Biological Conservation 72 (1995) 371-379
Elsevier Science Limited
Printed in Great Britain
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ELSEVIER
0006-3207(94)00049-2
CHANGES IN THE LANDSCAPE STRUCTURE OF A
SOUTHEASTERN WYOMING RIPARIAN ZONE FOLLOWING
SHIFTS IN STREAM DYNAMICS
James R. Miller*
Departmentof Fishery and Wildlife Biology, ColoradoState University,Fort Collins, Colorado80523, USA
Terri T. Schulzt
us Forest Service,Rocky Mountain Forest and RangeExperimentStation, Fort Collins, Colorado80526, USA
N. Thompson Hobbs
Mammals ResearchSection, ColoradoDivision of Wildlife, Fort Collins, Colorado,80526, USA
Kenneth R. Wilson
Departmentof Fishery and Wildlife Biology, ColoradoState University,Fort Collins, Colorado80523, USA
Donald L. Schrupp
Habitat Section,Colorado Division of Wildlife, Denver,Colorado80216-1000,USA
&
William L. Baker
Departmentof Geography,Universityof Wyoming,Laramie, Wyoming82071, USA
(Received23 September1993;revisedversion received24 March 1994;accepted18 Apri11994)
Abstract
Throughout westernNorth America,flood regimeshave
been altered as a result of large-scalewater impoundments and diversions,yet the effects on riparian landscape structure have not been quantified. Using aerial
photographs and a GIS, we examined changesin the
Rawhide Wildlife Management Area in southeastern
Wyoming between 1937 and 1990 after shifts in the
frequency and intensity of flooding of the North Platte
River. The river declinedin wettedarea by 75% between
1937 and 1990. Also, the areal proportion occupiedby
cottonwood (Populus spp.) stands with <30% canopy
closureincreasedwhile standswith> 70% canopyclosure
decreasedduring this period.. indicating a shift from
young, densestands to older, more open stands. Some
traditional measuresof landscapestructure (i.e. richness,
diversity, dominance, average patch perimeter length,
averagepatch shape), however,appearedinsensitiveto
these changes.Finally, the proportion of the landscape
that changedland typesdeclinedwith increasingdistance
from the river. We expectfurther modificationof the land* Pleasedirect all correspondence
to this author. Presentaddress:
Dept. of Biology, Colorado State University, Ft. Collins,
Colorado 80523,USA.
tpresent address:The Nature Conservancy,Boulder, Colorado
80302. USA.
scapestructure, associatedwith continued declinesin cottonwood recruitment and increases in cottonwood mortality.
Keywords: North Platte River, flood regime, Wyoming,
Populus spp., landscape structure.
INTRODUCTION
Environmental disturbance regulates many ecological
processes.Large-scale disturbances such as fires and
floods may cause significant mortality in populations,
exerting strong control on the structure and function of
biotic communities (see reviews by White, 1979;
Denslow, 1985; Caswell & Cohen, 1991). Virtually by
definition, riparian zones are subjectedto periodic disturbance by flooding. In many areas, particularly arid
ones, riparian systemsare strongly pulsed by flooding
events as a result of annual cycles in precipitation and
temperature (Ewel, 1979; Graf, 1988; Baker, 1990).
Much of the spatial pattern and many of the functional
properties of riparian systemscan be attributed to this
pulsed behavior. However, despite the apparent significanceof flooding and the recognition of riparian zones
as key landscape features and centers of biological
diversity (Naiman et al., 1993), the consequencesof
floods for theseareasare not nearly as well understood
as are the consequencesof disturbances in other
J. R. Miller et al.
systems, such as fire in forests (Kimmerer & Allen,
1982;Sousa, 1984;Mallik & Wein, 1986).
While riparian zones at lower elevations in western
North America comprise lessthan I % of the total land
area, up to 80% of terrestrial vertebrate speciesare dependent upon them for at least part of their life cycles
(Chaney et al., 1990). These riparian landscapesoften
consist of irregular patchesof distinct vegetation types
or age classesarrayed in a mosaic along a watercourse
and the structure of this mosaic is important to wildlife
species living in these landscapes(e.g. Gutzwiller &
Anderson, 1987).This type of landscapestructure has
been shown to result from periodic large floods that
destroy existing vegetation and create open patches
(Baker, 1990).These open patchesare particularly important as seed beds for cottonwoods (Populus spp.)
which typically do not regenerateinside mature forests
(Wilson, 1970;Bradley& Smith, 1986;Baker, 1988;Bums
& Honkala, 1990).
