Introductory Overview

The timescale structure of this book has served well to keep the attention of investigators focused on specific aspects of climate variability and ecosystem response. Indeed, judging by the responses received by the editors of this volume, when given a choice between focusing on one timescale or several timescales, the LTER community was far more comfortable dealing with just one scale. There are obvious reasons for this, not the least of which is that focusing on a single scale greatly simplifies things. The real world, however, does not focus on one timescale. Climatic events and ecosystem responses occur simultaneously at a variety of scales. We wished to explore the climatic variability and ecosystem responses at LTER sites across several different timescales, and the two chapters in this part attempt such an exploration. The chapters consider the temperate rainforest of the H. J. Andrews LTER site in Oregon and the tallgrass ecosystem of the Konza Prairie LTER in Kansas. For the Andrews rainforest, and to some extent the Pacific Northwest (PNW) in general, Greenland et al. (chapter 19) discuss climate variability and ecosystem response at the daily, multidecadal, and century to millennial scales. This discussion for the PNW is supplemented in chapters 6 and 13 of this volume by a consideration of the quasi-quintennial scale and an additional ecosystem response at the decadal scale. The forest ecosystem is more complex than the grassland ecosystem. Greenland et al. cover a wide variety of potential ecosystem responses for the PNW Forest, ranging from severe weather events, to pine cone production, to century- and millennial-scale forest fire frequency regimes and their variation. The focus of chapter 19 is on some of the framework questions of this volume. The questions specifically addressed include the following: What preexisting conditions affect the impact of the climatic event or episode? Is the climatic effect on the ecosystems direct or cascading? Does the system return to its original state? The authors also consider potential future climate change and its possible ecosystem effects. They found that timescale becomes important in addressing some of these questions. For example, at century to millennial timescales, it is suggested that there are likely to be no identical past analogs to the ecosystem at any point in time.

The Interdecadal Timescale—Synthesis

The five chapters of part III provide a broad overview of decadal-scale climate processes and their ecological effect in a variety of ecosystems. Written by authors with disciplinary backgrounds that encompass climatology, biometeorology, and ecology, the chapters range from cross-site climate analysis with little direct attention to ecosystem effects (e.g., McHugh and Goodin, chapter 11; Hayden and Hayden, chapter 14) to more intensive studies of direct climate/ecological interaction at single sites or over more defined geographical areas (e.g., Greenland, chapter 13; Juday et al., chapter 12; Milne et al., chapter 15). Separately, each of these chapters contributes to understanding some aspect of the interaction of climate and ecology. As an integrated whole, they encapsulate many of the cross-disciplinary problems confronted by LTER scientists as they explore the interaction of climate and ecology. Despite the widely varying topics addressed and the disparate backgrounds of the contributors, similar themes emerge in each of the chapters. Here, we elucidate these themes and place them within the framework questions that have guided this volume. Climatologists have long recognized the existence of cyclical or quasi-cyclical modes or patterns in the global circulation system. Typically, these patterns are characterized by variation in the strength or position of semipermanent pressure centers within the global circulation system. These variations occur at timescales ranging from seasonal to decadal, and such variability is frequently invoked as a causal mechanism for climatic trends or fluctuation at these various timescales. A variety of indexes have been constructed to characterize these pressure patterns and the teleconnections that result from them (see van Loon and Rogers 1978, Rogers 1984, and Trenberth and Hurrell 1994 for in-depth discussion of the derivation and interrelationships of atmospheric circulation indices). Evidence of some of these patterns recurs throughout each of the chapters, suggesting their importance in decadal-scale climate/ecology interactions at LTER sites. Although the chapters in this section concentrate on interdecadal variability, climate variability is a multiscale phenomenon in both space and time. Several authors acknowledge this, notably Milne et al. (chapter 15), McHugh and Goodin (chapter 11), and Greenland (chapter 13). Each of these chapters notes the importance of nondecadal variations, particularly the El Niño–Southern Oscillation (ENSO) phenomenon.

