In woody angiosperms, vessels within the xylem tissue are primarily responsible for water transport. Xylem vessel structure is important in determining both safety and efficiency of hydraulic transport and these xylem features are often discussed as representing a safety-efficiency tradeoff. The role of vessel diameter as related to this tradeoff has been abundantly studied, but vessel length remains mostly understudied.
We examined the relationship between vessel length and diameter to hydraulic safety and efficiency in an interior mixed chaparral shrub community. The study site was located in the southern Sierra Nevada at a mid-elevation site that experiences colder temperatures relative to the more commonly studied coastal chaparral communities.
We hypothesized that both vessel diameter and length would be correlated to xylem transport efficiency and safety. We predicted that species with longer mean vessel lengths and wider diameters would be more vulnerable to water stress-induced cavitation. Additionally, we predicted that species with longer and wider vessels would have greater xylem transport efficiency. We measured resistance to water stress-induced cavitation (P50), xylem specific conductivity (Ks), and mean vessel lengths and diameters of five dominant chaparral shrub species: Arctostaphylos glauca, A. glandulosa, Ceanothus vestitus, Garrya flavescens, and Quercus berberidifolia.
The five species differed greatly in xylem function, with Ks (kg m-2 s-1 MPa-1) ranging from 0.20 ± 0.04 to 2.99 ± 0.59 and P50 (MPa) ranging from -7.67 ± 0.75 to -0.24 ± 0.07. Mean vessel length was positively correlated with xylem vessel diameter (R=0.964, P=0.008) and Ks (R= 0.965, P=0.008), but was not significantly related to vulnerability to cavitation (R=0.814, P=0.094). Similarly, mean vessel diameter was correlated to Ks (R=0.981, P=0.003), but only marginally related to P50 (R=0.865, P=0.058). Finally, Ks and P50 were also correlated (R=0.917, P=0.029).
We found that species with longer vessels had more efficient xylem with respect to hydraulic function. The lack of relationship between vessel length and resistance to drought-induced cavitation suggests that vessel length does not impact hydraulic safety. Other traits such as pit characteristics, vessel connectivity, and tracheid presence may be interacting to reduce xylem vulnerability to cavitation in species with long vessels. Additionally, freezing has been shown to increase vulnerability to freeze-induced cavitation dependent on vessel diameter, but independent of vessel length. Our results support a tradeoff between hydraulic efficiency and safety within this interior mixed chaparral community. Vessel diameter and length were correlated to hydraulic function in these species.