Xylem traits were examined among 22 aridland shrub species from three plant communities, including the chaparral, coastal sage scrub, and Mojave Desert scrub.  The anatomical traits of vessel dimensions and pit area were compared to the functional traits of transport efficiency and safety from cavitation. The influence of evolution on trait relationships was examined using phylogenetic independent contrasts (PICs).  

Results/Conclusions

Xylem safety and efficiency were negatively correlated at the vessel level (r² = 0.34, P = 0.005 for vessel diameter and mean cavitation pressure). Pit area was correlated with cavitation resistance when species data were examined (r² = 0.42, P = 0.001), but PICs suggested that these traits have evolved independently of one another (P > 0.05). Differences in cavitation resistance that are not explained by pit area may be related to differences in pit membrane properties or the prevalence of tracheids, the latter of which may alter pit area through the addition of vessel-to-tracheid pits or through changes in xylem conduit connectivity. Some trait relationships are robust regardless of species ecology or evolutionary history and these trait relationships are likely to be the most valuable in predictive models that seek to examine anatomical and functional trait relationships. Examples of these ecologically and evolutionarily robust relationships include the relationships between xylem hydraulic conductivity and vessel diameter, between vessel diameter and vessel length, and between hydraulic conductivity and wood density.