Cottonwoods are an important component of lowelevation riparian landscapesin westernNorth America
and are often the most common tree found there. In the
Platte River basin, cottonwoods occurred historically in
widely scatteredpatchesalong the predominantly open
landscapesadjoining the river (Knopf & Scott, 1990).
Two properties limited the spatial distribution of these
trees. In addition to requiring open patches of alluvial
substrates for regeneration, such as those created by
floods, cottonwoods require water at or near the rooting
zone during the growing season,favoring establishment
on the river flood plain. However,flooding in theseother-
~~~
- - -~
- -
" ~.~;,North
Platte
""
Platte
Ri"',
NEBRASKA
'\
wise suitable areas periodically causeshigh mortality,
and this mortality opposeswidespreadestablishmentof
cottonwoodstands.Hence,the distribution of cottonwood
representsa compromise between its requirementsfor
high levelsof soil moisture and sunlight, and its vulnerability to floods (Currier, 1982;Knopf & Scott, 1990).
Throughout western North America, natural flood
regimeshavebeenalteredas a result of hydrologic development for agricultural, municipal, and industrial use
(Williams & Wolman, 1984;Knopf & Scott, 1990),with
dramatic consequences
for the fauna and flora of riparian zones(Bradley& Smith, 1986;Johnson, 1989;Knopf
& Scott, 1990).Thesealterations have contributed to a
decline and loss of cottonwood forests in many areas
(Rood & Mahoney, 1990), but at least temporary increasesin these forests in other areas (Williams, 1978;
Snyder & Miller, 1992). Knopf and Scott (1990) proposedthat changesin flood regimeswill alter the spatial
distribution, abundance,and age structure of riparian
woody vegetationin the Platte River headwaters.
Changesin disturbanceregimesmay result in changes
in the patch structure of the landscape.For example,a
number of measuresof landscapestructure (e.g. richness, shape, and Shannon diversity) changed over a
period of decadesas fires were suppressedin a midwestern U.S. landscape (Baker, 1992, 1993). Little is
known, however, about the effects of modified flood
regimes on riparian landscapestructure. Here, we examine the influence that altered flood regimesexert on
the patch composition of a riparian landscape. We
compare historic patterns with those seen after major
- ----
----
---
"
\
\
\
\
\
Fig. 1. The Rawhide Wildlife ManagementArea (RWMA) in relation to the North Platte River and its tributaries in southeastern
Wyoming. Numbers refer to major storage reservoirs(seeTable I).
Flood regimes and landscape structure
Table 1. Major storage reservoirson the North Platte River
upstreamfrom Torrington, Wyo (from Williams 1978)
Reservoir
I Pathfinder
2 Guernsey
3 Alcova
4Seminoe
5 Glendo
Completion
date
Maximum
storage
capacity (m3)
April 1909
1.26 X 109
July 1927
5.52 X 107
February 1938 2.35 X 108
April 1939
1.25 X 109
October 1957
9.84 X 108
Nearest
town
Alcova, Wyo
Guernsey,Wyo
Alcova, Wyo
Leo, Wyo
Glendo, Wyo.
Fig. 2. Annual peak streamflow between 1916 and 1992 for
the North Platte River below Whalen Diversion, about 30 km
upstream from the study area.
shifts in the frequency and intensity of flooding of the
North Platte River in southeasternWyoming.
STUDY AREA
We examined the Rawhide Wildlife Management Area
(RWMA), a cottonwood-dominated riparian landscape
along the North Platte River. The RWMA is comprised of approximately 313 ha located 10 km northwest of Torrington, Wyoming (Fig. 1).
The North Platte River is a major tributary of the
Platte River. Formed primarily by snow melt in the
Rawah and Medicine Bow Mountains, the North Platte
flows 1050km north then east, draining an area of approximately 80,000km2 before it joins the South Platte
River on the high plains of Nebraska (Fig. 1) (Kirchner
& Karlinger, 1983). Extensive development of dams
and reservoir storage occurred along the North Platte
River during the early and mid-1900s(Table 1, Fig. 1).
