|Mature white spruce in Alaska|
Picea glauca (white spruce) is a species of spruce native to the northern temperate and boreal forests in North America, from central Alaska to as far east as the Avalon Peninsula in Newfoundland, and south to northern Montana, Minnesota, Wisconsin, Michigan, northwestern Pennsylvania, upstate New York, Vermont, New Hampshire, and Maine; there is also an isolated population in the Black Hills of South Dakota and Wyoming. It is also known as Canadian spruce, skunk spruce, cat spruce, Black Hills spruce, western white spruce, Alberta white spruce, and Porsild spruce.
The white spruce is a large coniferous evergreen tree which grows normally to 15 to 30 metres (49 to 98 ft) tall, but can grow up to 40 metres (130 ft) tall with a trunk diameter of up to 1 metre (3.3 ft). The bark is thin and scaly, flaking off in small circular plates 5 to 10 centimetres (2.0 to 3.9 in) across. The crown is narrow - conic in young trees, becoming cylindric in older trees. The shoots are pale buff-brown, glabrous (hairless) in the east of the range, but often pubescent in the west, and with prominent pulvini. The leaves are needle-like, 12 to 20 millimetres (0.47 to 0.79 in) long, rhombic in cross-section, glaucous blue-green above with several thin lines of stomata, and blue-white below with two broad bands of stomata.
The cones are pendulous, slender, cylindrical, 3 to 7 centimetres (1.2 to 2.8 in) long and 1.5 centimetres (0.59 in) wide when closed, opening to 2.5 centimetres (0.98 in) broad. They have thin, flexible scales 15 millimetres (0.59 in) long, with a smoothly rounded margin. They are green or reddish, maturing to pale brown 4 to 8 months after pollination. The seeds are black, 2 to 3 millimetres (0.079 to 0.118 in) long, with a slender, 5 to 8 millimetres (0.20 to 0.31 in) long pale brown wing.
White spruce is the northernmost tree species in North America, reaching just north of 69°N latitude in the Mackenzie River delta. It grows between sea level and an elevation of 1,520 metres (4,990 ft). Its northern distribution roughly correlates to the location of the tree line, which includes an isothermic value of 10 °C (50 °F) for mean temperature in July, as well as the position of the Arctic front; cumulative summer degree days, mean net radiation, and the amount of light intensities also figure. White spruce generally is found in regions where the growing season exceeds 60 days annually.
The southern distribution corresponds to the July isotherm of 18 °C (64 °F) around the Great Lakes; in the Prairie Provinces its limit is north of this isotherm. During the summer solstice, photoperiod values range from 17 hours at its southern limits to 24 hours above the Arctic Circle.
White spruce is a climax canopy tree in the boreal forests of Canada and Alaska. It generally occurs on well-drained soils in alluvial and riparian zones, although it also occurs in soils of glacial and lacustrine origin. The understory is dominated by feather mosses (Hylocomium splendens and Pleurozium schreberi, Ptilium crista-castrensis, and Dicranum spp.), and occasionally peat moss. In the far north, the total depth of the moss and underlying humus is normally between 25 to 46 centimetres (9.8 to 18.1 in), although it tends to be shallower when hardwoods are present in the stand.
White spruce grows in soils with pH values of 4.7—7.0, although they have been found in soils as acidic as 4.0 in subalpine fir forests in the Northwest Territories. A presence of calcium in the soil is common to white spruce found in northern New York. White spruce most commonly grows in the soil orders of Alfisols and Inceptisols. Soil properties such as fertility, temperature, and structural stability are partial determinants of the ability of white spruce to grow in the extreme northern latitudes. In the northern limits of its range, white spruce is the climax species along with black spruce; Birch and aspen are the early succession species. Wildfires typically occur every 60 to 200 years, although they have been known to occur as infrequently as every 300 years.
White Spruce will grow in USDA Growing Zones 3-7, but is not adapted to heat and humidity and will perform poorly in a hot climate. The tree attains its greatest longevity and growth potential in Zones 3-4.
White spruce occurs on a wide variety of soils, including soils of glacial, lacustrine, marine, and alluvial origins; overlying basic dolomites, limestones and acidic Precambrian and Devonian granites and gneisses; and Silurian sedimentary schists, shales, slates, and conglomerates (Halliday 1937). The wide range of textures accommodated includes clays (Wilde et al. 1949, 1954; Nienstaedt 1957, Rowe 1972), even those that are massive when wet and columnar when dry (Jameson 1963), and sand flats, and coarse soils (Forest Section L 4d, Rowe 1972). Very large white spruce are found on soils consisting of layers of rich silts interspersed with layers of organic matter, the result of periodic spring floods (J.M. Robinson, Canadian Forest Service, Ottawa, Ontario, personal communication, 17 Sept. 1969). Its occurrence on some organic soils is not characteristic, except perhaps on shallow mesic organic soils in Saskatchewan and in association with black spruce on organic soils in central Yukon (Nienstaedt and Zasada 1990).
