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How does disturbance shape Canada's forests?

Annually, insects, diseases, wildfires and drought affect more than 18 million hectares of forests in Canada. By killing trees, natural disturbances increase light penetration and disturb the soil to varying degrees, depending on the type of disturbance and its severity. These changes in turn allow new trees to sprout, grow and start a new forest succession – how whole forests grow back.

Succession shapes landscapes by producing various types of forests, with trees of different ages and species that contribute to biodiversity. By killing trees, natural disturbances help cycle nutrients and also provide habitat for numerous insects, birds and animals that feed on, or live in, dead wood. Natural disturbances generate a patchwork of different kinds of forests across Canada.

Natural disturbances from abiotic and biotic causes

Natural disturbances can be caused by abiotic factors (i.e. the non-living components of the environment, such as temperature, water or wind) or biotic factors (i.e. the living parts of ecosystems, such as insects, fungi or bacteria). Abiotic disturbances are caused by natural forces such as wind, water or fire. Biotic disturbances are caused by high populations of an organism, such as spruce budworm or mountain pine beetle. Abiotic and biotic disturbances have different characteristics.

Abiotic disturbances

  • are caused by extreme weather conditions and are nearly impossible to predict
  • last only a few hours or days and kill most trees during this time
  • kill healthy trees
  • affect all, or nearly all, tree species
  • physically disturb soils

Biotic disturbances

  • can be predicted through efficient monitoring
  • may last a decade, so trees take several years to die
  • kill weak trees because they are weakened over several years before death
  • usually affect a limited number of tree species
  • do not physically disturb soils, but insect outbreaks may enhance soil nutrient cycling through caterpillar feces
 
A large black beetle with antennae longer than its body resting on a tree trunk.
Whitespotted sawyer beetle

Natural disturbances and biodiversity

Contrary to popular belief, burned forests are not deserts for biodiversity. For example, during the weeks or months following a wildfire:

  • Blueberries and morel mushrooms rapidly colonize burned forests, while other plants, such as jack pine, only regenerate in soils burned by wildfire.
  • The number of insect species found in recently burned forests is double the number found in unburned forests.
  • Smoke attracts many insects to burned forests, and several insect species live in burned, but still-standing, trees called snags. These burn-associated insects are rarely found in green forests.
  • Bark provides insulation against heat and, even in charred trees, the wood under the bark remains nutritious for many wood-eating insects following a fire. This feature is especially true in trees with thick bark or after fires of low-to-moderate intensity.
  • In the boreal forest, after long-horned beetles colonize burned snags, there are sharp increases in the numbers of black-backed woodpeckers, which eat the beetles.

In contrast, insect outbreaks do not cause such rapid changes at the stand level. Biodiversity remains similar to that found when insect populations were at an endemic level and in balance with their environment. For example, few long-horned beetles are found in trees killed by spruce budworm, compared to trees killed by fire. There are no specific “outbreak-associated” insects, whereas there are many “burn-associated” insects. However, biodiversity changes gradually as an outbreak progresses, such as for the mountain pine beetle or spruce budworm outbreaks, which have affected millions of hectares of forest over the past decade.

A black and white woodpecker with a yellow crown patch perched on a tree trunk outside its nest cavity.
Black-backed woodpecker

Natural disturbances and wood quality

Each kind of natural disturbance results in a unique group of insects colonizing the resulting dead trees. These are called primary colonizing insects, and they are the first insects that make the dead trees their home. And in turn, each kind of these primary colonizing insects affect the wood quality of the dead trees differently.

For example, following a wildfire, the whitespotted sawyer beetle digs deep holes into wood, which rapidly reduces the timber value of the tree for salvage logging – harvesting trees after a natural disturbance. In contrast, trees killed by spruce budworm are colonized by insects and fungi that cause a progressive decline in wood fiber quality.

Ecologically, snag colonization by insects and fungi enhances wood decomposition but economically, this decreases wood quality and value. Salvage logging plans need to be adapted to each type of disturbance to avoid stands severely affected by wood-colonizing insects. This adaptation helps to maximize benefits for the sector, but also ensures biodiversity conservation because these stands are considered of high conservation value for biodiversity.

Sources and information
Photo credits
  • Whitespotted sawyer beetle photo by Jocelyn Lebel, Ministère des Forêts, de la Faune et des Parcs du Québec.
  • Black-backed woodpecker feeding his young stock photo by Frank Fichtmüller for iStockphoto/Getty images.

 

Table of contents — The State of Canada's Forests Report

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