Gina Crandell, one of this month’s guest bloggers, is visiting professor of landscape architecture and environmental planning at the University of California at Berkeley. She is also principal of Gina Crandell Landscape Architecture, a Boston practice working with campuses and land trusts.

What is an arboretum? Many campuses have them. Sometimes a whole campus is one. But beyond having plants, which may or may not be labeled, the answer isn’t always clear.

Sometimes an arboretum is thought of as “natural.” Often what is done there is simply subtractive—invasive species have been removed. But often an arboretum (as well as a campus) is a garden. When decisions about it are derived from a garden ethic, rather than a wilderness ethic or naturalism, it becomes much more pleasurable to experience.

If you haven’t read Second Nature: A Gardener’s Education, I recommend it. In it, Michael Pollan describes the wilderness ethic as a zero-sum game: Do nothing, because if you do something, you destroy the wilderness. But Mr. Pollan goes on to challenge the idea that the forest is stable and independent of people: “Not only is forest succession little more than a comforting narrative, even the concept of an ecosystem is only a metaphor, a human construct imposed upon a future in which no one knows, not even nature, what might happen.”

After listing the many possible scenarios that might cause a pine forest to become something quite different, he asks: “If our cigarette butts, Norway maples, and acid rain are going to shape this place, why not also our hopes and desires?” Mr. Pollan proposes the garden as a model for designing places like arboreta, because the garden depends on local answers, begins with culture, avoids purity or blame, and recognizes interdependencies. Otherwise it won’t grow.

An idea that is related to a wilderness ethic could be described as naturalism: Whatever you do, make it look like you didn’t, so it will look natural. This is the ethic that fears architecture in the design of the campus landscape because it expresses intention. Instead, trees are sprinkled around so they don’t look like anyone planted them. The associate executive director of buildings and grounds for a historically fine institution told me he throws a stone over his shoulder to decide where to plant trees. And it looks like it.

Naturalism is easy, but making architecture in a campus landscape or arboretum is hard—you have to make decisions. You have to have reasons, be able to explain them, and believe that in a decade a visitor won’t need a sign or an explanation to see the beauty and botany of a tree garden.

Imagine an arboretum where groves of new, possibly native, closely planted trees are inserted into a young, regenerating forest in dense, recognizable masses of a single species. For a moment, while you are walking, you find the yellow-orange cast of Acer saccharum “Legacy” surrounding you. Then the path returns you to the diversity of the regenerating forest, which is then followed by the dense, muscular gray trunks of a grove of reddish-purple-leafed Carpinus caroliniana . . . —Gina Crandell