Evidence for linkages between the root elementome and oak decline in Mediterranean systems invaded by soil‐borne pathogens

Abstract Climate aridification and the increasing incidence of exotic pests and pathogens are contributing to a concerning global rise in tree decline and mortality. The analysis of tree elemental composition (the elementome) offers a promising tool to detect physiological disruptions and identify early‐warning signals of decline. However, the role of elemental alterations—particularly in response to exotic pathogens—remains underexplored. Here we analysed the elemental signature of tree decline in Mediterranean savanna‐type ecosystems (dehesas) invaded by the aggressive exotic soil‐borne pathogen Phytophthora cinnamomi . Using a multi‐scale approach (regional and local), we assessed root elemental concentrations and stoichiometry in relation to crown defoliation, soil physicochemical properties and P. cinnamomi abundance in 500 adult Quercus rotundifolia trees across 30 sites in Southern Spain. Our results revealed a distinct chemical signature of tree decline in the root elementome, with patterns varying across spatial scales. At the regional scale, only sodium (Na) emerged as an indicator of site‐level decline. In contrast, the local scale revealed strong and consistent chemical alterations in symptomatic trees, including elevated calcium (Ca) concentrations and reduced levels of key nutrients (potassium K, phosphorus P, iron Fe, zinc Zn) and elemental ratios (K:Ca) compared to asymptomatic neighbours. Among all variables, the K:Ca ratio was the best discriminator between asymptomatic and symptomatic trees, showing significant associations with crown defoliation and P. cinnamomi abundance. Elevated Ca uptake in symptomatic trees may reflect a defensive response to pathogen infection but also contribute to nutritional imbalances that exacerbate decline. Synthesis . Our findings highlight the value of multi‐scale elementome analysis for identifying sites and trees vulnerable to decline. Specifically, we showed that nutritional imbalances (particularly the K:Ca ratio) can act as reliable early‐warning signals of pathogen‐driven decline in Mediterranean systems, enabling rapid management responses. We advocate further research on plant elementomes across diverse pathosystems to uncover broader patterns that could inform biosecurity programs aimed at mitigating the growing impacts of exotic pathogens worldwide.