Plant phenological data and tree-rings as palaeoclimate indicators in south-west Finland since AD 1750

Plant phenological data and tree-rings were tested for their palaeoclimatic value in south-west Finland since AD 1750. The information from fragmentary, partly overlapping, partly non-systematically biased plant phenological records of 14 different phenomena (a total of 3,144 observations) was combined into one continuous time series of phenological indices. All site- and phenomenon-specific series were standardized to present an average of zero and standard deviation of one. The mean phenomenon-specific series were then averaged as arithmetic means for annually resolved time series representing the variability in the particular plant phenomenon. Consequently, each phenomenon-specific mean series was based on spatially normalized site-specific index series. These series were compared to each other, living-tree and subfossil tree-rings, and to early and modern meteorological time series. Phenological indices showed strong positive correlation with February to June temperatures. On the other hand, the correlations between phenological indices and precipitation data were around zero. Analysis using time-dependent running correlations showed non-stationary relationship between the tree-rings and phenological indices and observed spring temperatures. The skill of phenological data for reconstructing the spring temperatures was statistically proved.