Arboriculture & Urban Forestry 45(5): September 2019 207 Figure 3. The model SSE values (Z-axis) as the function of model parameters. X-axis shows the starting date (DOY) and Y-axis the temperature sum threshold. For each pair of the latter two, the critical temperature threshold giving the smallest SSE value was picked, and this temperature threshold is shown on the a contour map on the floor of the graph. parameters estimated from the other half of the data, was 13 and 20%. The value is somewhat larger than when using phenological models for the flowering of birch (Linkosalo et al. 2008). All in all the increase in SSE in this study is quite small, indicating that the model with this parameterization can be safely extrapolated. The RMSE value of 14.2 days was considerably larger than the values achieved when fitting a TT model to phenological data for the flowering of birch (Siljamo et al. 2013), and also larger than for models fitted to data from a more restricted area, where val- ues of even below 2 days have been reported (e.g., Linkosalo et al 2008). We fitted the model also to a smaller geographical area (Germany), but found that the model RMSE value was only slightly reduced, so it seems that the large geographical area used in this study is not the reason for the rather large RMSE value of the model. We also checked the model pre- diction residuals against geographical variables, but found no significant correlations (results not shown). DISCUSSION Our results showed that bud development of Tilia in central and northern Europe starts in early April and at a low critical temperature threshold, which sug- gests that, after the bud development start date (t0 ), moving the Tilia trees from southerly nurseries to southern Finland may prove problematic. The plants would have already progressed in their bud develop- ment and may be vulnerable to frost damages in the colder conditions of Finland. Our results indicate that since the plants should be transported in full rest state, this is best done before April to avoid the possibility that bud development has started in the nursery trees. However, that would mean planting trees in winter conditions: ground frost melts on average, depending on site conditions, in mid-April to mid-May in south- ern Finland (Huttunen and Soveri 1993). Severe night frosts, which may cause damage to buds at advanced stages of development, are also frequent, as the aver- age daily temperature in March is still below freezing in the area (Pirinen et al. 2012) with average daily minimums in the range of -4 to -6° C (21 to 25° F) . Thus transplanting before April would be very unpractical, and lifting trees in the southern nurseries before April to transplant a month or two later would require considerable cold storage capacity. The increased cost and responsibility for arranging stor- age and possible loss of stock quality during storage (e.g., Lindqvist 2001) would need to be dealt with within the commercial chain. The critical temperature threshold (Tcrit ) for our model, -2° C (28° F), was much lower than is typi- cally seen on Thermal Time models predicting the onset of spring events, such as leaf unfolding and onset of flowering (Pirinen et al. 2012). Combined with the modeled starting date of temperature sum accumulation (t0 role of the starting date in determining the date of leaf unfolding. The starting date parameter (t0 ) in early April, this emphasizes the ) most likely reflects a feature in the light environment of the plant, such as meeting a critical day or night length. In our model, after the starting date is met, the bud development proceeds even on the coldest days, while develop- ment of course is faster on warmer days. Interestingly, the resulting starting date of early April falls close to spring equinox, the date when the day length is the same all over the globe. The model parameterization suggests an interpretation of Tilia bud development, that once the critical day length is met, the bud devel- opment rather rapidly proceeds towards the leaf unfolding. In other words, day length is an important factor defining the leafing date. In contrast to our finding of critical temperature threshold (Tcrit ) of -2° C (28° F) for most phenological models of boreal and temperate trees, the typical range for the critical temperature parameter is between 2° and 5° C (36° and 41° F)(e.g., Linkosalo et al. 2008). For these plants, the bud development may be consid- erably delayed due to cold weather, even after the ©2019 International Society of Arboriculture
September 2019
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