And there is a lot to think about. Our boys and our friends in their twenties can think about the young forest their kids will run around in, the big trees they may see as old men and women. But if you’re in your forties or fifties or older, these trees are not for you. Planting trees is a gentle way of acknowledging your mortality and celebrating it. The trees you are putting into the ground are a gift to the world. They are a thank-you present.
This is a good thing to think about at the end of the day. Gather your tools and your people together as the sun sets or the rain rolls in and look out at what you’ve done. There are little green sprigs scattered through the field. They will be a forest one day.
Blood, Sweat, Tears
When Susan’s father was planting trees with his family, back in the late 1930s and early ’40s, his mother had her very own planting system. As she finished putting a sapling in the ground, she would bend over it, wag a finger, and say, “Now, grow!”
And so they did. Forty-five years later, Carl built a house from some of those trees, cut during a thinning operation. More recently the Leopold family has donated posts, beams, and paneling to educational organizations in Wisconsin and beyond.
In the Tarboo Valley, we’ve looked at rings on recently harvested Douglas-firs and counted seventy on stumps that are 4 feet across. If our boys take care of themselves and are lucky, they may live to see trees that they planted get to that size. On the Olympic Peninsula, there are places where western redcedar trees are a thousand years old and where firs are more than 250 feet tall. At least a few of the little trees that we’re planting may get that old and that large. But between planting two-year-old saplings and walking or harvesting a mature forest, there is work to be done.
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One aspect of managing a newly reforested area is common to every restoration site, whether it’s a prairie or a rainforest or a grove of mangroves: controlling invasives. Invasives are plants that are exotic—meaning not native to the site—and aggressive enough to outcompete the native vegetation. Only a small fraction of exotic plants—something like 10 to 15 percent—become invasive; why some do and some don’t is a question biologists have been wrestling with for 150 years.
Initially, it seems paradoxical to observe exotic species outcompeting natives. After all, the natives have been living at the location for millennia and should be well adapted to local conditions. How do invasives outcompete them and take over? Sometimes the exotics use forms of chemical warfare that are new to the natives. Spotted knapweed, for example, emits an herbicide that native plants in North America have never been exposed to; the chemical it “sprays” may help it subdue susceptible competitors and take over sites. And unlike most plants, garlic mustard does not depend on fungi in the soil to supply its nitrogen and phosphorus in exchange for sugars. Instead, it secretes a fungicide that kills native soil fungi, causing their associated plants to weaken from malnutrition.
In many or most cases, though, the signature attribute of invasives is that they lack the herbivores and the pathogens—fungi, viruses, bacteria, and other disease-causing organisms—that native plants have to deal with. Think of the number of infectious agents and parasites you work to avoid, from colds and flus to mumps and food poisoning. The total is in the hundreds, and every organism on the planet has a similar array of viruses, bacteria, and other parasites that can infect it. To pathogens and parasites, a person or sword fern or cedar tree is a habitat: a paradise brimming with resources. Plants have insect enemies to worry about as well—some of which specialize in a single species, others that can eat a variety.
Trees and shrubs are constantly monitoring their own bodies and their surroundings for signs of danger. They can tell when their roots or leaves are being chewed, or when viruses have entered a wound and are infecting their cells. They can even tell when nearby plants are being chewed by caterpillars or other herbivores; in response they manufacture proteins that clog the guts of herbivorous insects, and they sequester these poisons in their leaves, ready for the attack to begin. If they detect a viral infection, they wall off the site and kill the remaining cells within, wiping out the viruses in the process. And if they are attacked by caterpillars themselves, they may emit molecules that vaporize and are sensed by species of wasps called parasitoids. The wasps follow the scent trail to the plant and lay eggs inside the bodies of the caterpillars. The wasp larvae then eat the caterpillars from the inside out, killing them. In effect, the plants have called in close air support from waspy helicopters.
