Trees and flora thread

Climate, especially temperature, is obviously a factor in determining distributions of northern flora. Halliday and Brown (1943)[50] suggested that white spruce's northern limit corresponds “very closely” with the July mean monthly isotherm of 10 °C in Ungava, but that the northern limit west of Hudson Bay was south of that isotherm. Other climatic factors that have been suggested as affecting the northern limit of white spruce include: cumulative summer degree days, position of the Arctic front in July, mean net radiation especially during the growing season, and low light intensities (Nienstaedt and Zasada 1990).[7] Topography, soil conditions, and glaciation may also be important in controlling northern limits of spruce (Drew and Shanks 1965).[53]



The southern limit of distribution of white spruce is more complex. From east of the main range of coastal mountains in British Columbia, the southern continuous limit of white spruce is the forest/prairie interface through Alberta, Saskatchewan, Manitoba, the northern parts of Minnesota and Wisconsin, central Michigan, northeastern New York, and Maine (Fowells 1965).[45] Sargent (1922)[38] and Harlow and Harrar (1950)[28] also included Vermont and New Hampshire; and, while Dame and Brooks (1901)[54] excluded New York and states further west, they included Massachusetts as far south as Amherst and Northampton, “probably the southern limit of the species” in that area. Nisbet (1905)[55] gave the range of white spruce as extending to “Carolina”, but he did not recognize red spruce as a species and presumably included it with white spruce.

White spruce is extremely hardy to low temperatures, provided the plant is in a state of winter dormancy. Throughout the greater part of its range, white spruce routinely survives and is undamaged by winter temperatures of −50 °C (−58 °F), and even lower temperatures occur in parts of the range (Fowells 1965, Nienstaedt and Zasada 1990).[7][45] Boreal Picea are among the few extremely hardy conifers in which the bud primordia are able to survive temperatures down to −70 °C, −94 °F (Sakai and Larcher 1987).[101]



Especially important in determining the response of white spruce to low temperatures is the physiological state of the various tissues, notably the degree of "hardening" or dormancy. A natural progression of hardening and dehardening occurs in concert with the seasons (Glerum 1985).[102] While different tissues vary in ability to tolerate exposure to stressful temperatures, white spruce, as with woody plants in general, has necessarily developed sufficient winter hardiness in its various tissues to enable them to survive the minimum temperatures experienced in the distribution range.



White spruce is subject to severe damage from spring frosts. Newly flushed shoots of white spruce are very sensitive to spring frost (Smith 1949, Rowe 1955, McLeod 1964).[103][104][105] This sensitivity is a major constraint affecting young trees planted without overstorey nurses in boreal climates (Sutton 1992).

Growth varies with site quality. In swamp and muskeg it shows progressively slower growth rates from the edges toward the centre. The roots are shallow and wide spreading, resulting in susceptibility to windthrow. In the northern part of its range, ice pruned asymmetric black spruce are often seen with diminished foliage on the windward side.[10] Tilted trees colloquially called "drunken trees" are associated with thawing of permafrost.[3][11]



In the southern portion of its range it is found primarily on wet organic soils, but farther north its abundance on uplands increases. In the Great Lakes States it is most abundant in peat bogs and swamps, also on transitional sites between peatlands and uplands. In these areas it is rare on uplands, except in isolated areas of northern Minnesota and the Upper Peninsula of Michigan.

The frequent fire return interval, a natural fire ecology, perpetuates numerous successional communities. Throughout boreal North America, Betula papyrifera (paper birch) and Populus tremuloides (quaking aspen) are successional hardwoods that frequently invade burns in black spruce. Black spruce typically seeds in promptly after fire, and with the continued absence of fire, eventually dominates the hardwoods.

Pinus banksiana ranges from 9–22 m (30–72 ft) in height. Some jack pines are shrub-sized, due to poor growing conditions. They do not usually grow perfectly straight, resulting in an irregular shape similar to pitch pine (Pinus rigida). This pine often forms pure stands on sandy or rocky soil. It is fire-adapted to stand-replacing fires, with the cones remaining closed for many years, until a forest fire kills the mature trees and opens the cones, reseeding the burnt ground.

Larix laricina is a small to medium-size boreal coniferous and deciduous tree reaching 10–20 m (33–66 ft) tall, with a trunk up to 60 cm (24 in) diameter. Tamaracks and larches (Larix species) are deciduous conifers. The bark is tight and flaky, pink, but under flaking bark it can appear reddish. The leaves are needle-like, 2–3 cm (3⁄4–1 1⁄4 in) short, light blue-green, turning bright yellow before they fall in the autumn, leaving the pale pinkish-brown shoots bare until the next spring.

Tamaracks are very cold tolerant, able to survive temperatures down to at least −65 °C (−85 °F), and commonly occurs at the Arctic tree line at the edge of the tundra. Trees in these severe climatic conditions are smaller than farther south, often only 5 m (15 ft) tall. They can tolerate a wide range of soil conditions but grow most commonly in swamps, bogs, or muskegs, in wet to moist organic soils such as sphagnum, peat, and woody peat. They are also found on mineral soils that range from heavy clay to coarse sand; thus texture does not seem to be limiting. Although tamarack can grow well on calcareous soils, it is not abundant on the limestone areas of eastern Ontario.



Tamarack is generally the first forest tree to grow filled-lake bogs. In the lake states, tamarack may appear first in the sedge mat, sphagnum moss, or not until the bog shrub stage. Farther north, it is the pioneer tree in the bog shrub stage. Tamarack is fairly well adapted to reproduce successfully on burns, so it is one of the common pioneers on sites in the boreal forest immediately after a fire.[7]



The central Alaskan population, separated from the eastern Yukon populations by a gap of about 700 kilometres (430 mi), is treated as a distinct variety Larix laricina var. alaskensis by some botanists, though others argue that it is not sufficiently distinct to be distinguished.