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59 Chapter 5 Broadleafs Growing among the Conifers Broadleafs in a Harsh Climate Although evergreens dominate the northern forests, a few broadleafs are found with them, and they are the subject of this chapter. The trees and shrubs considered here are those living in the harsh climate of the true north, not those in the mixed forests of the St. Lawrence valley and southern Great Lakes where climate and soils are hospitable to many broadleafs. The broadleafs described here play an important part in the coniferous forests. They belong to four genera: the willows (Salix), the poplars (Populus), the birches (Betula), and the alders (Alnus). Shrubs as well as trees of these genera are common in the evergreen forests. All have catkins (figure 5.1). A catkin consists of densely packed rows of flowers growing along a short stalk that may be upright or dangling. The individual flowers are tiny and devoid of petals. There are female catkins made up of female, seed-bearing flowers, and male catkins made up of male, pollen-producing flowers. The female catkins of alder differ from all the others in that they resemble a tiny cone: they are so conelike that they are often called “cones.” These four catkin-bearing genera belong to two families. The willows and poplars are in the willow family, Salicaceae, in which each plant (tree or shrub) bears either male or female catkins (not both), and in which the tiny seeds bear long fluffy hairs (the “cotton” of cottonwood, a poplar subspecies) so that even a gentle wind will carry them far from their 60 the world of northern evergreens parents. The birches and alders belong to the birch family, Betulaceae, in which each plant bears catkins of both sexes, and in which the seeds are winged. The flowers of these four broadleafs and the cones of conifers are adapted to life in high latitudes where wind-pollination is more likely to succeed than insect-pollination. The female flowers (in broadleafs) and the cones (in conifers) must be pollinated early in the year if their seeds are to ripen in the short growing season, and there is always the risk that the weather will be too cool for bees to be on the wing when they’re needed. Catkins, like conifers’ pollen cones, are adapted to wind-pollination, and the poplars, Figure 5.1. Twigs with catkins. (a) Birch. (b) Alder. (c) Poplar. (d) Aspen. broadleafs growing among the conifers 61 birches, and alders are wholly dependent on wind to pollinate them. But many of the willows rely on wind-pollination only in a cold spring. When spring is warm, pollen-covered pussy willows (which are the male catkins of some species of willow) produce nectar that attracts bees to act as pollinators . (This presumes that bees are sufficiently numerous. At the time of writing, bee populations are declining.) Of these four genera of conifer “companions,” the willows are probably the least important for their effects on the conifers. They are companions only in the sense that they grow, in suitable habitats, throughout the north country. The great majority of willows are shrubs rather than trees, and it’s rare for large numbers of them to occupy ground where conifers would otherwise grow. Many species grow only where water is nearby, along the shores of lakes, rivers, and streams; others, such as net-veined willow (Salix reticulata), are prostrate shrubs, equally at home in arctic tundra and boreal forest. The willow genus is so enormous (more than 80 species in our area), and the species so hard to identify, that to investigate them is a job for specialists. No more will be said of them here. What follows concentrates on broadleafs that have an appreciable effect on the welfare of conifers. Luckily, they are easy to identify. Poplars Members of the poplar family—that is, the trees known as poplars, cottonwoods, and aspens—are fast-growing, hardy trees. Life is usually rather short for each individual tree, but they propagate so rapidly that there’s little risk of their numbers decreasing as long as the environment they need does not diminish. Two poplar species, balsam poplar and trembling aspen, are adapted to endure the intensely cold climates of northern Canada and Alaska (see chapter 1) and can therefore grow in the company of similarly adapted conifers. They are found either as tracts of pure poplar or aspen forest or mingled with the conifers, almost everywhere in our area. The...


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