By 1980,these mainstem impoundments resulted in an
order of magnitude increasein water diverted from the
river, concurrent with an 80% reduction in peak flows
and an 85% reduction iT}mean annual flows relative to
pre-1900conditions (Williams, 1978;Fig. 2).
METHODS
Classification and mapping
We examined changes in the landscape structure of the
R WMA based on photo-interpretation
of 1: 20,000
373
scaleblack and white aerial photographs taken in June
1937 and June 1990. The photos were scanned using
the Map and Image ProcessingSystem (MIPS) into
gray-scaleimage data sets. These data sets were then
transferred to the GRASS geographic information system (USA-CERL, 1991) and registered to Universal
Transverse Mercator (UTM) projection coordinates.
The imageswere rectified to fit standard US Geological
Survey 1: 24,000 scale topographic maps and subsequently photo-interpreted.Our analysisfocusedon seven
land types that were readily discernablefor both setsof
photos and representedthe most important landscape
elements for purposes of this study. The land types
identified were river (here defined as wetted channel),
sandbar, herbaceous/bareground, willow (Salix spp.),
and cottonwood stands in three canopy-closureclasses:
<30%, 30-70%, and >70%. Patcheswere then digitized
and labelled for each year and the resulting maps were
converted to a raster format using a 5 m X 5 m pixel
resolution. The 1990 classification was ground-truthed
during March 1992.Field observationswere made over
the entire area on features influencing the interpreted
categories,with the appropriate changesmade to the
original classificationmap.
Spatial analysis
Transition analysis
We examined transitions among land types between
1937and 1990in two ways. First, we graphed the proportion of the total area of each land type in each year.
Second,we overlaid the 1937 and 1990 maps, creating
a third map. Pixel valuesin this new map representthe
type of change that occurred for each pixel between
1937and 1990.Shifts betweenland types were quantified from the resultant coincidencetable in GRASS.
Analysis of landscapestructure
We computed several measuresof landscapestructure
in order to quantify changes that occurred between
1937and 1990.All measureswere computed using the
Raster Landscape Ecological (r.le) Spatial Analysis
Package(Baker & Cai, 1992),designedfor the analysis
of landscape structure within GRASS. For each of
the two maps, we measured patch diversity in four
ways:
(1) Richness: the number of different land types
present.
(2) Shannon index (H '): combines richness and
evennessusing the formula:
m
H' = L pi*ln(pi)'
i=1
wherePi is the fraction of the land area occupied
by land type i and m is the number of land types
present. Larger values of H' indicate a more
diverselandscape(Turner & Ruscher, 1988).
(3) Dominance: emphasizesthe deviation from evennessusing the formula:
D
= In(n)-
H'.
J. R. Miller et at.
374
Fig. 3. The location of the North Platte River in 1937and 1990within the boundariesof the RWMA
where n is the number of land types present.
Larger values of D indicate a landscape dominated by one or a few land types while smaller
values of D indicate a landscape in which land
types are found in approximately equal proportions (O'Neill et al., 1988).
(4) Reciprocal Simpson's index (liS): measuresthe
probability of encountering two pixels of the
land type when taking a random sample of two
pixels, using the formula:
s
m
L pi
where the variables are the same as those in the
Shannon index, except that here larger values of
S indicate a decreasein landscapediversity.
In addition, we totalled the number of patches by
land type for each map. Finally, we computed some
measuresboth for the entire map and by land type:
(1) total patch perimeter; (2) mean patch perimeter;
(3) mean patch size; and (4) mean patch shape using
a corrected perimeter/areaindex (Baker & Cai, 1992).
The shape index can range from 1 (i.e. a circle) to
infinity, with higher values indicating shapeswhich are
increasinglyirregular (Baker & Cai, 1992).
