Catálogo de publicaciones - libros

The ecological and geological responses following the May 18, 1980, eruption of Mount St. Helens are all about change: the abrupt changes instigated by geophysical disturbance processes and the rapid and gradual changes of ecological response. The explosive eruption involved an impressive variety of volcanic and hydrologic processes: a massive debris avalanche, a laterally directed blast, mudflows, pyroclastic flows, and extensive tephra deposition (Lipman and Mullineaux 1981; Swanson and Major, Chapter 3, this volume). Subsequent, minor eruptions triggered additional mud-flows, pyroclastic flows, tephra-fall events, and growth of a lava dome in the newly formed volcanic crater. These geological processes profoundly affected forests, ranging from recent clear-cuts to well-established tree plantations to natural stands, as well as meadows, streams, and lakes. This book focuses on responses of these ecological systems to the cataclysmic eruption on May 18, 1980.

Volcanoes and volcanic eruptions are dramatic players on the global stage. They are prominent landscape features and powerful forces of landform, ecological, and social change. Vesuvius, Krakatau, Pompeii, and, in recent decades, Mount St. Helens hold an important place in our perceptions of how the Earth works and the incredible, destructive effects of violent eruptions. Perhaps less appreciated is the great diversity of interactions between volcanoes and the ecological systems in their proximity.

The diversity and intensity of volcanic processes during the 1980 eruption of Mount St. Helens affected a variety of ecosystems over a broad area and created an exceptional opportunity to study interactions of geophysical and ecological processes in dynamic landscapes. Within a few hours on the morning of May 18, 1980, a major explosive eruption of Mount St. Helens affected thousands of square kilometers by releasing a massive debris avalanche, a laterally directed volcanic blast, mudflows, pyroclastic flows, and widespread tephra fall (see Figure 1.1; Figures 3.1, 3.2; Table 3.1). These primary physical events killed organisms, removed or buried organic material and soil, and created new terrestrial and aquatic habitats. Despite these profound environmental changes, important legacies of predisturbance ecosystems, including live organisms, propagules, and organic and physical structures, persisted across much of the affected landscape. The physical characteristics of the volcanic processes (elevated temperature, impact force, abrasion, and depth of erosion and burial) in part determined the extent of mortality and the types and significance of biotic legacies in the posteruption landscape.

Tephra fall is the most widespread disturbance resulting from volcanic activity (del Moral and Grishin 1999), including the 1980 eruption of Mount St. Helens (Sarna-Wojcicki et al. 1981). Tephra is rock debris ejected from a volcano that is transported through the air some distance from the vent that produced it. Fine-textured tephra (less than 2 mm in diameter) is referred to as volcanic ash. Tephra may be transported far from a volcano and affect vegetation over thousands of square kilometers, well beyond the influence of other types of volcanic ejecta. Individual tephra deposits from volcanoes in the Cascade Range have been traced east into the Great Plains, and others cover much of the Pacific Northwest (Shipley and Sarna-Wojcicki 1983). Mount St. Helens has been the most frequent source of tephra in the Cascades for 40,000 years, producing dozens of tephra layers equal to or larger than the 1980 eruption, three experienced by trees alive in 1980 (1480, 1800, and 1980; Mullineaux 1996). The likely extent and magnitude of past volcanic eruptions are apparent in Cascade Range soils near or downwind from major volcanoes, soils that are largely formed from tephra (Franklin and Dyrness 1973), and in the large amounts of tephra in soils far east of the Cascade Range (Smith et al. 1968).

Debris avalanches occasionally occur with the partial collapse of a volcano, and their ecological impacts have been studied worldwide. Examples include Mt. Taranaki in New Zealand (Clarkson 1990), Ksudach in Russia (Grishin et al. 1996), the Ontake volcano in Japan (Nakashizuka et al. 1993), and Mount Katmai in the state of Alaska in the United States (Griggs 1918a, b, 1919). Analyses have shown that as many as 18 previously undetected debris avalanches have flowed from the Hawaiian island volcanoes (Moore and Clague 1992). Following the debris avalanche at Mount Katmai in Alaska, Griggs (1918c) found that the deposit depth influenced plant survival. As a volcano collapses, glaciers, rocks, soil, vegetation, and other material are moved with great force down the mountain. Debris avalanches are typically cool and can bury surfaces with as much as 200 m of material. They tend to follow the original topography, have abrupt edges, and produce steep, undulating topography that can persist for many millennia.

