The result of this acknowledgment of the limits of forests was somewhat heartening. In many parts of the temperate world, a substantial release of land from agriculture led to a resurrection of forests, as has been documented in England, France, China, Japan, and eastern North America. Land records and, later, aerial photographs show entire regions where forest stands emerged as cohorts. In these stands it is easy to see how species composition and structure reveal imprints of dynamic land-use changes and population growth. An example is found in New England, 98 percent of which was forested before Europeans arrived. By 1840, after the harvesting of timber for ship masts and charcoal, the expansion of colonization, and the rise of American industries, only 25 percent of these forests remained. Today forests cover approximately 75 percent of New England, but only a fraction of these new forests are due to natural regeneration. Similarly, by 1800 some form of silviculture had been established in both Asia and Europe, and by 1900 timber management was a global project of resource production, just as agriculture had been in previous centuries.
Management of timber can take many forms. The practice of clear-cutting stands to replace them with a single fast-growing species has maximized the use of timber material in managed forests but also has made them vulnerable to pest outbreaks and climate shifts. A new emphasis on longer-term sustainability is growing in importance across countries with temperate forests. The Menominee nation in Wisconsin has long practiced timber production in this way, managing their forests for hundreds of years with a “seven generation” approach. They plant, cultivate, and remove trees across a variety of landscapes and through different climate patterns, considering both profitability and sustainability. Their practices include single-tree harvesting; cultivating a combination of species that best corresponds to given soils, topography, and past forest composition; harvesting damaged trees; and leaving healthy trees in longer rotations than most commercial timber companies do. Such sustainable practices are now being recognized as critical to global temperate forest management and afforestation initiatives worldwide, especially as climate change and invasive pests and pathogens seriously threaten species-poor and hastily harvested forests.
Scientific research on forest growth and health has been instrumental in the recovery of temperate forests over the past century, as accurate descriptions of the ecological dynamics of forests have informed effective efforts to preserve and regrow them. Temperate forests were the first to have large monitoring plots installed; in Czechia, for example, careful measurements of tree location, size, and species date back to the late nineteenth century. Such monitoring programs can follow the dynamics of long-lived and complex systems. The Smithsonian Institution’s Forest Global Earth Observatory (ForestGEO) is a network of research plots where every stem is identified, mapped, and measured every five years. ForestGEO has extensive coverage across China, North America, and Europe, with large plots demonstrating the influence of land-use history, species diversity, and climate on the productivity and health of temperate forests. These research forests are now highly instrumented—everything from photosynthesis in the upper canopy to the chemical links between roots and soil microbes may be measured. These efforts are directed at learning how forests evolved to their current state and then using that knowledge to better predict how they may change in the coming decades.
A folktale may teach a lesson about life or society. The lesson of the tale of temperate forests is clear: to live sustainably, humans must consider how best to preserve, regrow, manage, and protect forests. Many societies have learned this lesson, but not all have changed their practices to reflect the knowledge gained. Tropical forests are now being treated in a way similar to that which led to the vast disappearance of temperate forests; further, the deforestation of the tropics is largely a response to the demands of the temperate zone, undertaken for the economic benefit of its societies, which need only look into their own histories to understand what the fate of the tropics may be. In a fast-changing world that will put ever more demands on land and resources, a global plan is needed for the implementation of management policy and practices that stem from a scientific understanding of how forests function. Initiatives to manage, monitor, and sustain forests have emerged as important features of local and global conservation efforts, but there remains a distressing disconnect between the clear arc of the tale of temperate forests and the path that society is on.
URBAN NATURE / HUMAN NATURE
PETER DEL TREDICI
From their humble beginnings as isolated settlements at the intersections of important transportation routes, today’s cities have evolved into pillars of the Anthropocene. They house half of the world’s human population and produce 70 percent of its fossil fuel emissions while occupying only 3 percent of Earth’s land area. In cities, human values—driven by socioeconomic factors—trump ecological values, such that people encourage the presence of organisms that make the environment a more attractive, livable, or profitable place to be, and vilify as weeds and pests those species that flourish in opposition to these goals.
