There are only a few native deciduous tree species with colourful summer foliage among the trees adapted to the severe northern climate in Finland . Ornamental trees and bushes imported to Finland from southerly latitudes are adapted to more temperate climate conditions. They are easily killed by midwinter temperatures sometimes as low as -40 degrees and sudden frosts during spring.
Several ornamental birches, alders and maples, mostly with dark red foliage have become commercially available. The studied decorative birches would bring additional variety to landscaping, because the leaf veins of the golden-veined (GV) birch are yellow and the leaves of the white-flecked (WF) birch are mixed green and white.
Cloning of the WF birch was proven to be easy by micropropagation even after cryopreservation. After two years from grafting, more than 90 percent of the grafts were alive. Correspondingly, the micropropagation success percentage of GV birch was less than ten and the survival percentage of the grafts was approximately 50.
The leaf colouration, which had been assumed to be inheritable, was lacking in the micropropagated progenies. Only the leaves of the oldest branches of the WF birch grafts showed a touch of white-fleckedness. The reason was found in the donor trees. It is typical for a WF donor birch growing on a gravely road bank to have strong year-to-year variation in foliar colouration, and the fleckedness is not expressed in the newest leaves of the current year’s growth. The amount of white-fleckedness in the donor tree was shown to be associated with the amount of spring precipitation and the surface level of the adjacent lake. Dry growth conditions increased the fleckedness. White-fleckedness in the older tree parts is explained by chlorophyll deficiency.
The results indicate that when the cloned WF birches, either grafts or micropropagated plants age, they express more flecked leaf colouration. They are well suitable for landscaping.
The GV donor birch growing on a dry rocky mound did not show corresponding year-to-year variation in leaf colouration. The reason for the gold-coloured veins is so far unknown. To facilitate the use of the GV birch for decorative purposes, further studies will be needed to find the factor controlling leaf colouration and establish its usefulness.