Relationship between cavitation resistance and functional traits
Surprisingly, we observed no significant relationship between P50 and functional traits (e.g. wood density, SLA, HV, LDMC; Fig S4 supporting information) (Hacke et al., 2001; Markesteijn et al., 2011; Schumann et al., 2019; Villagra et al., 2013). Of these functional traits, wood density has received more attention in relation to drought tolerance as greater structural investment (wood density) would prevent xylem implosion, and thus greater resistance to embolism (Hacke et al., 2001; Li et al., 2018; Markesteijn et al., 2011). However, some other studies have also reported no significant association between wood density and P50 (Larter et al., 2017; Trueba et al., 2017). The non-significant relationships between P50 and functional traits (e.g. WD and SLA) found by Trueba et al. (2017) may be because species were pooled from diverse communities and genera. Furthermore, there may be limited selective pressure for investment in structural strength since our study sites were well-watered, as studies have shown environment or site to be a determinant of wood density (Downes et al., 2006; Onoda et al., 2010; Roderick and Berry, 2002; Searson et al., 2004; Wimmer et al., 2002).
Leaf size traits were not important in pooling species apart in relation to stem P50, as both broad-leaved and terete leaved species are distributed within the arid and humid communities (Groom and Lamont, 1996). However, in combination with Huber value, leaf size may be important in highlighting species strategy to drought and across climate/biomes. For instance in arid biome, species with reduced leaf area to sapwood area employs avoidance strategy, while species with greater surface area within the same system will tend to prioritize the construction of xylem resistance to embolism (i.e higher stem P50) for survival. Interestingly, we did not observe significant relationships between hydraulic traits (HV and P50). However, the direction of the relationship was positive (R2 = 0.14, P value = 0.588) as expected (Carter and White, 2009; Markesteijn et al., 2011). The weak relationship suggests that not all species with higher P50 (tolerance) necessarily had higher HV (avoidance) (Fig S4, supporting information), as some species may either employ alternate strategies for survival.