202 transplanting trees have been studied, such as the rel- ative timing of shoot and root growth initiation (e.g., Richardson-Calfree and Harris 2005). To our knowl- edge, however, bud burst or dormant period timing in relation to transport of nursery trees across climatic zones has not been previously studied. According to EU legislation, when a public pur- chase of a sufficiently valuable lot of nursery trees is made, all businesses registered within the EU, irre- spective of the geographic region or distance to the planting site, have equal right to compete for the ten- der. For living plant material in springtime, this may be problematic. Within the EU, the thermal growing season starting date varies for over a month from south to north. Nursery trees are most successfully transplanted when they are fully dormant (e.g., Solfjeld and Hansen 2004). If the transplanting of the trees takes place too late in spring, then in the more southerly nurseries, the spring has advanced further, and trees may already start to develop their leaves. When brought to more wintry conditions in the north, they may be exposed to frost damage and drying, with high likelihood of increased transplanting stress and tree losses. On the other hand, if trees are dug up in southerly locations and brought to southern Fin- land too early in spring, the local soil may still be fro- zen (Soveri and Varjo 1977; Huttunen and Soveri 1993), preventing tree planting. Some of the problems related to this seasonal mis- match may be avoided by a period of cold storage, commonly used on smaller plantlets for forestry (McKay 1997; Lindqvist 2001) to match the timing of phenological events with local climate, but for large sized nursery stock, this would accrue consider- able, additional costs. Therefore the feasibility of directly transported plant material from various geo- graphical locations needs to be evaluated in the ten- dering process. Tools are needed to predict the spring development of trees in advance, to define the possi- ble source areas for nursery trees as a function of geo- graphical location and calendar date. The most cost-effective way would be to define the geographi- cal area and time window from where the plant trans- plantation is possible on an average year with minimal risk of plant damage and poor transplanting success. The most practical means of predicting the spring development of plants, like the leaf development of Tilia, is with modeling. Phenological models have been widely used to forecast phenological events, such as leaf unfolding and onset of pollen season ©2019 International Society of Arboriculture Linkosalo et al: Modeling Tilia Transplant Phenology (e.g., Hänninen 1990; Linkosalo et al. 2008; Schaber and Badeck 2003; Chmielewski and Götz 2016). Most models use air temperature as the key driver of phenological development, while there is variation in the mechanism that triggers the development. Most simple models use a plain calendar date which can be interpreted as representing a signal from the changing light environment (e.g., day length) in spring (Lin- kosalo and Lechowicz 2006), while more complex models also describe the development and release of winter dormancy and its impact on the spring devel- opment. Many studies of modeling boreal phenology suggest that the simple models omitting the dor- mancy phase are at least equal if not better than the more comprehensive ones (Linkosalo et al. 2008). Since urban Tilia trees are usually of clonal origin, it is even more applicable to use phenological model- ing, as one can assume that the plants have largely the same response to environmental cues, and therefore the same model with same parameterization should work satisfactorily in a wide geographical area. In this study, we used phenological observations col- lected throughout Europe and fitted a Thermal Time (TT) model that predicts the timing of leaf unfolding to the data. We then used the model to simulate the leaf unfolding on a temperature data grid covering the whole of northern and central Europe and a time period of 35 years. We used the results to produce maps showing the average and variation of Tilia bud development. We utilized the maps to define areas from where the Tilia nursery trees can be brought to southern Finland without risking poor transplanting success. MATERIALS AND METHODS We used a common Thermal Time model to predict the leaf unfolding dates for northern and central Europe. The model describes the development lead- ing to the occurrence of the phenological event by accumulating a temperature sum, and once this sum exceeds a pre-set threshold, the phenological event is predicted to take place. The model has three parame- ters: the onset date of temperature sum accumulation, t0 , a critical threshold for effective temperatures, TCrit and a temperature-sum threshold, SCrit , , for the event to take place. The temperature sum, S(t), is accumu- lated as:
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