Imagine the competitive advantage for invasives, then, which don’t have to expend all of this time and energy defending themselves. When researchers study plants that have become invasive by comparing their growth in a native habitat with their progress in a region where they’ve been introduced, the data usually support the predicted outcome: the invasive populations have fewer pests. They commit fewer resources to defense and more to getting big and gobbling up space. This is not ecological fair play. And there are consequences. A recent survey concluded that more than 20 percent of the endangered plant species in Canada are victimized by competition from invasives.
Introduced predators can be just as devastating, for the natives have no co-evolved defenses. Small, flightless birds called rails were no match for the rats introduced by humans to the Pacific Islands; David Steadman, curator of ornithology at the Florida Museum of Natural History, estimates that more than a thousand species of rails—essentially, at least one per island—were wiped out by rats who stowed away with the humans who colonized the Pacific.
Exotic diseases may be the most dangerous threat of all, though. Introduced strains of avian malaria wiped out dozens of native bird species in Hawaii; many of the survivors are reduced to relict populations at high elevation. Malaria is transmitted by mosquitoes, and the mountaintop habitats are too cold for mosquitoes to thrive. Similarly, the spread of West Nile virus is reducing bird populations in eastern North America; an introduced fungal pathogen is wiping out hundreds of species of frogs and salamanders all over the world; and white-nose syndrome is devastating North American bats.
Nonnative diseases are an issue for plants as well. Plant cells recognize pathogenic viruses and bacteria in much the same way as your immune system cells do. The process starts when proteins manufactured by our cells stick to specific parts of proteins from an invader. This binding event sets off alarm bells that lead to an effective defense response. But the proteins found on certain exotic fungi were never recognized by the sentinel proteins in American elms and American chestnuts because they were different from the proteins found on the native fungi. No binding occurred, no alarm bells rang, and two of the most important trees native to eastern North America were virtually wiped out. They never knew what hit them.
People have littered exotics all over the world. In Japan I’ve seen goldenrods from North America growing as invasives—each plant more than 6 feet high and featuring tough, woody stems. Hordes of them have taken over roadsides, forming dense monocultures—a far different situation from the scattered individuals found among dozens of other species in their native prairies. In Costa Rica, at the site where Carl led a successful effort to reforest abandoned pastures with native rainforest trees, the restoration crew had to battle grasses imported from Africa.
At Tarboo Creek the bad actors are holly trees, reed canary grass, and Himalayan and Eurasian blackberry bushes. Holly grows in the woods, reed canary flourishes on wet soils, and the blackberries are everywhere. Each of us has it in for one of them. Our older son cuts holly on sight; Susan can’t pass a clump of reed canary without stomping on it or trying to yank it out; I can machete or dig out blackberry for hours. Our younger son is a gentle soul. He lives and lets live—but only because his least-favorite invasive, Scotch broom, is rare at our place.
The reed canary and blackberry should eventually disappear as the trees grow and shade them out. But we will never be rid of the holly at Tarboo because it’s shade tolerant—it c
an grow under a forest canopy. We also have neighbors who own a copse of mature holly trees, each of which bears bushels of bright red fruit in winter. The berries make wonderful holiday decorations and are a favorite with the robins. The birds eat the berries and scatter them around our woods along with little packets of fertilizer—sowing the seeds of destruction.
Invasives are a bad business. Cheatgrass has forever altered the short-grass prairies and sagelands of the Rockies and Intermountain West; the economic losses to the ranching industry are incalculable. Kudzu has formed impenetrable monocultures in abandoned farmlands of the American South that otherwise would have succeeded to productive forest.
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When biologists catalog threatened species and analyze the causes of decline, they find a strong historical shift: until about 1950, most extinctions and other declines were caused by overhunting. Overfishing is still a concern for many marine species, and overhunting is causing problems for certain terrestrial mammals, like elephants, and birds in selected areas. But the general situation has changed in a fundamental way: the overwhelming majority of current and projected problems are due to habitat loss and invasive species.
Although invasives can be deadly, ecologists and restorationists are beginning to realize that they are also a fact of life. For better or for worse, humans have changed the geographic ranges of more species faster and more broadly than at any previous time since life began.
