The Arbornaut, page 34
Technically, Animal Planet television was not exactly correct to call them extremophiles, because tardigrades are classified as “extremotolerant” organisms, which is the secret of their capacity to endure environmental adversity. During droughts, they enter a dormant phase called cryptobiosis, transforming into a tiny desiccated ball called a tun. During other environmental extremes, they survive by swelling up like a balloon and floating into the atmosphere, scientifically termed anoxybiosis or commonly known as tardigrade rain, to seek better conditions for their lifestyle. Tardigrades have come back to life when a drop of water was administered to a tun in a dried plant collection over a hundred years old! Another was revived from an Antarctic moss sample that had been stored and frozen for over thirty years. They have flown as experimental organisms on NASA expeditions and not only survived, but also reproduced in outer space. During 2019, the Israeli Beresheet probe was intended to be the first private vehicle to land on the moon, until the robotic craft lost contact with mission control and crashed. Organizers believe its cargo, including a few thousand tiny water bears, may have survived when the draft released its payload onto the landscape. This means a population of tardigrade tuns could now exist on the moon, a harsh environment for mammals but entirely tolerable for the toughest animal on Earth! Technically, they need an atmosphere plus water to colonize, but can exist as tuns for a long time. Will scientists find water on the moon? Who knows, but if it does exist somewhere on the lunar landscape, water bears may be the first to detect it.
Our students addressed questions new to science: What is the density of water bears in different canopies? Are there hundreds per tree, or millions? Of the same species, or different? Over five summers, we collected 28,384 samples representing 37 species from 58 forests in 4 states, ranging from zero to 4,500 feet in elevation and from zero to 200-foot-high trees. Our students climbed 492 crowns and discovered 8 new species, plus set 26 new distribution records. What was especially amazing (called an “OH WOW factor” in field biology) is that 80 percent of every sample of lichen, moss, bark, or foliage collected contained at least one water bear! And it is likely the other 20 percent also had a water bear, but our microscopic techniques were not so exacting with amateur eyes. Because of the labor-intensive process to analyze each sample, most of our questions posed remained unanswered. It will probably take years to analyze enough samples to determine how many tardigrades, on average, live in the canopy. But based on preliminary findings, there are probably over a billion tardigrades per ten acres of forest. Over five summers, we focused on the local trees of Kansas, because it was most cost-effective with our limited budget in the research grant. But each summer we also visited one field station or national forest around the country, asking a curious question: Does water bear density differ in forests across the country, such as between oak-maple stands in Massachusetts and old-growth conifers in Oregon? To answer that, we visited several ecological field stations across America to collect samples. Preliminary counts indicate that Massachusetts temperate forests have more water bears than Kansas, Florida, or Oregon, but as with all ecological research, one summer of sampling is not conclusive. One finding emerged from all sites, including Kansas: higher densities of tardigrades live in the treetops than the understory. As with so many aspects of field biology, especially biodiversity, both funds and labor are limited, so the collections are slow to process.
