Viability of Engelmann spruce seed is rated good and the vitality persistent. The average germinative capacity of spruce is higher than for many associated species (102):
Species Average germinative
capacity Engelmann spruce 69 Subalpine fir 31-34 Lodgepole pine 65-80 Western white pine 44 Interior Douglas-fir 60-93 Western larch 57 Grand fir 46-57 Western hemlock 53-56 Pacific silver fir 20-26 White fir 30-37 Viable seeds of spruce that survive over winter normally germinate following snowmelt when seedbeds are moist and air temperature is at least 7° C (45° F). Field germination of spruce over long periods in Colorado have ranged from 0 to 28 percent of the sound seeds dispersed, depending upon the seedbed and environmental factors (9,73).
In the undisturbed forest, spruce seeds germinate and seedlings become established on duff, litter, partially decomposed humus, decaying wood, and mounds of mineral soil upturned by windthrown trees. Any disturbance that removes the overstory produces new microhabitats (80). Under these circumstances, germination and initial establishment are generally better on prepared mineral soil, and disturbed mineral soil and humus seedbeds because moisture conditions are more stable (27,35,41,73,94). However, initial survival of spruce on severe sites at high elevations in the Intermountain Region was higher on duff seedbeds than on mineral soil seedbeds (37). Spruce seedling establishment on burned seedbeds has been variable. Success is related to severity of burn, depth of ash, and amount of exposed mineral soil (29,80,91). Regardless of the seedbed, high initial mortality usually slows establishment of seedlings. Once established (at least 5 years old), the ability to survive is not increased by a mineral soil seedbed, but is favored by adequate soil moisture, cool temperature, and shade.
Engelmann spruce will germinate in all light intensities found in nature, but 40 to 60 percent of full shade is most favorable for seedling establishment at high elevations. Light intensity and solar radiation are high at elevations and latitudes where spruce grows in the central and southern Rocky Mountains, and seedlings do not establish readily in the open. Planted seedlings often develop a chlorotic appearance that has been attributed to solarization-a phenomenon by which light intensity inhibits photosynthesis and which ultimately results in death (82). Mortality can be reduced by shading seedlings. At low elevations and high latitudes in the northern Rocky Mountains, spruce can become established and survive in the open (17). Spruce can establish and survive better in low light intensities than its common, intolerant associates such as lodgepole pine, Rocky Mountain Douglas-fir, and aspen, but at extremely low light intensities it cannot compete favorably with such shade-enduring associates as the true firs and hemlocks (20).
Engelmann spruce is restricted to cold, humid habitats because of its low tolerance to high temperature and drought (25,45). However, solar radiation at high elevations heats soil surfaces [up to 66° C (150° F or more)] and increases water losses from both seedlings and soil by transpiration and evaporation (9,73,80).
Because of its slow initial root penetration and extreme sensitivity to heat in the succulent stage, drought and heat girdling kill many first-year spruce seedlings. Drought losses can continue to be significant during the first 5 years of seedling development, especially during prolonged summer dry periods (9,34,73).
Tree seedlings in the succulent stage are particularly susceptible to stem-girdling. The cortex is killed by a temperature of 54° C (130° F), but prolonged exposures to somewhat lower temperatures may also be lethal. On the Fraser Experimental Forest, heat-girdling caused much early seedling mortality on unshaded seedbeds (9,73). Soil-surface temperature exceeded 65° C (150° F) in the open on a north aspect and 71° C (160° F) on a south aspect at 3200 m (10,500 ft) elevation in June. Maximum air temperature during this period did not exceed 260 C (780 F). In western Montana, at low elevations, soil surface temperatures exceeded 71° C (160° F) on gentle north slopes several times during one summer (80). Early shade protection increased survival of newly germinated spruce seedlings; 30 to 50 percent of the seedlings were lost to heat-girdling on unshaded plots, compared to 10 percent on shaded plots. In southwestern Alberta, when newly germinated spruce seedlings were deprived of water, nearly three-fourths of the mortality on four different unshaded seedbed types was caused by heat-girdling (34). Surface temperatures as low as 45° C (113° F) caused heat girdling, but losses were not high until soil surface temperatures were above 50° C (122° F). Shading reduced heat-girdling on all seedbed types. Soil surface temperatures in excess of lethal levels for spruce seedlings, especially on burned seedbeds, have been reported in British Columbia (94).
