Ordination of Bird Data
Geographic and stand-age patterns-DCA ordiiation of
bird species’ abundancesfrom all 132 standsproduced low
eigenvalues of 0.16 and 0.13 for the first two axes, respectively. Stand scoresorganized by geographic location separated into relatively distinct groups, but scoresorganized by
sand age-classesoverlapped extensively (fig. 14B, C). Additional ordinations of each province separately,although not
shown. produced similar results: distinct clusters of stands
could be identified easily by geographic location (that is, by
subprovince) and not by stand age-class.
Bird communities that separatealong DCA axis 1 were in
two groups: standsfrom the central and southern Oregon
Cascades,and standsfrom the Oregon Coast Ranges,northem Oregon Cascades,and southem Washington Cascades
(fig. 14B). Along axis 2, bird communities from the southern
Washington Cascadeswere distinct from the Oregon Coast
Rangesand Oregon Cascades.Bird communities within the
three subprovinces of the Oregon Cascadesshowed the most
intra-province variation; however, the geographic area
sampledin this province was far more extensive than the
other two provinces (see frontispiece and fig. 1; table 1).
Bird communities in the north subprovince of the Oregon
Cascades(fig. 14B; left dashedcircle) were more similar in
speciescomposition and abundanceu) those of the southern
Washington Cascadesand Oregon Coast Range than in subprovinces of the central and southern Oregon Cascades.
191
All stands
Provinces
Age categories
/a
04
*
0
..-'
50
100
150
20
0
50
100
150
2
DCA 1
CR = orsgon coast Rang*
OR = Oregon Cascade Range
SW = Southern Washington Cascade
Table -pearman
rank correlations of DECORANA scores of
axes 1 and 2 with environmental and biological variables
Variable
Latitude
Longihlde
Elevation
Smd age
Aspect
Sl0pe
Understory vegetation DCA 1
(wet to dry”)
Understory vegetation DCA 2
(cool moist to warm moista)
Douglas-fir density
Westernhemlock density
Bark-forager relative density
Aerial-forager relative density
Canopy-foragerrelative density
Understory-forager relativedensity
Residentrelative density
Migrant relative density
Speciesrichness
Total abundance
DCA Axis 1
DCA Axis 2
-0.376
-0.171
0.455
-0.213
-0.127
0.034
0.524
0.664
0.568
0.153
0.009
-0.177
0.310
-0.513
-0.461
0.426
-0.303
-0.604
-0.165
0.344
0.375
-0.659
0.459
-0.086
0.747
-0.734
-0.273
-0.389
-0.051
0.068
0.590
-0.740
-0.394
0.367
0.034
-0.252
’ DCA ordinationscoresof cwcr of understowvegetationcollectedby
T. Spies.Imerpretntionof envimnmentllgradienufar eachaxiswere
made by T. Spies @em corn.) and(SpiesandFranklin.this volume).
192
Range
Correlation and interpretation of the DCA axes-Fiftyfive percent of the Spearman rank correlations between DCA
axis 1 and the environmental and bird abundance variables
were significant at P < 0.001 (rs >0.283 or ~4.283, n = 130)
and 67 percent were significantly correlated with DCA axis 2
(table 6). In general, axis 2 yielded higher correlation coefficients than axis 1; however, no single strong association for
either axis emerged from the variables measured (table 6).
The complex patterns seemed to follow a temperature and
moisture gradient, especially along axis 2, that was related to
elevation, latitude, longitude, and understory vegetation
(figs. 15, 16). Also strongly correlated with axis 2 were the
abundance of resident and migrant species, in positive and
negative directions, respectively (fig. 16). Abundance patterns along axis 1 were highly correlated with the abundance
of understory-foraging species (fig. 15).
Relationships to Habitat
All 32 multiple regression equations of relationships to habitat, whether for all provinces combined or each separately,
were significant at P 5 0.007 (table 7, seeappendix table 9).
Yet, in general, the linear relationships were weak. Only 47
percent of the regression models had an R2 > 0.40 and only
13 percent were ~0.50. The linear relationships of all provinces combined and each separatelywere strongestfor t,Je
understory foragers (rangeR = 0.36 to 0.63 and meanR2 =
0.54) (table 7) and weakestfor speciesrichness (rangeR =
0.18 to 0.35 and mean R2 = 0.26) (table 7). The lowest R2
was 0.18 for speciesrichness in the Oregon Coast Range
(table 7) and highest was 0.65 for canopy foragers in the
Oregon Cascades(table 7). In general, the proportion of
variance explained by the regression equations tendedto be
lower in the Oregon Coast Rangesthan the other provinces
(table 7).