Podzolized, brunisolic, luvisolic, gleysolic, and regosolic (immature) soils are typical of those supporting white spruce throughout the range of the species (Nienstaedt 1957). Soils supporting white spruce are most commonly Alfisols or Inceptisols (Nienstaedt and Zasada 1990). In the podzol region of Wisconsin, white spruce occurs on loam podzols, podzolized gley loams, strongly podzolized clays, gley-podzol clays, stream-bottom soils, and wood peat (Wilde et al. 1949). Moist sandy loams also support good growth (Harlow and Harrar 1950). On sandy podzols (Wilde et al. 1949), it is usually a minor species (Nienstaedt and Zasada 1990). Good development occurs on moist alluvium (Seeley, cited by Nienstaedt 1957; Jeffrey 1961, 1964; Lacate et al. 1965; Viereck 1973) on the banks of streams and borders of swamps (Sargent 1898, Kenety 1917, Rowe 1972). White spruce makes good growth on well-drained lacustrine soils in Alberta Mixedwoods (Heger 1971), on moderately-well-drained clay loams in Saskatchewan (Kabzems 1971), and on melanized loams and clays (with sparse litter and a dark-coloured organically-enriched mineral horizon) in the Algoma district of Ontario (Wilde et al. 1954).
White spruce becomes less accommodating of soil with increasing severity of climate. The distribution of white spruce in Labrador seems to depend almost entirely on the character of the soil (Sargent 1898), and between the southwestern shores of Hudson Bay and the northeastern regions of Saskatchewan, white spruce is confined to very local physiographic features, characterized by well-drained or fertile soils (Ritchie 1956). Robinson (Canadian Forest Service, Ottawa, Ontario, personal communication 17 Sept. 1969) called white spruce the black walnut of the north where “it only does well on the very best sites”, yet “white spruce near the mouth of the Mackenzie River can be found growing very well on not more than 12” [30 cm] of active soil over permafrost, provided there is no moss cover and the soil surface is warm. If it is covered with an insulating layer of mosses, especially sphagnum, the tree growth practically ceases”.
On dry, deep, outwash deposits in northern Ontario, both white spruce and aspen grow slowly (MacLean 1960). But, broadly, white spruce is able to tolerate considerable droughtiness of sites that are fertile, and no fertile site is too moist unless soil moisture is stagnant (Sutton 1968). Soil fertility holds the key not just to white spruce growth but to the distribution of the species. At least moderate fertility is needed for good growth, but white spruce occurs on many sites where nutrient deficiencies depress its growth more than that of black spruce, red spruce, Norway spruce, and the pines generally (Heiberg and White 1951, Lafond 1954, McLeod 1956, MacArthur 1957, Paine 1960, Swan 1960). White spruce performed much more poorly than Japanese larch in an afforestation trial on abandoned, hill-top farmland in New York State, an “adverse” site (Cook and Schierbaum 1948). Minimum soil-fertility standards recommended for white spruce sufficient to produce 126 to 157 m3/ha of wood at 40 years are much higher than for pine species commonly planted in the Lake States (Wilde 1966): 3.5% organic matter, 12.0 meq/100 g exchange capacity, 0.12% total N, 44.8 kg/ha available P, 145.7 kg/ha available K, 3.00 meq/100 g exchangeable Ca, and 0.70 meq/100 g exchangeable Mg.
Forest floors under stands dominated by white spruce respond in ways that vary with site conditions, including the disturbance history of the site (Nienstaedt and Zasada 1990). Composition, biomass, and mineral soil physical and chemical properties are affected. In Alaska, the accumulation of organic layers (to greater thicknesses in mature stands of spruce than those in hardwood stands on similar sites) leads to decreased soil temperatures, in some cases leading to the development of permafrost (Viereck 1970a, b, Viereck et al. 1983). “Along the Canol Road in the Yukon many of the white spruce had litter accumulation of as much as 12” [30 cm] near the tree trunks... but the pH of that litter was no higher than that of aspen [litter]” (Robinson, J.M., Canadian Forest Service, Ottawa, Ontario, personal communication 17 Sept. 1969). Acidity of the mineral soil sampled at an average depth of 17 cm in 13 white spruce stands on abandoned farmland in Ontario increased by 1.2 pH units over a period of 46 years (Brand et al. 1986).
