Many forested areas contain multiple species of trees. How do “mixed stands” of trees form? Why do some forest stands have stands of trees that are in discreet age classes?
A recent study of forests in Northern Minnesota links repeated outbreaks of insect defoliators to the forest composition. Populus tremuloides, (a.k.a. Quaking Aspen) is a “pioneer” tree that grows in recently disturbed areas. Following a period of clear cutting or other canopy disturbance, Populus tremuloides can receive enough sunlight to establish. Once aspens are established, shade tolerant tree species begin to grow under the canopy. Eventually these other tree species will grow above and shade the aspens. These tree species will replace some of the Populus tremuloides.
In some stands, outbreaks of tent caterpillars and other tree defoliators have detrimental effects on the shade tolerant trees and reopen the forest canopy to pioneer species such as quaking aspen. When defoliation occurs over multiple years, the pioneer species can establish. This process produces cohorts of same age aspen trees. Forest Tent Caterpillars have outbreak cycles with about a 5 -10 year periodicity and the outbreaks frequently persist for 3 or more years. Defoliators, such as tent caterpillars are considered destructive pests in many situations. However, for pioneer species such as quaking aspen, caterpillar defoliation of competitor trees can be beneficial.
The initial reaction of many people looking at a forest with heavy levels of insect defoliation is to worry about the trees. However, one tree’s defoliator is another tree’s opportunity. Insects have a huge role in selecting the tree species that compose our forests. Native defoliators such as forest tent caterpillar are a component of structuring our forests in the composition we find them today. Non-native, invasive insects are a worry because of the large effect that insects have on forest tree composition. The invasive species can alter tree species composition from its current state to a drastically different composition. We may or may not be happy with the results which is why efforts at preventing the importation of invasive species is so important. We worry less about the native defoliators because they are responsible in part for the current status of our forests.