The US Long Term Ecological Research (LTER) Network enters its fourth decade with a distinguished record of achievement in ecological science. The value of long-term observations and experiments has never been more important for testing ecological theory and for addressing today's most difficult environmental challenges. The network's potential for tackling emergent continent-scale questions such as cryosphere loss and landscape change is becoming increasingly apparent on the basis of a capacity to combine long-term observations and experimental results with new observatory-based measurements, to study socioecological systems, to advance the use of environmental cyberinfrastructure, to promote environmental science literacy, and to engage with decisionmakers in framing major directions for research. The long-term context of network science, from understanding the past to forecasting the future, provides a valuable perspective for helping to solve many of the crucial environmental problems facing society today.

Long-term research should play a crucial role in addressing grand challenges in environmental stewardship. We examine the efforts of five Long Term Ecological Research Network sites to enhance policy, management, and conservation decisions for forest ecosystems. In these case studies, we explore the approaches used to inform policy on atmospheric deposition, public land management, land conservation, and urban forestry, including decisionmaker engagement and integration of local knowledge, application of models to analyze the potential consequences of policy and management decisions, and adaptive management to generate new knowledge and incorporate it into decisionmaking. Efforts to enhance the role of long-term research in informing major environmental challenges would benefit from the development of metrics to evaluate impact; stronger partnerships among research sites, professional societies, decisionmakers, and journalists; and greater investment in efforts to develop, test, and expand practice-based experiments at the interface of science and society.

Scenario studies have emerged as a powerful approach for synthesizing diverse forms of research and for articulating and evaluating alternative socioecological futures. Unlike predictive modeling, scenarios do not attempt to forecast the precise or probable state of any variable at a given point in the future. Instead, comparisons among a set of contrasting scenarios are used to understand the systemic relationships and dynamics of complex socioecological systems and to define a range of possibilities and uncertainties in quantitative and qualitative terms. We describe five examples of scenario studies affiliated with the US Long Term Ecological Research (LTER) Network and evaluate them in terms of their ability to advance the LTER Network's capacity for conducting science, promoting social and ecological science synthesis, and increasing the saliency of research through sustained outreach activities. We conclude with an argument that scenario studies should be advanced programmatically within large socioecological research programs to encourage prescient thinking in an era of unprecedented global change.

The US National Science Foundation—funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after many years of study; (b) influenced management and policy decisions; and (c) evolved into research platforms supporting studies and involving investigators who were not part of the original design. Furthermore, this suite of long-term experiments addresses, at the site level, all of the US National Research Council's Grand Challenges in Environmental Sciences. Despite these contributions, we argue that the scale and scope of global environmental change requires a more-coordinated multisite approach to long-term experiments. Ideally, such an approach would include a network of spatially extensive multifactor experiments, designed in collaboration with ecological modelers that would build on and extend the unique context provided by the LTER Network.

Analyses of long-term records at 35 headwater basins in the United States and Canada indicate that climate change effects on streamflow are not as clear as might be expected, perhaps because of ecosystem processes and human influences. Evapotranspiration was higher than was predicted by temperature in water-surplus ecosystems and lower than was predicted in water-deficit ecosystems. Streamflow was correlated with climate variability indices (e.g., the El Niño—Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation), especially in seasons when vegetation influences are limited. Air temperature increased significantly at 17 of the 19 sites with 20- to 60-year records, but streamflow trends were directly related to climate trends (through changes in ice and snow) at only 7 sites. Past and present human and natural disturbance, vegetation succession, and human water use can mimic, exacerbate, counteract, or mask the effects of climate change on streamflow, even in reference basins. Long-term ecological research sites are ideal places to disentangle these processes.

The cryosphere—the portion of the Earth's surface where water is in solid form for at least one month of the year—has been shrinking in response to climate warming. The extents of sea ice, snow, and glaciers, for example, have been decreasing. In response, the ecosystems within the cryosphere and those that depend on the cryosphere have been changing. We identify two principal aspects of ecosystem-level responses to cryosphere loss: (1) trophodynamic alterations resulting from the loss of habitat and species loss or replacement and (2) changes in the rates and mechanisms of biogeochemical storage and cycling of carbon and nutrients, caused by changes in physical forcings or ecological community functioning. These changes affect biota in positive or negative ways, depending on how they interact with the cryosphere. The important outcome, however, is the change and the response the human social system (infrastructure, food, water, recreation) will have to that change.

The Diagnostic Question Cluster (DQC) project integrates education research and faculty development to articulate a model for the effective transformation of introductory biology and ecology teaching. Over three years, faculty members from a wide range of institutions used active teaching and DQCs, a type of concept inventory, as pre- and posttests to assess students' understanding of concepts about energy and matter across biological scales of organization. Surveys of the instructors indicated a substantial use of DQCs and active teaching, and nearly all of those faculty members participating in the research saw significant student gains and a large positive effect size between the pre- and posttests. Important programmatic components included reliable research-based conceptual questions and the associated active-learning exercises; formative examination of preinstruction data, including the students' written answers; a professional society for recruitment, workshops, and dissemination; progressive faculty growth over three years; and cooperative communities of practice. We propose that research-based conceptual inventories can be effective tools in faculty-development programs offered through biology professional societies.

Pesticides applied on land are commonly transported by runoff or spray drift to aquatic ecosystems, where they are potentially toxic to fishes and other nontarget organisms. Pesticides add to and interact with other stressors of ecosystem processes, including surface-water diversions, losses of spawning and rearing habitats, nonnative species, and harmful algal blooms. Assessing the cumulative effects of pesticides on species or ecological functions has been difficult for historical, legal, conceptual, and practical reasons. To explore these challenges, we examine current-use (modern) pesticides and their potential connections to the abundances of fishes in the San Francisco Estuary (California). Declines in delta smelt (Hypomesus transpacificus), Chinook salmon (Oncorhynchus tshawytscha), and other species have triggered mandatory and expensive management actions in the urbanizing estuary and agriculturally productive Central Valley. Our inferences are transferable to other situations in which toxics may drive changes in ecological status and trends.