The time series of flowering and leaf bud burst of boreal trees (1846–2005) support the direct temperature observations of climatic warming

The timing of spring phenological events of boreal trees, such as flowering and leaf bud burst, is controlled to a great extent by the ambient air temperature, and these events are already showing an advancement that can be attributed to climatic warming. In this paper we utilised this phenomenon to verify direct observations of climatic warming. We constructed eight phenological time series using observations covering 160 years of the leaf bud burst of two species and of the flowering of six species of native deciduous trees growing in Finland. To eliminate the effect of urbanization on local temperature, we rejected the observations made in densely populated areas. When analyzing the trends in the timing of leaf bud burst and flowering we found an advancement of these phenological events ranging from 3.3 to 11.0 days per century. We then converted the observed phenological trends into temperature trends by using thermal time-type models that link the timing of bud burst and flowering to the temperature conditions during bud development. Warming estimates derived from the phenological trends suggested that the mean spring temperature increase in Finland has been 1.8 °C per century, which is very close to the value of 1.5 °C per century indicated by long-term temperature records.