Specific pest concerns

Pest problems & stand development

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Forest Health

Saperda calcarata on aspen stem (Mike Ostry, USDA FS Northern Research Station)

Trembling aspen, bigtooth aspen and balsam poplar share most insects and diseases. Of the three, bigtooth aspen tends to be longer lived and has fewer health related problems than the other two species. All three species are considered short-lived. Aspens have thin bark and shallow root systems and are therefore easily wounded and subject to invasion by stain and decay organisms. Predicting stand longevity and overall health can be difficult for aspens largely because of the complicated interactions that occur between genetically diverse clones, stand age, stand density and site quality.

Detailed information on specific pest problems are described and discussed in the section titled Specific pest concerns . Pests are described based upon what part of a tree they damage. This section provides links to additional management guides and identification aides. Further pest information can also be obtained in a section on Pest problems and stand development. This section details specific pest problems that are most likely to occur during various stages in stand development.

Aspen clones are often distinguishable in autumn when the foliage changes colors and leaf abscission begins (2006, M. Ostry, USDA FS Northern Research Station)

Clones and aspen forest health

Most Lake States aspen stands have developed from root suckers, creating stands made up of a mosaic of genetically different clones. The genetic diversity among clones within stands has been a key to minimizing large scale damage from insect pests and diseases. Over time, superior clones expand and replace clones that cannot compete or that are susceptible to damage by insect pests and diseases. The incidence and severity of two major diseases, hypoxylon canker and white trunk rot, is strongly influenced by clone.

Tree and stand age and aspen forest health

Young aspen stands can have very high levels of stem mortality, sometimes exceeding 80 percent. But, as long as initial stand density is high, this mortality is generally viewed as having a beneficial thinning role. Many aspen stands will go through 2-3 natural thinning events during stand development.

Aspen trees are not long-lived and stand deterioration or breakup can occur anywhere from age 35-90. The age at which any given stand deteriorates is influenced not only by age but by clone, site quality, weather, damage from insects pests and diseases and other factors. This process starts with a slowing of growth at maturity, holes occur in the canopy as individual trees begin to die or break due to increasing prevalence of wood decay. As the canopy opens, trees are subject to increased exposure to wind, sunlight and evaporation. This increases tree stress reducing the vigor of the remaining trees. Stand deterioration can occur quickly, sometimes within 2-5 years the majority of the aspen canopy can be killed or broken by wind, snow or ice.

Defoliated aspen with a conifer understory (S. Katovich, USDA FS Northeastern Area State and Private Forestry)

Defoliation and aspen forest health

Aspen stands are often subject to periods of complete defoliation (leaf-loss) due to outbreaks of caterpillars or leaf beetles. This is more common on aspens than any other forest type in the Lake States. Forest tent caterpillar and large aspen tortrix are the most likely culprits, though a number of insects eat aspen leaves. These defoliation events can be massive in scale with millions of acres of defoliated trees in any given year. Aspen trees are very tolerant of defoliation and widespread tree mortality following these outbreaks is rare. However, intense and prolonged defoliation can play an important role in the rate of stand breakup in old-age stands.

	Young dense aspen stand (Mike Ostry, USDA FS Northern Research Station)
	Hypoxylon canker
	Old stand in break up stage

Stand density and aspen forest health

Numerous examples exist that illustrate the importance of tree density and the incidence of aspen diseases and insects. In poorly stocked stands aspen branches persist, often these branches are attacked by twig and shoot feeding insects. Wounds caused by insects provide entry points for fungi, including the fungus that causes hypoxylon canker. This can result in a high incidence of stem cankers in under-stocked stands.

The most important disease that often results in aspen mortality in the lake states is hypoxylon canker. It has been shown that stand density can have a major influence on the incidence and severity of this disease. Aspen along stand edges and in poorly stocked stands are more likely to have a higher incidence of hypoxylon canker. Managers applying strategies to increase growth and yield and shorten aspen rotations such as thinning stands may need to consider how reducing stem density within a stand may have the unexpected consequence of increasing disease incidence and severity.

Site quality and aspen forest health

Site quality plays an important role in the health of all of our forest tree species. But, it often becomes an overriding concern with aspen simply because aspen stands occur on such a wide variety of sites. As a pioneer species, trembling aspen can be found growing on very dry, outwash sands; to very wet, lowland forest types; to everything in between those two extremes. As a generalization, the best aspen growth and longest life span is correlated with more mesic, nutrient rich sites, and shorter life spans correlate with very dry and very wet sites.

Overall forest health strategy

It is possible to manage aspen stands to reduce insect and disease risk. The emphasis should be on long-term strategies focused on growing aspen on appropriate sites and in well stocked stands. Older aspen stands deteriorate, often quite quickly, and maintaining older aspen for long time periods is unrealistic. Opportunities to manipulate clones to favor superior, disease resistant clones should be considered.