Sampling intensity and species richness: Effects on delineating southwestern riparian plant communities

Forest Ecology and Management, 33/34 (1990) 335-349

335

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Sampling intensity and species richness: Effects on delineating southwestern riparian plant communities R o b e r t C. Szaro 1 a n d R u d y M. King 2 ' USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Forestry Sciences Laboratory, Arizona State University Campus, Tempe, AZ 85287-1304 (U.S.A.) 2USDA Forest Service, Mountain Forest and Range Experiment Station, 240 West Prospect, Fort Collins, CO 80526-2098 (U.S.A.)

ABSTRACT Szaro, R.C. and King, R.M., 1990. Sampling intensity and species richness: Effects on delineating southwestern riparian plant communities. For. Ecol. Manage., 33/34: 335-349. Riparian communities in the southwestern United States often occur in patches and isolated pockets along stream corridors, making classifications of community types highly dependent on sampling design and methodology. We investigated the relationship between sampling intensity and species richness at five sites having differing species richness. Riparian tree species were sampled at 40 plots, each 5 × 25 m in size, at each site. Using bootstrap subsampling techniques, we determined the relationships between number of plots and tree species richness in estimating density, basal area, and species ranks. Variability in species density ranks for the most frequent species stabilized, with about 10-20 plots. As species richness increased, so did the variability associated with estimating ranks. Studies based on only a few arbitrarily selected sample plots may not produce reliable results.

INTRODUCTION I n studies o f p l a n t c o m m u n i t i e s , it is c o m m o n p r a c t i c e to q u a n t i f y a n d classify d i s t r i b u t i o n s o f species ( M o h l e r , 1983 ). W h e t h e r p l a n t c o m m u n i t i e s actually o c c u r in d i s c r e t e u n i t s o r c o m m u n i t y v a r i a t i o n is c o n t i n u o u s d o e s n o t i n f l u e n c e t h e utility o f c l a s s i f i c a t i o n f o r m a n y p r a c t i c a l p u r p o s e s ( G a u c h , 1982 ). I n fact, d a t a t h a t are n a t u r a l l y c l u s t e r e d are r a r e in studies o f ecological c o m m u n i t i e s , a n d c l u s t e r i n g is i m p o s e d o n d a t a t h a t a r e r e l a t i v e l y c o n t i n u o u s ( G a u c h a n d W h i t t a k e r , 1981 ). T h e c l a s s i f i c a t i o n o f c o m m u n i t i e s is useful n o t o n l y b e c a u s e h u m a n s n a t u r a l l y t h i n k , a n d c o m m u n i c a t e , in t e r m s o f classes ( G a u c h , 1982 ) b u t also b e c a u s e it allows for the c o m p a r i s o n o f s i m i 3fPresent address: USDA Forest Service, Forest Environment Research, P.O. Box 96090, Washington, DC 20090-6090, U.S.A.

Elsevier Science Publishers B.V.

336

R.C. SZAROAND R.M, KING

TABLE 1 Mean density, mean basal area, and importance value for tree species at five riparian sites in Arizona (n=40) Study site/Tree species

No. plots present

Density (trees/125 m E)

Basal area (cm2/125 m 2)

Importance value

Cave Creek

Garrya wrightii Torr. Robinia neomexicana Gray Quercus emoryi Torr. Salix spp. Acer grandidentatum Nutt. Prunus virginiana L. Arbutus arizonica (Gray) Sarg. Juniperus deppeana Steud. Juglans major (Torr.) Heller Pinus strobiformis Engelm. Pseudotsuga menziesii ( Mirb. ) Franco Yucca schottii Engelm. Pinus leiophylla Schiede & Deppe var. chihuahana (Engelm.) Shaw Fraxinus pennsylvanica Marsh. ssp. velutina (Torr.) G.N. Miller Cupressus arizonica Greene Pinus ponderosa Lawson Quercus arizonica Sarg. Quercus hypoleucoides Camus. Platanus wrightii Wats.

