Figure 3.

Climatic memory in Norway spruce. Bud phenology is depicted throughout the yearly cycle (summer, autumn, winter, and spring), with a focus on the epigenetically altered timing of bud burst between cool and warm epitype trees. Exposure of Norway spruce embryos to different temperatures (18 and 28 °C) induces lasting epigenetic change that affects bud phenology. When the resulting “cool” and “warm” epitype trees grow together under identical natural conditions (“common garden”), they have significantly different timing of bud burst in the spring, even though the epitypes are genetically identical. Bud swelling, bud burst, and shoot elongation start significantly earlier in the spring and early summer in the cool vs the warm epitype (“active growth”). These phenological differences have so far persisted for 20 years in the field. The new buds that are set in summer (“bud set”) are visually indistinguishable in the two epitypes for the rest of the yearly cycle (“inactive growth”).