Our most daunting sampling escapade involved climbing in the Pacific Northwest, where the average height of a mature western conifer is about two hundred feet. Our students were already adept at climbing fifty-foot-high oak and ash in Kansas, but four times more distance to climb is a lot of perspiration. To ensure absolute safety, we recruited an arborist who was particularly qualified to rig tall trees. After flying from Kansas to San Francisco, California, our western expedition started with a walk on an ADA-accessible trail under the coastal redwoods (Sequoia sempervirens) in Muir Woods. That evening, I asked the students to write in their journals about how they felt to first set eyes on some of the world’s tallest trees. One commented, “Looking at the size of these trees and the magnitude of their trunks makes me realize how small I am in the world, and especially in the universe.” How true. The Pacific Northwest is home to the world’s tallest species, yet fewer than 10 percent of Americans have ever seen a redwood. More people have climbed Mount Everest than scaled a redwood! Despite their stature and geographic location in one of the most science-savvy regions of the world, almost nothing is known about their canopies. It is not easy to scale these giants, with such enormous height and lack of side branches for positioning a rope. Other tall denizens of Pacific Northwest conifer stands include giant sequoia (Sequoia gigantea), western hemlock (Tsuga heterophylla), Douglas fir (Pseudotsuga menziesii), western red cedar (Thuja plicata), noble fir (Abies procera), and Pacific silver fir (Abies amabilis), all equally daunting to climb. With the advent of high-powered slingshots and extra comfortable harnesses, forest scientists can now safely explore the upper reaches of the crowns. Our students sampled in several different crowns and a week later, back in the lab, discovered that these moist canopies represented ideal habitat for water bears, with one dominant tardigrade, Pilatoibus oculatus, comprising 52 percent of their collections. Redwoods often live surrounded by fog, and that moisture undoubtedly offers a welcoming ecosystem for water bears. Their crowns not only produce energy from sunlight in the conventional fashion by drawing water up from their roots, but occasionally shortcut the system by directly absorbing fog into their foliage through stomata. In the past decade, the botanist Todd Dawson at University of California, Berkeley, and his students discovered this unique pathway where fog offers an advantage for photosynthesis, explaining why these trees thrive in foggy coastal areas along the Pacific coast. In addition to providing water directly to the needles, fog surrounds the crowns and reduces evaporation, increasing the water efficiency of redwoods and other neighboring vegetation. These tall trees not only directly absorb water into their foliage via fog, but also take in moisture in the conventional fashion, from the roots to the leaves via a vast network of xylem cells. Like a straw in a milkshake, the xylem tissue operates with a suction mechanism that draws water upward as a key ingredient for photosynthesis in the foliage. Water moving up a tall redwood represents one heck of a big straw!
Current climate-change models predict decreases in fog for California’s coast as well as the advent of more frequent droughts. The Arizona State University biologist Greg Asner conducted aerial surveys of forests before and after the extreme droughts of the years 2014 through 2016, working in collaboration with the researchers Anthony Ambrose and Wendy Baxter, who climbed the trees and provided ground-truthing by confirming the results of the imagery with an up-close examination. By repeated canopy measurements from within the canopy and from afar, Greg, Anthony, and Wendy found that individual canopies transpired upward of five hundred to eight hundred liters (132 to 211 gallons) of water per day! This enormous figure far exceeded estimates based on ground-based calculations alone. Since then, teams in airplanes have surveyed Northern California forests to monitor the impacts of drought, assessing losses in water content using laser-guided spectroscopy and satellite-based models. The results indicated an astounding number of dead crowns, and severe water loss in over 30 percent of a one-million-hectare forest, comprising approximately fifty-eight million large trees. As climate change increasingly causes extremes in both rainfall and fog, such moisture fluctuations represent an enormous threat to the future health of these otherwise resilient giants.
For scientists like me who study herbivory, the redwoods are puzzling because they are almost devoid of foliage feeders, despite the enormity of their salad bar. In fact, they have escaped predation over millennia, which is almost entirely unique in the world of ecology where at least one insect usually evolves to defoliate every species of plant. Paul Fine at the University of California, Berkeley, is currently studying the toxins of redwood foliage and bark, to figure out how they have remained immune to insect attack over thousands of years. A few historic observations have cited very mild impacts from cone moths and roundheaded borers attacking cones and seeds; foliage nibbled or sucked by aphids, scales, mealybugs, leaf beetles, and a few others; twigs attacked by bark beetles and twig borers; buds nibbled by tip moths; and occasional bark stripping by bears. Unfortunately, in 2020 the first mortality of a giant sequoia from insect attack was reported in Sequoia National Park, California. This tree was over two thousand years old, and its neighbors had recently died from drought and fires, leaving the remaining individuals more vulnerable. Bark beetles had invaded the upper branches and caused the first documented death of a sequoia from insect attack, presumably because the trees were exceptionally vulnerable due to climate extremes. Sadly, similar deaths are occurring with other conifers throughout the region. As Dr. Christy Brigham, who oversees the forests of Sequoia and Kings Canyon National Parks, admitted to The Guardian, “This is not how giant sequoias die. It’s supposed to stand there for another 500 years.” As climate change becomes more extreme, it is beginning to impact even the world’s biggest, longest-lived giants.
