Variation in, and reproductive capacity of, Betula pendula and B. pubescens

While B. pendula can be distingushed from B. pubescens by its diploid number of chromosomes, 28 compared with 56, separation by morphology depends upon a blend of variable characteristics, including leaf hairiness, occurrence of warts on branches, roughness of bark, pendulous habit and, most consistently, leaf shape. Individuals of B. pendula , like those of B. pubescens , are mostly self-incompatible. The dates of flowering of individuals within a population of trees may differ markedly. However, female flowers usually mature sooner than male flowers on the same tree. The pollen of Betula spp. can be wind-dispersed considerable distances. Seed production varies greatly from year to year. Within a seedlot, the onset of germination may differ by as much as 7 days. When side by side at latitude 56°N, trees from seed collected at southerly sites (lat. 50°N) in the natural ranges of B. pendula and B. pubescens grew taller than those from northerly (lat. 69°N) locations. However, when compared at latitude 69°N, the more northerly collections were the more vigorous. The seasonal onset of growth of different B. pubescens seedlings depended upon their origins. Seedlings from latitude 56°N commenced growth when days were 14h long; those from 63° and 70°N started when days were 16 and 20 h long respectively. Whereas northerly collections ceased growth when daytime temperatures dropped below 9°C, southerly collections continued to grow. The stem extension of B. pubescens seedlings grown from seeds collected at different altitudes (in the range of 200–1000m) was inversely related to the altitude of collection when compared at the same low altitude. Within a population, some B. pubescens seedlings grew significantly more than others when supplied with small amounts of nutrients, the differences disappearing when amounts of nutrients were increased. In Britain, B. pendula was usually more severely attacked by the rust fungus, Melampsoridium betulinum , than B. pubescens whereas in southern Sweden, B. pubescens has been reported as the more severely attacked. At both locations, there were conspicuous within-species differences. More fruitbodies of sheathing mycorrhizal fungi were produced in association with southerly, than with northerly, collections of both B. pubescens and B. pendula —384 compared with nil fruitbodies per tree, 4 years after planting.