Pistachio (Pistacia vera L.) is one of Iran's most significant horticultural and economic products, with high strategic value in non-oil exports. Khorasan Razavi province, following Kerman, ranks second in both cultivation area and production of dry pistachios in the country. However, global climate change, characterized by rising temperatures and altered precipitation patterns, poses a significant threat to the sustainability of pistachio cultivation. One of the most critical factors for the successful growth and fruiting of pistachio trees is the fulfillment of their chilling requirements during the winter dormancy period. Insufficient winter chill can lead to a cascade of physiological problems, including delayed and irregular flowering, asynchronous bloom between male and female cultivars, reduced fruit set, and ultimately, substantial yield losses. Different pistachio cultivars have unique chilling needs, ranging from 600 to 1200 hours below 7.2°C. Therefore, selecting the appropriate variety based on the specific climatic conditions of a region is essential for sustainable and profitable production. This study aims to conduct a comprehensive spatial analysis to produce and evaluate a climatic suitability map for pistachio cultivation in Khorasan Razavi province. By leveraging high-resolution reanalysis data and multiple chilling models, this research provides a crucial agrometeorological tool for regional planning and farm-level decision-making.



- Materials and methods

This study focuses on Khorasan Razavi province, located in northeastern Iran, which exhibits diverse climatic conditions from semi-arid to cold mountainous regions. To overcome the limitations of sparse and often incomplete meteorological station data, this research utilized high-resolution (0.1° x 0.1°, ~9 km) hourly gridded temperature data from the ERA5-Land reanalysis product, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). The data covers a 31-year statistical period from 1991 to 2021. The chilling accumulation for pistachio was calculated for the critical dormancy period from November 1st to February 4th. Three distinct and widely used models were employed to estimate chilling accumulation: (1) the Chilling Hours (CH) model, which counts hours with temperatures between 0 and 7.2°C; (2) the Utah model, which assigns different weights to various temperature ranges, including negative effects for warm temperatures; and (3) the Dynamic model, which conceptualizes chill accumulation as a two-step process and is considered more suitable for warmer climates. The spatial distribution of chilling accumulation for each model was mapped across the province. Furthermore, long-term trends in chilling accumulation over the 31-year period were analyzed using the non-parametric Theil-Sen's slope estimator, with trend significance assessed by the Mann-Kendall test. Finally, to provide a risk assessment framework, probability maps were generated to identify areas with different likelihoods (75% and 90%) of meeting the chilling requirements for various pistachio cultivars.



- Results and discussion

The analysis of chilling requirement patterns in the province revealed significant spatial and temporal variability. Based on the Chilling Hours (CH) model, most parts of the province, with the exception of some limited northwestern areas, are capable of satisfying the chilling requirements for pistachio. A majority of locations demonstrated the ability to provide over 800 chilling hours, suitable for commercial cultivars like 'Ahmad Aghaei' and 'Owhadi'. The spatial distribution of probabilistic values showed that areas with a 75% probability of meeting high chilling requirements (1000 to 1200 hours) are concentrated in the western and southern parts of the province, such as Bardaskan and Khalilabad counties, making them highly suitable for the 'Akbari' cultivar. With a 90% probability, large areas of the province have the potential to meet chilling needs in the 800-1000 hours range, indicating low-risk zones for investment. The trend analysis indicated a statistically significant increasing trend in chilling hours in the eastern and western parts and some central areas of the province, with an annual increase of about 2 to 6 hours. This suggests an improvement in climatic suitability for higher-chill cultivars in these regions. Analyses based on the Utah model provided a more conservative estimate, highlighting the southern parts of the province as having the most effective chilling accumulation due to fewer mid-winter warm spells. The Dynamic model results, showing that most regions received more than 50 chill portions, confirmed the general suitability of the province for breaking dormancy. The convergence of results from all three models strengthens the validity of identifying the southern and western plains as prime locations for pistachio cultivation.



- Conclusion

This research provides a detailed and robust assessment of the climatic suitability for pistachio cultivation in Khorasan Razavi province. The findings indicate that large parts of the province, particularly the southern and western regions, possess a high and reliable potential for pistachio cultivation, accommodating a range of cultivars from moderate to high chilling needs. The identified increasing trend in chilling hours in several regions suggests a potential for future expansion of suitable cultivation zones. The generated probability maps serve as a valuable risk management tool for stakeholders. The study highlights the critical importance of using high-resolution climate data and multiple agro-climatic models for accurate zoning and strategic planning. The results can guide policymakers in promoting sustainable agricultural development, assist farmers in selecting appropriate cultivars to minimize risk and maximize yield, and inform investment decisions in the agricultural sector. Future research should focus on integrating these findings with soil and water availability data and employing climate change projection models to assess long-term viability.

Keywords: Climatic Suitability, Chilling Requirement, Probabilistic Analysis, Reanalysis Data, Pistachio, Khorasan Razavi.