A 200-Year Perspective of Climate Variability and the Response of White Spruce in Interior Alaska

The two most important life functions that organisms carry out to persist in the environment are reproduction and growth. In this chapter we examine the role of climate and climate variability as controlling factors in the growth of one of the most important and productive of the North American boreal forest tree species, white spruce (Picea glauca [Moench] Voss). Because the relationship between climate and tree growth is so close, tree-ring properties have been used successfully for many years as a proxy to reconstruct past climates. Our recent reconstruction of nineteenth- century summer temperatures at Fairbanks based on white spruce tree-ring characteristics (Barber et al. in press) reveals a fundamental pattern of quasi-decadal climate variability. The values in this reconstruction of nineteenth-century Fairbanks summer temperatures are surprisingly warm compared to values in much of the published paleoclimatic literature for boreal North America. In this chapter we compare our temperature reconstructions with ring-width records in northern and south-central Alaska to see whether tree-growth signals in the nineteenth century in those regions are consistent with tree-ring characteristics in and near Bonanza Creek (BNZ) LTER (25 km southwest of Fairbanks) that suggest warm temperatures during the mid-nineteenth century. We also present a conceptual model of key limiting events in white spruce reproduction and compare it to a 39-year record of seed fall at BNZ. Finally, we derive a radial growth pattern index from white spruce at nine stands across Interior Alaska that matches recent major seed crop events in the BNZ monitoring period, and we identify dates after 1800 when major seed crops of white spruce, which are infrequent, may have been produced. The boreal region is characterized by a broad zone of forest with a continuous distribution across Eurasia and North America, amounting to about 17% of the earth’s land surface area (Bonan et al. 1992). The boreal region is often conceived of as a zone of relatively homogenous climate, but in fact a surprising diversity of climates are present. During the long days of summer, continental interior locations under persistent high-pressure systems experience hot weather that can promote extensive forest fires frequently exceeding 100 kilohectares (K ha). Summer daily maximum temperatures are cooled to a considerable degree in maritime portions of the boreal region affected by air masses that originate over the North Atlantic, North Pacific, or Arctic Oceans.

Climate and Hydrologic Variations and Implications for Lake and Stream Ecological Response in the McMurdo Dry Valleys, Antarctica

Because polar regions may amplify what would be considered small to moderate climate changes at lower latitudes, Weller (1998) proposed that the monitoring of high latitude regions should yield early evidence of global climate change. In addition to the climate changes themselves, the connections between the polar regions and the lower latitudes have recently become of great interest to meteorologists and paleoclimatologists alike. In the southern polar regions, the direct monitoring of important climatic variables has taken place only for the last few decades, largely because of their remoteness. This of course limits the extent to which polar records can be related to low latitude records, even at multiyear to decadal timescales. Climatologists and ecologists are faced with the problem that, even though these high latitude regions may provide important clues to global climatic change, the lengths of available records are relatively short. The McMurdo Dry Valleys Long-Term Ecological Research (MCM LTER) program was established in 1993. This program built on the monitoring begun in the late 1960s by researchers from New Zealand, who collected records of climate, lake level, and stream discharge in the Wright Valley, Antarctica. Griffith Taylor’s field party obtained the first data related to lake level in 1903 as part of Scott’s Discovery expedition. Analysis of the more recent data from the New Zealand Antarctic and MCM LTER programs when compared to the 1903 datum indicates that the first half of the twentieth century was a period of steadily increasing streamflows, followed in the last half of the century by streamflows that have resulted in more slowly increasing or stable lake levels (Bomblies et al. 2001). Thus, meteorological and hydrological records generated by the MCM LTER research team, when coupled with past data and the ecological information currently being obtained, provide the first detailed attempt to understand the connection between ecosystem structure and function and climatic change in this region of Antarctica. In addition, the program helps to fill an important gap in the overall understanding of climatic variability in Antarctica.