A considerable range of soil pH is tolerated by white spruce (Nienstaedt 1957). Stone, in personal communication with Nienstaedt (1957), reported white spruce on some soils as acid as pH 4.5 and on others as alkaline as pH 7.5 in the surface layers. Thrifty stands of white spruce in Manitoba have developed on soils of pH 7.6 at only 10 cm below the surface, and pH 8.4 at 43 cm below the surface (Stoeckeler 1938, USDA Forest Service 1938); rooting depth in those soils was at least 81 cm. An abundant calcium supply is common to most white spruce locations in New York state (Nienstaedt and Zasada 1990). Chlorosis was observed in young white spruce in heavily-limed nursery soils at about pH 8.3 (Stone, cited by Nienstaedt 1957). Wilde (1966) gave 4.7 to 6.5 as the approximate optimum range of pH for white spruce in Wisconsin, but optimum growth seems possible at pH levels up to 7.0 and perhaps higher (Sutton 1968). Alluvium on the floodplains of northern rivers shows pH levels from 5.0 to 8.2 (Zasada et al. 1977). High-lime ecotypes may exist (Pelletier 1966), and in Canada Forest Section B8 the presence of balsam poplar and white spruce on some of the moulded moraines and clays seems to be correlated with the considerable lime content of these materials (Rowe 1972, Stiell 1976), while calcareous soils are favourable sites for northern outliers of white spruce (Hustich 1953).
Mature stands of white spruce in boreal regions often have well-developed moss layers dominated by feather mosses, e.g., Hylocomium splendens (Hedw.) B.S.G., Pleurozium schreberi (Brid.) Mitt., Ptlium crista-castrensis (Hedw.) De Not., and Dicranum Hedw. spp. rather than Sphagnum Dill. spp. (La Roi and Stringer 1976, Viereck 1987). The thickness of the moss–organic layer commonly exceeds 25 cm in the far north and may approach twice that figure. The mosses compete for nutrients and have a major influence on soil temperatures in the rooting zone. Permafrost development in parts of Alaska, Yukon, and the Northwest Territories is facilitated by the insulative organic layer (Viereck 1970a, b, Gill 1975, Van Cleve and Yarie 1986). The role of windthrow in maintaining diversification of the bryophyte flora in boreal spruce forests has been described by Jonsson et al. (1990) and Jonsson and Dynesius (1993).
The interaction among permafrost, fire, and white spruce was described by Jack Robinson (J.M. Robinson, Can. Dep. For., For. Res. Branch, Ottawa, Ontario; notes in File Project H-123 on Symposium on the management of black spruce in Ontario, March 1962):
- “At Norman Wells there are excellent white spruce stands on the islands of the Mackenzie River and a narrow band of good [white] spruce near the river banks. Behind this, the forest changes to a muskeg-type black spruce with a deep moss cover and permafrost close to the surface of the mineral soil. Up the slopes of the Franklin Mountains a similar condition was found until an area covered by a recent severe fire was reached. This had burnt all of the moss cover to the mineral soil surface and there was no permafrost [here] at the end of July at a depth of 42 inches [107 cm]. Farther up the slope and above the burnt area the sphagnum, scrub black spruce and permafrost was again found…A gravel bar on the hillside caused most probably by an old beach line bore fast growing white spruce. This good stand probably resulted from a previous fire...”
Viereck et al. (1983) studied the vegetation, soils, and forest productivity in a range of forest stands in the taiga of interior Alaska. The stands were arranged on an environmental gradient from a trembling aspen stand on a dry, steep, south-facing bluff, to open black spruce stands underlain by permafrost on north-facing slopes. The coldest site was a mixed white spruce and black spruce woodland at the treeline. Mesic upland were represented by successional stands of white birch, trembling aspen, and highly productive stands of white spruce. Several floodplain stands represented the successional sequence from productive balsam poplar and white spruce to black spruce stands underlain by permafrost on the older terraces. Viereck et al. (1983) described the environmental gradient by using 2 soil factors, soil moisture and annual accumulated soil degree days (SDD), which ranged from 2217 SDD for the warmest trembling aspen stand to 480 SDD for the coldest permafrost-dominated black spruce site. Oils varied from alfic cryochrepts on most of the mesic sites to histic pergelic cryochrepts on the colder sites underlain by permafrost. A black spruce stand on permafrost has the lowest tree standing crop (1586 g/m2)and annual productivity (56 g/m2) whereas a mature white spruce stand had the greatest tree standing crop (24 577 g/m2) and an annual productivity of 540 g/m2, little more than half that of the successional balsam poplar stand on floodplain alluvium.