We can’t go back and undo the changes or the damage. Even if we had the time and the money, invasives are too well entrenched to be eliminated entirely. The only real solution is for evolution by natural selection to run its course. If a lucky mutation in an American elm allows it to recognize and respond to its introduced fungus, or if individuals from native fungal species experience chance mutations that make them immune to garlic mustard’s poison, they will be fruitful and multiply. At some point, a virus native to North America will have a mutation that allows it to infect cells in the stems of reed canary grass, smiting it. When some beetle sustains a mutation that allows it to digest Himalayan blackberry sap, that fortunate insect will inherit the earth, or at least a small part of it. I hope I live to see the day.
In the meantime, restorationists cope. Occasionally I work through a blackberry patch with nippers in one hand to cut the stems and an herbicide-laden brush in the other to wipe the stumps. But more frequently we cut and recut. We are trying to keep the invasives down to a dull roar, buying time and space for the natives that we are introducing to get their feet on the ground and overgrow the exotics. We play the role of umpire, trying to level the ecological playing field.
Planting native species is a way to practice poetry, but controlling invasives is all prose. The work makes you sweat; the thorns and prickles draw blood; their persistence brings you to tears.
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The first restoration project Susan and I worked on together was near the Leopold Shack—the abandoned farm in central Wisconsin that Carl and his parents and siblings had transformed a generation before. On our walks around the place we found a little postage stamp of native prairie: a 4-acre site where a few native grasses and wildflowers were struggling to hold on, and where some swamp white oak trees were growing. It was a relict savanna—a plant community made up of a prairie understory with scattered trees.
The neighborhood where I grew up in southern Wisconsin and the campus where I was a college student in southern Minnesota had been covered with burr oak savanna when Europeans arrived. Burr oaks are stout, slow growing, thick barked, and deep rooted. All of these characteristics helped the trees withstand intermittent droughts and the fires set by native people from the Sioux, Fox, and Winnebago nations to kill invading trees and encourage grass for deer, bison, and elk. Because they grew in the open, the fire-resistant burr oaks developed wide, craggy, umbrella-like crowns. The combination of burr oaks and prairie grasses created parklands in midwestern North America akin to the Maasai Mara National Reserve in east Africa.
People respond to these environments instinctively. As our eyes sweep across a landscape dominated by savanna, it’s as if our subconscious recites, “Here is pasture for my cattle; wood for my hearth and home. The terrain is easy to travel and teeming with game. There is nowhere for my enemies to hide. The ponds and rivers have fresh, clear water, teeming with fish. Here I will live.”
We evolved in savannas and are hardwired to love them. That’s why landscape architects are always trying to reproduce them, using exotic trees and mowed grasses in corporate and college campuses. I even wonder sometimes about the savanna-like appearance of strip developments in suburban settings. Instead of grassland, there are expanses of concrete. Instead of acacia trees, there are scattered, spindly signs advertising gas stations or franchise restaurants.
When Susan and I found this bit of ragtag but actual savanna near the Leopold Shack, it was being invaded by a shrub called prickly ash. Although prickly ash is native to North America and isn’t considered an invasive, it started to take over vast swaths of the Wisconsin River floodplain after white settlers pushed the native people out. The whites didn’t follow the traditional practice of setting fires to clear out woody shrubs and young trees and encourage the prairie flora, and lack of fire led to a dramatic increase in prickly ash. Its takeover of the savanna wasn’t quite complete, however. Under and between the dense thickets of prickly ash stems, we found wisps of cordgrass, bluejoint grass, and other native species. But what made the place special was the swamp white oak trees.
The burr oak savanna that once grew in my boyhood neighborhood is rare now—vast areas have been converted to farms and suburbs, while other sites succeeded to continuous forest after Europeans arrived and controlled wildfires—but swamp white oak savanna is almost nonexistent. Swamp white oaks have such a limited geographic range that the habitat type was never common, and today this version of savanna is essentially extinct. We thought of Susan’s grandfather, who justified his restoration efforts in the 1930s by observing, “The first step in intelligent tinkering is to save all the parts.” Swamp white oak savanna was a part that had almost been lost, so we wanted to save the little patch near the Leopold Shack.
