Provenance and Individual Variation in Climatic Hardiness of Scots Pine in Northern Finland
In the optimum zone of Scots pine (Pinus sylvestris L.) in northern Finland, seed matures when 77 percent of the local average annual heat sum is reached (Sarvas, 1967, 1970). In absolute units, the temperature sum required for seed maturation is much higher for southern provenances than for northern ones. In the northern marginal areas, 950 growing degree days (d.d.) at a 5 °C threshold is the lowest average heat sum to which the annual development cycle of Scots pine is genetically adapted. In places where the average annual heat sum falls below this limit, for example, north 65° to 66° N. latitude in Finland (Fig. 1), all provenances and individuals have roughly the same minimum heat requirement of 950 degree days for complete ripening of the seed (Ryynänen, 1982; Sarvas, 1967, 1970).
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