Objective This study aimed to establish a standardized phenological coding system for the entire growth cycle of Pinus koraiensis based on the BBCH scale, and to identify quantitative descriptive indicators applicable to the developmental processes of various organs during vegetative and reproductive growth, thereby providing a unified and reliable phenological observation framework for the precise management of grafted P. koraiensis nut-production forests.

Method The study was conducted on an 11-year-old grafted P. koraiensis orchard at the Experimental Forest Farm of Yanshou County Forestry Bureau in Heilongjiang Province. Regular observations of morphological changes in buds, leaves, branches, flowers, and fruits were conducted over three consecutive growing seasons (March to October) from 2022 to 2024. By integrating images, measurement data and quantitative criteria, major and minor phenological stages were classified according to the BBCH scale.Monthly maximum, minimum and average temperature data, along with precipitation data for the corresponding period, were extracted from the Climate AP v3.0 software to generate a schematic illustration depicting the relationships between phenology and climate.

Result (1) The vegetative growth process comprises four distinct stages: vegetative bud development (PGS 0), needle development (PGS 1), shoot development (PGS 3) and dormancy (PGS 9), with each stage exhibiting unique morphological traits and quantifiable growth manifestations. (2) The reproductive growth process encompasses four stages: flower bud development (PGS 5), flowering (PGS 6), fruit development (PGS 7) and fruit maturation (PGS 8), each characterized by distinct morphological features. Notably, cone growth is constrained in the current year, with the primary growth occurring in the subsequent year. (3) The BBCH coding system employs a two-digit recording method for the four stages of vegetative growth, whereas a three-digit recording method is utilized for the stages of floral bud development, flowering and cone maturation during reproductive growth. Additionally, the fruit development stage is recorded using a two-digit coding method. During the coding process, quantitative grading criteria for the secondary stage classification of each organ were established, utilizing the needle length development index (10%, 20%, 50%, 70%, 90%), the shoot length development index (10%, 30%, 50%, 90%),and the cone volume development index (15%, 30%, 50%, 70%, 90%). (4) The BBCH phenological coding exhibits a correlation with climatic conditions, with the principal BBCH stages demonstrating temporal synchrony with monthly mean temperatures and precipitation levels.

Conclusion This study developed a phenological coding system for P. koraiensis utilizing the BBCH scale, which encompasses eight principal growth stages and fifty-one subsidiary phenological phases. The seasonal distribution of BBCH phenological stages exhibits a significant synchronicity with the interannual variations in climatic factors, providing a standardized phenological basis for guiding precise management of P. koraiensis nut-production forests.