1 1 3 3 4 8 11 11 12 14 14 17

0.05 0.03 0.08_+0.04 0.48_+0.17 0.28+0.09 0.23+0.05 0.38+0.12 0.40_+0.06 0.35_+0.04 0.43_+0.06 0.73_+0.10 0.45_+0.06 0.78_+0.09

0.5 0.9 39.6_+ 14.9 4.0_+ 1.0 24.8_+ 10.4 62.4_+ 18.7 212.4+ 66.6 179.2+ 51.2 50.5_+ 10.4 82.4_+ 47.6 356.5_+133.2 62.0_+ 7.6 432.7_+232.6

0.5 0.4 1.7 3.0 2.7 3.5 6.3 7.1 5.6 6.5 11.2 7.1 13.2

24

1.27_+0.19

109.6_+ 26.1

14.3

28 31 37 39 40

2.63_+0.67 1.80+0.21 4.03-+0.33 5.58_+0.70 4.05_+0.37

1823.0+326.8 1832.9-+295.6 1801.4+209.8 592.1_+ 92.7 2319.1_+227.9

38.4 36.1 47.0 42.0 53.2

1 1 3 3 6

35 35

0.03 0.03 0.08+0.04 0.08+0.04 0.57+0.09 0.53+0.17 0.28+0.07 1.05+0.14 1.43+0.25 0.78+0.10 1.00+0.15 1.45_+0.23 2.30+0.23 3.85_+0.32

2.3 5.5 157.5+ 42.7 0.5+ 0.1 8.2+ 2.0 31.4+ 23.8 18.1+ 3.8 127.0+ 49.2 196.1 + 61.8 244.1+ 65.2 57.7+ 10.9 1692.5+298.1 6821.4_+813.0 629.1 + 88.2

0.6 0.6 2.8 1.7 5.6 5.9 6.3 11.9 15.6 12.7 14.2 31.0 76.6 39.3

37

6.35_+0.65

3713.1+520.1

75.3

6

Bear Canyon

Pinus strobiformis Engelm. Quercus oblongifolia Tort. Populusfremontii Wats. Rhamnus californica Esch. Salix ssp. Prunus virginiana L. Juniperus deppeana Quercus hypoleucoides Juglans major Quercus arizonica Quercus rugosa Nee. Platanus wrightii Wats. Curpessus arizonica Fraxinus pennsflvanica ssp. velutina Alnus oblongifolia Torr.

7 11 13 16 16 20 26

Molino Canyon

Acergrandidentatum Garrya wrightii Morus microphylla Buckl. Ptelea angustifolia Benth.

1 1 1 1

0.03 0.05 0.03 0.08

1.9 1.4 0.5 0.4

1.0 1.2 1.0 1.4

SAMPLING AND SPECIES RICHNESS: DELINEATINGRIPARIANPLANTCOMMUNITIES

Tree species/Study site

Baccharis sarothroides Gray Populusfremontii Vauqueliniacalifornica (Tort.) Heller Juniperusdeppeana Prunus virginiana Rhamus crocea Nutt. CeltisreticulataTorr. Juglansmajor Platanuswrightii Fraxinuspennsylvanica ssp. velutina Quercusoblongifolia

No. plots present

Density (trees/125 m 2)

337

Basal a r e a

Importance

(cm2/125 m 2)

value

2 2 2 5 5 5 13 13 24 28

0.05 _+0.03 0.05_+ 0.03 0.18_+0.05 0.20_+0.03 0.25 _+0.06 0.33_+ 0.05 0.75_+0.15 1.15_+0.20 2.63_+0.33 4.08_+0.62

0.4_+ 0.1 4.0 + 2.0 54.7_+ 24.5 216.2_+ 153.0 56.1 _+ 14.4 15.2_+ 6.3 94.9_+ 19.1 179.4_+ 53.1 1248.7_+168.6 462.9_+ 90.9

2.0 2.0 4.4 10.9 7.3 7.0 18.7 24.2 72.1 68.0

30

1.70_+0.21

1660.5_+204.1

78.9

Madera Canyon

Acer negundo L. var. interius

1

0.10

1.2

1.1

(Britt.) Sarg.