While we do compareour observationswith the effects
of dams on downstream riparian zones described in
other studies, we do not use inferential statistics to
extend our observationsbeyond the RWMA. Doing so
would require that we randomly samplefrom the population of landscapes along, for instance, the North
Platte River. However, becausewe observedthe entire
'population of pixels' within the landscapewe studied,
differencesbetween 1937and 1990are establishedwithout any sampling error. Whether such differencesare
biologically significant dependson making referenceto
a particular processof interest (Hurlbert 1984; Kolasa
& Rollo 1991).
Buffer analysis
We determinedthe magnitude of land type changeas a
function of distance from the river. Eight 100 m-wide
buffer zoneswere created, radiating out from the 1937
river channel(zone 1 = the river itself, zone 2 = 1-100 m
from the river, zone 3 = 101-200m from the river, etc.).
For this analysis, we included the area outside the
RWMA boundaries in order to represent more accurately the changewithin eachzone.Theseadditional lands
were still relatively undeveloped,basedon inspection of
the 1990aerial photographs and the 1992field survey.
RESULTS
Fig. 4. The proportion of the RWMA in each land type in
1937and 1990.
Transitions
The relative proportions of area of the seven land types
changed between 1937 and 1990. The North Platte River
shifted, lost a major meander (Fig. 3), and declined in
wetted area by 75% (Fig. 4). The area of cottonwood
patches increased by approximately 16%, but the proportion of land area in the cottonwood canopy-closure
classes was more evenly distributed among classes in
1937 when compared with 1990 (Fig. 4). By 1990, the
proportion of land area covered by cottonwood stands
with <30% cottonwood canODV closure had nearlv
Flood regimes and landscape structure
37~
Table 2. For eachland type in 1937,the percentageof area that has changedto anotherland type in 1990
(The last row and column representthe total number of hectaresin each land type in 1990and 1937,respectively.)
1937
1990
<300/0
30-70%
>70%
cottonwoods cottonwoods cottonwoods
<300/0cottonwoods
30-70% cottonwoods
>700/0cottonwoods
Willows
Sandbar
Herabaceous/bareground
River
1990(ha)
58
52
22
0
29
10
29
50
8
10
35
11
23
2
0
14
0
10
108
79
19
doubled, while the proportion of land area with >700/0
cottonwood canopy closure declined by nearly 50%
during this period (Fig. 4).
The most dramatic land type shift evident in our
overlay analysis involved the river. Only 13% of the
area that was river in 1937remainedriver in 1990(Fig. 3
and Table 2) and, of the portion that changed,approximately 30% becamesandbar and 44% becamecottonwood (Table 2). The total loss in river area was 54 ha
while the total gain was only 8 ha (Table 2). Of the
area that was sandbar in 1937,more than 77% became
cottonwood by 1990 (Table 2). Of the 1990 land area
with 30-70% and> 70% cottonwood canopy closure,
approximately 43% was either river and/or sandbar in
1937(Table 2).
Landscapestructure
Richnessdid not changefrom 1937to 1990.The Shannon index (2.86 versus 2.93), dominance (1.66 versus
1.65), and the reciprocal Simpson index (1.21 for both)
were also similar for the 1937and 1990.landscapes.
The number of patches in each land type did not
change drastically between 1937 and 1990 (Table 3).
Willows
Sandbars Herbaceous! River
bare ground
I
2
7
99
3
2
5
6
7
8
0
8
11
30
13
8
7
0
6
61
9
4
1
4
0
5
4
13
37
43
16
1937
(ha)
60
~1
34
2
44
30
62
The largest relative increasein the number of patches
occurred for the willow type, while the <30% cottonwood canopy closure type showed the largest decrease
in the number of patches.While the number of patches
for the <30% cottonwood canopy-closure decreased,
the mean patch size increaseddramatically as did the
variability (Table 3). The willow and bare ground!
herbaceous mean patch sizes also increased slightly
from 1937to 1990.
The total length of all perimeters and the overall
mean perimeter length for all land types decreased
slightly from 1937 to 1990:94,065 m to 90,650 m and
1270m to 1209m, respectively.The largest changesin
mean patch perimeter occurred for the <30% cottonwood canopy closure class, which increased by 82%,
the> 70% cottonwood canopy closure class, which decreasedby 63%, and the willow class, which increased
by 39% (Table 3). The variability in mean perimeter
length increased for all land types except the >70%
cottonwood canopy closure class.
