Divergent responses to acid deposition and calcium addition in a subtropical secondary evergreen broadleaved forest: Experimental evidence from transpiration and water use efficiency dynamics

Chronic acid deposition has induced severe soil acidification, leading to the depletion of calcium (Ca) and the accumulation of phytotoxic aluminum (Al). Ca is essential for sensing water stress, strengthening cellular structures, and enhancing plant water use efficiency (WUE), while Al impairs root function. Consequently, the altered soil Ca and Al dynamics are likely to produce divergent impacts on plant water use and WUE, yet the underlying mechanisms remain poorly understood. To investigate this, we conducted a manipulative field experiment simulating soil acidification (Acid) and calcium amendment (Ca) in a secondary evergreen broadleaved forest in South China from 2022 to 2024. We continuously monitored sap flow-based transpiration and analyzed the δ¹ ³C-derived intrinsic WUE (WUEi) in four dominant species: Pyrenaria macrocarpa, Quercus myrsinifolia, Aporosa dioica, and Castanopsis fissa. Additionally, we conducted periodic ecophysiological measurements, including soil and leaf stoichiometry, leaf water potential, and stomatal conductance. The results showed that species-specific responses to Acid and Ca treatments. P. macrocarpa exhibited no significant change in transpiration under the Acid treatment in 2022 and 2023 but showed a significant decline in 2024, suggesting a delayed toxicity effect from accumulated Al. In contrast, A. dioica consistently showed increased transpiration under the Acid treatment, potentially reflecting an adaptive strategy to maintain nutrient uptake and support photosynthesis in acidic soils. Both species showed a significant increase in WUEi with Ca amendment, positively correlating with soil Ca content and leaf Ca/Al ratio. In comparison, Q. myrsinifolia and C. fissa displayed no significant physiological responses to either treatment. These findings highlight the divergent strategies adopted by co-occurring dominant species in response to acid deposition and Ca amendment. Prolonged acid deposition may threaten species like P. macrocarpa, while species with high photosynthetic and water demands, such as A. dioica and C. fissa, may face increased risk of hydraulic failure under the combined stressors of acidification and drought.