Most global climate change scenarios predict an increase in the frequency and intensity of extreme weather and climate events. Ecologists and other scientists engaged in ecosystem research recognize that these extreme events will impact their systems, often with effects disproportionate to the extent and duration of the event itself. Despite the importance of extreme events, there is little knowledge or agreement on what constitutes and extreme event, and how they shape organisms, communities, and ecosystems. Defining an extreme event from a climatic perspective is not difficult; it is simply a significant departure from the normal state of the climate system, irrespective of its impact on life or any aspects of the earth’s ecology (Alexander, 1993). Climatic extremes occur in a variety of time, space, and phenomenal dimensions. For example the severe droughts of the 1930s and 1950s were multiyear episodes of elevated temperatures and decreased precipitation affecting a substantial portion of the North America, whereas Hurricane Hugo in 1989 lasted only a few days and directly impacted a relatively small land area in the Caribbean Basin and southeastern United States. Yet each was severe, according to Alexander’s definition. From an ecological perspective, Alexander’s definition explicitly excludes ecological impacts and is therefore of limited to use to ecosystem studies. Gutschick and BassiriRad (2003) suggest that extreme events (both climatic and biotic) may play a key role in shaping the physiology, ecology, and evolution of individual organisms, and offer a definition based on organismal responses of acclimation and hysteresis (which they also term ‘de-acclimation’). Gutschick and BassiriRad’s definition of extreme events is organism-based and not restricted to climatic impact, but with some modifications, their concept might be scaled to the ecosystem level. Thus, we might define an extreme climatic event at the ecosystem level as a weather or climate event in which the acclimatory capacity of the ecosystem is substantially exceeded. The challenge for operationalizing this definition lies in recognizing that the acclimatory capacities of ecosystems vary widely, and that determining the defining parameters for acclimation can depend on the nature of the ecosystem in question. For example, in a forested system extreme event impacts might consist of limb loss or tree toppling, both of which might be expected to occur in response to very heavy precipitation or wind events, such as might accompany a major storm event. In contrast, an extreme event in an Antarctic dry valley might consist of a very slight temperature increase accompanied by increased stores of liquid water. Each of these events would be initiated by weather or climate events, but the relative severity of the initiating climatic event would contrast widely. Occurrence of extreme events might also be tempered by abiotic or biotic antecedent conditions in the ecosystem. So, a snow event might cause extreme damage to a forest system already weakened by insect infestation, fire damage, or previous weather condition, whereas an event of the same magnitude might not have the same effect in the absence of such antecedent conditions. Indeed, defining the baseline conditions by which to judge the severity of a climatic event may not be equally straightforward in all ecosystems. Grasslands, which evolve in extreme and variable climate conditions, are adapted to such extremes and it is therefore difficult to define a threshold against which to assess the impact of an event. In contrast, managed ecosystems (such as cropland) have well-defined baselines related to potential biomass yield that provide ready benchmarks against which to gauge the impact of an extreme event. Proposed Research The potential importance of extreme events in ecosystem studies and the relative paucity of knowledge about them represent a unique opportunity for research within the LTER network. The LTER network encompasses sites representing a variety of ecosystems and, more importantly, a deep reservoir of knowledge of the workings of those ecosystems. In addition, the network provides a mechanism for communicating both data and ideas. We propose to use these unique capabilities of the network to help define extreme events for each of the ecosystems represented by the various LTER sites. Put simply, we will study climatic extreme events from the perspective of ecology, rather than atmospheric science. This will involve development of a set of “calling questions” designed to define criteria for determining what constitutes an extreme event for various ecosystems, and when these systems are exhibiting an extreme response. We will gather information useful for defining; (1) what the acclimatory thresholds of a system are, (2) what ecosystem variables or properties are most important for determining thresholds, (3) what are the types and duration of weather/climate events that might cause the system to exceed these thresholds, (4) how preexisting conditions might affect acclimatory thresholds, (5) what “de-acclimation” paths might exist in the system. To facilitate gathering this type of information, these questions will be used to interview interested and willing researchers at each of the LTER sites. Interviews will consist of open-ended conversations, rather than questionnaires. We believe this approach will yield more useful information. A vital characteristic of this proposed research is that we will go beyond a catalog of extreme event effects at each of the 24 sites. Instead, our goal is network level synthesis by defining a “typology” of response to extreme events (using the responses to the calling questions) and then classifying and analyzing LTER sites (and the ecosystems they each represent) according to this typology of extreme event response. The variables for classifying sites will to a large extent be gleaned from the question responses, but we hypothesize that the typology of extreme events can be expressed in some sort of multi-dimensional space, where various dimensions might include frequency and duration of events, type of event (precipitation, temperature, of both), and perhaps the energy associated with the event. Project collaborators will then produce a series of monographs describing each type of extreme event response and what ecosystems are associated with it. Ultimately, these monographs will be collected and published in an edited volume. To accomplish this research, we proposal to do the following: 1.) Development of a set of calling questions to define extreme events at each site from an ecosystem perspective. 2.) Contact interested persons at each LTER site via the site Climate Committee member and interview them concerning the nature of extreme events at their site using the calling questions as a guide. 3.) Collation and categorization of responses to the interviews 4.) Development of a typology of climatic extreme events based on ecoclimatic parameters but using ecosystem response as the defining characteristic. 5.) Classification of the ecosystems represented by various LTER sites based in the typology described in #4. 6.) Synthesis of extreme event response across the LTER network using the framework defined in #5. Development of the calling questions is already underway. A key step in this research is the collation and categorization of question responses and the development of a typological framework based the responses to these responses (numbers 3 and 4). The finds we request will be used to conduct a workshop at which these activities will be accomplished.