Arbutus arizonica Garrya wrightii Rhamnus californica Prunus virginiana Juglansmajor Quercusernoryi Populusfremontii Quercusarizonica Quercushypoleucoides Juniperusdeppeana Platanuswrightii

1 1 2 4 5 5 13 37 37 39 39

0.03 0.08 0.13 + 0.02 0.18 + 0.04 0.20_+0.07 0.23_+0.07 0.33_+0.08 3.85_+0.45 4.80_+0.64 3.63_+0.38 6.13_+0.51

1.6 0.9 2.2 + 1.1 20.6 + 7.6 27.4_+ 9.0 33.6_+ 16.9 823.8_+125.9 1460.0+235.4 956.7+ 190.9 2039.0_+302.3 3426.2_+347.9

0.7 0.9 1.8 3.3 4.1 4.2 18.1 56.3 55.4 62.8 91.3

4 36 40

0.10 + 0.04 12.23+ 1.35 4.80 + 0.40

79.2 + 18.3 866.3+ 106.6 4290.9 + 331.6

7.1 132.9 160.0

Queen Creek

Populusfremontii TamarixpentandraPall. Salix gooddingii Ball

lar areas when developing land-management plans. Yet, often it is difficult to identify communities at the outset of a vegetation study (Mohler, 1983). Therefore, some method of classifying plant communities is necessary after data are collected. Multivariate methods have become commonplace in phytosociology, and there is a rapid development of quantitative phytogeography and biogeography (Crovello, 1981; Minchin, 1987; Andersson, 1988 ). But of prime importance in determining any classification or examining floristic distributions is deciding upon the number of observations necessary for adequately delineating a given stand. No analysis is any better than the original data upon which it is based. Obtaining an adequate sample is a particular problem when deal-

338

R.C. SZARO AND R.M. KING

Cave Creek

Madera Canyon

40

40

30

30

20 ~

20

10

10

0

1 2 3 4 5 6 7 8 91011

4of

0

1 2 3 4 5 6 7 8 91011

0

30

30

•! ~

20

,oil 20

10

10

E

0

Z

Bear Canyon

1 2 3 4 5 6 7 8 9 1011

Queen Creek

0 E

1 2 3 4 5 6 7 8 9 1011

Molino Canyon 40 30 20 10

_,i,,_

1 2,3 4 5 6 7 8 91011

Number of Species Fig. 1. Species/plot frequency histograms at five riparian sites in south-central Arizona (n = 40 per site).

ing with long strips of patchy mosaic communities such as those found along riparian systems in the arid southwest (Brown, 1982). Large, almost homogeneous stands, are common on the broad floodplains but become increasingly rare as valleys narrow and elevation increases. The number of observations necessary to describe a given site may vary with the homogeneity of the stand and the scale on which it is based (Gibson and Greg-Smith, 1986). Variability in species richness is only one of several characteristics that can produce distortions in any analysis (Dargie, 1986 ). This study was designed to determine the number of plots necessary to: ( 1 ) estimate tree species richness; (2) determine tree species ranks; and (3) provide reliable estimates of tree species density and basal area.

SAMPLINGANDSPECIESRICHNESS:DELINEATINGRIPARIANPLANTCOMMUNITIES Madera Canyon

Cave Creek 1.8

1.8

~

0.9

1.5 1.2 0.9

o.e

0.6

o3

0.3 0

1.5 1.2

O0 tt-

._o

0

10

nr" oO

20

30

1.8

f~

1.5

0

t-

30

40

Queen C r e e k

~

1.5 1.2 0.9 0.6 0.3 0

0.9 0.6 0.3

I,-I,,,,

"0

20

1.8

10

LU "0 ('~

10

40

1.2 (/~ '.,I-(~

t

Bear Canyon

,(~

339

20

30

10

40

20

30

40

Molino Canyon 1.8

1.5

L'~

1.2

~J~

0.9

tI --

Average

--

Total

0.6 0.3 0

10

20

30

40

Number of

plots

Fig. 2. Bootstrap estimates of the standard error of total and average species richness as a function of number of plots sampled. METHODS

