Landscape Heterogeneity – A Network Perspective
Frederick J.
1
Swanson ,
Julia A.
2
Jones
1. USDA Forest Service, Pacific Northwest Research Station, Corvallis OR 97331 Email: Fred.Swanson@orst.edu
2. Department of Geosciences, Oregon State University, Corvallis, OR 97331 Email: jonesj@geo.orst.edu
Abstract
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.
1. Networks and Related
Phenomena Relevant to This
Discussion
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).
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)
Design: Kathryn Ronnenberg
“small world”
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)
dam/reservoir
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).
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).
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).
branching hierarchical
4. Ambient environmental
gradients may impede movement
of organisms and propagules
through networks (Colleen
Hatfield, PhD UNM).
2. Landscape Interactions Involving Networks
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).
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).
Relevant Papers from the Andrews
LTER group (of many from the global
literature):
Forman RTT and a cast of thousands. 2002. Road Ecology.
Island Press.
1 - patch to net; 2 - net to patch; 3 - net to net
Mona Creek watershed, Western
Cascades, Oregon
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.
patch cuts
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.