Air and soil temperatures (below the surface) are not usually directly responsible for seedling mortality, but they affect growth. In a growth chamber study of Engelmann spruce seedlings under 30 different combinations of day and night temperatures, the greatest height and root growth, and top and root dry matter production was with a diurnal variation of 19° C (66° F) (air and soil) day temperatures and 23° C (73° F) (air and soil) night temperatures (45). Shepperd (92), using the same night temperature regime, raised the day soil temperature to 23° C (72° F) and significantly increased root growth.
Frost can occur any month of the growing season where spruce grows. It is most likely to occur in depressions and cleared openings because of cold air drainage and radiation cooling. Newly germinated spruce seedlings are most susceptible to early fall frosts. In a greenhouse and laboratory study, new seedlings did not survive temperatures as low as -9.5° C (15° F) until about 10 weeks old (71). Terminal bud formation began at 8 weeks; buds were set and needles were mature at 10 to 12 weeks after germination.
After the first year, seedlings are most susceptible to frost early in the growing season when tissues are succulent. Shoots are killed or injured by mechanical damage resulting from tissue freezing and thawing. Frost damage has been recorded in most years in Colorado (81). In light frost years, damage was minor, but heavy frosts either damaged or killed all new shoots of open-grown seedlings.
In early fall, the combination of warm daytime temperatures, nighttime temperatures below freezing, and saturated soil unprotected by snow are conducive to frost-heaving. On the Fraser Experimental Forest, Colorado, these conditions generally occurred about 1 out of 2 years (9,73). Frost-heaving has been one of the principal causes of first-year seedling mortality on scarified seedbeds on north aspects (9). Furthermore, seedlings continue to frost-heave after four growing seasons. Shading has reduced losses by reducing radiation cooling.
The moisture condition of the seedbed during the growing season largely determines first-year seedling survival. On some sites in the central Rocky Mountains, summer drought causes great first-year mortality, especially in years when precipitation is low or irregular. On the Fraser Experimental Forest in the central Rocky Mountains, drought and desiccation caused more than half the first-year seedling mortality on south aspects, and nearly two-thirds of the total after 5 years. On north aspects during the same period, drought accounted for about 40 percent of first-year seedling mortality, and more than half the mortality at the end of 5 years (9).
In the northern Rocky Mountains, late spring and early summer drought is a serious threat most years to first-year seedlings. In western Montana, all seedlings on one area were killed by drought in a 2-week period in late summer when their rate of root penetration could not keep pace with soil drying during a prolonged dry period (80). Late spring and early summer drought is also a serious cause of first-year seedling mortality in the southern Rockies. Drought losses can continue to be significant throughout the Rocky Mountains during the first 5 years of seedling development, especially during prolonged summer dry periods (9,73).
The moisture provided by precipitation during the growing season is particularly critical to seedling survival during the first year. A greenhouse study of the effects of amount and distribution of moisture on seedling survival (simulating common summer precipitation patterns in north-central Colorado) showed that under favorable seedbed and environmental conditions: (1) at least 2.5 cm. (1 inch) of well distributed precipitation is needed monthly before seedlings will survive drought; (2) with this precipitation pattern, more than 3.75 cm (1.5 in) of monthly rainfall is not likely to increase seedling survival; but (3) few seedlings will survive drought with less than 5 cm (2 in) of rainfall monthly when precipitation comes in only one or two storms (18).
