Response of native (Quercus robur L.) and alien (Quercus rubra L.) species to water stress and nutrient input in European temperate ecosystems.

Drought and nutrient-poor soils can increase the invasive potential of non-native species, further changing the ecosystems they invade. The high adaptability of these alien species, especially in their efficient use of resources, improves their resilience against abiotic stress. Here, we evaluated the response of the North American Quercus rubra L. (RO) and the European Quercus robur L. (EO) oak species to drought and nutrient scarcity as single and combined factors. Both species were grown under well-watered or alternating short dry-wet phases, with or without the addition of phosphorous (P) and labelled nitrogen (N). Leaf gas exchanges and stem water potential were measured; moreover, leaf chemical characterization was carried out. Under concurrent low fertility and drought, both species reduced gas exchanges and stem water potential, although RO recovery was faster than EO. Nutrient inputs did not modulate RO's physiological response; however, P supply increased its uptake of the more available N forms (¹⁵NH₄ ¹⁵NO₃). The different leaf contents of N and P demonstrated that EO has lower nutrient use efficiency compared to RO. Nevertheless, P addition significantly mitigated the drought effects on EO, highlighting the crucial role of this nutrient in aiding EO's recovery under stress conditions. RO invasive potential may be linked to its superior adaptability and resource-use efficiency under combined abiotic stress. Nevertheless, EO competitiveness can be improved through targeted nutrient management.