Fifty-two vegetation variables were selectedfor the 32 regression equations (table 7). Tbe variables selecteddiffered mxkedly between each province for each of the eight dependent
variables. Only three vegetation variables were associated
with the samedependentvariable in more than one province:
density of live deciduous trees was positively associatedwith
abundanceof migrant bid species(table 7); evergreen shrub
cover was negatively associatedwith resident bid species
(table 7); and fern cover was positively associatedwith
understory foragers (table 7).
Live deciduous-treevariables-namely, density, basal
area, and importance value-were selectedin at least one
regression model for every species-richnessor abundance
dependentvariable tested,except for the bark foragers (table
7). The live deciduous-weevariables were selectedmost
frequently with the data sets of both the Oregon Coast
Rangesand all provinces together and were always positively related to species-richnessand bird abundance.Other
variables selectedoften in the regression models were two
dead-wood variables-mean snag diameter and density of
snags 10 to 50 cm in d.b.h. and 5 to 15 cm tal-that were
positively and negatively associatedwith the dependent
variables, respectively.
Flrat-axls decorana acorea
based on bird abundance data
193
47,
*-
70
r = -0.734
*o
*
l
‘.
* . ..
. . . . .r
. . . . *. -*
‘II
-
t$!
15 1
.
-
-.
i
--
Second-axls deccwana scores
based on bird abundance data
,“”
,_”
Second-axis decorana scores
based on bird abundance data
_“I
Table ‘I-Multiple
regression using all possible subsets of independent vegetation variables with
dependent variiibles of bird species richness, total abundance, abundance of residents and migrants,
and abundance of bark, aerial, canopy, and understory foragers
Regression
coefficient
T-statistic
Two-tailed
significance
12.996
0.359
4.025
a010
0.047
-0.017
13.15
2.74
-2.61
-2.5
2.38
-2.12
16.846
-0.338
0.245
21.20
-2.43
2.01
.a00
15.542
-1.492
-0.569
0.148
-0.008
11.98
-2.34
-1.95
1.27
-1.20
.004
14.059
-1.841
-0.022
-0.010
1.076
6.33
-2.28
-1.95
-1.82
0.76
.mo
6.952
-0.659
0.303
0.008
0.040
35.61
-5.78
3.87
2.06
1.35
.om
.OMl
.om
7.774
0.125
-0.035
0.285
-0.032
20.03
2.88
-1.57
1.39
-1.31
Contibntion’
toI?
‘All provinces
(dp = 5, 117; R*' = 0.25;Pd= 0.000)
Intercept
Basal area of live deciduous trees
Percentage ground cover of moss
hnp~rtance value of live western redcedd
Mean height of live dominant trees
Density of western hemlock snags >1.5 m tall
.OOO
.OO7
,010
,014
,019
,036
.046
.042
,038
,035
,028
Oregon Coast Ranges
(df= 2, 40; R2= 0.18; P = 0.007):
Intempt
Density of decay-class-2 log<
Basal area of live deciduous trees
,019
.051
.115
.079
Oregon Cascades
(df= 4, 43; R2= 0.25;P = 0.002):
hltW%pt
Percentage cover of ferns
Density of snags >50 m d.bb. and S-15 m tall
Density of live trees 50.99 cm d.b.h.
lmpmmce value of live western redcedar
.024
,057
0.211
0.235
.088
,061
,026
,023
Southern Washington Cascades
(df = 4. 34: R2 = 0.35; P = 0.001):
IntCICEp
Density of snags lo-50 cm d.b.h. and 5.15 m tall
Densitv of western hemlock snaes >1.5 m tall
Imp&c,
value of live west&redcedar
Density of live @es 2549 cm db.h.
,029
,059
,078
,455
,089
,065
,057
,010
Total abundance:
All Pmvinces
(df= 4. 118: R* = 0.41:P = O.OCQ
lIlt.%Xpt
Density of snags
Basal area of live
Percentage CoYa
Basal .wea of live
10.50 cm d.b.h. and 5-15 m tall
bigleaf maple
of plants 4.5 m tall
pine trees
,041
,179
.I62
.073
,021
.oQ9
Oregon Coast Ranges
(df= 4. 38: RZ= 0.25:P = 0.004):
Intercept
Basal area of live deciduous trees
Density of live incense-cedar
Volume of decay-class-5 snags
Density of live Douglas-fir
,004
,006
,125
,172
,198
,148
,344
,034
,030
195
Table 7-cootinued
Dependent variable/independent variables
Regression
coefficient
T-St&tiC
Two-tailed
significance
Contibution’
tORZ
Oregon Cascades
(df= 3,44; R2= 0.24;P = 0.002):
htEZCf+X
Mean height of live dominant trees
Relative do,nina~,ce of Douglas-fir snag volume
Density of live trees 50.99 cm d.b.h.