It is not easy to study the canopies of the world’s tallest trees. Not only redwoods in California, but the dipterocarps in Malaysia, great kapoks of the Amazon, and even the cardboard trees of Africa posed great challenges for safety and scientific accuracy. Ropes and slingshots are not effective for their upper reaches, and walkways are not always feasible to engineer. A fourth unique tool in our arbornaut’s toolkit that has been utilized for the highest crowns, including in the Pacific Northwest, is a construction crane. Albeit expensive to operate because it requires a highly paid professional driver, the crane provides easily repeated access to the uppermost reaches of the highest crowns, because its crane arm rises above. One crane requires about a million dollars to acquire and install (usually purchased secondhand from a construction company), in addition to operating expenses. As a reality check, that cost is minuscule compared to the budgets of NASA or a particle accelerator, which require billions of dollars, but the budgets for field biology are significantly less than for outer space exploration or physics. The Wind River Canopy Crane was operated in the Pacific Northwest by the University of Washington during the late twentieth century, allowing unique access to some of the world’s tallest trees. I was fortunate to be one of a handful of arbornauts who used this crane for research. Our team sampled insect damage in the tops of Douglas fir, western hemlock, Pacific yew, and western red cedar. We created a sophisticated and highly accurate sampling regime, generating 101 random points in the canopy with a three-dimensional computer model, and then used the crane bucket to access each sampling point over the period of one week and conduct replicated measurements of insect damage to the foliage. The results showed the least insect damage of any forest type in the world, with an average of 0.3 percent leaf area consumption. This is next to nothing, especially as compared to tropical foliage in Australia and the Amazon averaging 15 to 30 percent leaf area consumption. Half of the sample points had no detectable herbivory on the conifer needles, making this one of the world’s most resistant foliage. These conifers contain effective defense chemicals and appear to stay ahead of the rapid adaptation of insects to digest such toxins. Because one old-growth Douglas fir contains well over one million leaves, accurate subsampling to measure herbivory was key to obtaining accurate results.
That same Oregon crane bucket was also ideal for sharing the treetops with middle school students who could never have climbed up 150 feet on their first attempt. As part of the JASON Project virtual expedition series, I took approximately ten sixth graders on a mini expedition into the enormous Wind River crane bucket to survey some of the world’s highest foliage. One youth from the Bronx gasped with excitement when he saw a banana slug on a branch as our gondola car rose higher, and he grinned from ear to ear. Cranes made it easy to share the eighth continent with even the most landlocked citizens, or with the least likely arbornauts. Approximately ten canopy cranes exist around the world, including two in Panama, several in China, one in Australia, some in European temperate forests, and a few in planning stages. Unfortunately, the Pacific Northwest crane has ceased operation after funding cuts and a history of political skirmishes between loggers and environmentalists. Now China leads the way with three cranes, and Germany boasts two, all dedicated to canopy research.
I had my first experience operating a crane when I went to survey herbivory in the tropical forests of Panama. How exhilarating to stand in the bucket up close to a large cluster of iguanas sunbathing in the upper branches of Swartzia simplex (later calculated as 10.1 percent herbivory), and to sample hundreds of sun leaves in one day using the comfort of a four-foot-by-four-foot crane bucket to maneuver between crowns. Compared to single-rope techniques, conducting research from a crane bucket is easy; the toughest challenge in Panama for me was learning to communicate directional vocabulary in Spanish to a crane operator via walkie-talkie. As with most leaf research, I am a stickler for not bumping the branches or tearing my subjects—so I kept reminding the driver to steer delicately. Cranes require almost no perspiration, but they are too expensive for most field teams to afford, and sampling is limited to the reach of the crane arm. Even walkways offer more flexible canopy access because bridges and platforms can be moved or expanded at a relatively low cost, plus offer access twenty-four hours per day without the expense of hiring professional drivers. But cranes have inspired canopy research in the Pacific Northwest as well as in Panama and Germany.
