Geomorphic Change and Vegetation Development on the Muddy River Mudflow Deposit

Abstract

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 PacificNorthwest,mudflows are locally important geomorphic disturbance events governing shortand 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 andHarmon 1983; Adams andDale 1987;Wood and delMoral 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). The comparatively high disturbance intensity but spatially variable nature of volcanic mudflows provide unique opportunities to study complex interactions between geomorphic processes and ecological succession (Beardsley and Cannon 1930; del Moral 1998; Kroh et al. 2000). Nonetheless, few studies examined plant succession on mudflow deposits before the 1980 eruption of Mount St. Helens (Frehner 1957). Research subsequent to that eruption has shown that plant succession on mudflow deposits is highly variable in response to local substrates, plant reproductive strategies, distances to seed sources, and chance dispersal events (Halpern and Harmon 1983; del Moral 1998). “Biological legacies,” such as floated logs, remnant snags, and shallowly buried residual plants, also play important roles in vegetation development on mudflow deposits (Frehner 1957; Franklin et al. 1985; Frenzen et al. 1988; Halpern and Harmon 1983; del Moral 1998; Kroh et al. 2000; Weber 2001). Vegetation succession on mudflow deposits can follow an initialor relay-floristics model (sensu Egler 1954) or some combination of the two (del Moral 1998) and can lead to the compositional convergence or divergence of neighboring communities (Franklin et al. 1985;Wood and delMoral 1987;Kroh et al. 2000). Although earlier studies of vegetation recovery on mudflowdeposits provide important insights into the dynamics of herbaceous plant communities, few of these studies examined succession over decades after a disturbance (Frenzen et al. 1988; Kroh et al. 2000). In this chapter, we present a case study of geomorphic and vegetation responses at four sites along the Muddy River to large (up to 107 m3) mudflows triggered by the May 18, 1980 eruption of Mount St. Helens. Our objective is to describe and qualitatively compare geomorphic changes and vegetation development along distinct reaches that represent a range of mudflow-induced disturbance intensities and environmental settings. We address three questions: