# Combining temperate fruit tree cultivars to fit spring phenology models

## TL;DR

This paper introduces a new method to build spring phenology models for fruit trees by combining data from multiple cultivars of the same species.

## Contribution

The novel 'combined-fitting' approach allows joint analysis of cultivars with limited data, improving model development and comparison.

## Key findings

- Combined-fitting did not improve prediction accuracy over cultivar-specific models.
- Cultivar-fit models showed large variation in parameter estimates among cultivars of the same species.
- Combined-fitting provides a practical solution for modeling with limited datasets and enables cultivar comparison.

## Abstract

Phenological datasets for temperate fruit trees are often short, fragmented and geographically restricted, which hampers the development of cultivar-specific spring phenology models. To address this, we propose a novel calibration approach (“combined-fitting”), which pools observations from several cultivars of the same species, distinguishing between shared and cultivar-specific parameters. This method requires fewer observations per cultivar and allows jointly analyzing cultivars of the same species. We evaluate combined-fitting using the PhenoFlex framework, comparing it to a baseline model and to models that are fitted only with data for single cultivars (“cultivar-fit”). Our analysis is based on flowering data from nine almond, six apricot and six sweet cherry cultivars across Mediterranean (Spain, Morocco, Tunisia) and German climates. The combined-fit model failed to achieve higher prediction accuracy compared to the cultivar-fit and the baseline approach, as evidenced by similar root mean square errors across the data splits and calibration dataset sizes. When comparing the estimated parameters of the chill and heat accumulation submodels, we observed a large variation among cultivars of the same species in the cultivar-fit models. In contrast and by design, the combined-fit yielded only one parameter set for cultivars of the same species. Our findings demonstrate that integrating data from multiple cultivars can yield spring phenology models with high accuracy. Even though the combined-fit approach did not outperform the cultivar-fit approach, combined-fitting offers a practical solution for spring phenology modeling with limited datasets and facilitates comparison across cultivars of the same species.

The online version contains supplementary material available at 10.1007/s00484-025-03068-2.