Variation in species cavitation resistance across life history traits
We observed significant species-specific variation in cavitation resistance among Hakea species (Table 2), demonstrating that species within the genus vary broadly in their capacity to tolerate high levels of water stress. Differences in species drought tolerance (stem P50) are largely attributed to the differences in the xylem structure; e.g. pit membrane porosity and thickness, and conduit size (Choat et al., 2012, 2008; Delzon et al., 2010; Li et al., 2016; Maherali et al., 2004; Sperry et al., 2006). At maturity, xylem conduits are dead with no possible acclimation to environmental change, making estimates of drought tolerance via embolism resistance very important to make reliable prediction under future climatic changes (Choat et al., 2012).
Species habitat preference and survival under disturbances (e.g. fire and drought) within the Hakea genus have previously been reported to be related with species life history and leaf form (Groom and Lamont, 1996). In contrast to our expectation, there were no significant differences between resprouters and non-resprouters, as well as broad and terete leaved species (Table 2). Previous studies have reported contrary findings in relation to resprouting ability and drought tolerance (Vilagrosa et al., 2014; Zeppel et al., 2015). The differences between these studies and our study may be due to the fact that Zeppel et al (2015) considered a large database of stem P50with species across different genus, indicating that resprouters were more drought tolerant than non-resprouters, at least within Angiosperms. On the other hand, Vilagrosa et al. (2014) used field observation and stem hydraulic measurements focusing on 12 co-occurring woody species within the Mediterranean system found that resprouters were less drought tolerant than non-resprouters (i.e. seeders), in direct contrast to Zeppel et al. (2015). In contrast to these studies, we focused on a single genus across multiple biomes finding no difference in drought tolerance with resprouting ability. This finding is partially supported by Groom and Lamont (1996), who found resprouting ability amongHakea species within the Mediterranean biome of southwest Australia was not associated with aridity. The differences may be due to varying drought strategies (i.e. tolerators, avoiders) for survival among genera and biomes.