Although we did not use the canopy crane for collecting, our water bear findings from the Pacific Northwest are the first records of this phylum from these tall trees, but they may require a decade to fully analyze. Regardless of the slow process of classifying and publishing the findings, our mobility-limited students can be proud of their achievements, and I hope more than one of them will pursue a career in field biology! A star student named Rebecca, who was shy and reminded me of myself, later accompanied me to the Amazon jungles, where she undertook research in tropical trees. Loading and unloading her wheelchair from small dugout canoes was not easy, but the trip fulfilled a lifelong dream for her to experience the Amazonian rain forests—and she is now a seasoned arbornaut! Not bad for a reticent young lady with limited mobility working alongside her treetop supervisor, who still sometimes acts like a wallflower, even in adulthood.
Coastal Redwood
(Sequoia sempervirens)
A FRANCISCAN MISSIONARY, FRAY JUAN CRESPÍ, authored the first written record of redwood trees in his diary on October 10, 1769, near Monterey Bay, California. Imagine those early explorers struggling to advance their brave teams through the treacherous Northern California terrain of ancient Paleozoic and Mesozoic rocky slopes, only to enter a valley of enormous reddish trunks. Crespí was the only Franciscan monk to traipse from Baja California northward to what is now San Francisco, as the official expedition diarist, and he wrote of “very high trees of a red color, not known to us … In this region there is a great abundance of these trees and because none of the expedition recognizes them, they are named redwood for their color.” From his simple descriptive journal entry, a humble missionary gave rise to the name of America’s tallest and most iconic species.
In geological nomenclature, redwoods are called “paleoendemics,” meaning a species with an extant range that represents a remnant of its former distribution in the fossil record. This species is, in a sense, a living fossil. First recorded among the fossils of the Mesozoic Era some 150 million to 200 million years ago, redwoods had a nearly circumpolar arctic distribution as well as extensive mid-latitude range and were common throughout the western United States, Canada, Europe, Greenland, and China. As the climate became cooler and drier almost two million years ago during the Quaternary, remaining stands shrank to their current distribution along the Pacific coast of North America. Extensive logging practices during the nineteenth century reduced these fragments even further to 120,000 precious acres, according to Save the Redwoods League at the time of their centennial in 2019.
Early redwood science was primarily based around extracting timber. Many forestry reports cited estimates of wood volume for different old-growth stands, plus their variation in structure and species composition. In 1934, the forester W. Hallin published the largest volume of wood per unit area; he recorded 10,856 cubic yards per hectare, which represented 178 stems. (Based on the average home requirement of 2,480 board feet in 2011, that is enough timber for 188 homes!) Other twentieth-century research included the interface of stand dynamics and environmental conditions, as biologists sought to understand how the trees grew so tall. Despite an overarching interest in harvesting these giants, biological curiosity also grew, along with a strong conservation ethic. Only in the last few decades have microclimate, photosynthesis, soil mats on branches, light requirements, and canopy biodiversity become hot topics, but initial research was predominantly limited to ground-based observations.
One of the newest discoveries about redwoods involves their water hydraulics. Most foliage absorbs carbon dioxide in leaf surfaces through openings called stomata, and in turn releases water in a process called transpiration. This triggers a siphon (or straw-like) train of water molecules pulled up from the roots through the xylem cells, a virtual water channel up the trunk. In short, trees serve as the exhale to our inhale, filtering carbon dioxide from the air and exchanging it for oxygen. This photosynthetic process whereby trees can produce energy from the sun using hydrogen and oxygen from water is especially productive in redwoods, because their height allows for so many layers of foliage to operate simultaneously. Leaf area index has been measured at 14.2, meaning each square yard of forest floor contains 14.2 layers of foliage overhead. If one were to dangle a string down from the top of the canopy, it would intercept 14.2 needles! That makes for a lot of photosynthesis going on overhead! (Contrast this statistic with New England deciduous forests, which have a leaf area index of approximately 4 to 6, meaning a string dangling through their canopy depth would intersect only 4 to 6 leaves.) Other creative water strategies in these crowns include the direct intake from fog into leaf stomata, as described in the previous chapter.
