Support for a fiber inclusive model of xylem cavitation resistance

Vessel mechanical traits, such as theoretical vessel implosion resistance ([t/b]_h²), are important to resistance of negative pressures in the xylem; however, vessel traits alone are insufficient in explaining resistance to extreme pressures in some species.  In these species, xylem fibers may be important in reinforcement of vessels against bending or implosion.  Indeed, in some plant communities such as the California chaparral, fiber wall area correlates with cavitation resistance (r² = 0.42) while (t/b)_h² does not (r² = 0.00) when analyzed using phylogenetic independent contrasts.  Strong correlations between cavitation resistance and xylem density also support a role for fibers in cavitation resistance, especially considering that density is largely a measure of fiber and not vessel properties and vessel and fiber properties can vary independently.  Of 13 arid shrub species for which seasonal vulnerability curves were measured, eight displayed significant seasonal shifts in cavitation resistance of as much as a two-fold increase from the wet to dry season.  Among several Rhamnaceae shrubs of southern California, we found that mechanical strength against stem breakage shifted seasonally to become stronger during the dry season.  Seasonal shifts in cavitation resistance and biomechanics support a role for fibers in cavitation resistance.  Early in the growing season, xylem vessels may be surrounded by an immature fiber matrix which is unable to buttress newly functional vessels against implosion when exposed to extreme negative pressures.  These data suggest a greater role for fibers in vessel cavitation resistance and conductive safety than previously considered.