Limited evidence for provenancing in two co-occurring species with contrasting drought strategies

There is concern about an increase in drought induced dieback in forests as the climate becomes warmer and drier. In response, land holders are looking to new strategies in the restoration of forests. One such approach is provenancing; importing genotypes from seperate populations that may be adapted to different climates. However, in many cases, populations are facing similar pressures, and there may not be a population from a significantly ‘drier and hotter’ region. In these circumstances, provenancing may be reliant on disjunct populations that face similar climatic pressures having different adaptations to hot and dry climates. We tested drought responses, traits and strategies in provenances of two Eucalyptus species, E. melliodora and E. microcarpa, from the drier ranges of their respective distributions. The species co-occur in woodland communities in south eastern Australia, but E. microcarpa extends into regions that are drier and hotter. We measured a) chronic drought responses in a 17 week drought experiment of provenances planted in-ground in a rain exclusion shelter, b) transpiration responses to acute drought in a glasshouse experiment, and c) growth and seasonal water relations in a common garden field experiment. Both species had high within-provenance intraspecific variation in many drought traits, but similar adaptations to drought between provenances. The two species, despite co-occurring, had contrasting drought strategies. E. melliodora had a drought avoidant strategy, with much greater allocation of biomass to root growth and highly sensitive stomata. The comparatively greater root growth resulted in successfully avoiding drought and having comparatively better growth outcomes in the chronic drought experiment. In contrast, E. micropcarpa was much more drought tolerant and had greater hydraulic function at greater water deficits during acute drought. However, both species had almost identical growth outcomes over a five year period when planted in a provenance trial in the field. Despite the two species co-occurring and coming from the same section (Adnataria) within the Eucalyptus genus, they had significantly contrasting drought strategies. Therefore, understanding a species’ drought strategy may be important when considering which traits may confer an adaptive advantage to drought.