CORVALLIS, Ore. - Recent studies show that thinning of young forests can benefit the development of old-growth characteristics and the diversity of plants and animals, but only if methods are used that protect and promote the development of shrubs, hardwoods, and large or old trees.
The findings, which were made by researchers from the U.S. Geological Survey and Oregon State University, hold special significance for the management of many young forests - with trees about 40-60 years old - that cover vast portions of the Pacific Northwest.
The conclusions are based on a number of related studies funded in recent years by the USGS.
According to John Tappeiner, a professor at OSU and retired USGS forest scientist, millions of acres of old-growth forests in the Pacific Northwest were clearcut in past decades and densely replanted with uniformly spaced tree seedlings. The original management goal of most plantations was to produce high yields of timber and associated wood products.
"This management goal dramatically shifted for millions of acres of young forests with adoption of the Northwest Forest Plan in 1994," Tappeiner said. "Many dense, young forests were incorporated into a network of large conservation reserves intended to provide habitat for plants and animals typically associated with older forests."
Although researchers and land managers had assumed that these dense, young forests would, in time, grow to resemble the old-growth forests they replaced, a group of researchers have accumulated a wide range of evidence suggesting that this may not occur unless the young forests are selectively thinned to allow the remaining, uncut trees to grow under less-dense conditions.
Crowded young trees develop differently from more open-grown individuals, the scientists found. Widely spaced trees have larger crowns and diameters than closely spaced trees of the same species and age. Dense young forests typically have more than 200 trees per acre at 50 years of age, but studies of 90 old-growth forests revealed an average of just 6-8 large trees (over 40 inches in diameter) per acre.
Other findings of the research include:
"Taken together, these studies suggest that thinning may have positive results for plants and animals if the methods used protect shrubs, hardwoods, large trees, and old trees," Tappeiner said.
Pat Muir, a professor of botany at OSU, said it is also important to consider that the sites studied were thinned only 15 to 20 years ago, with a primary objective of commercial tree harvest.
"As a group we found indications of benefits for some plants and animals less than two decades after thinning, even though the thinning was conducted without bearing in mind the effect on these organisms, and some benefits of thinning may not be seen for decades," Muir said. "I suspect even greater benefits would be evident if thinning were conducted with a long-term goal of enhancing biodiversity."
In these research projects, the USGS and OSU scientists contrasted the responses of plants and animals in three types of forest stands in western Oregon: young stands thinned by commercial techniques 15-20 years ago; young unthinned stands; and old-growth stands.
The organisms selected for study have complex interdependencies that are only partially understood, the scientists say, such as providing food, nesting material, habitat, or pollination.
Other contributors to this research included Joan Hagar, a doctoral candidate at OSU; Bruce McCune, an OSU professor of botany and plant pathology; Nathan Poage, a contractor for the USGS; Jeff Miller, an OSU professor of entomology; and Eric Petersen, previously a doctoral candidate at OSU.
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John Tappeiner, 541-737-3055