City Trees

from the shortlist—that's when I can get mean- ingful homeowner buy-in." That time invested up front pays off when the homeowner cares enough about the tree to mulch and water it during estab- lishment and to advocate for it going forward. Tree replacement and site vacancies give Kesterson the chance to chip away at monocultures. For example, American sweetgum (Liquidambar styraciflua) is the fourth most common tree in the city inventory. There are thousands of them, often the sole species on whole blocks. (They were overplanted by predeces- sors because of their fall color and quick growth habits, and are extremely common throughout California.) "Obviously, we're never going to clear-cut 50 sweetgum trees on a block," Kesterson says. It's all about replace- ment as sweetgum trees are lost to storm damage or because they've become hazards, which Kesterson says work out to no more than 1-5 trees in each block each year. Berkeley also has monocultures of London plane (Platanus × acerifolia) and Bradford pears (Pyrus call- eryana 'Bradford') that Kesterson is looking to diversify. "As recently as the early 2000s we still thought we could just find a better species, but not change the plan," Kesterson says. Recent failed attempts at tree monocul- tures with especially poor public appeal include 'Purple Robe' black locust (Robinia), which has a short lifespan and produces thorny root sprouts as the tree declines); western hackberry (Celtis occidentalis), which may be the worst host for aphids in the area; and 'Raywood' ash (Fraxinus oxycarpa) all of which have been declining at about 15 years in. "These all present unique chal- lenges which are magnified when you see more than ten in a row," Kesterson says. "Our goal is to have up to seven species in play on each block, planted stra- tegically for as much visual coherence as possible." As replacements for the blocks of sweet gum, Kesterson and team are planting a collection of similar-looking species from various botanical backgrounds to maxi- mize genetic, size, and growth-pattern diversity. These include Persian ironwood (Parrotia persica), sourgum (Nyssa sylvatica), and 'October Glory' red maple (Acer rubrum), the cultivar of red maple that's currently least represented in the city. Graves oak (Quercus gravesii) 36 CityTREES Volunteers and city staff getting some new California buckeye (Aesculus californica) Persian ironwood (Parrotia persica) like the one in the foreground is one of the many tree species used to gradually replace a monoculture block of American sweetgum (Liquidambar styraciflua)—seen on the right side of the street, topped and under utility lines.

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