Seasonal Nutrient Dynamics in Flowers and Leaves of Sweet Cherry (Prunus avium L.) Across Altitudinal and Agro-Environmental Gradients: Implications for Site-Specific Fertilization

Sweet cherry (Prunus avium L.) production in mountainous Mediterranean regions is characterized by strong altitudinal gradients, heterogeneous soils, and short phenological cycles, which complicate nutrient management and the interpretation of conventional foliar analyses. This study aimed to evaluate the seasonal dynamics of macro- and micronutrient concentrations in flowers and leaves of sweet cherry orchards located in the Jerte Valley (Spain) in order to refine nutritional diagnosis under mountain conditions. Flower samples at full bloom and leaf samples at 30, 60, 90, and 120–150 days after full bloom (DAFB) were collected from early, mid-season, and late-maturing cultivars during two consecutive seasons. Nutrient concentrations were analyzed using linear mixed models and multivariate analyses. Sampling date and cultivar maturation cycle were the main factors explaining nutrient variation. Despite large differences in yield, fruit quality, and irrigation inputs among orchards and seasons, the seasonal trajectories of nutrient concentrations remained consistent within cultivars. This consistency suggests phenological development and internal nutrient cycling may play an important role in shaping seasonal nutrient patterns, while local management, soil, and environmental conditions may modulate nutrient supply. Nitrogen, phosphorus, and potassium concentrations declined progressively throughout the season, with N decreasing from approximately 3.8–4.0% in flowers to 2.0–2.1% at 120–150 DAFB, whereas calcium and magnesium increased with leaf age. Micronutrients, particularly manganese and iron, showed marked temporal variability. The lowest overall median nutrient variability was observed at 60 DAFB (CV = 10.1%), supporting the 60–90 DAFB interval as the most informative window for foliar diagnosis in mountainous orchards, whereas the generally weak flower–leaf relationships indicated that flower analysis has limited diagnostic capacity and may serve only as a preliminary early-season screening tool for selected nutrients. Comparison of measured foliar nutrient concentrations in the Jerte Valley agroecosystem with published sweet cherry reference values revealed the need for region-specific calibration. While N concentrations broadly matched established benchmarks (2.5%), phosphorus (0.21%), potassium (1.95%), and iron (70 mg kg−1) showed consistently negative Deviation from Optimum Percentage (DOP) values relative to published reference values (DOP = +1%, −38%, −18%, and −75%, respectively). This work highlights that phenology-based nutritional diagnostics can improve nutrient monitoring and provide a framework for developing site-adapted fertilization strategies in mountain sweet cherry production systems.