Natural variability in Olea europaea L. reveals diverse functional strategies to drought under a CO2-enriched atmosphere

The wild olive (Olea europaea L.) is one of the most iconic Mediterranean forest tree species and the ancestor of many cultivated olive varieties. In this study, twelve wild olive genotypes from three distinct subspecies (subsp. europaea, subsp. guanchica, and subsp. maroccana) were grown under ambient ([CO₂]_amb) and enriched CO₂ atmospheres ([CO₂]_enr), subject to two watering regimes (WW - Well-watered/WS - Water-stressed). We measured water use at leaf and whole-plant levels, along with growth traits, leaf gas exchange, and leaf isotopic carbon (δ¹³C) and nitrogen composition (δ¹⁵N), to differentiate functional strategies among genotypes in response to CO₂ and soil water availability. Growth under [CO₂]_enr lessened water stress effects by increasing water use efficiency (WUE). The improved WUE came from reduced water consumption at both leaf and plant levels under [CO₂]_enr, although sensitivities to soil water content varied among genotypes. Growth-related traits responded more to water stress than to CO₂. Patterns of N¹⁵ isotopic composition (δ¹⁵N) and total leaf nitrogen content on a mass and area basis (N_mass and N_area) varied among genotypes in response to atmospheric CO₂ and water stress. A negative relationship between δ¹³C and δ¹⁵N indicates a complex interaction between carbon and nitrogen metabolisms under the influence of water availability and CO₂ supply. Our study shows that (1) the response of wild olive to atmospheric CO₂ enrichment is strongly influenced by resource availability, especially water, and (2) it is necessary to consider genotype- and subspecies-specific responses when predicting vegetation response in a CO₂-enriched world.