The mean shapeof all patches,basedupon the mean
corrected perimeter/areaindex, was not very different
for the 1937and 1990maps: 1.78and 1.87,respectively.
Table 3. Patch characteristicsfor 1937and 1990with standarddeviationsin parentheses
<30"/0
Cottonwood
30-700/0
Cottonwood
>700/.
Cottonwood
Willow
Sandbar
Herbaceous!
bare ground
24
25
8
10
1.83
(1.71)
1.48
(1.26)
3.8
(1.8)
4.32
(3.88)
713
(320)
743
(420)
1641
(970)
1539
(1150)
1.66
(0.26)
1.83
(0.48)
2.26
(0.85)
2.13
(0.73)
376
J. R. Miller et al
..
.,
<
"0
c
0
E
80
Q:
-
0
1-100
101-200 201-300 301-400 401-500 501-600 601-700
Distance from 1937 river (m)
Fig. 5. The proportion of area that has changed land types
between1937and 1990as a function of distancefrom the river.
When comparisons were made by land type, there was
little difference between years for the mean patch
shapes(Table 3).
Buffer analysis
The proportion of the landscape that changed land
type between 1937 and 1990 declined with increasing
distance from the 1937 river channel up to 400 m (Fig. 5).
Approximately 65% of the area within 1-100 m of the
river changed land type between 1937 and 1990: nearly
twice the proportion of area that changed within
301-400 m (Fig. 5). At distances exceeding 400 m, once
again the magnitude of change increased as a side channel
was crossed.
DISCUSSION
Transitions
The geomorphology of the Platte River throughout its
basin has changed considerably over the past century.
Other studies have shown that the current channel
width is much less than it was in the mid-19th century
below major reservoirsor diversion dams on the North
Platte (Williams, 1978; McDonald & Sidle, 1992), the
Platte (Williams, 1978; Eschner et al., 1983),and portions of the South Platte (Nadler & Schumm, 1981;
McDonald & Sidle, 1992).Likewise, we found a decline
in the surfacearea of the river in the RWMA, although
the geomorphology of this reach of the North Platte
has not been specifically studied. Other changes in
channel morphology, including a decreasein braiding
and an increase in sinuosity, also have been reported
for the North Platte (Williams, 1978). In contrast, the
river has decreasedin sinuosity in the RWMA, due
primarily to the loss of a major meander.
Accompanying reductions in river size elsewherein
the basin, vegetation invaded the former channel and
became established on the Platte, North Platte, and
South Platte floodplains (Williams, 1978; Eschner et
al., 1983), with perhaps most of the expansion occurring during the 1930s(Crouch, 1979a; Eschner et al.,
1983). However, in the 1950s. cottonwood mortality
appears to have increased on portions of the South
Platte in northeastern Colorado (Crouch, 1979b). Furthermore, a more extensive study of the South Platte River
revealed that the area of bottomland cottonwood
forests declined slightly between 1941 and 1979 from
33% to 30% (Snyder & Miller, 1991). Greater depletion
of flows on the Arkansas River in eastern Colo~ado is
thought to be responsible for even greater declines in
cottonwood area on this river between 1940 and 1980
relative to the South Platte (Snyder & Miller, 1991).
Livestock grazing, the invasion of fire-prone exotic plants,
and agricultural expansion may also be contributing to
recent declines in cottonwood area in the Platte River
system (Crouch, 1979b; Snyder & Miller, 1991). The
evidence thus suggests that upstream reservoir storage,
water diversion, and other human land uses lead to
downstream changes in riparian landscapes.
While river impoundments may cause dramatic
changes downstream, different components of the
riparian system may respond at different rates (Petts,
1987). Although there have been dramatic reductions in
the size and extent of the river in the R WMA between
1937 and 1990, the total area of cottonwoods has increased from 175 to 205 ha.
