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Before European settlement, vast areas of deciduous forest in what is now the eastern United States were dominated by oak species. Among these species, white oak (Quercus alba) reigned supreme. White oak tended to grow at lower elevations but was distributed across a broad range of sites, from wet mesic to subxeric, and grew on all but the wettest and most xeric, rocky, or nutrient-poor soils. A comparison of witness tree data from early land surveys and data on modern-day forest composition revealed a drastic decline in white oak throughout the eastern forest. By contrast, other dominant oaks, such as red oak (Quercus rubra) and chestnut oak (Quercus prinus), often exhibited higher frequency in recent studies than in surveys of the original forest. The frequency of red oak witness trees before European settlement was quite low, generally under 5% in most forests. Red oak's distribution was apparently limited by a lower tolerance to fire and drought and a greater dependence on catastrophic disturbances than that of white oak. During the late 19th and early 20th centuries, much of the eastern forest was decimated by land clearing, extensive clear-cutting, catastrophic fires, chestnut blight, and then fire suppression and intensive deer browsing. These activities had the greatest negative impact on the highly valued white oak, while promoting the expansion of red oak and chestnut oak. More recently, however, recruitment of all the dominant upland oaks has been limited on all but the most xeric sites. Thus, the dynamic equilibrium in the ecology of upland oaks that existed for thousands of years has been destroyed in the few centuries following European settlement.
Changes in Earth's surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.
Human development of coastal watersheds has greatly increased nutrient loading and accelerated estuarine and coastal eutrophication. These waters are also affected by climatic perturbations (e.g., droughts, hurricanes, floods), which may be increasing. The ecological effects of these stressors are often most evident at the microbial level, where the bulk of primary production and biogeochemical cycling occurs. Phytoplankton dominate coastal primary production and thus may be indicative of eutrophication and other major perturbations underlying coastal ecosystem change. Using photopigments that are diagnostic for phytoplankton functional groups (chlorophytes, cryptophytes, cyanobacteria, diatoms, and dinoflagellates), we examined the relative responses of these taxonomic groups to nutrient and hydrologic alterations and evaluated their use as indicators of ecological change in the Neuse River Estuary, North Carolina, and Galveston Bay, Texas. Photopigment indicators can be routinely incorporated in water-quality monitoring programs to assess environmental controls on ecosystem structure and function over varying spatial and temporal scales.
Darwin's finches on the Galápagos Islands are particularly suitable for asking evolutionary questions about adaptation and the multiplication of species: how these processes happen and how to interpret them. All 14 species of Darwin's finches are closely related, having been derived from a common ancestor 2 million to 3 million years ago. They live in the environment in which they evolved, and none has become extinct as a result of human activity. Key factors in their evolutionary diversification are environmental change, natural selection, and cultural evolution. A long-term study of finch populations on the island of Daphne Major has revealed that evolution occurs by natural selection when the finches' food supply changes during droughts. Extending this finding to the past, we discuss how environmental change has influenced the opportunities for speciation and diversification of finches throughout their history: The number of islands has increased, the climate has cooled, and the vegetation and food supply have changed.
Tropical reef corals are ecologically important examples of mutualistic symbioses whose success is defined by the interwoven biologies of their symbiotic partners. These associations are exquisitely regulated, yet the equilibrium is sensitive to environmental disturbances, which cause a breakdown in symbiotic communication, loss of algae from the host, concomitant paling of coral coloration known as coral bleaching, and, if the conditions persist, death of the coral. Faced with the prospect of catastrophic coral mortality associated with global warming and related environmental shifts, researchers have focused their efforts on coral bleaching; although significant progress has been made in this area, understanding of the basic biology of these associations remains poor. Here we discuss several issues that have potentially contributed to this knowledge gap and conclude that without a sound understanding of the basic biology of these important symbioses, it will be very difficult to elucidate the mechanisms that drive coral bleaching.
Estimates of forest wood volume and biomass, and estimates of the surface area of asphalt, concrete, roof, and open water, were generated from a single set of airborne laser-profiling data acquired during the summer of 2000. Estimates of aboveground dry biomass for different types of land cover in each county or state were converted directly to estimates of standing carbon. A portable, inexpensive laser that measures forest canopy height and canopy cover was also used to identify and map mature forest stands that might support the Delmarva fox squirrel (Sciurus niger cinereus), an endangered species. Merchantable volume estimates were within 24 percent of US Forest Service estimates at the county level and within 15 percent statewide. Total aboveground dry biomass estimates were within 19 percent of US Forest Service estimates at the county level and within 16 percent statewide. Various types of impervious surfaces (roofs, concrete, asphalt) and open water were tallied along the flight lines to estimate areal coverage statewide, by land cover and county. Laser estimates of impervious surface area were within 28 percent of satellite-based estimates at the county level and within 3 percent at the state level. Laser estimates of open water were within 7 percent of photointerpreted geographic information system (GIS) estimates at the county level and within 3 percent of the GIS at the state level.
The Lewis and Clark journals contain some of the earliest and most detailed written descriptions of a large part of the United States before Euro-American settlement. We used the journal entries to assess the influence of humans on wildlife distribution and abundance. Areas with denser human population, such as the Columbia Basin and the Pacific Coast, had lower species diversity and a lower abundance of large mammals. The opposite effect was observed on the Plains. We believe that overhunting before Euro-American contact and the introduction of the horse, which heightened the effects of hunting, may have been major contributors to the historical absence of some species that are present in the archaeological record. The results show considerable human influence on wildlife even under relatively low human population densities. This finding has major implications for conservation biology and ecological restoration, as human influence is often underestimated when considering presettlement conditions.