The depth of freezing and speed of thawing of soil under dense stands of Norway spruce in boreal Komi (one of the federated republics of Russia), west of the Urals, were studied by Deryugin (1989) on fresh and moist sites. The soil froze deepest (average 47 cm) on the fresh site, but on the moist site averaged only 7 cm in depth of freezing. Nearby arable land froze to a depth of 12 cm, birch forest to 32 cm. Snow depth and soil moisture content were the most influential factors governing depth of freezing. In birch forest, soil thawed mostly before the snow melted, whereas in the spruce forest the soil thawed mostly after snowmelt. In dense spruce forest, the rate of soil thawing averaged 1.8 cm per day, which was 1.1 cm per day less than in birch forest, where thawing was completed 27 days earlier than in spruce forest.
Floodplain deposits in the Northwest Territory, Canada, are important in relation to the development of productive forest types with a component of white spruce (Jeffrey 1964). The most recently exposed surfaces are occupied by sandbar vegetation or riparian shrub willows and alder (Alnus incana); with increasing elevation, the shrubs give way successively to balsam poplar and white spruce forest. In contrast, older floodplains, with predominantly Brown Wooded soils, typically carry white spruce–trembling aspen mixedwood forest.
Populus balsamifera–Equisetum hiemale and P. balsamifera–Equisetum pratense occur on point-bar deposits close to the river, with the former somewhat more favourable to the establishment of white spruce seedlings than the latter. P. balsamifera–Equisetum pratense and P. balsamifera–Picea glauca–Equisetum pratense forest are vulnerable to stand deterioration in consequence of their unreceptive conditions for white spruce establishment. Open, decadent stands are common. Jeffrey (1964) suggested possible pathways of physiographic-vegetational change on point-bar deposits of recent Liard River floodplains in the Northwest Territories.
The terraces, ancient and modern, along the Liard River, have their origin as floodplain deposits, subsequently dissected by erosion, and occur at various elevations up to 300 m above river level. The most common forests on the terraces are mixedwood and mixed broadleaf, and generally lack white spruce except on steep slopes that connect one terrace level to another. White spruce–balsam poplar forest occurs on small localized floodplains of secondary streams (Jeffrey 1964).
Interrelationships among nutrient cycling, regeneration, and subsequent forest development on floodplains in interior Alaska were addressed by Van Cleve et al. (1980), who pointed out that the various stages in primary succession reflect physical, chemical, and biological controls of ecosystem structure and function. Thus, each successional stage has a species combination in harmony with site quality. Short-circuiting succession by planting a late successional species such as white spruce on an early successional surface may result in markedly reduced growth rates because of nitrogen insufficiency. Without application of substantial amounts of fertilizer, use would have to be made of early successional alder and its site-ameliorating additions of nitrogen.
Neiland and Viereck noted that “The slow establishment and growth of spruce under birch stands [in Alaska] may be partially due to effects of shading and general competition for water and nutrients, but may also be more directly related to the birch itself. Heikinheimo (1915 cited by Lutz 1956), found that birch ash inhibited white spruce seedlings, and Gregory (1966) found that birch litter has a smothering effect on spruce seedlings” (Neiland and Viereck 1977).
On dry upland sites, especially south-facing slopes, the mature vegetation is white spruce, white birch, trembling aspen, or a combination of these species. Succession follows in 1 of 2 general patterns. In most cases, aspen and birch develop as a successional stage after fire before reaching the spruce stage. But, occasionally, with optimal site conditions and a source of seed, white spruce will invade with the hardwoods or within a few years thereafter, thereby producing even-aged white spruce stands without an intervening hardwood stage.
- Picea glauca var. glauca (Typical or Eastern white spruce). From Newfoundland west to eastern Alberta, on lowland plains.
- Picea glauca var. densata (Black Hills white spruce). The Black Hills in South Dakota.
- Picea glauca var. albertiana (Alberta white spruce). The Rocky Mountains in Alberta, British Columbia and northwest Montana.
- Picea glauca var. porsildii (Alaska white spruce). Alaska and Yukon.
The two western varieties are distinguished by pubescent (downy) shoots, and may be related to extensive hybridisation and/or intergradation with the closely related Engelmann Spruce found further south in the Rocky Mountains. White spruce also hybridises readily with the closely related Sitka Spruce where they meet in southern Alaska; this hybrid is known as Picea × lutzii.
A dwarf cultivar, P. glauca var. albertiana 'Conica', is a popular garden plant. It has very slender leaves, like those normally found only on one-year-old seedlings, and very slow growth, typically only 2–10 centimetres (0.79–3.94 in) per year. Older specimens commonly 'revert', developing normal adult foliage and starting to grow much faster; this 'reverted' growth must be pruned if the plant is to be kept dwarf.
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