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.