Dynamic modelling of effective crown ratio in Larix olgensis plantations under long-term climate conditions

Effective crown ratio (ECR) is an important indicator of crown functional status, branch survival and crown recession in managed Larix olgensis plantations used for industrial timber production. It can help identify crown decline, optimise the timing of artificial pruning and guide stand-density regulation. Climate change and stand competition affect the formation and maintenance of crown structure, but age-dependent ECR trajectories and their long-term environmental responses remain insufficiently understood. Based on stem analysis and knot-dissection data from Larix olgensis plantations in Heilongjiang Province, this study reconstructed historical ECR changes across tree ages, developed a climate-sensitive dynamic model that incorporated tree age, stand density, tree form and long-term climatic factors, and compared model performance under alternative random-effect structures. The results showed that ECR declined continuously with increasing tree age, indicating that effective crown recession is a stable structural change during plantation development. Height-diameter ratio, reference evaporation, extreme maximum temperature, mean annual temperature and precipitation as snow significantly affected ECR dynamics, and interactions among climate variables further improved model explanatory power. Comparisons among random-effect structures showed that tree-level random-effect models better captured individual variation in ECR dynamics, indicating marked differences among sample trees in initial effective crown level, age-related decline rate and climate-response intensity. Scenario simulations showed that higher reference evaporation and precipitation as snow reduced the maintenance level of ECR, whereas higher mean annual temperature, extreme maximum temperature and height-diameter ratio helped maintain higher ECR under the observed cold-temperate climatic range. Adjacent-age differencing further showed that ECR decline was not uniform, but contained rapid recession windows at specific ages. By integrating knot-based ECR reconstruction, long-term climatic variables, density-regulation terms and tree-level random effects, this study improves dynamic crown modelling for Larix olgensis and provides preliminary, region-specific support for crown-structure regulation, pruning timing and precision management of industrial timber plantations under climate change.