From this discussion it is clear that the LTER program provides a platform from which a huge amount of information emerges on the topic of climate variability and ecosystem response. Based on the information in this book, many avenues of research on this topic will be important in the future.
1. We must continue to obtain more information at each LTER site on climate as a disturbance factor of ecosystems. Each new piece of information on this topic alters our perspective of the principles that emerge from this field. We need to develop tools that are sensitive to both atmospheric and ecosystem variability. In addition, we must attempt to anticipate the correct combination of system properties to be observed to be able to demonstrate in detail system resonance or another kind of behavior.
2. We must continue to be aware of the need for cross-site comparison. One corollary to this is that we must strive to design our experiments, and to collect our data, in such a manner to facilitate intersite comparison. The LTER network has a unique infrastructure for being able to make such comparisons as long as a certain amount of preplanning is accomplished. We have the potential to formulate hypotheses related to climate variability and ecosystem response for groups of LTER sites with common properties. The network has an ever-increasing number of site-years of sampling for different disturbance processes. The hurricane event is a case in point. Currently, nine LTER sites in the Caribbean and the East Coast of the United States are well positioned to observe the effects of tropical storms that make landfall. Four LTER, or former LTER, sites have been directly impacted by tropical storms. In 1938 an unnamed hurricane passed over the Harvard Forest LTER site. Hurricane Hugo passed over the Luquillo, Puerto Rico, LTER site and the North Inlet, South Carolina, former LTER site in September 1989 and Hurricane Opal passed over the Coweeta, North Carolina, site in October 1995. At the time of this writing, the subnetwork of hurricane-vulnerable LTER sites collectively represented about 160 site-years of direct, LTER-supported observation of both hurricanes and ecosystem responses and perhaps more than 2000 site-years of archival records of hurricane occurrence. This mininetwork is well configured geographically and temporally to obtain a good sample of large and extreme events and to consider questions about regional patterning of disturbance and ecosystem responses across a range of ecosystem types. Current sites provide opportunities for observation in ecosystems, including tropical and temperate forests, coastal barrier islands and wetlands, and an urban site.
Some scientists believe the U.S. East Coast is entering a period of several decades when the frequency of hurricanes making landfall will increase (Goldenberg et al. 2001). LTER scientists at potentially affected sites should make contingency plans to study new storms and their ecological impact by standardizing some of the methodologies that have been used in earlier studies. Preexisting conditions and common impact indicators should be carefully specified. Hurricanes are not the only possible focus of study. Other individual LTER sites or groups of sites may sample other climate disturbance processes. Diagrams such as figure 1.2 may be used to identify such groups of sites. As the LTER program extends into the future, it may also be possible to use available climate forecasts, such as those for the state of the ENSO, to design experiments that use the natural climate extremes of this quasi-quintennial phenomena. A visionary might even conceive of the ability to forecast the state of the interdecadal-scale PDO and its resulting ecosystem responses.
Another question related to cross-site comparison concerns the possibility that some sites might be more susceptible to climate variability than others. At first sight, the Sevilleta LTER site in New Mexico might be said to be more susceptible to an El Niño climate signal of a similar size than the Andrews site in the PNW. Could it be, for example, that the sites on the extreme outside edge of the cluster of LTER sites shown in figure 1.2 are likely to show a more marked response to climate variability than those sites near the center of the cluster? A related matter is the question of "redundance" of climate variability. There are many situations where the variability of a climate variable is of little importance to the ecosystem. For example, at the Andrews LTER, as long as no flooding occurs, greater than average January precipitation does little except run off from a system that is already fully charged with water physically and biologically. Where it is not already obvious, the identification of climate variability redundance would permit investigators to focus their resources on other parts of the ecosystem.
3. We must use our increasing knowledge at LTER sites and our cross-site comparisons to identify important generalities, often related to process, that are more specific in nature than our comments about the importance of scale. For example, we should pay more attention to critical thresholds such as those related to the ice/liquid boundary or to plant rooting depth. Other thresholds might include the precipitation duration-intensity necessary to trigger landslides or thresholds related to the phenology associated with achieving good seed crops. Another general concept has to do with the residence time of communities and individuals at a site and the sensitivity of a site to climate disturbance. One way to achieve the identification of these generalities is to hold workshops on them individually. Experience from some of the workshops that led to this volume has shown us that important concepts will emerge from such workshops.
4. We must start to develop multidimensional approaches to the issues of climate variability and ecosystem response. It is a rare case that there is only one aspect of climate variability occurring at any given time. At Coweeta, for example, both droughts and windstorms occur from time to time, but they each favor the development of different types of microhabitats. Drought effects, such as increased standing necromass, favor the development of some microhabitats, whereas windthrows favor others. The two situations exist simultaneously in the forest, and investigators must find ways to treat the parallel ecosystem responses and their possible interactions.
5. We must begin to confront the climate signal detection problem. The detection of climate signals embedded in the ecosystem and realized as cascades of time-varying ecosystem properties is exceptionally complex. The problem requires the application of the full spectrum of analytical tools available to scientists and an ever-growing resource of long-term data, especially on ecosystem dynamics. Endeavors in this area will represent another fortunate congruence between climatol-ogists and ecologists. This is especially so for paleoclimatologists, who have long used a wide variety of proxy ecological data such as tree rings. However, as difficult and sophisticated as the interpretation of tree rings is, we envision the problem of climate signal detection as being much more complex because it involves multiple levels in the ecosystem cascades. There is also the problem of the overprinting of a variety of climate impacts as one moves from shorter to longer timescales. Part of the task in climate signal detection is to gain a clearer picture of how phenomena at focal scales are affected by phenomena at adjacent or other scales. Hierarchy theory (Ahl and Allen 1996) will be one of the analytical tools for this task.
6. We must seriously consider how ecosystems may respond to global trends. In particular, we need to understand how ecosystem response may have either positive or negative feedback on a climate change. For example, shifts in polar ecosystems (melting of sea ice and permafrost, changes in snow cover, etc.) will, in turn, have an impact on climate. An understanding of such feedback mechanisms is of enormous ecological and social importance.
7. We must continue to refine the principles that emerge from the studies in this book. Quantitative modeling studies backed by carefully collected field data will help achieve this goal. More quantification will also help address some of the framework questions of this study that have been largely neglected.
8. At least as far as this list is concerned, one of the exciting realizations emerging from this volume is the possibility of ecosystems moving into resonance with climate variability at the quasi-quintennial and longer timescales. This seems to be a very fruitful idea worthy of further development and investigation. The growing maturity of LTER sites places researchers in a good position to examine the existence of such resonance in ecosystems other than those we have identified. In the cases we have already identified, the subtleties of the resonance may be examined more thoroughly. Such investigations exemplify the central core of the character of LTER research. This is true both in the sense of capitalizing on long-term research already completed and in the sense of opening up exciting new areas of investigation not envisioned when the LTER program began.
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