The late Alex Shigo was my teacher and forest companion. He is known to many as the father of modern arboriculture, and everyone who cares about trees owes him a debt of gratitude. Even those who found his teachings difficult to embrace were moved to see their own positions in a new light. Shigo still has detractors, but many more admirers. Everyone who plants or prunes has been influenced by his work or that of his philosophical heirs.
One day in the woods near his New Hampshire cottage he proclaimed: “In nature there are no pathogens.” Like many of his koan-like teachings, it took me quite a bit of hiking and pondering to get my head around this idea. I finally came to understand that “pathogen” is a label that arborists and others might use to describe organisms that interfere with their clients’ expectations, and that many of these organisms are essential to native ecosystems.
The damage caused by these native organisms (or “pests”) is frequently the result of human activity (including tree “care”) that has either stressed the tree or hindered natural controls. The non-native insects and diseases that are often so devastating have been introduced by human activity, not to mention the rampant exotic plants that have damaged habitats across the continent. In this respect, the true tree pests have human faces and have brought with them invasive ideas as well as pests and infections.
Seeing the Forest for the Fish
I found large areas of Alaska’s Tongass rainforest to be relatively free of pestiferous humans and their rapacious machines. In the 1970s I spent my college summers teaching native kids in remote Tlingit and Tshimshian villages. Many of the small islands in this archipelago were yet untouched by logging or development, and the virgin stands of spruce and Douglas fir grew right up to the water’s edge. I loved my solitary morning walks, the “tonic of wildness” that Henry David Thoreau prescribed. Some mornings I had bears on my left, whales on my right and eagles overhead. But paradise is seldom perfect; by late July the mosquitos and deer flies loved me as much as I loved the plenteous wild berries I ate along the way. And the forest reeked of rotting fish.
To appreciate the Tongass, like any wild place, requires a broad understanding of symbiosis. This term is usually reserved for close and obviously beneficial relationships between two or more organisms like the algae and fungi that compose lichens or the fungi that infect roots to form mycorrhizae. But symbiosis also pertains to relationships we might view as benign or harmful, or that are not very obvious. Symbiosis simply means “living together,” and a good grasp of how nature works often means refraining from imposing our agendas on the complex relationships that comprise ecosystems.
One might not immediately assume that trees and fish need each other or that a key component of rainforest conservation is protecting bear populations. Yet, in the Tongass, the lives of trees, bears and salmon are woven together. Scientists have traced the movement of nitrogen from salmon to tree canopies, revealing a complex symbiotic web. In turn, salmon spawn in streams protected by forest watersheds; the lives of trees and fish are intimately linked. And every one of the creatures of land and sky that eats salmon seems to leave some rotting remains behind in the woods, providing nitrogen and other essential elements to roots and mycorrhizal fungi. As a result of their accidental arboriculture, the bears smelled as foul as their dispositions—making it easier to avoid them.
Despite the smell and bug bites, it was easy to love all creatures great and small in a place that looked like a photo from a Sierra Club calendar. In contrast, commercial tree care commonly comes down to deciding which creatures are beneficial and which ones should be exterminated, often with poor or incomplete information. Some products promise with great swagger to kill all the bugs in your yard!
When native trees are severely damaged by native insects, fungi and other creatures that feed on them, it is often because the natural predators and competitors of these “pests” and “pathogens” are absent or few. This is one reason why diversity of plant, fungus and animal populations are so critical in forest and landscape. Many frequently used pesticides are toxic to both the “pests” and the creatures that could provide biological control, and continuous use can create resistant pest populations. In the long run, it is much more sensible, practical and environmentally responsible to cultivate diversity rather than to engage in chemical warfare.
Here be Bagworms
Bagworms teach the importance of biodiversity. They can be serious defoliators throughout the Great Plains on various species of conifers and broadleaf trees. They are difficult to control chemically, especially after they are ensconced in their tough satchels of silk and vegetation. Timing is everything; they are most susceptible during the “crawler” stage of development. This is also when they are most likely to be eaten. A songbird can eat hundreds of naked larval crawlers in an hour. Flycatchers, vireos, warblers and other insectivores snatch the flying males. Chickadees and titmouses can even eat the adult females and larvae right out of the bag and love to do so in winter.
Bagworms also fall prey to parasitic ichneumonid wasps. These nonstinging wasps lay their eggs in bagworm bags, where the hatchling larva feed on the living contents. The adult wasps feed on pollen and nectar, particularly of plants in the aster family. Studies have shown that planting daisies and other types of asters near host plants can reduce bagworm populations by attracting these wasps. Asters also bring the added benefit of attracting birds that feed on their seeds, and many of these—including chickadees—feed on adult and larval bagworms as well.
On our small acreage in midtown Omaha, bagworms are worth more alive than dead. We’ve planted lots of native habitat over the years, and on the rare occasion that I find more than a few bagworms, I just leave them alone. My family enjoys the rich avian display in every season. I wouldn’t mind a few more bagworms. What some consider pests we count on for bird food.
