Increased Tree Growth After Clean Air Act Is Implemented

In a study of growth rings formed in the stems of eastern red cedar (Juniperus virginiana) trees in the central Appalachian Mountains, scientists found that mature trees ranging from 150 to 300 years of age grew faster in the 1980s than in the several prior decades. This apparent recovery comes about ten years after enactment of the Clean Air Act. The U.S. Congress passed the Clean Air Act in 1970 in order to reduce air pollution that, in some areas of the country such as the Ohio River Valley, the Appalachians and New England, was found to be adversely affecting forest growth, lake water quality and human health.

A red cedar tree growing on a relatively fertile site.

A red cedar tree growing on a relatively fertile site.

Air quality standards that limited the ambient levels of six pollutants were set by the U.S. Environmental Protection Agency. The pollutants of concern were (and, are today), sulfur dioxide, ozone, nitrogen dioxide, carbon monoxide, lead and fine airborne particulate matter. All of these are found in the emissions from the smoke stacks of electric power plants, especially those that burn coal. Acid rain, which is formed downwind of coal-fired power plants, also was recognized as a concern. Acid rain forms when sulfur dioxide is converted in the atmosphere to its acidic ionic form, sulfate, a major contributor to the acidification of aquatic ecosystems and forest soils.

Earlier in the 20th century, red cedar wood growth declined for decades following the 1930s when the economic depression in the U.S. led to decreased electricity generation and associated air pollutant levels. Scientists had long known that forests downwind of Midwestern power plants were exposed to elevated air pollution but the implications for tree growth were less understood.  Other factors known to limit tree growth such as climate and competition were  eliminated,  leaving decreased pollutant levels as the best explanation of the inter-decadal growth pattern.

A red cedar log cut in half and then turned on a lathe to show sapwood and red heartwood

A red cedar log cut in half and then turned on a lathe to show sapwood and red heartwood

A tree ring forms in the woody stem of a tree by the juxtaposition of larger-celled springwood (or earlywood) cells that form in the spring of the year next to the smaller-celled summerwood produced later in the growing season of the previous year. The alternation is repeated annually leading to the familiar series of rings apparent in a cut tree trunk. The age of the tree can be estimated by counting the number of rings while the favorability of seasonal growing conditions, including air quality, can be inferred from the width of the rings.

Eastern red cedar is a relatively short-lived tree reaching a maximum age of around 300 years. Its  seeds which  are dispersed by  birds such as catbirds, cedar waxwings, robins and mockingbirds, are found within fleshy, berry-like cones, which reveal the species’  true identity as a juniper.

Mature cones of eastern red cedar.

Mature cones of eastern red cedar.

Other North American trees called cedar such the western red cedar (Thuja plicata,) or the white cedar (T. occidentalis) produce dry, leathery cones,  occur in totally different habitats, and are not closely related to junipers. Red cedar is tolerant of drought and often colonizes abandoned rangeland, limestone outcrops and formerly cultivated soils. Its most frequent habitat is well-drained sites on limestone especially where it is too dry and exposed for tall forests which, in more fertile locations, can shade out the red cedar. This habitat preference is ideal for examining past tree growth as light intensity, a major influence on growth, can be assumed to have been not growth-limiting and to have been relatively similar across the years in trees growing in the open.

Thomas, R.B.,S. E. Spal,K. R. Smith and J. B. Nippert 2013. , doi: 10.1073/pnas. 1308115110. (Proc. of National Acad. Sci.)

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