The UC Davis Arboretum, home to one of North America's finest and most diverse collections of oak trees, has launched an effort to protect and enhance the Shields Oak Grove while at the same time manage and enhance the wildlife of the arboretum.

It is a new challenge for arboretum managers, who during the past five years have watched in amazement as their prized grove has evolved from a small breeding area for herons in 2000 into one of the largest nesting areas in Yolo County. Indeed, UC Davis wildlife ecologists estimate that more than 2,400 egrets and herons nested or roosted in Shields Oak Grove last year, producing more than 850 chicks.

And while the nesting birds have developed a following of their own among visitors and bird-watchers — much like the oak collection has its own fans and students — arboretum officials and outside consultants are growing concerned that the birds and their excrement, or guano, are taking a toll on the health of the oak trees where the birds have built their nests.

Guano from the birds is having a negative impact on the soil chemistry, and has led to trees losing their leaves and dying back within the grove, especially where bird activity is high, said John Lichter, a professional consulting arborist. "These changes and impacts to the oaks threaten their health and may ultimately cause their death."

Arboretum Director Kathleen Socolofsky calls Shields Oak Grove "the best-kept secret of the arboretum, and we're trying to make it a more attractive and educational destination. To do so, we need to manage in a comprehensive manner if we are to protect and enhance trees and other life in the grove."

To that end, Socolofsky, after consulting with Lichter, as well as campus wildlife ecologists and biologists, horticultural experts, environmental planners and other UC Davis faculty and staff, has taken the following steps:

• Before the nesting season began last month, workers removed remnants of last year's nests from the oaks' canopies and removed twigs from the ground beneath the trees. By doing so, they hope to reduce nesting activity by removing some of the most accessible building materials. But the workers have not and will not disturb any of this year's ongoing nesting activities in the trees.
• As soon as the breeding season is over this summer and the birds leave, the arboretum staff will oversee the thinning of the grove. In all, about 25 of the grove's nearly 350 trees — a little less than 10 percent — will be removed. The trees will be removed because they have grown too close together and, as a consequence, are deformed. By thinning the grove, the remaining trees will have more room to grow and develop in their natural form.
• Throughout the breeding season, bird activity will be monitored to ensure the birds are not disrupted — all nesting species are federally protected — and to determine what effect if any the removal of the nesting materials had on nesting and roosting.

Campus experts say the challenges posed at the arboretum are not unique to UC Davis.

"More and more, declining forested habitat is causing colonial birds to target valuable stands and bringing them into conflict with humans," said Melanie Allen Truan, director of biomonitoring and research for the UC Davis Museum of Wildlife and Fish Biology.

The Shields Oak Grove is, by any measure, a valuable stand. It is a living library that contains the largest collection of mature oaks in the southwestern United States — 346 trees representing 89 types of oak species, varieties and hybrids, some of which are rare and endangered. The grove includes species from around the world but is particularly rich in western North American and Mediterranean oaks. Researchers, students and recreational visitors regularly walk "the rows" of this outdoor library for both scientific study and relaxation.

John Tucker, professor emeritus of botany and former arboretum director, donated $250,000 to the arboretum in 2000 to establish the John M. Tucker Oak Collection Endowed Fund, to maintain and enhance the grove as an educational resource. Tucker has been apprised of and is in agreement on the protection and enhancement plan. But the grove is not only prized locally; it is a nationally recognized treasure.

The arboretum in the last three years has received two prestigious federal grants totaling $106,000 from the Institute of Museum and Library Services to improve the horticultural care of the oak collection. (As a match, the UC Davis Grounds Division has pledged an additional $45,000, and the arboretum an additional $61,000, to care for and enhance the oak collection.)

In addition, the prestigious North American Plant Collections Consortium has invited the arboretum to partner with other arboreta across the country to build a national collection of oaks. To become a certified national-collection holder, the arboretum officials must commit to maintain the oak collection at the highest curatorial and horticultural standards.

At the same time, though, the grove is becoming an increasingly popular nesting and roosting area for four species of birds — the black-crowned night heron, snowy egret, cattle egret and great egret. All four are among the birds protected from harm by the Migratory Bird Treaty Act, which prohibits any disruptions to the birds during the breeding cycle.

"Heron and egret populations were decimated by plume hunters in the early part of the 20th century, leading to the current federal protections," said Truan, a wildlife ecologist. "While populations are recovering, suitable breeding sites are in short supply. That's why the Shields Oak Grove represents an important resource for these beautiful birds."

In January, Truan and Amanda Castañeda, a UC Davis wildlife specialist, released an extensive arboretum wildlife management and enhancement plan that helped form the basis for a comprehensive healthy ecosystem approach.

Emily Griswold, horticultural curator for the arboretum, said the birds pose a unique challenge to the health of the trees — and to the people who come to see them. "We have read about cases where large breeding colonies of egrets and herons have killed mature stands of oaks in as little as two years," Griswold said. "We are very concerned about the rapid accumulation of guano, both for the health of our trees and our visitors."

In addition, the arboretum has had to postpone the installation of educational exhibits and a trail system in the grove because of the human health concerns posed by the bird guano. The guano, which coats anything under the trees in the most popular nesting areas, can contain disease-causing bacteria such as E. coli, Salmonella and Chlamydia psittaci. The bacteria can be spread to humans through breathing or hand-to-mouth contact.

Socolofsky is committed to the success of this multidisciplinary approach to managing the arboretum's ecosystem. She is quick to point out that the California Aggie earlier this year named the arboretum the best place to take a date and the most picturesque, and that The Davis Enterprise has said the arboretum is one of the best places in Yolo County to take an out-of-town visitor. She does not take these accolades lightly; she does not want to lose them.

"Visitors to UC Davis are drawn to the arboretum," Socolofsky said. "It is the front door to our campus."

Information

To view the entire UC Davis Arboretum Wildlife Management and Enhancement Plan, and other related materials, visit: http://arboretum.ucdavis.edu.