Heavy bagworm infestations are most common on isolated host trees growing in turfgrass lawns, but are rare in the wild. If you feel the need to reduce bagworms or if they are causing significant damage, pick off the ones you can reach by hand (or hire some kids to do this) and let the wasps and birds have the rest. Plant more native habitat. Some of the most commonly used pesticides labeled for bagworm control are toxic to a wide variety of insects and birds. Does it make sense to spray trees with chickadee-killer? Instead of bug spray, landscape trees need a good measure of Thoreau’s wild tonic. Humans need it, too.
On Walden Lawn
Like many nascent tree huggers, I grooved on the writings of Henry David Thoreau when I was required to read “Walden” in eighth-grade English class. I continue to be moved by his work, but was shocked to learn that for all of his teaching about the value of wilderness, he actually spent a relatively small amount of time in anything that resembled one. The Walden woods, about a mile from his family home, served as public park for picnics and cutting firewood. It was on a major rail line. Even while living at Walden pond, Thoreau routinely walked into town to dine with family and friends. He had lots of visitors and threw large parties in his little cabin by the pond. His actual “wilderness” lifestyle was so civilized that nature writer David Quammen has identified Thoreau as the first in a long line of Thoreau impersonators.
My first impulse was to announce to my fellow naturalists that Thoreau was a fraud. But after some reflection, I realized that the historical facts about his life actually gave him more credibility. I was easily inspired on my early morning walks in the remote Tongass. It might be obvious in such a place that “in wildness is the preservation of the world,” but to come to this understanding in a frequently visited park like Walden, just outside a growing population center in mid-19th-century Massachusetts, takes a little more insight. In fact, I think it makes Thoreau’s philosophy much more relevant and practical.
Thoreau found wilderness right in the middle of human settlement. His final literary work, unfinished at the time of his death in 1862, celebrates the “rich and fertile mystery” of nature. “Wild Fruits” was edited and published by Thoreau scholar Bradley Dean, and begins: “Most of us are still related to our native fields as the navigator to undiscovered islands in the sea.” He laments the fact that the citizens of Concord cultivate exotic and foreign shrubs in their yards, while the wild plants growing in the uncultivated fields and woods escape their notice. This recently rediscovered manuscript describes and praises the native and naturalized plants around his suburban home, including everything from wild oaks to imported dandelions. Thoreau ends his final work with the admonition that “each town should have a park, or rather a primitive forest, … for instruction and recreation.”
Trees Gone Wild
Humans have a long-standing ambivalence toward the kind of “primitive forests” that Thoreau recommended. I saw evidence of this in the forest surrounding New Skete Monastery, perched halfway up a mountain in New York’s Taconic range. My friend Brother Stavros showed me two giant red oaks that towered over the beeches, birches, maples and younger oaks that grew around them. As we pondered these trees, we discovered decayed stumps from other huge oaks and realized that these were “boundary oaks” that had been planted more than a century earlier to mark a property line.
The other trees in these woods were second or third growth, having sprouted after the logging and pasturing on this mountainside had been abandoned. This is the story of human settlement: forests were seen as sinister and savage places that hosted evil forces, or at least stood in the way of divinely ordained progress. For the New England colonists, to clear the land was to do God’s will. Thankfully, the monks of New Skete regard the preservation of the forest as sacred work.
But here lies the irony. After forests were cleared for New England farms, native oaks were replanted to delineate property and to shelter human dwellings. Obviously this schizoid imperative continues all across North America; land is stripped of topsoil and vegetation to create landscapes of concrete, exotic turf and grafted designer trees. It seems that for many Americans, the only good tree is a dead tree replaced by new-and-improved tree, destined to languish in a wasteland of fertilized and sanitized lawn. But what is the alternative? What would the neighbors say?
Fortunately, growing healthy trees doesn’t always require smelly bears and rotting fish heads. Nutrients in nature are recycled by roots and soil fungi and countless creatures that pass organic material through their digestive systems. The variety of plant species in a community, each having different needs, grow roots in various densities and depths in soil—drawing essential elements into their tissues and depositing them on the surface when these tissues are shed or when the plant dies. These plants also capture energy from the sun and exude sugars directly in to soil at varying depths, thus feeding vast numbers of soil organisms behaving wildly.
Trees did not evolve in landscapes dominated by lawn and are ill-equipped for suburban life. Progressive tree care requires a movement away from green sterility and toward a wilderness in soil. One can begin the process by planting native plant communities, returning leaves, clippings and other dead vegetation to plants and soil to which they belong, and by avoiding chemical fertilizers, pesticides and herbicides. Growing wildness provides habitat and food for the birds that delight us and for the worms, bugs and microbes that make plants green and flowers colorful. All of these creatures will help to create the kind of symbiosis that will turn pests and pathogens into partners. This is the “rich and fertile mystery” embraced by Thoreau and Shigo and woodsy mountain monks. Make it grow outside your door.