Water relations during drought and the recovery from drought in two Eucalyptus species with contrasting water-use strategies

In recent decades, climate change has led to drought and heat-induced mortality events in forest ecosystems worldwide. Our current understanding of physiological mechanisms leading to drought-induced tree death is limited, which prevents us from accurately predicting responses of forest to a warmer and drier climate in the future. Since carbon and water relations are inevitably linked through stomatal control, it has been suggested that trees can die from hydraulic failure or carbon starvation. This PhD project aimed at exploring the effects of drought on carbon and water relations in seedlings of two Eucalyptus species. The two species, Eucalyptus marginata Sm. and Eucalyptus wandoo Blakely, were chosen because they are known to exhibit contrasting water use strategies. Eucalyptus marginata is a relatively isohydric species and has been shown to reduce its water loss in response to dry conditions to maintain higher leaf water potentials. Eucalyptus wandoo, on the other hand, develops distinctly low water potentials before substantially reducing its stomatal conductance and it is thus considered a relatively anisohydric species. Consequently, the two species were expected to show distinct responses to drought, which could affect carbon relations.

In the first experiment, I aimed to test the capacity of Eucalyptus seedlings to repair embolisms when they were rewatered after a 20-day drought treatment, and to study the coordination of embolism repair with the recovery of leaf water potential (ΨL), relative water content and stomatal conductance (gs). During the drought treatment, both species lost over 70% of their conductivity through embolisms. Upon rewatering, both species showed a fast recovery of ΨL (from c. -3 to -1 MPa) and relative leaf water content, as well as an almost 30% decrease in the percentage loss in conductivity (PLC), but gs did not recover to control levels in E. wandoo within eight hours after rewatering. A fast recovery of stem hydraulic conductivity and leaf water potentials is thus not always followed by a recovery of transpiration.

In the second experiment, the two species were exposed to a long and progressive drought, to study embolism repair in relation to carbon pools upon rewatering. Contrasting water-use strategies in the two species were evident from differences in the lowest observed ΨL and timing of stomatal closure. Seedlings of both species, however, accumulated large amounts of soluble sugars in leaves, stems, and bark including cambium but not in roots, while starch pools remained largely unaffected. Within 18 hours after rewatering, PLC in the more isohydric species, E. marginata, recovered from over 50% to levels of well-watered controls (~20%) along with a complete recovery in ΨL and gs. In E. wandoo, ΨL mostly recovered while gs remained as low as in droughted seedlings. Unexpectedly high PLC in rewatered E. wandoo seedlings were most likely a result of ~50% higher maximum sapwood-specific conductivity, compared to droughted seedlings, but similar initial sapwood-specific conductivity, which may indicate that hydrogels had a role in the recovery of hydraulic conductivity or that the removal of embolism during pressurisation was inadequate in droughted seedlings. In both species, soluble sugar pools decreased to levels close to well-watered controls upon rewatering. I suggest that an accumulation of soluble sugars could be significant for osmoregulation and embolism repair in Eucalyptus seedlings during drought.

The aim of the third experiment was to assess how drought in two different soil types affected water relations, in particular the hydraulic system, in two Eucalyptus species with contrasting water-use strategies and soil preferences. Both species accumulated embolisms in response to soil water deficit but Eucalyptus wandoo displayed a more than 20% higher xylem vulnerability in the sandy compared with the loamy soil while xylem vulnerability was the same in E. marginata. I propose that adaptations to soil types in their native habitat led to higher vulnerability in E. wandoo due to an inability to adjust its water use to higher critical soil water potentials in sandy soils. The survival of some tree species in areas with favourable future climates could thus be threatened if their hydraulic system is affected by the soil substrate.

In summary, Eucalyptus seedlings showed a fast, but not always complete, recovery of most hydraulic parameters in response to rewatering after drought. I found no evidence for carbon starvation. Rather, increased soluble sugar concentrations, irrespective of water use strategy, may contribute to drought tolerance through a role in osmoregulation and embolism repair.