In most modern cities, the native vegetation that originally occupied the site is long gone. In its place, one typically finds a cosmopolitan array of species—some planted intentionally, some growing spontaneously—that are adapted to the ecological conditions generated by the city itself. In urban areas in northeastern North America, it has been estimated that 25 to 40 percent of the spontaneous vegetation is nonnative, a figure that rises to 70 percent when one looks at only the densely populated core regions. Just as they are for people, cities are melting pots for plants, and questions about where they came from become irrelevant after a few generations.
From a functional perspective, most vegetated urban land can be classified into one of three broad categories: remnant native landscapes, which are left over from the earliest days of urban settlement and are composed mainly of native plants; managed horticultural landscapes, which are composed of horticultural plants cultivated for specific purposes (e.g., ball fields, parks, gardens, street trees); and abandoned or neglected landscapes, which no one takes care of and which are dominated by plants that flourish without human intervention. Depending on the socioeconomic conditions of a given city, this last land-use category can make up from 5 to 40 percent of the total area. While most people have a negative view of spontaneous urban plants, they are actually performing many of the same ecological functions that native species perform in nonurban areas. In short, they help make cities more livable by absorbing excess nutrients that accumulate in wetlands; reducing heat buildup in heavily paved areas; controlling erosion along rivers and streams; mitigating soil, water, and air pollution; providing food and habitat for wildlife; and converting the carbon dioxide produced by the burning of fossil fuels into biomass.
Cities display a suite of environmental characteristics not typically found in natural habitats. The most significant is the ongoing physical disruption and land fragmentation associated with the construction and maintenance of infrastructure. Ongoing construction destabilizes native plant communities by altering soil and drainage conditions, thereby creating opportunities for the establishment of disturbance-adapted, early successional plants. Going hand in hand with disturbance is the covering of most urban land with pavement and structures that shed water. The density of these impervious surfaces is greatest in the center of the city and decreases as one moves out to the edges, while the amount of open ground typically follows the reverse pattern. Compounding the problem of imperviousness is the issue of soil compaction produced by pedestrian and vehicular traffic. This reduced porosity inhibits the flow of air and water into the soil and can be particularly serious for native trees and shrubs whose shallow roots require a constant supply of oxygen for proper growth. When the pavement in a town exceeds 25 to 30 percent, it can be considered urbanized from the biological perspective, independent of the density of its human population.
In most cities, the quality of the soil, like that of the vege
tation, is a mixed bag. One can certainly find existing pockets of native soil that support remnant native ecosystems, but most cities, especially those along coasts, have large areas filled in with construction rubble to create more land. Roughly 17 percent of Boston as it currently exists is built on fill soil, as are significant parts of New York City near the ocean. Such filled land, by definition, can never support a native ecosystem—which is not to say that it cannot support a functional cosmopolitan ecology.
Another distinguishing characteristic of urban environments, and a function of their abundance of impervious surfaces, is their high temperatures relative to the surrounding nonurbanized land, a phenomenon referred to as the urban heat island effect. Because buildings and pavement absorb and retain heat during the day—to say nothing of cars, air conditioners, heating units, and electrical equipment, which also generate heat—the annual mean temperatures of large urban areas can be between 2 and 5°F (1–3°C) higher than those of the surrounding nonurban areas. On cloudless summer nights, the temperature difference between the center of a large city and the nearby countryside can be as much as 18°F (10°C). This means that cities are already providing people with a preview of what climate change will look like on a much broader scale in the not-too-distant future.
