pathways can lead to old-growth forests. One trend is consistent among all
of them—increasing structural complexity as a forest develops.
of today's old-growth forests started hundreds of years ago after some
kind of catastrophic change—perhaps a large forest fire or windstorm—destroyed
the previous forest. Click on this link to learn more about the pathways
that lead to old-growth forest.
change is a normal part of all ecosystems, including old-growth forests. Small
changes go on all the time. A snag falls
over, or an animal is born.
Bigger changes such as fires, windstorms, or insects kill some trees and
create openings in the forest where new trees can grow, adding to the complexity
Three qualities are important when an ecosystem responds to
changes—composition, forest structures, and processes.
- Composition—The complexity of the old-growth forest creates many
habitats. These habitats support thousands of species, including soil arthropods,
spiders, insects, mites, millipedes, lichen, fungi, mosses, small mammals,
and large mammals. This high level of biodiversity means that many species
carry out each ecological process.
- Forest structures—A forest has live
and dead structures. The structures left after a disturbance are called
biological legacies. After a fire or windstorm,
the dead trees become snags of fallen trees on the ground. These dead trees
shelter many plants and animals, protect the soil, and enrich the soil
as they decay. Biological legacies ensure that many species survive a fire
disturbance, and the legacies help rebuild the ecosystem.
are changing all the time; they are dynamic. Forests continue their ecological
processes through all the changes, a quality known as resilience.
Click on the heading '"processes" to learn more about them.