Biology of the Black Spruce
| By Johnny Caryopsis | (Click links for more images.) |
Black Spruce, a major treeof the boreal forest.
The black spruce (Picea mariana) is one of 8 species of coniferous or evergreen trees that are native to Manitoba. This group of plants belong to a larger group called the Gymnosperms. These are plants that have true seeds (as opposed to the spores, used by so-called "lower plants"), but the seeds are exposed on the surface of the seed-bearing structure. The most familiar seed-bearing structure, though not the only kind in this group, is the "cone", from which the group draws one of its other common names, the conifers (cone-bearers).
The word gymnosperm is derived from two Greek words: "gymnos", meaning naked or lightly clad, and "sperm", a seed. So, the gymnosperms are the "naked-seeded" plants. The bulk of plants that we regularly encounter, most flowers, trees and shrubs, are from the other group of plants that have true seeds, the Angiosperms. "Angio" is from the Greek "ang or angos" referring to a jar or container. The angiosperms are the "enclosed-seeded" plants.
The other common name of this group of plants is "evergreens". The term evergreen derives from the habit of these plants not to shed their leaves (needles) at one particular time, as do deciduous plants. The needles of evergreens are just thick, waxy, needle-shaped leaves. Individual needles stay on the tree branch for more than one year, although each is eventually shed and replaced with new needles. Needles are shed continuously, though not all at once, as in deciduous trees that drop their leaves for the winter. One exception, however, is the tamarack (Larix laricina) which drops its soft needles each fall. It is a deciduous evergreen. Tamaracks turn colour and drop their needles like a deciduous tree in autumn.
Tamarack tree and needles.
But what are the benefits of being evergreen? Needles, as opposed to the flattened, fleshy leaves of deciduous species, are well adapted for water conservation, thus giving evergreens an advantage over deciduous trees in conditions where soils are poor or non-existent (bare rocks and sand) and there is low water holding capacity. Deciduous trees need lots of water in the spring to "inflate" their newly emerging leafs. Evergreens can take advantage of water whenever it is available. And by having their leaves already formed in the spring they can start photosynthesizing earlier than deciduous plants, and can continue to produce food after other trees have lost their leaves, if favourable conditions persist later into the fall. In northern latitudes, high altitudes and areas with poor soil, evergreens often become the dominant tree species.
Description
The black spruce tends to be a medium sized tree growing to about 20 m in height and usually less than 50 cm trunk diameter. It has short needles (1-2 cm) that are densely packed around the branch tips. They are a dull blue-green colour, giving the overall tree a dark blue-green appearance. In Manitoba, black spruce may be confused with two other conifers, white spruce (Picea glauca) and balsam fir (Abies balsamea). Balsam fir may be distinguished from both spruces by its flattened arrangement of needles, its smooth, resin spotted bark and its upward pointing cones which break apart when mature. The spruces have needles surrounding the branchlets, scaly bark on their trunks and have downward hanging cones which persist long after the seeds have matured.
To tell a black spruce from a white spruce, examine the small branchlets, the cones and the habitat. Black spruce has tiny reddish hairs on the outermost branchlets and short needles (to 1.5 cm); white spruce has no hairs and has longer needles (to 2.5 cm). Black spruce cones are smaller (to a maximum of 3 cm) and rounder than white spruce cones (to 5 cm long). And finally, black spruce usually grows in low lying areas (bogs or edges of wetlands), whereas white spruce prefers upland sites. Black spruce will often grow in association with other moisture preferring species like tamarack, white cedar (Thuja occidentalis) and paper birch (Betula papyrifera), while white spruce is more often found with jack pine (Pinus banksiana), balsam fir and aspen (Populus tremuloides).
Comparison of black and white spruce twigs and cones.
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When it grows in dense stands, in bogs or other low lying areas, a black spruce often loses most of its lower branches. These die as they become increasingly shaded by neighbouring trees. Often the top of the tree will appear densely branched, with the rest appearing rather spindly. Intense cone-clipping by red squirrels (the cone bearing branch tips are cut off and collected from the ground) is thought to play a role in pruning these uppermost branches of black spruce, promoting branching close to the main trunk and creating this dense growth.
Red squirrel, sitting in a spruce tree.
When it grows in more open conditions, black spruce is one of the classic "cone shaped" evergreen trees. (That's cone, as in the geometric shape, not the scaly seed-bearing structure.) Many spruce (Genus: Picea) and fir (Genus: Abies) trees, and some pines (Genus: Pinus) exhibit a very regular and symmetrical growth form that results in them appearing conical in shape. In silhouette they look like a narrow isosceles triangle with the apex pointing up. When most people think of a "pine" tree, this is the shape they think of, but most pines don't actually end up this shape. They tend to be less regular in their growth form, with upward pointing branch tips and often have twisted or branching trunks. It is actually the spruce and fir trees that most often exhibit the regular "pine" tree shape. (Sorry to be such a "nudge", but you know I'm a stickler for details like this.)
The conical shape of many evergreens results from the simplicity of the way they grow. Most have one central, unbranching stem, or trunk, that grows straight up. Branches arise from the stem in whorls (actually, closely arranged alternate pairs), with several pointing outwards from the same point on the stem, at regular intervals. These grow out horizontally, perpendicular to the trunk, or at a slight upward angle. As the branches grow out from the stem, they tend to divide or branch only in the horizontal plane. (Only in the case where the top of the tree is damaged or broken off, will the tips of a branch start to grow up. One or more may eventually become the new trunk of the tree.) So, each set of branches encircling the trunk grows out like an ever expanding ring. The older the branches (the first to be sent out near the ground), the wider the ring, and as you proceed up the trunk the progressively younger branches have had less time to grow and spread. The overall effect is a symmetrical cone shaped tree.
Carry on for the Biology of the Black Spruce - Part 2
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