.I00
5.251
0.032
4.008
0.031
9.18
2.88
-1.49
0.86
,006
,142
.396
.134
,036
.012
3.194
0.014
-0.114
1.599
2.28
2.16
-1.90
1.85
.029
.038
,066
,073
,086
,066
,063
3.328
0.199
-0.375
-0.089
0,368
1.25
3.36
4.41
A.06
-2.94
2.43
2.33
.JOl
.oOO
.OW
.004
,016
.022
.093
,079
,042
,028
,026
6.23
17.09
,000
4.007
0.087
0.006
-0.033
-3.22
2.21
2.00
-1.30
,003
,033
,053
.200
,134
,063
.052
,022
4.735
4.136
0.005
0.021
a.006
9.65
-3.86
2.34
2.30
-1.42
,000
.oOO
,024
,027
.164
,185
.068
.065
,025
4.87
0.016
0.244
-0.105
4.068
14.06
2.97
2.59
-2.18
-1.34
,000
,005
,014
,036
,191
.124
,094
,067
,025
Southern Washington Cascades
(df= 3, 35; R2= 0.3O;P= 0.001):
IntffEpt
Percentage COYeIof plants <OS m tall
Percentage raya of live deciduous tx.es 18 m tall
Mean d.b.h. of snags
Resident swies:
All pro&es
(df= 5. 117: Rx=
0.42:P = 0.000):
Basal area of live deciduous trees
Density of snags lo-50 cm d.b.h. and 5-15 m tall
Percentage cover of evergreen shrubs
Biomass of decayclass- snags
Basal area of live needle-leaved evergreen trees
Oregon Coast Ranges
(df= 4, 38; RZ= 0.45;P = 0.000):
IntWSpt
Basal area of live deciduous trees
Importance value of live western redcedar
Density of live Douglas-fu
Oregon Cascades
(df= 4.43; R2= 0.42;P = 0.000):
Relative ckninance of Douglas-fr
snag volume
Southern Washington Cascades
(df= 4.34; R2= 4.47;P = 0.000):
IntW.Spt
Percentage cover of plants co.5 In tall
Density of snags >50 cm d.b.h. and S-15 m tall
Percentage cover of evergreen shrubs
Percentage cover of live deciduous trees S8 m tall
196
Table 7-continued
Dependent vruiable/iidependent
variables
Regession
coefficient
T-Stati&
Two-tailed
significance
Contribution’
toR2
aupro;inces
(df = 4, 118; R2= 0.44;P = 0.000):
Intercept
Den&v of snaes lo-50 cm d.b.h. and 5-15 m tall
Importance V.&e of live Pacific silver fu
Relative frequency of western hemlock snags
Importance value of live deciduous trees
4.69
-0.563
4.008
-0.015
0.004
26.86
-5.15
-3.87
-2.73
2.18
.M)O
.oOO
,000
,307
.03 1
,123
,070
,035
,022
4.407
-0.056
0.091
0.010
0.001
16.86
-3.39
1.88
1.64
0.73
,000
,002
,070
,111
,471
,205
,063
,048
009
3.95
4.008
0.066
-0.012
0.045
14.17
-2.56
2.08
-1.83
1.30
,000
,014
,043
,074
,200
,107
,070
,054
,027
5.449
4.009
0.017
-1.878
0.011
2.79
-2.45
2.36
-1.88
1.51
,009
,020
,024
,069
,141
,101
,094
,059
,038
0.869
-0.539
0.085
1.033
0.050
0.584
1.04
-3.37
3.14
2.97
2.43
2.01
,300
,001
.I02
.I04
,017
,047
,060
,052
a47
,031
,021
1.114
0.070
1.219
-0.5 12
0.004
0.89
2.52
2.23
-2.02
1.98
,377
,016
,031
,051
,055
,090
,071
,058
,055
Oregon Coast Ranges
(df= 4, 31; RZ= 0.38;P = 0.001):
Density of live incense-cedar
Percentage cover of shrubs 0.5-2.0 m tall
Percentage ground cover of moss
Density of live deciduous trees
Oregon cascades
(df= 4. 43; R2= 0.24;P = 0.003):
Intercept
Immrtance value of live Pacific silver fir
Density of live trees 50.99 cm d.b.h.
