Cold temperature and aridity shape the evolution of drought tolerance traits in Tasmanian species of Eucalyptus

Abstract

Perennial plant species from water-limiting environments (including climates of extreme drought, heat, and freezing temperatures) have evolved traits that allow them to tolerate these conditions. As such, traits that are associated with water stress may show evidence of adaptation to climate when compared among closely related species inhabiting contrasting climatic conditions. In this study, we tested whether key hydraulic traits linked to drought stress, including the vulnerability of leaves to embolism (P50 leaf) and the minimum diffusive conductance of shoots (gmin) were associated with climatic characteristics of fourteen Tasmanian eucalypt species from sites that vary in precipitation and temperature. Across species, greater cavitation resistance (more negative P50 leaf) was associated with increasing aridity and decreasing minimum temperature. In contrast, gmin showed strong associations with aridity only. Among these Tasmanian eucalypts, evidence suggests that trait variation is influenced by both cold and dry conditions, highlighting the need to consider both aspects when exploring adaptive trait-climate relationships.

Petiole XLA (xylem to leaf area ratio) integrates hydraulic safety and efficiency across a diverse group of eucalypt leaves

“…Ψ50 leaf was determined for each of the four to five branches sampled per species and then averaged to give a species mean (±SE). These Tasmanian species' Ψ50 leaf values are also recorded in a related study (Hartill et al, 2023).…”

“…Similar relationships between cavitation vulnerability, sapwoodspecific conductivity and vessel size have been observed in stems of ecologically diverse eucalypts (Peters et al, 2021), suggesting that a hydraulic safety-efficiency trade-off exists throughout the xylem network in eucalypt trees. They are also consistent with strong trait-climate relationships observed in eucalypts where species from arid, as well as freezing environments, tend to be less vulnerable to cavitation (Hartill et al, 2023;Li et al, 2018Li et al, , 2019Peters et al, 2021) and exhibit xylem with narrower vessels (Bourne et al, 2017;Peters et al, 2021;Pfautsch et al, 2016) than species from milder and more mesic environments. Additionally, while leaf size scaled positively with petiole XA, vessel size and hydraulic conductance, consistent with previous studies and hydraulic scaling theory (see Gleason et al, 2018;Levionnois et al, 2020), variation in leaf size was not significantly related to leaf hydraulic vulnerability.…”

“…In the current study, low cavitation vulnerability was observed in species from low rainfall environments as well as from high rainfall environments associated with freezing temperatures (see Hartill et al, 2023). In these environments, species require increased xylem investment for a given LA (low XLA petiole ) by building xylem with wide, highly conductive (but more vulnerable) vessels.…”

“…We observed an inverted u‐shaped relationship across species between Ψ50 leaf and the mean annual precipitation at each species sampling site (Supporting Information: Figure ), showing that species with low xylem cavitation vulnerability were sampled from low and high rainfall environments. Importantly, these high rainfall environments, especially in Tasmania, are associated with winter freezing temperatures that select species with low xylem cavitation vulnerability (see Hartill et al, 2023).…”

“…Levels of significance: *p < 0.05. low and high rainfall environments. Importantly, these high rainfall environments, especially in Tasmania, are associated with winter freezing temperatures that select species with low xylem cavitation vulnerability (seeHartill et al, 2023).…”