Physiological responses and morphological adjustments in provenances of two woodland species in a chronic drought experiment.
Drought-induced tree dieback is impacting forest and woodland ecosystems. One approach to mitigate this is to plant better adapted genotypes of species from warmer and drier climates. This requires understanding drought adaptation both interspecifically and intraspecifically, which we investigated in this common garden experiment. Six genotypes from two related Eucalyptus species, E. microcarpa and E. melliodora, were selected from geographically disjunct locations, five of which encompass the drier ranges of each species. One year old saplings were planted in soil under a rainout shelter. After a five-month adjustment period, irrigation was withheld for 17 weeks to emulate drought, while control trees were irrigated twice weekly. Predawn (Ψpd) and midday (Ψmd) water potentials and stomatal conductance (gs) was measured weekly. Trees were harvested at the end of the drought period, and biomass and leaf osmotic potential were measured. We observed significant differences between species, and only limited evidence for intraspecific variation. Neither species developed substantial water deficits during the drought, but both species exhibited osmotic adjustment. Ψpd and Ψmd were significantly lower and gs was higher in droughted E. microcarpa in comparison to droughted E. melliodora, however the differences were small. Decreases relative to control trees in biomass were greater in E. microcarpa (57%) than E. melliodora (41%). Root:shoot ratios in droughted trees were comparatively greater than well-watered trees in both species, however, dry root weight was greater in E. melliodora across both treatments. Only E. microcarpa had significantly lower SLA (10%) and tree height (26%) in comparison to well-watered trees. Provenances of E. microcarpa from drier regions exhibited less negative Ψpd, less osmotic adjustment and comparatively lower height in droughted vs well-watered trees. Our results highlight the complexity of local drought adaptation and that morphological changes may be as important as physiological trait responses for drought resistance.