The species most liable to be struck are oaks, poplars and pear trees; beech trees are exceptionally safe.
The poplars are almost entirely confined to the north temperate zone, but a few approach or even pass its northern limit, and they are widely distributed within that area; they show, like the willows, a partiality for moist ground and often line the river-sides in otherwise treeless districts.
As in all poplars, the catkins expand in early spring, long before the leaves unfold; the ovaries bear four linear stigma lobes; the capsules ripen in May.
In the Mountain Region at the bases of the mountains are oaks, hickories, chestnuts and white poplars: above these are hemlocks, beeches, birches, elms, ashes, maples and limes; and still higher up are spruce, white pine and balsam; and all but a comparatively few of the higher mountains are forest-clad to their summits.
The materials made use of are driftwood, green willows, birch and poplars; also mud and stones intermixed in such a manner as contributes to the strength of the dam; but there is no particular method observed, except that the work is carried on with a regular sweep, and that all the parts are made of equal strength.