Geomorphic disturbances are widely recognized as important processes that influence plant-community development and landscape-scale vegetation patterns [e.g., Veblen and Ashton (1978), Garwood et al. (1979), Swanson et al. (1988), and Malanson (1993)]. In volcanically active areas such as the Pacific Northwest, mudflows are locally important geomorphic disturbance events governing short- and long-term ecological conditions. Volcanic mudflows can scour and inundate river valleys with large volumes of debris (Janda et al. 1981; Pierson 1985; Vallance and Scott 1997; Scott 1988; Vallance 2000; Kovanen et al. 2001) and influence plant succession tens of kilometers downstream from their points of origin (Halpern and Harmon 1983; Adams and Dale 1987;Wood and del Moral 1987; Frenzen et al. 1988). In addition to altering plant succession, large volcanic mudflows can initiate a cascading chain of secondary disturbances that further modify the landscape and affect subsequent ecological responses (see Swanson and Major, Chapter 3, this volume).

Our studies of succession on mudflows and pumice surfaces at Mount St. Helens support the view that plant succession is determined as much by chance and landscape context as by the characteristics of the site itself. Early primary succession is dominated by the probabilistic assembly of species, not by repeatable deterministic mechanisms. Before most plant immigrants can establish, some physical amelioration in the form of nutrient inputs or the creation of microsites may occur. As vegetation matures, there is a shift from amelioration to inhibition (Wilson 1999), but the magnitude of this shift varies in space and time. Species-establishment order is not preordained as stated by classic succession models (Clements 1916; Eriksson and Eriksson 1998). Life-history traits influence both arrival probability and establishment success, and the best dispersers are usually less adept at establishment. Therefore, interactions between site amelioration and proximity to colonists affect the arrival sequence and initial biodiversity. Unique disturbance events combine with usually low colonization probabilities to produce different species assemblages after each disturbance at a site. Early in primary succession, individuals just accumulate. However, over time, interactions begin that cause species to be replaced.

The variety of disturbance mechanisms involved in the 1980 eruption of Mount St. Helens (e.g., heat, burial, and impact force) and the resulting diversity of vegetation responses have provided abundant opportunities for disturbance-zone-specific research (see other chapters in this volume). As evidenced by the research reported in this volume, tremendous amounts of knowledge can be acquired from studies that focus on vegetation responses within individual disturbance zones, such as the debris-avalanche deposit. As the responses to the eruption continue to develop, however, it becomes increasingly important to understand the larger context for specific study sites: What responses are common among disturbance zones? And what responses are distinctive to certain disturbance mechanisms and sites? Can the lessons from a local area be generalized throughout the disturbed area or even throughout a zone dominated by one disturbance type? These questions can be addressed in at least two ways:

The eruption of Mount St. Helens, on May 18, 1980, affected an area of 600 km within which communities of animals and plants sustained a wide range of impacts, depending on proximity to the volcano and local topography. The most extreme destruction occurred in the area immediately north of the crater, now known as the Pumice Plain (see map, Figure 9.1), where the eruption apparently destroyed the entire biota over tens of square kilometers. Our interest concerned the response of arthropods to the eruption and changed landscape, particularly in the most intensively disturbed area and one remote site. Early questions to address included the following:

Arthropods are important components of ecosystems because of the roles they play in pollination, herbivory, granivory, predator—prey interactions, decomposition and nutrient cycling, and soil disturbances. Many species are critical to the structure and functioning of their ecosystem, although some (particularly insects) are considered pests in farmlands and forests because of their detrimental effects from feeding on foliage and transferring pathogens to trees and crops. Arthropods also constitute a high-protein prey resource for vertebrate wildlife (especially small mammals, birds, reptiles, and amphibians), thus contributing to the existence and stability of these wildlife species. As such, studies of arthropod population dynamics and changes in species assemblages following natural disturbances are important for understanding ecosystem responses. In the case of the Mount St. Helens volcanic eruption, studies of arthropods not only can provide information on natural history and ecology of many different species but also are relevant for evaluating theories of disturbance ecology and postdisturbance successional processes.