In this case, the restoration plan was straightforward. The first step was to cut the prickly ash out; the second was to start a regular program of prescribed burns. The fires would keep the prickly ash down and out and encourage natives to fill the vacated space. Once the burning regimen was under way, we could seed in new prairie species to increase diversity.
Step one took a day—a long day. We recruited a gang of friends and relatives for a work party one hot summer Saturday. A cousin of Susan’s and I each made a vanguard, running handheld brush cutters—long bars with a two-cycle engine on one end cabled to a circular saw blade on the other end—into the tangles of prickly ash. The rest of the crew worked behind us. As we scythed back and forth with the saws, the crew bucket-brigaded the cut stems out and threw them into great stacks—thorny versions of Monet’s haymows.
It was brutal work. Prickly ash is weaponized with thorns, and even with long sleeves and leather gloves, Susan’s cousin and I were scratched and clawed and bleeding. At lunch, when we’d completed about half of the cutting, he collapsed against a grassy bank. His shirt was streaked with mud and soaked with sweat; his horn-rimmed glasses had been knocked askew; his prickly ash gashes were glowing red. He looked at me and pleaded, “Now, tell me again: Why are we doing this?”
I didn’t have much of a response then, but a year later, we did. Friends helped us burn the patch early in the autumn after the work party, and the following summer, scattered among the charred and rotting prickly ash stumps, Susan and I found a spectacular show of bottle gentian—an uncommon forb with a flower so purple it should be worn only by royalty. The gentians hadn’t been able to bloom there for decades. Susan’s Savanna is now one of the pearls of the Leopold Memorial Reserve.
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In the Tarboo Valley, the prickly ash equi
valents are the exotic shrubs called Himalayan and Eurasian blackberry. Peter Bahls has a name for blackberry removal: “The worst.” He hires crews of twenty-somethings to cut it and then dig it out in areas where plant-a-thons are scheduled for the following winter. The nwi crew members end up as sweaty, scratched, haggard, and muddy as Susan’s cousin and I were. Indeed, a family friend who did blackberry removal in the Seattle parks system told a story that captured the toll blackberry can take. She was in Seattle’s University District after work one evening, waiting for a bus home, and got panhandled from behind by a street kid. When she turned to face him, he backed away, horrified. He took in her blackberry scratches, dirty face, and disheveled clothing and said, “Oh, I’m so sorry, man. Hey, good luck.”
When I spend a weekend cutting blackberry at our place, it’s often in thickets where the blackberry canes are growing over the tops of the other shrubs, 8 feet in the air. I show up at work on Monday morning with scratches on my forehead and scalp. I get a few “What happened to you?” queries, but there isn’t much to say. Invasive control is down and dirty.
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On every trip to Tarboo Creek, our older son reminds us to do something constructive and something destructive. If we rip into a blackberry patch in the morning, we plant trees or put up a wood duck box or take a long walk to check on saplings in the afternoon. The walks through the restoration area are especially rewarding. You are visiting loved ones with your loved ones—checking on their health and well-being.
Ten-plus growing seasons after the initial plant-a-thon at our place, many of the trees in the grassy old pasture have just begun to grow in earnest. Their root systems have finally gotten big enough to compete effectively with the overlying grass net. For years their trunks and branches had a cramped and stunted look, as if they were hunched over in pain. Some were barely 2 feet tall five years after planting, not even double the size they were on planting day. But closer to the creek, where the grass was partially shaded by the alders that remained near the stream channel, and where the low-lying soils don’t dry as thoroughly in late summer, the trees began to flourish almost immediately. Within a few years their leaders were casting upward 3 feet a year; within the decade the tree canopy had closed and the creek was shaded and cooled.
Saving Tarboo Creek Page 10