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Snag and Oak response to Forest thinning on the U.S. Naval Observatory

Typical old ponderosa pine snag on the U.S. Naval Observatory in Flagstaff, Arizona (M. Bayless)Background:
Standing dead trees (snags) are considered an integral habitat component of cavity-nesting birds and other wildlife in the ponderosa pine forests of the southwestern United States. The species, size, bark retention, and condition influences the value of a snag as wildlife habitat. Removal of snags has been linked to declines in both diversity and density of cavity-nesting birds and tree roosting bats in southwestern forests. Snags also serve as nesting and perching platforms for numerous raptor species. Conversions of forests from old growth to even aged stands shortens the rotation age from centuries to decades, thereby reducing the size and age of the trees left in the forest. This reduction drastically decreases the amount, size, and quality of dead and dying trees that will result in the presence of snags in the future. Oak trees are an important component of the ponderosa pine forest ecosystem. These mast-producing trees are a valuable food, nesting, and denning resource for numerous wildlife species.

Currently the U.S. Naval Observatory in Flagstaff is treating approximately 210 acres of ponderosa pine forest using a 20 foot tree spacing prescription to reduce fire danger in the area of the Observatory.

Oak regeneration on the U.S. Naval Observatory in Flagstaff, Arizona (M. Bayless)For this study, ponderosa pine snags are considered to be standing dead trees greater than 4 inch diameter at breast height (dbh) and greater than 7 feet tall, with an angle greater than 45 degrees from the ground. We located and measured all ponderosa pine snags on the Naval Observatory. For each snag we documented species, size (height and diameter), and decay class. We also noted woodpecker use, beetle infestation, and bark sloughing as a surrogate for bat roost suitability. In addition we established 30 plots to monitor long-term oak regeneration. At these plots, we inventoried all oak stems smaller than 4 inchesat the base within a 30 foot radius.

Thinning the forest may alter the reproductive response of oaks, as well as the type, longevity, and density of snags. The purpose of this project was to collect baseline measurements of oaks (for example, size distribution, condition, density, etc.) and conduct a census of snags within the ponderosa pine forest on the Naval Observatory. We permanently marked all snags and oak plots on the installation. This will facilitate the long-term monitoring of these snags where each can be sampled through time to assess decay and retention rates, and oak regeneration can be monitored as it relates to the prescribed forest treatment.

For more information contact:
Mylea Bayless, Arizona Game and Fish Department, 5000 W. Carefree Highway Phoenix, AZ 85086-5000 .
Phone: (928) 213-9591 E-mail:

Michael Ingraldi, Ph.D., Arizona Game and Fish Department, 5000 W. Carefree Highway Phoenix, AZ 85086-5000 .
Phone: (928) 523-5625 E-mail:

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