Avoidance of water stress as a possible strategy to persist in the arid biome
Huber value (HV, ratio of sapwood to leaf area) is a measure of carbon investment in xylem tissue per unit leaf area (Eamus et al., 2006; Gotsch et al., 2010; Pérez-Harguindeguy et al., 2013). Knowledge of the HV gives insight into the strategy species employed to survive in varying climate, as such reduced leaf area to sapwood area ratio implies avoidance strategy (Eamus et al., 2006). Species in the arid biome may employ drought avoidance strategy in addition to tolerance to persist in the arid biome (Fig 3). This was indeed true as HV was significantly related to climate (MAP, AI), such that species with higher HV tended to occupy areas with reduced rainfall, inferring greater demand for water transport (Choat et al., 2007; Gleason et al., 2012). Studies have also shown variation in drought tolerance traits within communities irrespective of the precipitation level (Maherali et al., 2004; McCulloh et al., 2019). This is true, as we observed wide variation in P50 within the Arid-Arid communities (or vegetation-biome, Table 3) driven by the low stem P50value of H. eyreana . We also observed the wood density ofH. eyreana (1.23 ± 0.09) to be about 3-fold smaller than H. leucoptera and 4.1-fold smaller than H . grammatophyllawhich also possesses a higher P50, suggesting thatH. eyreana may probably employ a different strategy (e.g. drought avoidance) to survive in the arid biome. Hakea eyreana may have different trait coordination or trade-offs among traits along the water transport pathway under field scenarios or in response to the experimental (well-watered) condition (Brodribb et al., 2017; Li et al., 2018; McCulloh et al., 2019). Traits including life history (e.g. resprouting ability and leaf form), growth form (e.g. liana vs tree), stomatal regulation, soil water depth, and root depth may also offset the need for developing more negative stem P50(Bartletta et al., 2016; Meinzer et al., 2009; Padilla and Pugnaire, 2007; Skelton et al., 2015). We found evidence that drought avoidance provided with terete leaf form and reduced leaf area to sapwood ratio (HV) may enable species with low drought tolerance (e.g. H. eyreana ) the ability to persist in the arid biome. The plots of HV and P50 against aridity showing different life-histories (Fig 3) are informative in understanding different drought strategies. These traits may balance the need for carbon capture and growth with the demand for developing xylem resistant to drought (high P50, -MPa) for colonization and persistence in the arid biome.