Forest Lifecycle Stages

Just like people, forests evolve and change over time. With or without human intervention, forests progress through a lifecycle – they’re “born,” they grow, and they eventually die, at which point the cycle begins again.

Forests of every age play important roles. Each stage comes with advantages for people, plants and wildlife.


Age 0-15

Where sections of older forest have burned or been harvested, a young forest has room to emerge. In the case of harvesting, Alberta’s forestry companies are required to replant the area with a similar mix of trees and help the new forest establish itself. This responsibility lasts for a minimum of 14 years, or longer if needed for the forest to become self-sustaining.

Since the trees are still small and haven’t yet formed a canopy, the floor of a young forest is open to the sun. This allows a variety of plants to grow around the trees, and many animals find food or habitat in these open areas.


Deciduous trees, like trembling aspen, grow back very quickly on their own – they don’t need to be replanted. In fact, these trees can easily dominate a young forest, taking up the available sun and space. Coniferous trees don’t come back as easily on their own, and they grow much more slowly. Replanting conifers and helping the seedlings survive in their new home is part of what forestry companies do in the 14 years after harvest.


Plants that need sun – like grasses, honeysuckle and wild blueberry – thrive in young forests. The flowers and fruits of these plants are important to the survival of many insects, birds and animals.


Young forests provide food and habitat that sustains many forms of wildlife. White-tailed deer, elk, snowshoe hares, moose, foxes and beavers all find what they need in young forests. Grizzly bears also rely on young forests for food and often build dens near them – strategically creating more “edge habitat” for grizzlies, where young forests sit on the edge of older forests, has been a key part of re-establishing Alberta’s grizzly bear population.


Age 15-85

The middle stage of a forest’s life is the longest. Over the course of this stage, the forest continues to mature and evolve.

By this time, the forest is self-sustaining. Trees are established and a canopy forms, though at this stage it hasn’t fully closed. Trees grow larger over the course of this stage – reaching the prime of their life – and gradually, as the canopy becomes denser, less sun streams through to the forest floor.


By this time, the competition between deciduous and coniferous trees has evened out. In time, the odds start to reverse – although they take longer to get established, coniferous trees are specifically adapted to Alberta’s cold climate. The waxy coating on the needles of coniferous trees means they lose less water when exposed to high winds, and the fact that they keep their needles year-round is an advantage over deciduous species that lose their leaves every fall.


The middle stage of the forest lifecycle is a period of transition for plants. As sunlight becomes more limited, species that thrive in full sun begin to fade, and are replaced with understory plants better adapted to lower light. Mosses are an example of plant species that start to spread across the floor of a middle-to-older-aged forest.


Middle-aged forests provide a combination of cover and open space. This appeals to animals like The Canada Lynx, wolves, chipmunks, squirrels and rabbits, among others. The plants these smaller animals rely on for food are still available, but the increasing density of trees provides places to hide and burrow.


Age 85+

The trees in mature boreal forests have long since reached full size and tend to be densely packed together. Mature forests play a very important part in our province’s ecosystem, for example by providing habitat for caribou and holding the large amount of carbon mature trees have absorbed over their lifetime. However, when forests reach this advanced age, trees become more susceptible to fire, disease and insect infestations. Trees also die of old age eventually, even without fire or other natural disturbances.

In Alberta, the forest industry works with government, researchers and other partners to carefully manage and sustain mature forest. That starts with understanding the unique features of our forests and the types of trees that live there. Most tree species native to Alberta might live a maximum of 150 years under ideal conditions, but historical fire patterns show that most Alberta forests would burn every 50-100 years without intervention. This would make most of Alberta’s forests less than 50 years old, with patches of mature forest scattered in between.

Forest fires can be a good thing – where older forest has burned, young forests have room to grow, and the forest lifecycle begins again. But fire comes with significant risks. The carbon that trees captured over their lifetime is released back into the air, along with other harmful compounds. Water supply can be damaged by fire, and communities caught in the fire’s path can be destroyed. Left unchecked, a fire will also just burn until it burns out – there’s no limit to how much forest is consumed, or what parts of the forest are affected.

Strategic harvesting is a tool that mimics the benefits of fire, without these risks. Unlike fires, harvesting is carefully planned and controlled – the size, shape and location of harvest areas is strategically chosen for maximum benefit and minimum risk. In part, this means we harvest less than 1% of Alberta’s forest each year – much less than would burn without human intervention. 

It also means leaving buffer zones around important features like wetlands, rivers and cultural sites. The areas that are harvested help the forest, too – creating spaces between stands of mature forest can slow the spread of fire and disease when they do occur, which makes our mature forest less vulnerable. The result is that Alberta now has more mature forest than it otherwise would – instead of burning every 50-100 years, the majority of Alberta forests are more than 50 years old, and an unnaturally high proportion are over 80 years old.

The trees that are harvested are used to create products and materials that people use every day, and the carbon the trees absorbed over their lifetime stays locked in the things we produce. This doesn’t happen when trees burn or decay – when that happens, the carbon is released back into the atmosphere. Through effective forest management, Alberta maximizes a number of benefits: fire and insect risks are mitigated, carbon stays sequestered in wood products, newly replanted areas absorb even more carbon, and the overall amount of mature forest in the province is much higher than it would be naturally.


Deciduous trees tend to die and decay faster than coniferous ones. When trees die and fall, they form woody debris on the forest floor – this nourishes the ground and provides food and nesting material for insects and animals. This material is often left in place after a harvest to support the regeneration of the forest.


Thick carpets of moss and other dense, low-growing plants are a hallmark of mature forest. Fungi also thrive here, as do lichens, which are an organism made up of algae and fungus in a symbiotic relationship.


Cavity-nesting birds, which make their homes in the hollows of dead trees, like to live in mature forests. Rodents also do well here, which means that raptors and owls do well too. Importantly, caribou make their homes in mature forests, as lichen is an important winter food source – maintaining caribou habitat is one reason that harvesting of mature forests needs to be done so carefully.