| Summary: | In recent decades, it has become clear through meteorological observations that the climate is getting warmer. Particularly spring is getting warmer and seems to start earlier. Since in the far north the growing season is shorter than in the temperate latitudes, this advance of spring is more prominent. The ecosystem in the northern areas is more vulnerable to this disturbance in the spring, because many natural processes risk being out of balance with each other. In order to properly understand these current changes of the growing season, it is necessary to look back in time at the pre-industrial mode of climate, in order to understand how the dynamics of early spring manifested themselves before the modern period of warming. Since there is little to no reliable meteorological data available from before the industrial revolution, one is forced to derive the state of the climate of the past from so-called proxies, in this case biological thermometers. With the fossil remains of these biological thermometers, one can deduce what the temperature must have been in the past, for example. For these proxies to work properly, they must be calibrated and tested. In this PhD research, the degree of undulation in the circumference of the cell wall of the epidermal cells of the leaf of birch trees was used as a proxy for the intensity of the growing season, with the advance of spring as main influence. Dwarf birch cell circumference was previously known to work well as paleo-thermometer. During this PhD research, it was also established that the downy birch acts well as a proxy based on the circumference of the leaf cells. The proxy based on these birch species has been applied to experiments in Finland, Poland, and Greenland with the aim of demonstrating that the advance of spring will disrupt the ecology. The usefulness of the proxy has also been demonstrated by proving that it is sensitive to past fluctuations in the large-scale atmospheric system that affects the weather in the areas around the North Atlantic. This ...
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