Based on extensive research in Europe, scientists have determined that the typical urban plant is well adapted to soils that are relatively fertile, dry, unshaded, and alkaline. Through a twist of evolutionary fate, many of these species have evolved life-history traits in their native habitats that have “preadapted” them to flourish in cities. Marble or brick buildings, for example, are analogous to naturally occurring limestone cliffs. Similarly, the increased use of deicing salts along walkways and highways has resulted in the development of high-pH microhabitats that are often colonized by either grassland species adapted to limestone soils or salt-loving plants from coastal habitats. Finally, the hotter, drier conditions one finds in cities favor species that come from exposed, sunny habitats in nature. Preadaptation is a useful idea for understanding the emergent ecology of cities because it helps to explain why some plants and not others grow on piles of construction rubble, chain-link fence lines, highway median strips, pavement cracks, and compacted turf.
Any discussion of urban ecology would be incomplete without a consideration of the cultural significance of the plants that grow in cities. Indeed, the changing composition of spontaneous urban vegetation over time reflects the constantly shifting value judgments, socioeconomic cycles, and technological advances that shape the evolution of cities. The shift from horses to automobiles in the early twentieth century, for example, effectively transformed land once used for hayfields and pastures into roads and parking lots, thereby reducing available habitat for many grassland plants and animals.
While most biologists view invasive plants as a serious biological problem, the fact remains that their initial introduction and distribution were usually the result of deliberate decisions that reflected the economic, ornamental, or conservation values of the day. Between the 1930s and the 1960s, various federal, state, and local agencies encouraged—and often subsidized—the cultivation of plants such as kudzu, multiflora rose, and autumn olive for erosion control and wildlife habitat purposes. It should come as no surprise that they became major problems forty years later, after millions of them had been planted. Indeed, the spread of nonnative species across the landscape is as much a cultural as a biological phenomenon, a fact often overlooked by advocates of strict ecological restoration.
The interacting forces of urbanization, globalization, and climate change have led to the formation of novel associations of plants that have become the de facto native vegetation of the city. These plants not only reflect the city’s socioeconomic history but also project its future trajectory. Given the environmental uncertainty that is a hallmark of the Anthropocene, now is the time for people to acknowledge the role that spontaneous urban vegetation can play in helping to clean up the ecological mess that we have made of the planet.
ATMOSPHERICS AND THE ANTHROPOCENE
KELLY CHANCE
On a lovely September night in 1885, Samuel Pierpont Langley measured the Moon’s light throughout a lunar eclipse. Those infrared measurements and Langley’s research on the temperature of the Moon and on solar heat provided the physical chemist Svante Arrhenius with data that led him to propose the theory of greenhouse warming of Earth. Published in 1896, Arrhenius’s theory includes an early mathematical formulation of the atmospheric greenhouse law that is still useful today.
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Greenhouse warming. As Arrhenius predicted, global warming is real, and it is here. The prime physical cause of warming is that a blanket (in this case, a blanket of infrared-absorbing gases) retains warmth. Denying global warming means invoking feedback effects that are as large as the primary warming. Carbon dioxide (CO2), known in Langley’s and Arrhenius’s time as carbonic acid, is the main contributor to this blanket. It has reached an atmospheric concentration of 400 parts per million, up from about 280 in preindustrial times. Methane (CH4) is next in importance: its atmospheric abundance has almost tripled since preindustrial times. Modern methods of extracting natural gas, which is mostly methane, cause still unquantified amounts of leakage, and gas and oil wells that are no longer in production may continue to be significant sources of methane. Synthetic chlorofluorocarbons (Freons) remaining since their phase-out, after it was determined that they cause stratospheric ozone depletion, and their hydrochlorofluorocarbon and hydrofluorocarbon replacements are smaller but significant greenhouse sources, as is nitrous oxide (N2O), which is about 40 percent anthropogenic in origin and is growing in concentration because of fertilizer use.
Global warming produces much more than a gradual rise in temperature. Thanks to the thermal expansion of seawater and glacial melting, sea levels are rising. Circulation changes induced by warming also cause anomalous weather, including droughts, flooding, and increased storm frequency.
