Reproductive ecology of birch: a review

The reproductive processes and environmental requirements for regenerating Betula alleghaniensis, B. papyrifera, B. pendula , and B. pubescens are reviewed. Their taxonomy is confusing because somatic chromosome number varies within species, and natural hybridization is common. Paper birch and the Eurasian birches are typical species of the boreal forest, whereas yellow birch grows no farther north than 49 degree , from Manitoba and Minnesota eastward across North America, and somewhat farther south than paper birch. Birches do not tolerate shade, except that seedlings prosper best at about half of full sunlight, and can endure up to 90% shade. Long photoperiods are required for genetic sources from high latitudes and from high altitudes. Growth is optimum at about 18 degree -22 degree C soil and air temperatures. The birches do not use water efficiently and are sensitive to both drought and flooding.