Summer precipitation may not always benefit seedling survival and establishment. Summer storms in the Rocky Mountains may be so intense that much of the moisture runs off, especially from bare soil. Moreover, soil movement on unprotected seedbeds buries some seedlings and uncovers others (80).
Understory vegetation can be either a benefit or serious constraint to spruce seedling establishment (2,35,83). Spruce seedlings become established more readily on sites protected by willows (Salix spp.), shrubby cinquefoil (Potentilla fruiticosa), fireweed, and dwarf whortleberry than in the open. Because these plants compete less aggressively for available soil moisture than those listed below, the net effect of their shade is beneficial to seedling survival. In contrast, mortality occurs when spruce seedlings start near clumps of grass or sedges or scattered herbaceous plants such as mountain bluebells, currants (Ribes spp.), and Oregongrape that compete severely for moisture and smother seedlings with cured vegetation when compacted by snow cover (83).
The only significant biotic factor affecting spruce regeneration on a long-term study on the Fraser Experimental Forest was birds. About 15 percent to 20 percent of the total mortality resulted from the clipping of cotyledons on newly germinated seedlings by grey-headed juncos (Junco caniceps) (9,73,75).
Damping-off, needlecast, snowmold, insects, rodents, and trampling and browsing by large animals also kill spruce seedlings, but losses are no greater than for any other species (20).
The number of seeds required to produce a first-year seedling and an established seedling (5 years old) and the number of first-year seedlings that produce an established seedling vary greatly, depending upon seed production, distance from source, seedbed, and other environmental conditions. In one study in clearcut openings in Colorado during the period 1961-1975, covering a wide variety of conditions, on the average 665 sound seeds (range 602,066) were required to produce one first-year seedling, and 6,800 (range 926-20,809) to produce a seedling 4 or more years old. An average of 21 first-year seedlings was necessary to produce a single seedling 4 or more years old, although as few as 4 and as many as 24 first-year seedlings survived under different conditions (74).
Aspect and cultural treatments can also affect establishment of Engelmann spruce. In another Colorado study (covering the period 1969-1982), an average of 18 sound seeds was required to produce a single first-year seedling on shaded, mineral soil seedbeds on a north aspect; and 32 sound seeds were needed to produce a 5-year-old seedling. In contrast, 156 seeds were required to produce a first-year seedling on shaded, mineral soil seedbeds on a south aspect, and 341 seeds to produce a 5-year-old seedling (8,9). Shearer (91), studying the effects of prescribed burning and wildfire after clearcutting on regeneration in the western larch type in Montana, also found that natural and planted spruce survived better on the north aspect than on the south aspect.
Environmental conditions favorable and unfavorable to the establishment of Engelmann spruce natural regeneration are summarized in Figure 1.
Favorable Unfavorable Seed crop
More than 600,000 seeds pe hactare (242,800/acre Less than 60,000 seeds per hectare (24,300/acre)
Aspect North South Temperatures
Ambient air more than 0° C (32° F) night and less than 25° C (77° F) day; maximum surface less than 30° C (86° F)
Ambient air less than 0° C (32° F) night and more than 25° C (77° F) day; maximum surface greater than 30° C (86° F) Precipitation
More than 10 mm (0.4 in) per week Less than 10 mm (0.4 in) per week Soil
Light-textured, sandy-loam Heavy-textured, Clay-loam Seedbed
50 percent exposed mineral soil, 40 to 60 percent dead shade, Duff and litter less than 5 cm (2 in), Light vegetative cover 10 percent or less exposed mineral soil, 10 percent or less dead shade, Duff and litter more than 5 cm (2 in), Heavy vegetative cover Survival
Seedlings more than 12 weeks old by mid-September, Low population of birds and rodents that eat seeds and seedlings, Protection from trampling, Snow cover when frost-heaving conditions exist Seedlings less than 12 weeks old by mid-September, High population of birds and rodents that eat seeds and seedlings, No protection from trampling, No snow cover when frost-heaving conditions exist