By analyzing long-term data we found that increasing levels of
O3 delayed plant phenology of birch and horse chestnut (see
Table 3). Birch species are typically regarded as O3 sensitive species Matyssek, 2001); thus, we suggest that the delay in spring
phenology with increasing O3 levels might be a consequence of the
species’ sensitivity. Although the variation of O3 is lower at urban
compared to rural sites (see Table 2), this environmental factor was
only relevant for full flowering of birch in stepwise regression analyses
when rural sites were excluded (see Table 5).