Several aspects of water development may explain
the rates of morphological and vegetational change on
the R WMA and other parts of the Platte basin. Irrigation return-flow and transmountain diversions have
increased mean low flows, resulting in a more continuous band of vegetation along the channel edges (Nadler
& Schumm, 1981; Eschner et al., 1983). Enhanced low
flows coupled with a decline in peak flows, a result of
impoundment construction and the drought years of
the 1930s, created favorable conditions for cottonwood
establishment in the narrowed riverbed (Williams,
1978; Nadler & Schumm, 1981; Currier, 1982; Eschner
et al., 1983). Subsequent floods could not reverse this
trend due to the vegetation expansion and bank stabilization. Similarly, McDonald and Sidle (1992) reported
that overall increases in riparian forests along the
North Platte and South Platte in Nebraska were closely
related to reductions in the active channel.
Baker (1992) proposed that the rate of response to
disturbance control, such as fire suppression or modification of flood regimes, may depend upon the state of
the landscape at the time control is initiated. The
RWMA contained abundant areas of dense, young
cottonwood stands as late as 1937, probably because
large flows continued on the North Platte through
most of the 1920s (Fig. 2) and also due to encroachment of cottonwoods into the formerly active channel
during the 1930s. Further colonization of the narrowed
channel probably followed the filling of Alcova, Seminoe,
and Glendo reservoirs (Table 1). This would account
for the number of patches in the 30-70% and >70%
cottonwood canopy closure classes remaining constant.
Although both of these cottonwood types increased
substantially in the former channel, they each declined
somewhat in the total proportion of the R WMA they
occupy. Concurrent with a reduction in the areal
Flood regimes and landscape structure
proportion of mid- and high-density patches of
younger trees, there has been a sharp increasein the
proportion of the cottonwood area occupied by lower
density patchesof older trees since 1937.As this trend
continues, a loss of cottonwoods can be expected,
perhaps within a short time period, as a substantial
fraction of the landscapeis now occupied by theseold
stands.
Surprisingly, substantial peak flows in 1955, 1971,
1973, and 1983 on the North Platte (Fig. 2) do not
appear to have reversedthe trend toward the predominance of older cottonwood stands on the landscape.
There is nothing obvious in the hydrographs for these
years that distinguishesthem from hydrographs for the
years between 1917 and 1929 that also had large
annual runoffs. However, severalfactors might explain
the lack of cottonwood recruitment following the 1970s
and 1980sfloods. First, the sediment budget has been
altered as a result of sediment trapping behind the
upstream dams. As a result, channel responseto large
flows may be the incision of a single channel rather
than meandering or braiding (Williams & Wolman,
1984).This responsewould limit the deposition of new
alluvial substrates suitable for cottonwood establishment and growth. Although channel incision was not
measured specifically on the RWMA, much of the
braided characterof the North Platte evident in the 1937
photos is indeed absentin the 1990photos. Second,the
RWMA experienced 20 years of reduced peak flows
before the first flood year (1955) occurred and also between 1955 and the next flood year (1971). Cottonwood standswere thus generally older, with larger trees
that are less likely to be drowned or uprooted by
floods. It may simply take larger floods to produce
open patches when older stands predominate. Finally,
the floods of the 1917-1929 period were much more
frequent (9 years with> 200 m3 per second peak
streamflow)comparedwith the 1971-1983period (3 years
with> 200 m3 per secondpeak streamflow). It may be
that frequent or ongoing high flows can produce more
patchessuitable for cottonwood recruitment. Resolution
of this question will require dating of patch origins and
reconstruction of the effects of individual floods (e.g.
Baker, 1990).
Landscape structure
The results of the transition analysis and a visual
inspection of the aerial photographs indicate that the
RWMA has undergone some dramatic changes between
1937 and 1990. Yet, these changes are not apparent
in the diversity indices of richness or evenness. The
Shannon index showed the greatest change - a very
modest increase of 0.07, indicating a slightly more
diverse landscape. Dominance actually decreased by
0.01, indicating a slight decrease in diversity. Other investigators have found these diversity measures useful
in examining changes in landscape structure in other
areas. For instance, Mills (1991) found that the diversity of riparian communities, based on the Shannon
index, generally increased downstream of Jackson Lake
377
Dam in western Wyoming between 1945 and 1989,
attributable primarily to increasedchannel stability and
the expansionof forest communities. Moreover, Baker
(1992) found that the suppressionof fires in the Boundary Waters Canoe Area of Minnesota increased the
Shannon index of landscapediversity from 2.9 to 3.5.
