Climate change is one of the most significant challenges facing the agricultural sector, with profound implications for horticultural production. This study aimed to investigate the impacts of climate change on the phenological stages of walnut trees in the major walnut-producing provinces of western Iran. To this end, meteorological data from 35 meteorological stations during the baseline period (1990–2014) and outputs from 15 CMIP6 global climate models under two Shared Socioeconomic Pathway scenarios (SSP2-4.5 and SSP5-8.5) were employed for two future time horizons: the near future (2021–2040) and the far future (2041–2060). To enhance prediction accuracy and reduce uncertainty, a weighted averaging approach using the best-performing models was applied. The findings indicate a significant temperature increase across the study region, with minimum temperatures projected to rise by 1.0 to 3.3°C and maximum temperatures by 0.8 to 2.1°C in future periods. These thermal changes will lead to substantial shifts in the walnut phenological calendar. Budbreak will occur 2 to 14 days earlier and flowering 3 to 15 days earlier compared to the baseline period. The onset of fruit growth will advance by 3 to 11 days in the near future and 8 to 17 days in the far future, while fruit maturation will occur 6 to 17 days and 16 to 29 days earlier, respectively. Consequently, the harvest date will be advanced by 7 to 33 days. In contrast, the winter dormancy period (9 to 29 days) and leaf senescence (8 to 37 days) will be delayed. Spatially, the most pronounced phenological shifts are projected for the cold regions in the north and east, while the smallest changes are expected in the warmer southern and southwestern areas. These uneven and premature alterations pose serious risks, including increased vulnerability to late spring frosts, heat and water stress during critical growth stages, and disruption to the product supply chain. The results underscore the urgent need to develop adaptive strategies, such as utilizing late-flowering cultivars, to mitigate the adverse impacts of climate change on walnut orchards in the region.
Introduction
Climate change, driven by rising greenhouse gases, poses a significant threat to global ecological balance, with agriculture being particularly vulnerable due to its direct climate dependency. Horticultural and nut crops like walnuts are especially sensitive to thermal fluctuations, which can jeopardize their yield and quality. Consequently, analyzing the impact of climate change on plant phenology is a crucial step for optimizing resource use and mitigating future risks. Existing literature, including regional studies in Iran and globally, underscores this vulnerability, demonstrating that rising temperatures advance key phenological stages—such as flowering and fruit maturation—in various tree species, including walnuts, pistachios, and apples. These shifts can disrupt dormancy cycles, increase exposure to late frosts, and alter water requirements.
However, a research gap exists in conducting a comprehensive, high-resolution assessment of climate change impacts across all major phenological stages of walnut trees specifically in western Iran, utilizing the latest CMIP6 climate models and SSP scenarios. Previous studies have often focused on specific regions, stages, or older model generations. This study aims to address this gap by systematically evaluating the effects of projected climate change on the seven key phenological stages of walnut trees in this vital cultivation area. It seeks to determine the extent and spatial variability of changes in the timing and length of these stages under different future scenarios. The research is guided by two primary questions: 1) Does the selection of different climate models influence the projected outcomes? and 2) What specific effects will climatic changes have on the duration and scheduling of the phenological stages of walnut trees in western Iran? By employing an ensemble of CMIP6 models under SSP2-4.5 and SSP5-8.5 scenarios for near- and far-future periods, this study provides a nuanced, forward-looking analysis intended for agricultural planners, policymakers, and environmental researchers to inform adaptive strategies and safeguard walnut production against imminent climatic threats.
