Plant phenological records in northern Finland since the 18th century as retrieved from databases, archives and diaries for biometeorological research

Plant phenological data from northern Finland, compiled from several sources, were examined as potential biometeorological indicators of climate change since the 18th century. A common feature of individual series was their sporadic nature. In addition to waning enthusiasm, wartime hardships and crop failures had caused gaps in recording observations during the 18th and 19th centuries. The present study’s challenge was to combine separate records, as retrieved from several historical archives and personal diaries, into a single continuous series. To avoid possible biases due to the variability of data availability each year, each phenomenon-specific mean series was transformed into normalized site-specific index series. These series were compared to each other and to a regional instrumental temperature series (years 1802–2011). The inter-phenomena correlations were high. Moreover, a strong biometeorological response of the phenological series, most especially to monthly mean temperature in May, and seasonally to the April through June temperatures, was identified. This response focused on slightly later spring months compared to the responses in an earlier study conducted for southern Finland. The findings encouraged us to compute a total phenological index series as an average of all available phenomenon-specific index series for northern Finland. The earliest phenological springs were found as a cluster in the recent end of the record, whereas the anomalously-late phenological spring could be found through the centuries. This finding could indicate that potential future warming could result in an earlier onset of phenological springs (i.e. as experienced by the plants), with a remaining possibility of late phenological springs. To conclude, it was shown that the indices are reliable biometeorological indicators of the April through June temperature variations and thus of the climate variability in the region.