A comparison of phenological models of leaf bud burst and flowering of boreal trees using independent observations.

We compared and tested Thermal Time, Sequential, Parallel and Flexible phenological models of leaf bud burst in birch (Betula pendula Roth. and B. pubescens Ehrh.) and flowering in bird cherry (Prunus padus L.) and rowan (Sorbus aucuparia L.). We used phenological records from Oulainen-Ohineva (64 degrees 13' N, 24 degrees 53' E) in central Finland from 1953 to 2002 to estimate model parameters. We tested the models with data collected in all but six years between 1896 and 2002 in southern and central Finland; we divided this dataset into two 50-year datasets. The use of three datasets enabled us to test the models with data that were independent of the parameter fitting data, facilitating robust evaluation of model performance. Several models that fitted the parameterization data well showed poorer performance when tested with the independent data. This may be because the models were over-parameterized and able to adapt to noise in the data in addition to the phenological phenomenon itself. Simple Thermal Time models performed best with independent data, and Sequential and Parallel models were similar in prediction accuracy. Although Thermal Time models simulated boreal phenological events under current climatic conditions, some precautions are needed with simulations of climatic warming. For example, changed conditions may increase the relative importance of chilling in the timing of bud burst under elevated temperature conditions, which could alter the performance of phenological models.