Material and Methods
This study employed an integrated observational and simulation-based design to assess the impact of future temperature changes on the phenological stages of walnut trees in five major walnut-producing provinces of western Iran (Kurdistan, Hamadan, Lorestan, Kermanshah, and Ilam). The foundational data consisted of daily minimum and maximum temperature observations from 35 meteorological stations for the 1990–2014 baseline period. For future projections, data from 15 CMIP6 climate models under two scenarios, SSP2-4.5 and SSP5-8.5, were used for two future periods: 2021-2040 and 2041-2060. Following the extraction and spatiotemporal alignment of the model and observational data, the performance of each climate model was evaluated against the baseline observations using multiple statistical indices (CC, RMSE, NRMSE, ANMBD, AARD). Subsequently, the models were weighted by applying normalization, ranking via the Pomeral-Romero method, and entropy-based weighting to create a weighted multi-model ensemble, thereby reducing projection uncertainty. In the next phase, the daily temperature data (both observed and projected) were converted to Julian dates. By applying established temperature thresholds and growing degree-day requirements for each of the seven walnut phenological stages, the occurrence date for each stage was calculated for all stations and time periods. Finally, the quantitative results of these temporal shifts were spatially analyzed and mapped using a GIS environment.
Results and Discussion
Analysis of walnut tree phenological stages during the baseline period (1990-2014) revealed that the growing season in the region varies from approximately March to November (9 months). Bud break occurs from mid-March in warm, low-altitude areas (e.g., Ilam) to early May in cold, high-altitude regions (e.g., Kurdistan, Hamadan), indicating a 50-day difference in the start of the growing season. Flowering occurs about 10 days after bud break, spanning from late March to mid-May. Other phenological stages also occur later from south to north and from lowlands to highlands; for instance, harvest takes place from late August in warm areas to mid-October in colder zones. Leaf senescence and the onset of dormancy begin earlier in cold regions (mid-October) and later in warm regions (mid-November).
Following the selection and weighting of the best-performing CMIP6 models for temperature simulation, projections were made under the SSP2-4.5 and SSP5-8.5 scenarios. The results indicate a temperature increase across the entire region, but this warming is markedly more intense in cold, high-altitude areas (e.g., Saqqez, Zarrineh) compared to warm, low-altitude zones (e.g., Dehloran). This asymmetric warming directly impacted the walnut phenological cycle.
After projecting the temperature changes, the changes in the phenological stages of walnut trees in two future periods (2021–2040 and 2041–2060) were investigated, based on the temperature changes obtained under the SSP5-8.5 scenario, which showed relatively greater temperature variations. In the future periods (2021-2040 and 2041-2060), under the SSP5-8.5 scenario, all growth stages are projected to occur earlier. Bud break will advance by 2 to 14 days, flowering by 3 to 15 days, fruit set by 3 to 17 days, and fruit maturation by 6 to 29 days compared to the baseline period. Consequently, the harvest date will also shift earlier by 7 to 33 days. The most significant shifts were observed in the cold, high-altitude areas of the north and east (Kurdistan and Hamadan provinces), while the smallest changes occurred in the warm southwestern parts. Conversely, the stages of leaf senescence (delayed by 8 to 37 days) and dormancy onset (delayed by 9 to 29 days) are projected to occur significantly later. These asynchronous shifts in the agricultural calendar for walnuts entail risks such as increased vulnerability to late spring frosts and disruptions to harvest scheduling and market logistics.
Conclusion
This study assessed climate change impacts on walnut phenology in western Iran. By employing a weighted multi-model ensemble of 15 CMIP6 models under the SSP2-4.5 and SSP5-8.5 scenarios, the projections indicate that minimum temperatures will increase by 1–3.3°C and maximum temperatures by 0.8–2.1°C by the middle of the 21st century. These changes will significantly advance key phenological stages: bud break by 2–14 days, flowering by 3–15 days, and harvest by 7–33 days, with the most pronounced shifts occurring in colder, higher-altitude northern and eastern regions. Conversely, leaf senescence and dormancy onset will be delayed by 8–37 days. The earlier onset of sensitive stages, particularly flowering, heightens the risk of damage from late spring frosts, while higher growing-season temperatures increase the potential for heat stress and greater irrigation demand. These findings underscore the urgent need for adaptive strategies in orchard management to mitigate future climatic risks to walnut production.