Picea glauca , white spruce

This review synthesizes the fire effects information and relevant ecology of white spruce in North America that was available in the scientific literature as of 2015. Details and documentation of source materials follow this summary. Introductory: The Introductory section discusses the taxonomy of white spruce, including hybrids and synonyms. Distribution and Occurrence: Although white spruce communities are widespread throughout the North American boreal region, the species is less prominent in the moist, eastern part of the continent and becomes increasingly prominent in the drier western and northwestern regions. Associated trees across much of its range include quaking aspen, paper birch, black spruce, and balsam poplar. White spruce typically occurs on riparian, upland, and treeline sites, although it occurs on a wide range of other sites as well. It grows best on well-drained soils but occurs on a variety of land forms and soil types, with many different plant associates. Botanical and Ecological Characteristics: White spruce crowns are usually densely foliated, and branches and needles are often retained low on the trunk. The arrangement of these vertically continuous branches may promote ignition and torching. White spruce stand structure ranges from very open to closed. Seed production is highly variable; mast years occur episodically, about every 10 to 12 years. Favorable seedbeds include mineral soil, thin organic soil, and rotten logs. White spruce establishment occurs throughout stand development, including early and late postfire succession. It is generally considered a mid- to late-successional species, but it occurs in all stages of boreal forest succession. Fire Effects and Management: Fire of any severity generally kills white spruce. After fire, white spruce typically establishes from seed from trees along fire edges or unburned trees within the burn area. Establishment depends on seed availability, seedbed conditions, fire characteristics, site and soil characteristics, and weather. White spruce abundance may be drastically reduced after fire because of high tree mortality and limited postfire recruitment. White spruce forests may be highly flammable, although less so than black spruce forests. Among white spruce sites, floodplain sites are less flammable than upland sites. Stands with more hardwoods are less flammable than those with fewer hardwoods. Across white spruce's distribution, fires tend to be more frequent in drier western regions than in wetter eastern regions. White spruce tends to be limited to areas that burn infrequently; white spruce communities often have less frequent fire than other boreal forest types. The longest fire-rotation intervals in the western boreal region are likely in floodplain white spruce stands, where they may be about 300 years. Stands experience crown, surface, and ground fires, and nearly all fires in white spruce communities are stand-replacement. Across a landscape, white spruce communities often display a mosaic of unburned patches and burned patches with stand replacement. Most fires in white spruce stands are small, but large wildfires occurring in extreme fire years account for most acreage burned. Across Arctic and boreal regions, the area burned, fire behavior, and fire severity in white spruce-dominated communities will likely increase with global climate changes, although predicted future fire occurrence varies spatially throughout the region. Prescribed fire is often used to consume logging slash, improve seedbed conditions, and promote regeneration after white spruce stands are logged. However, white spruce regeneration is variable and often inadequate, because prescribed fires often fail to consume sufficient organic material to create large areas of suitable seedbed. Management Considerations: A wide variety of wildlife species use white spruce communities as habitat. Some animals prefer specific postfire successional stages. For example, moose, woodpeckers, and northern hawk owls use early successional stages, while caribou typically use late-seral woodlands. While most wildlife species avoid white spruce browse, it provides important winter forage for some species. Many wildlife species eat the seeds. Climate change is expected to have varied effects on white spruce distribution, growth, and establishment. The pattern, direction, and timing of change depend on local, landscape, and regional climatic and site conditions. Under a warmer climate, many studies suggest that white spruce may expand into areas formerly underlain by permafrost or beyond its current latitudinal and/or altitudinal extent, increase in density at the forest-tundra ecotone, or experience increased growth rates. Other studies suggest that many white spruce communities may be negatively affected by a warming and drying climate.