Birches are among the most recognizable of trees, both for their white bark and for their graceful, slightly drooping twigs. Though not all birches have bark as light as the white birch, Betula pendula, they all have bark that separates into thin, papery plates, especially the paper birch, Betula papyrifera. Many birches are adapted to the coldest, most northern climates, being one of the first trees to colonize open land after glaciers melt.
Branches And Twigs
Birch twigs and branches are flexible, unlikely to break under a load of snow. They are deciduous, losing their leaves in the winter, which reduces water loss and eliminates the need to protect soft leaf tissue. The dormant buds that remain are fairly watertight.
The small, triangular, toothed leaves have minimal surface area, reducing water loss in drying winds while still taking advantage of the intense summer sunlight for photosynthesis. Without the advantage of larger leaves, the birch grows poorly in shaded areas.
In fall, as the leaves turn from green to yellow, a layer of weakness called an abscission layer forms between the leaf stem and the twig. Scar tissue forms at that point, preventing water loss, and the leaf then blows off the branch.
The trunk of the birch is covered with a thick, somewhat papery bark that protects the inner wood from damage and from drying out. This bark contains resinous oils that make it durable and resistant to decay. The wood within the bark is hard and strong, resistant to breakage in winter winds.
The root system of the birch is shallow and wide, giving it the ability to reach out for the moisture the tree needs. Since birches often grow in wet soil or on the banks of rivers, they need the oxygen found in the upper layers of soil. In drier conditions, the spreading root system utilizes all the moisture available.
Acclimation is the physiological process that occurs as days shorten – cellular transformations that allow tissues to survive the intense cold of a northern winter. These include changes in soluble fats within the cells, changes in proteins and an increase in cell permeability. The temperature at which cell damage occurs will vary from species to species, sometimes even from tree to tree, but there is always a killing temperature at which ice crystals form within the cells, destroying them.