Climate Variability in the North Central Region: Characterizing Drought Severity Patterns

This chapter examines the spatial and temporal variability and patterns of climate for the period 1972–1991 in the North Central Region of North America (NCR). Since the mid-1970s, climate has become more variable in the region, compared to the more benign period 1950–1970. The regional perspective presented in this chapter characterizes the general climatology of the NCR from 1972 to 1991 and compares the climate to a severe drought that occurred in 1988. This one-year drought was one of the most substantial in the region’s recent history, and it had a significant impact on the region’s agricultural economy and ecosystems. Petersen et al. (1995) characterize the 1988 drought with respect to solar radiation, and Zangvil et al. (2001) consider this drought from the perspective of a large-scale atmosphere moisture budget. A major reason for the seriousness of the drought in 1988 was the fact that May and June were unusually dry and hot (Kunkel and Angel 1989). Drought is defined as a condition of moisture deficit sufficient to adversely affect vegetation, animals, and humans over a sizeable area (Warwick 1975). The condition of drought may be considered from a meteorological, agricultural, and hydrologic perspective. Meteorological drought is a period of abnormally dry weather sufficiently prolonged to a point where the lack of water causes a serious hydrologic imbalance in the affected area (Huschke 1959). Agricultural drought is a climatic digression involving a shortage of precipitation sufficient to adversely affect crop production or the range of production (Rosenberg 1980). Hydrologic drought is a period of below-average water content in streams, reservoirs, groundwater aquifers, lakes, and soils (Yevjevich et al. 1977). All of these drought conditions are mutually linked. The objectives of this chapter are to (1) address the issues of climatic spatial scale to quantify variability of climate in the NCR, (2) examine the characteristics of the 1988 drought as it relates to characteristics of an ecoregion, (3) illustrate a means to quantify drought through a potential plant stress index, and (4) examine the link of regional drought to ecosystem processes. This analysis will provide background and methodology for ecologists, agriculturalists, and others interested in spatial and temporal characterization of climate patterns within large geographic regions.

Hurricane Impacts in New England and Puerto Rico

Hurricanes have a profound effect on many coastal ecosystems. Direct impacts often include wind damage to trees, scouring and flooding of river channels, and salt-water inundation along shorelines (Simpson and Riehl 1981; Diaz and Pulwarty 1997). In some areas, secondary impacts may include landslides triggered by heavy rains (Scatena and Larson 1991) or catastrophic dry-season fires resulting from heavy fuel loading (Whigham in press). This chapter will focus on the longterm impacts of hurricane wind damage at two LTER sites, the Harvard Forest (HFR) in central New England and the Luquillo Experimental Forest (LUQ) in northeastern Puerto Rico. These two sites, both located in the North Atlantic hurricane basin and occasionally subject to the same storms, provide interesting examples of tropical and temperate hurricane disturbance regimes. Wind damage from a single hurricane is often highly variable (Foster 1988). Damage to individual trees can range from loss of leaves and fine branches, which can significantly alter surface nutrient inputs (Lodge et al. 1991), to bole snapping or uprooting, which can significantly alter coarse woody debris and soil microtopography (Carlton and Bazzaz 1998a and b). At the stand level, damage can range from defoliation to individual tree gaps to extensive blowdowns, creating different pathways for regeneration (Lugo 2000). At landscape and regional levels, complex patterns of damage are created by the interaction of meteorological, topographic, and biological factors (Boose et al. 1994). Adding to this spatial complexity is the fact that successive hurricanes are not necessarily independent in terms of their effects. A single storm lasting several hours may have effects that persist for decades (Foster et al. 1998). And forest susceptibility to wind damage is strongly influenced by composition and structure, which in turn are strongly influenced by previous disturbance history (Foster and Boose 1992). Thus, the impacts of a single hurricane may depend in part on the impacts of earlier storms as well as on other previous disturbances and land use. Hurricanes, like other disturbances, both create and respond to spatial heterogeneity (Turner et al. 2003). To understand the long-term ecological role of hurricanes at a given site, we must consider these three sets of questions: (1) What is the hurricane disturbance regime?

