Climate is a major driver of variation in cavitation resistance
Climate has often been highlighted as the key driver of species variation in hydraulic traits (Li et al., 2018). Hydraulic traits appear to be adaptive with species that have shorter and narrower vessels tending to occupy drier biomes and have lower vulnerability to cavitation (Christman et al., 2009; Larter et al., 2017; Lens et al., 2011, 2009; Pockman and Sperry, 2000; Skelton et al., 2018; Sperry et al., 2008; Wheeler et al., 2007). In this study, species in the arid biome/climate (with the potential exception of H. eyreana ) generally had higher cavitation resistance (P50 below -6.75 MPa) compared with species in the humid biome (Table 3). This was indeed expected as species within the arid biome possess traits that confer greater drought tolerance (Choat et al., 2012; Li et al., 2019, 2018; Trueba et al., 2017). To understand if drought tolerance is genetically determined hydraulic trait, we plotted P50against mean annual precipitation (MAP), temperature (MAT), and aridity (AI) across the species distribution range (Fig 3). We found evidence supporting our expectation, that drought tolerance trait P50 is significantly related with climate (Bourne et al., 2017; Choat et al., 2012; Larter et al., 2017; Li et al., 2018; Maherali et al., 2004; Trueba et al., 2017). Our results revealed that rainfall and aridity were key drivers of the variation in species P50 within the diverse Hakea genus, such that species cavitation resistance increases with reduced rainfall of species climatic origin. Furthermore, the P50 trait variation measured in a common garden provides strong evidence that drought tolerance is genetically-determined and adaptive (Lamy et al., 2014; Li et al., 2018; López et al., 2016; Skelton et al., 2019). Hence, cavitation is a key factor shaping species distribution with respect to water availability (Brodribb and Hill, 1999).