Contrasting adaptive strategies of native and alien Quercus species to drought and nutrient scarcity: linking soil nutrient cycling with plant physiological and biochemical responses

Climate change and biological invasions are major drivers of environmental transformation, especially in temperate forest ecosystems. Invasive species such as Quercus rubra (red oak) are increasingly replacing native vegetation like Quercus robur (English oak), with significant implications for biodiversity, nutrient cycling, and ecosystem functioning. This PhD thesis investigates how drought and nutrient availability modulate the physiological, biochemical, and metabolomic responses of Q. rubra and Q. robur, with the goal of understanding their competitive dynamics under future climate scenarios. A series of mesocosm experiments were conducted using nutrient-poor soils collected from forests invaded by Q. rubra in northern Italy. By manipulating water regimes (including drought-rewetting cycles, moderate and severe drought) and nutrient inputs (nitrogen and phosphorus), we assessed the interplay between abiotic stress and nutrient status on plant performance. The results revealed that Q. rubra exhibits higher tolerance and faster recovery under drought stress compared to Q. robur, especially under nutrient-limited conditions. However, phosphorus supplementation improved the physiological and metabolic resilience of Q. robur, partially mitigating drought effects. Metabolomic analyses highlighted distinct stress-adaptive strategies between species, with Q. rubra investing more in root biochemical adjustments and Q. robur showing stronger dependence on phosphorus for maintaining photosynthetic function. These findings suggest that soil fertility restoration, particularly phosphorus input, may enhance the competitiveness of native species and represent a viable strategy to mitigate the spread of invasive trees in European forests under climate change.