August 3, 2009

Tasmanian blue gum is too aggressive

Take a close look because the Presidio of San Francisco is replacing all of these with cypress and pine trees. With 300 acres of eucalyptus, pine and cypress, the 123 year old Presidio is changing and redefining its historic forest.

Check out the Presidio Trust’s reforestation efforts here...

The forest is the most dramatic example of how people shaped the Presidio’s landscape. Its 60,000 trees provide an important wildlife habitat (the forest is home to more than 250 different species of birds) and contribute to the Presidio’s National Historic Landmark status.

In the late 1800’s the Army began the prodigious task of transforming the Presidio from mostly open dunes to a richly forested, park-like reserve, similar to New York’s Central Park. Following a plan developed by Major William A. Jones, the Army planted some 100,000 trees over 14 years along the Presidio’s ridges and entrance gates. The first trees, donated by Adolph Sutro, were planted in 1886, on Arbor Day. The eucalyptus, pine and cypress groves accentuated the post’s size, sheltered it from the winds and created a clear visual distinction from the surrounding city. It was the Army’s most impressive accomplishment in landscape architecture. No other military installation in the nation has ever undertaken landscape planning on such a grand scale.

The relatively short period of time during which the trees were planted however, created an “even-aged” forest. And while the eucalyptus have thrived, the cypress and pine have begun declining simultaneously. Presidio forester Peter Ehrlich is leading an ambitious effort to revitalize the forest over the next several decades.

“The goal,” says Ehrlich, “is to create an ‘uneven-aged’ forest that can be more easily sustained and will be a healthier forest in the long run.”

Each year the Trust replants two to three acres of pine and cypress. Since 2002, more than 2000 trees have been planted, with a careful eye says Ehrlich “towards preserving the qualities that define the forest’s character, like the orderly, military alignment of the trees.”

Staying one step ahead of nature is as Ehrlich puts it, “a daunting challenge.” One he has, at times, met with the latest scientific methods. Several years ago, in the face of an outbreak of pitch canker disease among Presidio pines, Ehrlich and his staff, along with foresters from U.C. Davis identified, then cloned Presidio pines that showed resistance to the disease. In 2004, more than 140 of the experimental, disease-resistant trees were planted. Five years later they appear to be thriving.

Current efforts are focused on finding a less aggressive and invasive replacement for the Tasmanian blue gum, a eucalyptus tree that can live to be 300 years old and is the most populous tree in the Presidio. The new tree Ehrlich says would look the same as a blue gum and be the same height, “only without the aggressiveness.”

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