Density of decayclass- logs
Percentage cover of live deciduous trees 58 m tall
sourhan Wahington Cascades
(df = 4. 34; ~2 = 0.36; P = 0.001):
InttXZZpt
Density of snags 1.549 m tall
Relative frequency of Douglas-fir snags
Basal area of live needle-leaved evergreen trees
Density of live deciduous trees
Bark foragers:
AU~provinces
(df = 5, 124; R* = 0.31;
P = 0.000):
Density of live trees 25-49 cm d.b.h.
Basal area of live bigleaf maple
Basal area of live needle-leaved evergreen trees
Basal area of live grand fir
Mean height of snags
Oregon Coast Ranges
(df=4.38: Rz=0.40:P=0.0W):
Intercept
Basal area of live bieleaf ma&
Basal area of live needle-lea&d evergreen trees
Density of live !xees 25-49 cm d.b.h.
Density of live western redcedar
197
Table 7-continued
Dependent variable/independent variables
Regression
coefficient
T-statistic
Two-tailed
significance
Connibution~
toR2
Oregon Cascades
(df= 3, 44; R2= 0.38; P = 0.000):
Intempt
Percentage cover of shrubs 2.0-4.0 m tall
Basal area of live needle-leaved evergreen trees
Relative dominance of Douglas-fir snags
0.967
AI.609
1.175
-0.006
1.13
-3.43
2.64
-1.66
,263
,001
.Oll
,105
,155
,092
,036
5.041
-1.226
0.194
6.00
-4.20
2.80
.ooQ
,000
,008
,278
,123
1.669
-0.497
1.358
0.355
3.92
-5.04
3.10
2.50
.oal
mo
.ow.
,014
.125
.047
,031
0.003
1.97
,051
,019
5.17
-0.036
-1.221
5.08
-2.83
-2.00
.008
,054
.160
.080
0.123
1.68
,103
,057
1.159
-0.429
1.68
3.28
-2.77
,101
.I02
,008
,157
,112
a.537
0.003
1.61
1.43
.077
.160
048
,030
1.800
0.280
0.012
aOQ3
15.56
3.52
2.45
-2.07
.ooo
.oOl
,019
,046
,197
,396
,068
Southern Washington Cascades
(df= 2, 36; R2 = 0.40; P = 0.000):
Density of live needle-leaved evergreen trees
Density of snags >50 cm d.b.h. and >15 m tall
Aerial foragers:
All provinces
(df= 4. 118; RZ= 0.40; P = 0.000):
Intercept
Density of snags 10.50 cm d.b.h. and 5.15 m tall
Mean height of snags
Percentage MVCI of ferns
Inpxtance value of live broad-leaved evergreen
Oregon Coast Ranges
(df= 3. 32; R* = 09o;P = 0.002):
InterCept
Density of live incense-cedar
Mean d.b.h. of live trees
Percentage cover of live evergreen trees ~8 m
tall
.ooo
Oregon Cascades
(df=4, 43; R2=0.32;P = O.ooO):
Intercept
Vohlme of
Density of
Equitability
classesg
Importance
Douglas-fir snags
snags 10-50 cm d.b.h. and 5.15 m tall
index of tree basal area by heightvalue of live grand fir
0.645
Southern Washington Cascades
(df= 3, 35; R* = 0.4o:P = O.OCQ:
Intercept
Basal area of live western yew
Density of live deciduous trees
Density of live western redcedar
198
Table 7-continued
Dependent variable/independent variables
Regzssion
coefficient
T-Std.&
Two-tailed
significance
Contributiona
tOR2
canopy foragers:
All provinces
(df= 4, 118; R2= 0.34; P = O.OWl):
Intercept
Basal area of live deciduous trees
Percentage ground cover of moss
Basal area of live pine trees
Density of snags 1.5-4.9 m tall
,000
.wo
4.647
0.205
4.011
0.072
-0.003
35.51
4.93
-3.76
2.88
-1.76
5.975
0.177
11.36
3.43
,002
,198
a.455
-2.56
,015
,111
4.827
-2.29
,029
,088
4.291
-0.021
0.129
0.043
22.33
-6.53
5.30
2.26
.OoO
.OOO
.OOO
4.383
-0.982
-0.101
0.011
-0.150
18.88
-2.50
-2.25
2.13
-1.56
,017
,031
,040
,127
,102
,082
,074
.040
1.388
0.175
0.201
1.784
-0.034
-1.715
1.99
6.07
5.26
4.79
-3.95
-3.67
.049
.OOO
,000
.oim
.OOQ
.OOQ
.117
.088
.073
.049
.043
6.235
0.643
-0.012
-0.951
4.21
2.50
-2.26
-1.72
.ooo
,000
.nl5
,081
,131
,076
,045
,017
Oregon Coast Ranges
(df = 3, 32. R* = 0.41; P = 0.000):
Basal area of live deciduous trees
Equitability index of tree density by diameter&.%e$
Percentage c4wer of berry (vacciniwl spp.)
shrubs
.wo
Oregon Cascades
(df= 3. 44; Rz = 0.6S:P = O.OC0):
Percentage cover of grass, herbs, and fems
Densitv of live tTees 50.99 cm d.b.h.
