Leaves and Catkins
Arctic Moor Birch is a small tree growing to 20 feet. It is native to Greenland, Scandinavia and protected inland valleys of Northern Europe. It is usually multi-trunked, with often interlacing, ascending branches. Twigs have numerous small resin glands. Leaf blade is ovate, rhombic-ovate, or suborbiculate-rhombic, 1-2 inches long, base cuneate to truncate, margins coarsely serrate or dentate, apex acute; surfaces abaxially moderately pubescent.
Infructescences 1/2 inch; scales pubescent to glabrous, often ciliate, central lobe oblong or narrowly triangular, apex acute to obtuse, lateral lobes divergent and ascending, about equal in length but somewhat broader. Samaras with wings about equal in diameter to body, broadest near summit, usually extended beyond body apically.
Birch grows in climates ranging from boreal to humid and tolerates wide variations in precipitation. Its northern limit of growth is arctic Canada and Alaska, in boreal spruce woodlands, in mountain and sub alpine forests of the western United States, the Great Plains, and in coniferous - deciduous forests of the Northeast and Great Lakes states.
Separate male and female flowers are borne on the same tree; the male in the form of a catkin, and the female in cone-like clusters that fall from the tree and are blown for long distances by the wind. In the fall, the foliage turns pale yellow.
The tree limit of Betula pubescens ssp. tortuosa (mountain birch) in the southern Swedish Scandes was monitored during 1972-1992. The study included various aspects of growth, vigour and reproduction of Betula. The main focus was on the character and mechanics of the tree limit/climate equilibrium system. The tree limit, which changed in response to a temperature rise early this century, remained constant in position and tree physiognomy did not change, although the past 50 yr or so have been colder. Indeed, growth and reproductive effort and capacity decreased in this period.
Obviously, most resources in Betula were used to resist stress in the mature phase, resulting in delayed recession of trees. Unless drastic warming occurs, stem dieback is predicted for the near future. Even a minor climatic disturbance would have an effect, because resources are gradually being depleted. Radial growth correlated most closely with the mean temperature in July. Particularly at the present-day tree limit, the variance in annual growth could be largely explained by climatic factors. The hypothesis is that long-term tree limit dynamics during the late Holocene is merely a matter of fluctuations in vegetative vigour and stature of old individuals. At the population level, response to climatic variability appears to be greatly delayed.
The birches have long been popular ornamental trees in North America, chiefly in the northern United States and Canada. Several are native Americans, but many species have been introduced from Europe and Asia. Our specimens include river birch, Dahurian birch, paper birch, Arctic birch, Manchurian birch, Manchurian alder, downy birch, Japanese white birch, and 10 other species.
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