Previous drought exposure leads to greater drought resistance in eucalypts through changes in morphology rather than physiology

Abstract

Over their lifetime, trees are repeatedly exposed to droughts. It is therefore important to understand if repeated drought exposure makes trees more or less drought tolerant. Here, we investigated the effect of repeated droughts on functional trait expression and tree function in Eucalyptus obliqua. Further, we tested whether previous drought exposure enabled trees to avoid leaf death for longer under a subsequent severe drought. Trees were subjected for one year to two drought-rewatering cycles (drought treatment), or well-watered conditions before imposing a severe drought. Trees in the drought treatment reduced their overall leaf area and biomass, whereas leaf-level anatomical, morphological and physiological traits remained mostly unaffected. There were no differences in water potential at the turgor loss point, leaf xylem vulnerability to embolism, leaf size, maximum xylem vessel diameter or cell wall thickness between treatments after the conditioning period. When exposed to a subsequent severe drought, trees previously exposed to drought were more drought tolerant due to a lower water potential at leaf death and tree-level morphological rather than physiological adjustments. Trees previously exposed to drought were smaller and used less water, which delayed leaf death for 39 days compared to 22 days for the well-watered trees. Our study indicates that previous drought exposure can facilitate tree-level morphological adjustment which potentially enhances survival of E. obliqua trees during subsequent drought events.

Perspectives on Drought Preconditioning Treatments With a Case Study Using Western Larch

“…Furthermore, long-term drought preconditioning appears to have enhanced the physiological vigor of western larch seedlings, as water limitation increased the mass, count, and length of new roots produced during aeroponic growth (none of which were strongly correlated with initial lateral root mass). These results agreed with those of Pritzkow et al (2021) , who subjected seedlings of Eucalyptus obliqua to long-term drought preconditioning and found that preconditioning induced drought-adaptive reductions in foliar biomass. Pritzkow et al (2021) reported that drought preconditioning did not influence plant water-relations or anatomy, but the smaller aboveground mass of preconditioned seedlings was associated with reduced water use and increased seedling survival under dry outplanting conditions.…”

“…These results agreed with those of Pritzkow et al (2021) , who subjected seedlings of Eucalyptus obliqua to long-term drought preconditioning and found that preconditioning induced drought-adaptive reductions in foliar biomass. Pritzkow et al (2021) reported that drought preconditioning did not influence plant water-relations or anatomy, but the smaller aboveground mass of preconditioned seedlings was associated with reduced water use and increased seedling survival under dry outplanting conditions. The present study established that long-term drought preconditioning of western larch promotes the development of seedling traits associated with establishment success in dry field conditions.…”

Lack of hydraulic recovery as a cause of post‐drought foliage reduction and canopy decline in European beech

“…The observed limitations of hydraulic function and foliage development undoubtedly have serious consequences for trees. Importantly, reduced foliation and partial branch dieback may also serve as a post‐drought acclimation response that rebalances root water uptake and transpirational water loss through canopies over longer developmental timescales (Rood et al ., 2000; Bréda et al ., 2006; Trugman et al ., 2018; Pritzkow et al ., 2021). This is in good agreement with recent studies linking observations of heterogeneous leaf or branch mortality across the canopy of drought‐stressed trees with hydraulic dysfunction, and based on this, partial leaf or branch mortality has been proposed to act as buffer against complete dehydration and tree death during prolonged drought (Davis et al ., 2002; Cardoso et al ., 2020).…”

In the Hot Seat: Behavioral Change and Old-Growth Trees Underpin an Australian Songbird’s Response to Extreme Heat

“…The world's dryland areas are predicted to become increasingly arid, with worsening droughts (IPCC, 2021), which reduce vegetation cover and can permanently alter habitat structure (Pritzkow et al, 2021). At our study site, densely leaved, green shrubs-identified as critical thermal refuges in other terrestrial habitats (Carroll et al, 2015;Anthony et al, 2020;Ruth et al, 2020)-were simply not present during the data collection period due to 2 years of severe drought.…”

“…In previous summers, we had observed many small passerines (although not Jacky Winters) sheltering within such shrubs at high temperatures. The loss of this microhabitat, combined with the reduction in leaf area that eucalypts suffer during drought (Pritzkow et al, 2021), is likely to seriously impair the ability of avian species to survive heat wave events.…”