Basal &ea of live pine trees
,029
.319
,211
,038
southern Was~gton cascades
(df= 4. 34; R = 0.38;P = O.OCO):
lllt.%C.Spt
Biomass of decay-class-5 snags
Percentwe cover of live deciduous trees <8 m tall
Percentage ulver of plants co.5 m tall
Basal area of live western yew
,000
Understory foragers:
All provinces
(df = 5, 117; R* = 0.61; P = O.C+O):
Intacept
Pacentage COYa of plants 0.5-2.0 m tall
Basal area of live deciduous trees
Mean d.b.h. of snags
Density of live pine trees
Mean number of live twe species
&egon Coast Ranges
(df = 3, 32; R2 = 0.36; P = 0.001):
Intercept
Percentage cover of ferns
lmpmance value of live incense-cedar
Basal area of live needle-leaved evergreen trees
,018
,031
,095
,115
,094
,055
199
Table 7-contloued
Dependentvariablefidependent variables
Regression
coefficient
T-Stati&
Two-tailed
significance
Conbibutionn
tOR2
Oregon Cascades
(df = 4, 43; R* = 0.63: P = 0.000):
Intercept
PeKxmagecoverof ferns
Impmce value of live pine trees
Mean height of live dominant trees
Biomass of logs
0.738
0.575
-0.007
0.027
0.607
1.40
3.76
-3.69
3.52
2.52
.I68
,001
,001
,001
,016
,110
,106
,096
,049
1.439
0.016
Al.114
1.380
1.55
3.65
-3.01
2.49
,130
,001
.005
,018
,155
,105
,072
4.067
-1.66
,107
,032
southem wahingmn Cascades
(df = 4,34; Rz = 0.56; P = 0.000):
Intercept
Percenta8ecover of Plants <OS m tall
Percenta8ecover of evergreenshrubs
Mean d.b.h. of snags
Percentagecova of live deciduous trees $8 m
tall
Discussion
Regional Patterns
Our results suggestedthat bird abundancepatterns were regionally distinct (fig. 14B) and that abundancewas substantially higher in the Oregon Coast Rangesthan in the other two
provinces (figs. 6-13). even though >70 percent of the bird
specieswere detectedin two or mope.provinces. Location of
the study standsby latitude, longitude, and elevation; inconsistenciesin sample design; and varying degreesof forest
fragmentation appearedm be partly responsible for the
differences among regions.
In the ordination analyses,regional abundancepatterns
(fig. 14B) were best explained by correlations with latitude,
longitude, and elevation (table 6: figs. 15, 16). They were
negatively associated with latitude and elevation and posi-
tively associatedwith an east-to-westlongitudinal gradient
(fig. 2). Elevation was essentially overlooked when stands
were selectedand varied substantially within and among provinces. Partial correlation using elevation as a constant was
impractical for explaining abundancepatterns. The southemmost standsin the Oregon CascadeRangeswere among the
highest in elevation and generally had more birds than stands
to the north, while stands in the Oregon Coast Rangeswere
generally the lowest in elevation but were higher in abu’ndance than standsin the CascadeRanges (table 1, fig. 1).
Although we detecteda relatively high correlation between
longitude and abundance(fig. 2), we assumedlongitude per
se had little effect on bird abundancewithin one broad vegetation zone (seeHamel and others 1986). The increase in
abundancefrom east (CascadeRange) to west (Oregon Coast
Ranges)was likely more directly related to the moderate
climate of the Oregon Coast Rangesrelative to the colder
Cascades,or to variation in elevation among the provinces.
Shortening the interstation distance to 50 m to accommodate
the smaller stand sizes in the Oregon Coast Rangesmay have
resulted in higher bird counts than in standswith longer interstation distances.Field observerswere trained not to count
individual birds more than once on a given day; however,
double counting was possible. We attempted to minimize
thesebiases by using only detections ~50 m from a counting
station in all three provinces, except during the analysis
specifically on individual species.
High speciesrichness and abundancein Oregon Coast Range
standsmay also be a result of forest fragmentation that has
increasedthe diversity of stand agesover the landscapeand
may have in turn increasedthe packing of individuals within
200
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