We selected riparian plant communities at five sites in south-central Arizona that had relatively homogeneous stands of vegetation. Each site had been previously examined and classified by Szaro and Patton (1986) and Szaro (1989). The original site classifications and their locations were as follows: ( 1 ) Platanus-wrightii/Fraxinus-pennsylvanica community type, South Fork of Cave Creek, Cochise Co., 1605-1635 m in elevation; (2) Platanus-wrightii/Fraxinus-pennsylvanica community type, Molino Canyon, Pima Co., 1220-1280 m in elevation; (3) Platanus wrightii community type, Madera Canyon, Pima Co., 1515-1605 m in elevation; (4) Salix gooddingii community type, Queen Creek (behind Whitlow Ranch Dam), Pinal Co., 640-650

340

R.C. S Z A R O A N D R.M. K I N G

Cave Creek Standard Error

8[-

Mosn

10

I

- PLWR

- QUAR

, 6 [. , i

8

QLHY -. CUAR --RPO - FRPE

t:i

6

L?.,

10

20

3o

40

0

10

20

30

40

Bear Canyon

nr"

81 StandardError

Mosn

10

c-

- ~b(~

a

6 4

U) ('--

2

d) £3

o

10

20

30

40

0

10

20

30

40

30

40

Molino Canyon 10

Mo,n

'f Stan=,E,ror

8 6

4 2

"

....................................

1221211 211

0 10

20

30

40

', 'z

2 '' OF . . . . . . . . . . . . . . . . . . . 0 10 20

Number of Plots Fig. 3. Average rank and variability in rank based on density across bootstrap replications for the most prevalent species at Cave Creek, Bear Canyon, and Molino Canyon as a function of sample size. Species codes are: ALOB, Alnus oblongifolia; CERE, Celtis reticulata; CUAR, Cupressus arizonica; FRPE, Fraxinus pennsylvanica ssp. velutina; JUMA, Juglans major, PLWR, Platanus

wrightii; PIPO, Pinus ponderosa; POFR, Populus fremontii; Quercus hypoleucoides; a n d QURE, Quercus rugosa.

QUAR,

Quercus arizonica;

QUH¥,

m in elevation; and (5) Alnus oblongifolia community type, Bear Canyon, Pima Co., Coronado NF, 1650-1720 m in elevation (for a detailed description of each community type see Szaro, 1989). The vegetation sampling technique was a modification of that described by Daubenmire (1952) and Daubenmire and Daubenmire (1968). For this study, trees and shrubs were sampled using forty 5 X 25-m plots having reasonably homogeneous overstory and understory vegetation. All plots were measured in April-May 1988. At each site, the first plot was located at ran-

SAMPLING AND SPECIES RICHNESS: DELINEATING RIPARIAN PLANT COMMUNITIES

Madera Canyon i StandardError

,,Mean.,,,

341

-JUDE - PLWR QUAR - QUHY -- POFR

""'-,,.,......... 4 4

t--

0

0 10

20

30

nc

tt~

40

.

0

Queen Creek 8

lO Mean a

.

.

.

.

.

.

.

10

Standard

.

.

.

.

.

20

.

.

.

.

.

.

.

30

40

- TAPE - SAGO POFR

Error

6

e

2

/

0 0

10

20

30

40

0

10

20

30

40

Number of Plots Fig. 4. Average rank and variability in rank based on density across bootstrap replications for the most prevalent species at Madera Canyon and Queen Creek as a function of sample size. Species codes are: JUDE, Juniperus deppeana; PLWR,Platanus wrightii; POFR, Populusfremontii;

Quercus arizonica; QUHY, Quercus hypoleucoides; SAGO, Salix gooddingii; a n d TAPE, Tamarix pentandra.

