dam/reservoir. Landscape Heterogeneity – A Network Perspective Frederick J. Swanson 1 , Julia A. Jones 2 1. USDA Forest Service, Pacific Northwest Research Station, Corvallis OR 97331 Email: [email protected]
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Design: Kathryn Ronnenberg
3. Landscape Heterogeneity Involving Networks
Issues of structure and function of heterogeneous landscapes are commonly addressed in the context of landscapes viewed as patchworks of vegetation, soil, and habitats. In some cases it is useful to consider landscape heterogeneity in terms of roles of various types of branching, hierarchical network structures and their interactions with patchworks or environmental gradients. Networks can be natural (e.g., streams, ridgelines, riparian zones, animal travel routes) or human-constructed (e.g., roads, powerlines). Networks can fragment landscapes (e.g., roads functioning as barriers between inter-road habitat patches) and create ecotonal zones of distinctive habitats (e.g., sites of network-patchwork interaction in riparian zones), hence contributing to landscape heterogeneity, or may foster connectivity through landscapes, hence minimizing effects of heterogeneity. Networks themselves are subject to fragmentation (e.g., reservoirs on river networks), thus increasing heterogeneity in a system otherwise characterized more by gradual environmental gradients. Heterogeneity within networks is also created by disturbances, such as the patch-dynamic character of debris flows in mountain river networks. We present a simple typology of examples of landscape heterogeneity associated with network structures in landscapes.
A. Heterogeneity through networks
B. Heterogeneity created by network interactions with inter-network patches or other networks
1. Networks can be connectors through landscapes, thus minimizing heterogeneity, but geomorphic constraints may create heterogeneity in longitudinal gradients (e.g. in the River Continuum Concept)
1. Networks can fragment landscapes if they serve as barriers, filters, sinks.
2. River networks may be fragmented by reservoirs, which can interrupt movement of water, sediment, organic matter, nutrients, organisms (Dynesius and Nilsson, 1994, Science).
1. Networks and Related Phenomena Relevant to This Discussion
2. Variation in pattern of network-patchwork elements through a network can create a heterogeneous pattern of distinctive ecotonal environments (e.g., co-occurrence of bare soil in roadside area with high light levels in adjacent patch favoring establishment of exotic plants).
A. Branching, hierarchical networks vs. “small-world” networks with many connections among nodes
B. Branching hierarchical network structures can be natural (e.g., streams, ridgelines) or human-constructed (e.g., roads, railroads, powerline corridors).
3. Disturbances relegated to channels and riparian zones may create a patch dynamic in networks (e.g., debris flows or gravel bar dynamics in a river network).
C. The function of a network my be integral to its creation (e.g., streamflow created and maintains the stream network; a road network created for transport among land-use units) or incidental to its creation (e.g., dispersal of exotic plants through road networks)
3. Network-network intersections can alter patterns of heterogeneity within networks (e.g., introduction of exotic plants from road nets to stream and riparian nets).
4. Ambient environmental gradients may impede movement of organisms and propagules through networks (Colleen Hatfield, PhD UNM).
2. Landscape Interactions Involving Networks
Relevant Papers from the Andrews LTER group (of many from the global literature):
Network – Network Interactions
One network type may draw from or contribute to another type at points of intersection (e.g., road runoff contributing to flow in stream networks, roads trapping sediment moving down streams).
Network – Patchwork Interactions
Critical patch size may set network density (e.g., drainage area needed to support the head of a stream sets stream network density, basic land use unit (e.g. residential lots size, farm field, forest cutting unit) defines road network density).
Scaling issue – our main interest is where network and patchwork are of similar dimensions (e.g., mean patch diameter similar to inter-nodal distance).
Forman RTT and a cast of thousands. 2002. Road Ecology. Island Press.
Jones JA, Swanson FJ, Wemple BC, Snyder KU. 2000. Road effects on hydrology, geomorphology, and disturbance patches in stream networks. Conservation Biology 14:76-85.
Snyder KU. 2000. Debris flows and flood disturbance in small, mountain watersheds. MS thesis. Oregon State Univ.
Swanson, FJ, JA Jones, GE Grant. 1997.The physical environment as a basis for managing ecosystems. pp. 229-238 (Chapter 15) In: Kohm, KA and JF Franklin. Creating a forestry for the 21st century. Island Press, Boulder, CO.
Swanson FJ, Johnson SL, Gregory SV, Acker SA. 1998. Flood disturbance in a forested mountain landscape: interactions of land use and floods. BioScience 48:681-689.
Wemple BC, Swanson FJ, Jones JA. 2001. Forest roads and geomorphic process interactions, Cascade Range, Oregon. Earth Surface Processes and Landforms. 26:191-204.
1 - patch to net; 2 - net to patch; 3 - net to net
Mona Creek watershed, Western Cascades, Oregon