Improving phenological events identification in trees using manually measured dendrometer data: conventional approaches vs. the novel Two-Stage Threshold Approach

Przemysław Andrzej Jankowski^*Rafael CalamaJorge AldeaMatías GarcíaGuillermo MadrigalMarta Pardos^*

• Institute for Forest Research (ICIFOR, INIA-CSIC), Madrid, Spain

The accurate detection of phenological events, such as growth onset, cessation, and seasonal transitions, is essential for understanding tree growth dynamics, particularly in Mediterranean forests where bimodal growth patterns are common. While microcore analysis remains the most precise method, its labour-intensive nature has led researchers to rely on dendrometer-based approaches. However, traditional methods using dendrometer dataparametric growth curve fitting with phenological event detected as relative thresholds -are often biased by hydration-related fluctuations and may not accurately capture complex growth patterns. In this study, we compared commonly used detection methods, including Gompertz and Richards parametric functions, generalized additive models (GAM), and the Zero-Growth (ZG) concept, with a novel approach: the Two-Stage Threshold Approach (2STA). Our results show that 2STA consistently outperformed existing methods in identifying spring onset, summer cessation, and autumn onset, exhibiting deviations within the expected measurement uncertainty (±7 days). The method's reliance on direct growth rate transitions, rather than smoothed curves or arbitrary percentiles, reduced errors associated with hydration-related stem fluctuations. However, year cessation was best estimated using a 95% relative growth threshold from the Richards function. These findings highlight the potential of 2STA as a robust, objective method for phenological event detection using manually collected band dendrometer data, particularly for species with complex intra-annual growth patterns. Future research should refine climate-based thresholds for cambial activity and validate the method across a wider range of species and environmental conditions.