Climate Variability in Tallgrass Prairie at Multiple Timescales: Konza Prairie Biological Station

Climate is a fundamental driver of ecosystem structure and function (Prentice et al. 1992). Historically, North American grassland and forest biomes have fluctuated across the landscape in step with century- to millennialscale climate variability (Axelrod 1985; Ritchie 1986). Climate variability of at decadal scale, such as the severe drought of the 1930s in the Central Plains of North America, caused major shifts in grassland plant community composition (Weaver 1954, 1968). However, on a year-to-year basis, climate variability is more likely to affect net primary productivity (NPP; Briggs and Knapp 1995; Knapp et al. 1998; Briggs and Knapp 2001). This is especially true for grasslands, which have recently been shown to display greater variability in net primary production in response to climate variability than forest, desert, or arctic/alpine systems (Knapp and Smith 2001). Although the basic relationships among interannual variability in rainfall, temperature, and grassland NPP have been well studied (Sala et al. 1988; Knapp et al. 1998; Alward et al. 1999), the linkages to major causes of climate variability at quasi-quintennial (~5 years) or interdecadal (~10 year) timescales in the North American continental interior, such as solar activity cycles, the El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the North Pacific Index (NP), are less well understood. In this chapter, we will examine how interannual, quasi-quintennial, and interdecadal variation in annual precipitation and mean annual temperature at a tallgrass prairie site (Konza Prairie Biological Station) may be related to indexes of solar activity, ENSO, NAO, and NP, and in turn how these indexes may be related to aboveground net primary productivity (ANPP). Specifically, we present (1) period-spectrum analyses to characterize the predominant timescales of temperature and precipitation variability at Konza Prairie, (2) correlation analyses of quantitative indexes of the major atmospheric processes with Konza temperature and precipitation records, and (3) the implications of variation in major atmospheric processes for seasonal and interannual patterns of ANPP. The Konza Prairie Biological Station (KNZ), which lies in the Flint Hills (39º05' N, 96º35' W), is a 1.6-million-ha region spanning eastern Kansas from the Nebraska border to northeastern Oklahoma (figure 20.1). This region is the largest remaining tract of unbroken tallgrass prairie in North America (Samson and Knopf 1994) and falls in the more mesic eastern portion of the Central Plains grasslands.

Millennial and Century Climate Changes in the Colorado Alpine

Ecosystems are the products of regional biotic history, shaped by environmental changes that have occurred over thousands of years. Accordingly, ecological changes take place at many timescales, but perhaps none is more significant than the truly long-term scale of centuries and millennia, for it is at these timescales that ecosystems form, break apart, and reform in new configurations. This is certainly true in the alpine regions, where glaciations have dominated the landscape for perhaps 90% of the last 2.5 million years (Elias 1996a). In the alpine tundra zone, the periods between ice ages have been relatively brief (10,000–15,000 years), whereas glaciations have been long (90,000–100,000 years). Glacial ice has been the dominant force in shaping alpine landscapes. Glacial climate has been the filter through which the alpine biota has had to pass repeatedly in the Pleistocene. This chapter discusses climatic events during the last 25,000 years (figure 18.1). At the beginning of this interval, temperatures cooled throughout most of the Northern Hemisphere, culminating in the last glacial maximum (LGM), about 20,000–18,000 yr b.p. (radiocarbon years before present). The Laurentide and Cordilleran ice sheets advanced southward, covering most of Canada and the northern tier of the United States. Glaciers also crept down from mountaintops to fill high valleys in the Rocky Mountains. In the Southern Rockies, the alpine tundra zone crept downslope into what is now the subalpine, beyond the reach of the relatively small montane glaciers. By about 14,000 yr b.p., the glacier margins began to recede, leading eventually to the postglacial environments of the Holocene. It is now becoming apparent that the climate changes that drove these events were surprisingly rapid and intense. This chapter examines the evidence for these climatic changes and the biotic response to them in the alpine zone of Colorado. To reconstruct the environmental changes of this period, we must rely on proxy data, that is, the fossil record of plants and animals, combined with geologic evidence, such as the age and location of glacial moraines in mountain valleys. As of this writing, the principal biological proxy data that have been studied in the Rocky Mountains are fossil pollen and insects.

