Calcium addition modulates water use strategy of tree species with contrasting Al-tolerance in South China secondary forest

Long-term acid deposition in South China has led to increased aluminum (Al) toxicity, while plants typically adopt diverse Al-tolerance strategies to adapt to acidified soils. Ca addition is commonly applied to alleviate acid deposition and associated Al stress, yet it remains unclear whether increased Ca availability modulates plant water use strategies among species with contrasting Al tolerance. We conducted a field experiment in a secondary evergreen broadleaved forest to simulated acid deposition and Ca addition. We quantified leaf transpiration, intrinsic water use efficiency (WUEi), and key leaf functional traits of two Al-excluder species and two Al-accumulator species, aiming to assess the relationships among leaf Ca/Al ratios, leaf traits, and plant water use strategies. Results showed that the simulated acid deposition had limited effects on leaf traits and water use, indicating a high tolerance to acid stress in all species. Whereas, Ca addition significantly induced divergent physiological responses and modified plant water use strategies. The Al-excluders initially exhibited conservative traits but shifted toward a more acquisitive strategy under Ca addition, as evidenced by increased specific leaf area and decreased leaf dry mass content. In contrast, Al-accumulator species showed more conservative adjustments, reflected by the increased WUEi and reduced stomatal density or the development of smaller stomata. These results indicate that Ca availability, rather than additional acid input, is the main driver of leaf functional adjustment in this acidified forest. Furthermore, our findings suggest that Al tolerance strategy mediates tree responses to the changing soil Ca and Al dynamics, providing key insights for forest management under acid deposition scenarios.