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Another problem has proven more intractable. "We first noticed it in 1999," Sirois explains. "We'd see something like a brown stain that would go up the side of the trunk of the tree. It mostly seemed to start at the bottom, although we've found some trees where it looks like the top branches are being affected first." In the areas of the stain, the bark dries and cracks, and one by one, branches lose their leaves and die. From a distance, it can look as though the tree is rusting to death.

In a grove, "You may see one tree get it, and then a little while later another tree in the grove will get it, and a little while later another tree will get it," Sirois says. "But in general, we haven't seen whole groves disappear." As a preventive measure, park personnel have sprayed the bark of some trees with an antifungal chemical. "It's certainly not hurting, and a lot of the trees we've treated have not come up with any kind of disease," Sirois says. "But it's too early to say whether it's helping." Also, the fungal spray doesn't work once the tree is diseased; no infected tree has ever recovered.

Most frustrating has been the inability to figure out what's causing the malady. "Several possibilities have been thrown around," Sirois says. "But nothing has really matched up to what we have." He says authorities have conclusively ruled out the funguslike organism that causes sudden oak death. "There's also a disease called Mundulla yellows that they have down in New Zealand and Australia. It's pH-related, and we thought maybe that would have something to do with it." But the pH of the soil around affected trees hasn't "really fit the bill," Sirois says. "We thought of Xylella -- the bacteria that's currently affecting the wine crop and oleander. But that was tested and ruled out." This past winter, the advance of the mystery ailment seemed to slow, a possible consequence of the cold weather, Sirois speculates. "But last summer and spring were pretty bad. We lost quite a few large trees. I would say 30 to 50."

The primary victim has been the sugar gum (Eucalyptus cladocalyx), although a number of honey-scented gums (Eucalyptus melliodora) and silver dollar gums (Eucalyptus polyanthemos) have also come under attack. The towering sugar gums also happen to be the most prevalent tree in Balboa Park, a fact that was documented when the first-ever Balboa Park Tree Survey was conducted in 1998. "It was an old-style survey, prior to GPS," Sirois elaborates. Brass pins were driven into the ground every 250 feet; then teams counted every tree with a diameter of two inches or more within each grid block. The final tallies revealed 15,271 trees (including 348 separate species). Almost 5000 of the total were some variety of eucalyptus.

That large percentage worried Sirois and the park horticulturist at the time, Kathy Puplava. "If you grow a monoculture, you're susceptible to losing the whole tree population," Sirois explains. "Back East, Dutch elm disease wiped out all the elm trees." So in the wake of the survey, the two developed a park reforestation plan that emphasized the goal of making Balboa Park's tree population more diverse. Sirois says probably 30 species have been added to the park in the years since then. But he adds that it hasn't been easy to find substitutes for the really big eucalyptuses.

Trees that are skyscrapers in some areas often don't reach the same heights in San Diego's dry climate. The coast redwoods planted in Balboa Park's Redwood Circle during the 1920s stand only about 80 feet tall, compared to the 300-plus feet they attain in Northern California, where summer fogs bring moisture to the uppermost branches and free the trees from the need to transport water up that high from their roots. Sirois says another limiting factor is the park's soil, much of it hardpan and impenetrable below a depth of just a few feet. "We don't have the deep alluvial soils that they have in Pasadena," he says. "You see huge, magnificent trees there."

Some species do grow as tall in San Diego as the eucalyptus. The star pine (Araucaria heterophylla) and bunya-bunya (Araucaria bidwillii) do, Sirois says, but their silhouettes look nothing like the broccoli-esque forms of the sugar gums and lemon gums. (Bunya-bunyas have outstretched, almost prehistoric-looking branches, while star pines grow into the classical shape of Christmas trees.) Torrey pines can reach great size. "But as far as function -- meaning having that willowy top but very tall -- there really aren't any trees that match the eucalyptus, and that's something we've struggled with."

Sirois has talked with the urban forester from the City of Los Angeles about what L.A. is doing to replace tall-canopy trees. "One they're trying is the pecan tree. So we've planted a few of those in the park." The turpentine tree (Syncarpia glomulifera) "gets pretty tall," Sirois says. "It's close to the eucalyptus family." Familiar eucalyptuses will also continue to be planted, he adds. "We've planted sugar gums in the last six years. But we're just a little more selective in where we plant them. We're not putting them right next to buildings anymore." The risk of branches dropping off or trees falling is too great.

Sirois says the park's reforestation plan recognizes the importance of diversifying not just the types of trees but also their ages. When the 1915 and 1935 expositions were being readied, "Thousands and thousands of trees were planted all at once," he reminded me. Most weren't that old, perhaps four to five years, so they're maturing around the same time. Sirois thinks the fundamental explanation for the recent eucalyptus woes could just be that the trees are reaching the end of their natural lifespans here and in their waning years are susceptible to disease.

To ensure that future generations of Balboa Park managers don't face the same problem, "You really want to continue planting all the time," Sirois says. "That's critical to maintaining a healthy urban forest with old trees, middle-aged trees, and new trees." In practice, "We generally try to plant around 200 trees a year." When a stormy year destroys more than 300 trees, as happened in the winter of 2003-2004, "we'll try to accelerate the replanting then. But as a standard, we try to keep it around 200."

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