One explanation for the static nature of the diversity
indices when applied to the RWMA may be that apy
increasein dominance by a given land type is offset by
a decreasein the dominanceof another land type. The
increase in dominance for the <30% cottonwood
canopy closure class is apparently not sufficient to
causea meaningful changein the indices.
In the RWMA, there was little overall change in the
number of patches, mean patch size, mean patch
perimeter length, or mean patch shape.The only land
types that changeddramatically in mean patch perimeter
length are {he> 70% cottonwood canopy closure class,
which decreasedbecause the number of patches remained the samewhile the proportion of area that they
occupied decreased,and the <30% cottonwood canopy
closure class. The latter land type showed the greatest
increasein both mean patch perimeter and mean patch
size, and the variability associatedwith these metrics.
For this land type, the large increasein thesemeasures
can be explained by a decrease in the number of
patchescoupled with a large increasein the proportion
of area occupied. Someof thesepatcheshave obviously
grown in size, probably due to a merging of higher
density cottonwood patchesas they age and tree density
decreases.
Buffer analysis
The results of the buffer analysisemphasizethe importance of the river as an agent of change in the landscape.A large portion of the area adjacent to the river
changed land type between 1937 and 1990. However,
this effect diminished with increasingdistance from the
river, up to 400 m. The increasein the proportion of
area changedbeyond 400 m is primarily due to a sidechannel of the North Platte which flows parallel to the
main channel into Rawhide Creek. This tributary is
just outside the RWMA boundary and lies in the
501-600 m zone. The vegetation along this channel
changed from fairly densepatchesof. cottonwood and
willow in 1937 to smaller, more isolated patches in
1990.Thus, we seean influencein the landscapesimilar
to that of the main channel of the North Platte,
although on a somewhatsmaller scale.
CONCLUSIONS
Coincident with alterations in flow regimes toward
reducedpeak annual flows and enhancedlow flows, the
landscape structure in the RWMA has changed substantially. Traditional quantitative measuresof landscapestructure, however, were insensitive to the types
of changesobserved in this study. Observed shifts in
cottonwood canopy-closureclassesare consistent with
the idea that reducedflooding and consequentreduced
378
J. R. Miller et al
mortality have allowed existing stands to age while the
rate of recruitment of new stands has diminished. This
latter trend is somewhat obscured by the colonization
of the former river channel by cottonwood. However,
in the absenceof further decreasesin river width or a
modification of flow regimes to mimic historical flood
patterns, we expect this phaseof cottonwood establishment in the former channel to be short-lived. Instead,
we expect the landscapeto be increasingly dominated
by older, more open cottonwood stands concurrent
with further declines in recruitment and heightened
mortality. The future distribution of cottonwoods in
the RWMA may eventually be even more scattered
than that of the pre-developmentlandscape.
Lowland riparian zones are at once the rarest and
the most important landscape feature for the maintenanceof biological diversity in westernNorth America.
In addition to controlling key ecological processes
(Naiman et al., 1993),riparian zones support a disproportionately large number of specieswhen one considers
the tiny relative area that thesesystemsoccupy. Furthermore, riparian zoneshave been subjectedto substantial
modification by humans over the last 100 years. Subsequent woodland expansioncreatedcontinuous corridors,
facilitating dispersal and ultimately permitting faunal
mixing of previously isolated speciesbetween the eastern and western U.S. (Knopf, 1986; Knopf & Scott,
1991).We are now entering a new phase in which projected declinesin cottonwoods, already evident in some
areas, may lead to the loss of other speciesin riparian
zones, such as cavity-nesting birds (Knopf & Scott,
1991).Understanding the ways in which human activities affect riparian zones, over both the short and long
term, is fundamental to effective managementfor biological diversity in westernNorth America.
ACKNOWLEDGEMENTS
We wish to thank Brad Compton, Kathleen Doyle, Jon
Hak, Jim Honaker, Jen-Chu Lin, Asres Tsehay,
Tamara Wambeam, and Cathy Wilson for assistancein
ground-truthing the classification of patch types and
for commentson the research.Commentsby P. J. Currier,
L. Joyce, M. L. Scott, R. L. Beschterand R. A. Leidy
greatly improved earlier drafts of this manuscript.
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