The LTER program conducts and facilitates ecological research at 24 sites in the United States and the Antarctic. More sites are likely to be added to the LTER network in the future. There is also an important and growing International LTER (ILTER) program (LTER Network Office 1998). The U.S. LTER research sites operate as a network with a network office located at the University of New Mexico at Albuquerque. The network is a collaborative effort involving more than 1100 scientists and students. The current 24 LTER sites are located in various biomes throughout the United States and Antarctica (figure 1.1; Callahan 1984; Franklin et al. 1990; Van Cleve and Martin 1991; http://lternet.edu/). One of the missions of the LTER program is to conduct a cross-site synthesis. LTER research, like much Global Change research, focuses mostly on timescales of months to centuries. The operation as a network enables LTER to address large-scale questions concerning ecological phenomena such as the variations in stream organic matter budgets across the United States (Webster and Meyer 1997). The network also creates opportunities for comparisons between ecosystems across regional, continental, and global gradients such as organic matter decomposition (Long-Term Intersite Decomposition Experiment Team [LIDET] 1995). The network operation also allows scientists to distinguish system features controlled by absolute and relative scales. Neither the large-scale questions, such as what the decomposition rates are across the country, nor questions of absolute and relative scale, such as how decomposition rates vary along soil moisture gradients within LTER sites, can usually be answered without a detailed specification of the climate of LTER sites. The importance of cross-site synthesis has been expressed by an external review of the program as follows: "The power of the network approach of the LTER program rests in the ability to compare similar processes (e.g., primary production or decomposition of organic matter) under different ecological conditions. As a result, LTER scientists should be able to understand how fundamental ecological processes operate at different rates and in different ways under different environmental conditions" (Risser and Lubchenco 1993).
Two other features of the LTER program are important in the present context. First, the program prides itself on its interdisciplinary nature. The wide range of ecosystems studied demands that these studies be made in an interdisciplinary manner and that no single subdiscipline dominate. The LTER program also prides itself on its environmental information management system. This information man-
agement system, and its climate data component, is regarded as a model for such systems worldwide (Michener et al. 1998; Baker et al. 2000).
The LTER program encourages coherence in ecological research over the long term to take advantage of the fact that many ecosystem processes operate at long time scales and show directionality and periodicity. Studies that have recognized this (e.g., at Hubbard Brook [Likens and Bormann 1995; Likens et al. 1996]) have made fundamental contributions to ecology. Within these sites it was found that human-derived as well as natural perturbations act over a long time period.
Studies at the LTER sites are organized around five core themes: (1) pattern and control of primary production, (2) spatial and temporal distribution of populations selected to represent trophic structure, (3) pattern and control of organic matter accumulation in surface layers and sediments, (4) patterns of inorganic input and movement through soils, groundwater, and surface waters, and (5) patterns and frequency of disturbance. Although climatic aspects affect all these themes, the role of climate is paramount in the last theme.
The LTER sites (table 1.1; figure 1.1) were not selected primarily to give good geographic coverage. They were selected first based on the quality of research proposed at the site. As a result, the sites together do not necessarily provide a systematic spatial coverage of the country or its climate and biomes. The network was not designed to replicate the spatial cover of meteorological observations given by the National Weather Service stations. The temporal rather than the spatial emphasis of the LTER network is one of the reasons why this book takes on a structure categorized by timescale.
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