How forests grow

Forests are dynamic ecosystems—they’re always growing and changing.  Patterns of forest succession are relatively predictable within given site conditions, but highly variable from site to site.  For instance, in the sub-boreal forests of northeastern Minnesota, widespread aspen and birch forests are being invaded by native spruce and fir trees.  Over time as the short-lived aspens and birches die, shade-tolerant conifers will take over.  This process is readily visible driving along Highway 61 along Lake Superior’s north shore.

However, many factors can affect the process of forest succession.  Variation in climate, soils, seed sources, natural disturbance, and wildlife populations can all affect a forest’s succession.

Blowdown in Boundary Waters wilderness, 1999
1999 BWCA blowdown. Flickr photo by esagor. Click for original.

Natural disturbance
Natural disturbances such as windstorms and severe wildfire can kill most or all existing vegetation on a site.  Although windstorms and wildfires may appear to have similar effects, they in fact have very different impacts on the forest.  Windstorms tend to destroy the mature trees in the canopy and “release” existing seedlings in the understory, which then thrive and quickly replace the pre-disturbance stand.  Severe wildfire, on the other hand, tends to kill both mature trees and understory seedlings.

These two different natural disturbances therefore lead to very different future forests.

Just as different natural disturbances have different effects on the forest, different forest management activities allow foresters to carefully modify the condition of a stand.  In fact, applied forest ecology is the basis of modern forest management.  Planting trees, harvesting some or all of the trees on a site, burning, and direct seeding are only a few of the ways that foresters manage the process of forest development or succession.

White spruce under jack pine, Brainerd MN. Click for a better view.
White spruce thriving under aging Brainerd, MN hardwoods. Flickr photo by esagor. Click for original.

Forest succession
Shortly after a major disturbance like a severe wildfire or clearcut, an early successional forest will become established.  Whether planted or naturally regenerated, early successional stands are densely populated with seedlings early in their development.  Picture a dense “doghair” aspen stand or a thick red pine plantation.

Early successional stands provide unique and important wildlife habitat.  Several common game species, including white-tailed deer and ruffed grouse, depend on early successional forests for food, cover, or both.

Over time, as individual trees grow larger, competition for growing space intensifies.  During this process, some species will self-thin, with more competitive trees taking over growing space and less competitive trees dying out.

Other species, however, will not self-thin.  If grown in pure (single-species) stands, these species will stagnate and become unhealthy and vulnerable to insects, disease, or windthrow.  A common example in Minnesota is the unthinned red pine plantation.

If not left stagnant for too long, these stagnant stands can be reinvigorated through careful thinning or stand improvement operations.  By increasing the growing space available to the best trees on the site, thinning can greatly improve the stand’s vigor, resilience, and value, both for wildlife habitat and potentially for financial returns.  Read more about thinnings in our thinning and stand improvement post.

In the absence of major disturbance, as stands age individual or small groups of trees will die and fall.  These small gaps tend to be invaded by shade-tolerant species.  Over a very long time period, these shade-tolerant species will continue to invade and come to dominate the stand.  The resulting stand will include trees of a variety of species and a variety of ages.  This type of forest is known as an old-growth forest.

Eli 's work addresses Minnesota forest ecology & management. He's based in St Paul.

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