem, tradeoffs among them are likely to arise. Determining the relationships among these xylem traits is critical to our understanding of these tradeoffs and the implications of xylem trait differences among species. We investigated tradeoffs among functional traits associated with water transport, biomechanical support, and carbohydrate storage in 32 species of shrubs that co-occur in the chaparral community of southern California. We also examined how these traits differed for species with evergreen and deciduous leaf habits. To characterize water transport, we measured xylem specific hydraulic conductivity (Ks), a measure of water transport efficiency, and the water potential at 50% loss of hydraulic conductivity (P50), a measure of vulnerability to water stress-induced xylem cavitation. Biomechanical support was quantified by measuring stem mechanical strength as the modulus of rupture (MOR) and as xylem density. Carbohydrate storage was measured as the percent starch present in stem xylem.
Results/Conclusions
Xylem specific hydraulic conductivity (Ks) was correlated to P50 (R = 0.65, p < 0.0001) such that species with greater resistance to water stress-induced cavitation had lower water transport efficiency. Neither of these water transport traits was correlated to either of the functional traits characterizing biomechanical support, MOR or xylem density (p > 0.09). Neither Ks nor P50 was significantly correlated with starch storage, but the absence of species with both high resistance to cavitation and high starch storage suggests a potential tradeoff between these traits. Finally, neither of the biomechanical support traits was correlated to starch storage (p > 0.74). A comparison of evergreen and deciduous shrubs showed that evergreen species had a more negative P50 (i.e. greater stress resistance) and a lower Ks that were marginally significant (t = 1.95, p = 0.06; t = 1.94, p = 0.06, respectively). Evergreen and deciduous species did not consistently differ in MOR, xylem density, or starch storage (p > 0.47). In chaparral shrubs, xylem functional traits related to biomechanical support varied independently of those characterizing water transport and carbohydrate storage, but variation in water transport and carbohydrate storage functional traits was constrained.