Century- to Millennial-Scale Climate Change and Ecosystem Response in Taylor Valley, Antarctica

The view of climate change during the Pleistocene and the Holocene was very much different a mere decade ago. With the collection and detailed analyses of ice core records from both Greenland and Antarctica in the early and mid-1990s, respectively, the collective view of climate variability during this time period has changed dramatically. During the Pleistocene, at least as far back as 450,000 years b.p., abrupt and severe temperature fluctuations were a regular occurrence rather than the exception (Mayewski et al. 1996, 1998; Petit et al. 1999). During the Pleistocene, these rapid and large climatic fluctuations, initially identified in the ice core records, have been verified in both marine and lacustrine sediments as well (Bond et al. 1993; Grimm et al. 1993), suggesting large-scale (hemispheric to global) climate restructuring over very short periods of time (Mayewski et al. 1997). Similar types of climatic fluctuations, but with smaller amplitudes, have also occurred during the Holocene (O’Brien et al. 1995; Bond et al. 1997; Arz et al. 2001). What were the biological responses to these changes in temperature, precipitation, and atmospheric chemistry? We must answer this question if we are to understand the century- to millennial-scale influence of climate on the structure and function of ecosystems. Because the polar regions are thought to be amplifiers of global climate change, these regions are ideal for investigating the response of ecological systems to, what in temperate regions might be considered, small-scale climatic variation. Our knowledge of past climatic variations in Antarctica comes from different types of proxy records, including ice core, geologic, and marine (Lyons et al. 1997). It is clear, however, that coastal Antarctica may respond to oceanic, atmospheric, and ice sheet–based climatic “drivers,” and therefore ice-free regions, such as the Mc- Murdo Dry Valleys, may respond to climate change in a much more complex manner than previously thought (R. Poreda, unpubl. data 2001). Since the initiation of the McMurdo Dry Valleys Long-Term Ecological Research program (MCM) in 1993, there has been a keen interest not only in the dynamics of the present day ecosystem, but also in the legacies produced via past climatic variation on the ecosystem. In this chapter we examine the current structure and function of the dry valleys ecosystem from the perspective of our work centered in Taylor Valley.

Decadal Climate Variation and Coho Salmon Catch

When temporally smoothed data are used for the period 1925 to 1985 there is a close inverse statistical relationship acting at an interdecadal timescale between the Pacific Northwest (PNW) air temperatures and Coho salmon catch off the coast of Washington and Oregon. This relationship is now well known, although not fully explained, but at the time of its discovery in 1994 it was part of advances being made by several research groups on interdecadal-scale climate/ecological changes in the PNW (Greenland 1995). The discovery and later, related findings may be usefully examined within the context of the framework questions of this book (see chapter 1) because it provides a very interesting example of climate variability and ecosystem response found, in part, by Long-Term Ecological Research (LTER) investigators. The logical progression for this chapter is first to review a little of the relationship between Coho salmon and climate and then to explain how a study at one LTER site led to a finding with regional implications. An update of the findings at interdecadal-scale climate/ecological changes in the PNW is then appropriate, followed by a discussion of the topic with the framework questions of this book. The PNW is defined, for the purposes of this chapter, as the area of Washington and Oregon west of the crest of the Cascade Range. The term decadal is used loosely in this chapter to refer to changes that focus on time periods of about 10 to 30 years in length. Salmon live part of their lives in terrestrial, freshwater environments and part in marine, saltwater environments. The salmon life history starts with fertilized eggs remaining in gravel in freshwater stream beds and hatching after 1–3 months. One to five months later, fry emerge in the spring or summer. Juvenile fish are in freshwater from a few days to 4 years, depending on species and locality. After the juveniles change to smolts, they can migrate to the ocean, usually in spring or early summer, often taking advantage of streamflows driven by snowmelt. The fish spend 1–4 years in the ocean and then return to their freshwater home stream to spawn and die. More specifically, the typical life cycle for Oregon Coho spans 3 years (18 months in freshwater and 18 months in the ocean).