White, black and Engelman spruce are native spruce trees in Alberta, and along with introduced spruce species ( Blue, Norway, Serbian, and many other cultivars) are one of the most dominant coniferous species in natural as well as in urban areas. Spruce seeds are not just a source of future trees and forests but also a food source for numerous species of animals, including seed-eating birds, small mammals, and insects.
The year when there is high cone and seed production is also called “ mast year” or “mast event”. Spruce seed production is highly variable from year to year, and it depends on geographical location, age, environmental conditions, and genetics. White spruce produces good crop every 8-12 years but there are several exceptions when it produces crop every 2-6 years and in some extreme cases every year ( mostly due to constant stress that trees experience). Blue spruce is a good to prolific seed producer, producing full cone crops every 2 to 3 years. Black spruce usually produces cones at a younger age than white spruce and produces some seeds every year.
Under the normal natural forest conditions, spruce trees start producing viable seeds between 30-40 years of age but in some urban areas trees, they may start producing seeds at age 4.
Large spruce trees growing in open areas tend to produce large cones with higher seed production. Dominant trees growing in natural forests also produce more cones than trees that are shaded. Seed cones are typically located in the upper one-third of the tree crown.
Climate conditions are the most influential on annual spruce seed production. Most of the research indicates that large seed crops are associated with drought ( warm, hot, and dry weather) from previous growing seasons ( 2021 drought in prairie provinces). In urban centers, spruce trees produce seeds more frequently due to other environmental stresses such as heat (urban heat island),poor soils, lack of moisture and pollution.
Cone and Seed production
The number of cones per tree is highly variable but can range from 100 to 2000 per tree. The total number of seeds per cone varies significantly among trees and ranges from 32 to 130 with approximately 500,000 seeds per kilogram.
However, high cone and seed production does not necessarily mean high seed viability. Seed viability is lower during warmer temperatures; while seed viability is higher when the seedfall year was both warm and wet.
Most of the spruce seeds fall in September while a small number of seeds are usually dispersed in August. Initiation and pattern of seed dispersal depend on the weather with most of the cones opening during dry weather. Cool, wet, or snowy weather delays the onset of dispersal and causes cones to close after dispersal has begun.
Wind is a primarily dispersal method but the distance of flight for individual seeds was variable and depended on conditions at the time of dispersal. Water is also a method of dispersal. Squirrels also disperse seeds ( they can cache up to 16,000 cones) as they cut and cache cones before the cones open, but seeds collected by squirrels are unlikely to contribute to regeneration because caches are not suitable seedbeds and seeds rapidly lose viability after 1 or 2 years.
The seeds are winged, small, lightweight and most of them fall within 2 tree heights from parent trees. In open areas, seeds have greater dispersal and in late-dispersal period seeds may be blown over crusted snow and ice. In general, seeds that fall closer to the parent tree are more likely to be viable than seeds that are dispersed farther away. Most importantly, adequate seed sources must be near suitable seedbeds during favorable weather conditions.
Seed Banking and Germination
White spruce does not have serotinous cones, and seeds do not persist in the soil. Soil seed banking is not common because seeds do not remain viable in the soil for a long time.
Spruce seeds may remain viable for about 1 year while the viability is dropping steadily after the seeds ripen. High viability generally occurs in mast years with high seed production. Seed that is clean and dried may remain viable for up to 10 years in storage.
Seed grows best on well drained, moist, silty soils, but occurs on many soil types. Spruce can germinate on mineral and moist soil, organic soil, rotten logs, and moss, however spruce seedlings establish best on mineral or thin organic soils that are disturbed by either fire or site floor preparation/disturbances. Urban areas with turf grass as well as leaf litter, make poor seedbeds because they dry out easily. The seedlings will tolerate acid soils of pH 4.5 but have a very low tolerance to soil salinity.
Cone and seed production of various spruce trees is variable and depends on environmental conditions as well as genetics. Overall seed viability is high during the mast year, but it rapidly declines within 1 to 2 years. Taking advantage of mast year seed production involves site preparation in the fall or early in the spring. This is the best way for successful natural spruce regeneration.
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