Landscape Heterogeneity

1
Landscape Heterogeneity A Network
Perspective Frederick J. Swanson1, Julia A.
Jones2 1. USDA Forest Service, Pacific Northwest
Research Station, Corvallis OR 97331 Email
Fred.Swanson_at_orst.edu 2. Department of
Geosciences, Oregon State University, Corvallis,
OR 97331 Email jonesj_at_geo.orst.edu
Design Kathryn Ronnenberg
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.
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)
small world
branching hierarchical
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
1476-85. 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 48681-689.
Wemple BC, Swanson FJ, Jones JA. 2001. Forest
roads and geomorphic process interactions,
Cascade Range, Oregon. Earth Surface Processes
and Landforms. 26191-204.
1 - patch to net 2 - net to patch 3 - net to
net
Mona Creek watershed, Western Cascades, Oregon
patch cuts