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Patterns and Processes Geomorphic Features of the Earth's Surface Regulate the Distribution of Organisms and Processes

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Patterns and Processes Geomorphic Features of the Earth's Surface Regulate the Distribution of Organisms and Processes Landform Effects on Ecosystem Patterns and Processes Geomorphic features of the earth's surface regulate the distribution of organisms and processes F. J. Swanson, T. K. Kratz, N. Caine, and R. G. Woodmansee nderstanding the form, be- types. The static and dynamic physi- havior, and historical context Landform effects provide cal characteristics of landscapes high- U of landscapes is crucial to un- lighted in the identification of these derstanding ecosystems on several temporal and spatial classes are important considerations temporal and spatial scales. Land- when analyzing individual ecosystems forms, such as floodplains and allu- perspectives for and making comparisons among eco- vial fans, and geomorphic processes, examining soil, systems. We will not consider here such as stream erosion and deposi- other classes of geomorphology-eco- tion, are important parts of the set- vegetation, and aquatic system interactions, such as effects of ting in which ecosystems develop and fauna and flora on rates of geomor- material and energy flows take place. ecosystems and phic processes. Over the long term, geomorphic pro- for interpreting This article is based on ongoing cesses create landforms; over a efforts to compare ecosystems among shorter term, landforms are boundary ecosystem processes the 15 diverse sites studied in the conditions controlling the spatial ar- Long -Te r m E co 1o gi c a 1 Re s e arch rangement and rates of geomorphic (LTER) program (Callahan 1984). processes. (Hack and Goodlett 1960). These sites include an old-growth co- Ecosystems respond to both land- Fascinating interactions among nifer forest in the Cascade Range of forms and geomorphic processes. The geomorphic processes, landforms, Oregon, Rocky Mountain alpine tun- history of geomorphic processes may and biota occur on various temporal dra in Colorado, lakes in forest land be expressed directly in the composi- and spatial scales. On a fine spatial of Wisconsin, and shortgrass steppe tion and structure of vegetation, scale, for example, parts of individual in Colorado. Within this diversity we where geomorphic events and vegeta- plants may retard soil erosion or may find valuable generalities. tion develop together. Geomorphic be damaged by earth movement. On a processes operating before the estab- much greater scale, the geographic Landscapes, landforms, and lishment of existing vegetation, or distribution and height of landmasses geomorphic processes those subtly coexisting with the vege- broadly control distributions of tation, may have their greatest influ- plants and animals through influences A discussion of effects of landforms ence on vegetation through control- on environmental gradients of tem- on ecosystems is hindered by a lack of ling patterns of soil properties across perature and moisture and on corri- concise, widely accepted definitions a landscape, as in toposequences dors of migration during environmen- of key terms and concepts. Naveh tal change. Rigorous examination of (1982) traces some of the history of F. J. Swanson is a research geologist at the interactions among geomorphology, usage and perception of landscape, a USDA Forest Service, Pacific Northwest ecosystems, and landscapes at the particularly crucial but poorly de- Research Station, Corvallis, OR 9733 1. spatial scales of hectares to thousands fined term. From roots in the Bible, T. K. Kratz is an assistant scientist at the of square kilometers is required for secular literature, and art emphasiz- Center for Limnology, University of Wis- further understanding of landscape ing a highly idealized view of nature, consin, Madison 53706. N. Caine is a ecology (Risser et al. 1984) and eco- landscape has gained increasing use in professor in the Department of Geogra- fields such as physical geography, phy, University of Colorado, Boulder system structure and function. 80309. R. G. Woodmansee is the director In this article, we identify and ex- ecology, and landscape architecture. of the Natural Resources Ecology Labora- plore four classes of effects of land- Landscape commonly refers to the tory, Colorado State University, Fort Col- forms on ecosystems, providing ex- form of the land surface and associat- lins 80523. amples from a range of ecosystem ed ecosystems at scales of hectares to 92 BioScience Vol. 38 No. 2 many square kilometers. Landform is usually used at a finer scale and more specifically, such as a landform carved out by a landslide or created by sediment deposition forming a gravel bar. Landscapes are composed of landforms and ecological units, such as patches (Forman and Godron 1986). By geomorphic