Temperature sum models in plant spring phenology studies: two commonly used methods have different fields of application

In studies of plant spring phenology, temperature sum models are traditional tools. They are used to quantify plant development in terms of accumulation of temperature-dependent developmental units, such as Growing Degree Hours, GDHs. A key parameter in these models is the threshold (or base) temperature, Tthr, representing the lower thermal limit for the development to occur. The parameter can be either estimated when the model is fitted into the data or fixed a priori. Here we examine the limitations of both methods and identify fields of applications for each of them.