Air temperature was recorded at each of the 38 birch observation
sites using loggers (HOBO U23-001, Onset Computer Corporation,
Bourne, MA, USA) which were placed in a radiation shield
and mounted at a height of 3 m on the northern side of the tree. In
contrast, no site-specific temperatures for hazel and horse chestnut
flowering were measured; temperature data of the nearest birch
monitoring site was used instead. The distance of hazel sites to the
next meteorological station ranged between 20.0mand 5.8 km and
was on average 1.4 km. For flowering of horse chestnut the distance
was on average 1.6 km (min ¼ 70.0 m, max ¼ 9.5 km).