IV.5 Boundary Dynamics in Landscapes
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IV.5 Boundary Dynamics in Landscapes Debra P. C. Peters, James R. Gosz, and Scott L. Collins OUTLINE 1. Introduction 2. Conceptual models of boundary dynamics 3. Properties of boundaries 4. Dynamics of boundaries in response to climate and disturbance 5. Landscape dynamics 6. Consequences of boundary dynamics for animal populations 7. Approaches to detect boundary locations 8. Applying boundary dynamics to management Landscapes consist of a mosaic of distinct vegetation types and their intervening boundarieswithdistinctcharacteristics. Boundaries can exist along abrupt environmental gradients or along gradual changes that are reinforced by feedback mechanisms between plants and soil properties. Boundaries can be defined based on the abundance, spatial distribution, and connectivity of the underlying patches. There are three major types of boundary dynamics that differ in the direction and rate of movement of the boundary in response to climatic fluctuations: stationary, directional, and shifting. Future conditions in climate and the disturbance regime, including land use, may fundamentally alter the type of boundary as well as its location and composition through time. GLOSSARY boundary (ecotone). Transition area where spatial changes in vegetation structure or ecosystem process rates are more rapid than in the adjoining plant communities corridor. Edge that promotes movement or allows unimpeded movement of organisms between local populations directional transition. Location of a boundary between two areas that moves unidirectionally through time edge. Well-defined area between patch types; often a barrier, constraint, or limit to the movement of animals and plants patch. Discrete, bounded area of any spatial scale that differs from its surroundings in its biotic and abiotic structure and composition shifting transition. Boundary location that shifts back and forth with no net change over time state. Defined by either the dominant species or composition of species, and associated process rates stationary transition. Boundaries that are stable with little movement through time 1. INTRODUCTION Landscapes consist of a mosaic of distinct vegetation types and intervening boundaries (or ecotones) with different characteristics from the adjacent communities. An ecotone is a transition zone in time and space. Ecotones along spatial gradients in edaphic and climatic factors (e.g., elevation, soil texture, precipitation) have a long history in ecological studies. Gradual environmental gradients underlying vegetation transitions also exist, often related to positive feedbacks between plants and their environment. Ecotones are increasingly recognized as important elements of dynamic landscapes because of their effects on the movement of animals and materials, rates of nutrient cycling, and levels of biodiversity. Because dramatic shifts in location of vegetation types can occur at ecotones, these can also be important for management and as indicators of climate change. 2. CONCEPTUAL MODELS OF BOUNDARY DYNAMICS Ecotones along spatial gradients, such as the treeline along an elevation gradient, have long been recognized by ecologists as important elements of landscapes. The term zone of tension between two plant community types dates to Clements (1904). Focused research on a broader definition of ecotones began in earnest in the late 1980s and early 1990s. Theoretical models were developed that laid the foundation for future research that considered ecotones as transitional areas between different vegetation types (Gosz, 1993). Many of these models included a hierarchy of spatial scales to depict ecotones, from plant edges to populations, patches, landscapes, and biomes. Most research initially focused on transition zones between biomes that cover large spatial extents, such as between grasslands and forests or between different grassland types. At the biome scale, ecotones were viewed as consisting of a collection of patches where the number and size of patches vary spatially across long distances (hundreds of kilometers). In general, average patch size was predicted to decrease as the distance from a biome or core population increased because of the loss of suitable habitats with increasing distance. Under conditions of increasing resource availability as distance to the biome edge decreased, patches were predicted to coalesce and shift the spatial location of an ecotone through time. More recent conceptual models have focused on ecotones at landscape scales and have questioned this general relationship between patch size and distance from a core population. Some models focus on the properties of boundaries that influence the rate and pattern of movement of organisms, matter, and energy between adjacent areas (e.g., Wiens, 2002). In other cases, models have focused on boundary dynamics and the use of patch dynamics to provide new understanding about the structural and functional properties of ecotones. Patch dynamics theory has been integrated with hierarchy theory to relate pattern, process, and scale within...