Air pollution. The day following Langley’s eclipse measurements, London experienced its earliest snowfall on record, heralding the beginning of an extremely long, snowy, and smoggy winter. The city was infamous for its pea-soup fogs, which occurred when emissions from the burning of low-grade coal, notably inexpensive and plentiful sea coal, combined with ambient fog, usually in humid conditions with temperatures below 5°C. These noxious fogs had already been a problem for centuries when John Evelyn produced his report Fumifugium: Or, the Inconveniencie of the Aer and Smoak of London Dissipated, Together with Some Remedies Humbly Proposed for King Charles II in 1661. They still infested London in 1905, when the term smog was coined for them, and they culminated in the four-day Great Smog of December 1952, when twelve thousand people are estimated to have died as a direct result of this pollution.
London fog is an exemplar of “reducing-type” pollution: it is both highly sulfurous (sulfur dioxide, SO2, is produced in abundance by burning dirty coal) and high in particulates such as coal ash and sulfuric acid. Los Angeles smog is another exemplar: it is oxidizing pollution, or photochemical smog, produced by the action of sunlight on nitrogen oxides emitted from combustion, and it is rich in ozone (O3) and liquid-phase aerosols. Global urban pollution varies in type between these extremes and with pronounced seasonality, and it may include significant particulate sources such as desert dust or black carbon from forest fires and other biomass burning. But tropospheric pollution is not confined to urban areas. Given the ubiquity of emissions and the transport of ozone and aerosol precursors, unhealthy concentrations can occur over extended rural areas. Nitrogen oxides from combustion (in power plants, for example) and volatile organic compounds are the major rural tropospheric ozone sources. Forests stressed by heat—and perhaps by ozone itself, in a pernicious feedback loop—can produce prodigious amounts of ozone and aerosols.
Tropospheric ozone, nitrogen oxides, sulfur oxides, aerosols, carbon monoxide, and lead are the U.S. Environmental
Protection Agency’s criteria pollutants. Among them, ozone and aerosols cause the most atmospheric concern globally at present because they have the most serious health consequences. Aerosols with particulate matter whose diameter is 2.5 microns or less (PM2.5) are of particular concern for cardiovascular disease and respiratory health. Ozone is especially harmful to people with asthma or heart disease and may cause or exacerbate respiratory illness. Forest and crop damage by ozone is well documented and serious: it retards photosynthesis, sometimes leading to the “early autumn” syndrome, and makes some plants more susceptible to disease.
The ozone layer. Both the Iliad and the Odyssey mention the smell of lightning-produced ozone. By the mid-nineteenth century, unstable, highly explosive ozone gas had been isolated. The stratospheric ozone layer was discovered in 1913 and was soon being measured in detail on an ongoing basis. It is described by the 1930 Chapman chemical mechanism involving oxygen species and sunlight, with the later addition of water-vapor-initiated ozone-reduction chemistry. The ozone layer shields life in the lower atmosphere, where pollution and global warming predominate, from chemical bond–breaking ultraviolet radiation, which is particularly harmful to animals and plants. Not until the early 1970s did a handful of brilliant scientific studies establish that the ozone layer is subject to damage from anthropogenic activities, especially production of the precursors of free-radical nitrogen and halogen chemical sources (NOx and ClOx). Also beginning in the 1970s, ground-based measurements by the British Antarctic Survey showed rapid, deep declines in stratospheric ozone during austral spring. This discovery was confirmed by National Aeronautics and Space Administration measurements by the Nimbus 7 satellite, which mapped the extent of the depletion. It was found to correspond to the Antarctic polar vortex. Satellites have continuously mapped global ozone and the Antarctic ozone hole ever since. Intensive investigations, including expeditions to Antarctica, have shown that stratospheric halogen chemistry—especially affected by chlorofluorocarbons, particularly Freons, whose use has now been largely stopped—created the ozone hole.
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