processes we refer to mechanical transport of organic and inorganic material. In addition to surface erosion and mass movement, geomorphic processes include trans- port of material in solution in surface and subsurface water and biogenic soil movement by animals and root throw. A drawback of the term geo- morphic process is the implication that the process is necessarily shaping landforms. Frequently, however, geo- morphic process refers to transfer of material or disturbance of biota with- out regard to development of land- forms or the time scale in which that Figure 1. Examples of the four classes of landform influence on ecosystem patterns and processes. a. Class 1. Topographic influences on rain and radiation (arrow) shadows. b. occurs. We use the term in this general Class 2. Topographic control of water input to lakes. Lakes high in the drainage system sense. may receive a higher proportion of water input by direct precipitation than lakes lower in the landscape where groundwater input (arrow)predominates. c. Class 3. Landform- Effects of landforms on constrained disturbance by wind (arrow) may be more common in upper-slope ecosystems locations. d. Class 4. The axes of steep concave landforms are most susceptible to disturbance by small-scale landslides (arrow). We consider four classes of landform effects on ecosystem patterns and pro- cesses (Figure 1): fluence other environmental and biot- ker and Niering 1965). Implicit in the 0 Class 1: Landforms-by their eleva- ic factors. Ecosystems are considered relations between patterns of vegeta- tion, aspect (direction in which land static in class 1, but dynamic in class- tion and landforms are the influences surface faces), parent materials, and es 2 and 3. In class 4, we regard both of elevation and aspect on solar ener- steepness of slope-influence air landforms and ecosystems as gy and water regimes at patches with- and ground temperature and the dynamic. in complex ecosystems. As moist air quantities of moisture, nutrients, These four classes are valuable as a masses flow over hills and mountains, and other materials (e.g., pollut- framework for discussion rather than higher precipitation commonly falls ants) available at sites within a as a rigorous classification scheme. In at higher elevations (orographic ef- landscape. the following discussion, we give ex- fects), but such simple patterns may a Class 2: Landforms affect the flow amples of each class, address the ag- be confounded by the effects of rain of organisms, propagules (e.g., gregated effects landforms have in and shadow, fog and cloud belts, and seeds and sproutable root frag- determining the patterns of natural timing of snow accumulation and ments), energy, and material (water, landscapes, and consider implications melt. Neither the simple nor the com- dissolved material, and organic and for landscape ecology. Some effects of plex effects of landforms on environ- inorganic particulate matter) landforms on ecosystems at the spa- mental gradients, however, can com- through a landscape. tial scale of interest for this discussion monly be separated from the effects of 0 Class 3: Landforms may influence do not fall neatly into one of these landforms on movement of materials the frequency and spatial pattern of four classes. and energy. nongeomorphically induced distur- bance by agents such as fire, wind, Landforms .and environmental gradi- Landforms and movement of materi- and grazing. ents. Elevation and aspect are envi- al, organisms, propagules, and ener- 0 Class 4: Landforms constrain the ronmental gradients that have been gy. Landforms regulate movement of spatial pattern and rate or frequen- widely recognized in mapping and in material, organisms, propagules, and cy of geomorphic processes that al- gradient analysis of patterns of vege- energy across a landscape by defining ter biotic features and processes. tation across landscapes ranging from gravitational gradients, influencing a few hectares to thousands of square flow paths of wind, and forming bar- In classes 1-3, we consider landforms kilometers (Billings 1973, Hack and riers and corridors for movement. to be unchanging boundaries that in- Goodlett 1960, Kessell 1979, Whitta- The role of landforms in controlling February 1988 93 movement of material is implicit in forms strongly control the location Concentrations of silica, for exam- studies of nutrient cycling and sedi- and rate of processes that move both ple, are several orders of magnitude ment routing within drainage basins, soil and sediment. For example, steep, greater in groundwater than in pre- along transects crossing topographic concave hillslopes are landform units cipitation; biological productivity of features, and in other landform set- that are the predominant site of small, lakes high in the flow system can tings involving flow paths. The flow rapid landslides (Dietrich and Dunne become seasonally limited by
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