QUAR,

dom and then all successive plots were located linearly moving upstream. All plots were located with the long axis parallel to the stream channel and the short axis adjacent to the next plot. Plots were placed on both sides of the stream and confined to a 500-m stream segment to minimize elevation differences. Stands were selected to avoid ecotonal effects identified by abrupt changes in topography, microenvironment, or species composition. Within each plot, all trees and shrubs > 2.5 cm diameter at breast height (Dbh) were recorded. Stand data were summarized, and importance values (MuellerDombois and Ellenberg, 1974) determined. Plant species nomenclature follows Lehr (1978). We used bootstrap resampling from the 40 plots sampled at each site (Efron, 1982) to answer questions about appropriate plot numbers. This approach assumes that the 40 plots actually sampled provide an adequate representation of the variability present on each site. The effects of plot numbers on estimates were simulated by drawing various-sized samples from each site's 'population' of 40 plots. For each fixed number of plots to be sampled, multiple replications were formed by randomly sampling with replacement from

342

R.C. SZARO AND R.M. KING

Queen Creek - Density 10% kderval

Queen Creek - Basal Area 10% Interval

1

(,0

1

0.8

0.8

0.6

0.6

0.4

0,4

0 ,~"

O.2

0.2

/

E

LLI O.

/

o

0

10

20

30

40

10

25% Interval

40

30

40

25% Interval

o.8

0.8

~

o.6

0.6

(~

0.4

O

30

1

~

~ ',,I.,,-

20

0.4

/

/

0.2

0.2

o

10

~

~

40

i t

0

10

20 50% Interval

50%~val I 1 0.8 I...

Q.

-~"--

0.8

/ r o.4 l 0.6

~

0.6

/

0.4

J;

0 0

- - SAGO -- TAF~

0.2

0.2 10

20

30

40

10

20

30

40

Number of Plots Fig. 5. Sampling effort needed to achieve various levels of precision estimated by recording the proportion of bootstrap estimates within intervals 10, 25, and 50% about the actual overall mean density and total basal area at Queen Creek. Species codes are: SAGO,SAIL):gooddingii; and TAPE, Tamarix pentandra. the actual plots for each site. Preliminary analysis indicated that 500 bootstrap replications were adequate to produce repeatable results. The bootstrap replications were summarized to provide information on several sampling questions. Total numbers of species observed at a site is often used as an index of diversity. Variability in total and average n u m b e r of species across the bootstrap replications was estimated to examine the effect o f sample size on the stability of these parameters. Total species richness was the total n u m b e r o f different species observed on all plots within a sample. Average species richness was the mean n u m b e r o f species observed per plot within a sample. Since most classification analyses are based on d o m i n a n t vegetation, the

SAMPLING AND SPECIES RICHNESS: DELINEATING RIPARIAN PLANT COMMUNITIES

Madera Canyon - Basal Area

Madera Canyon - Density

10% Interval

10% Interv~ 1

1

0.8

0.8 0.6

~0 ~0

0.6

J~t D,/r~

I

0.4

","

:.

. ,;~.~L/"~" . ' ,

0.4

:/..., ~ ~ :'..............

0.2

0.2

0

E

10

LU Q.

20

30

40

0

10

20

30

40

25% interval

25% Interval

1 f'

L'~

I,....

0.8

~

0.e

0

0.4

~[~

0.2

0

~ i~./- .

!:".,', ,. ~,i

/", ,"" ,,.,j. -...

J

0.8

,

// //f //"".

0.4

0

4-o I,...

~

1

EL 0.a 0I,....

,

10 . . . .

,

20 . . . .

30 ....

l

j.,,"-"

0

0

40

10

50% Interval

20

30

40

50% Interval

1 ...:.::::: ............... 0.8

t?,'

0.6

_/J"

O. 6

0.2

....

0

. ,

',~:' "-'

,-

c"

I~

343

~

0.6

'

,,

..."

"

0.4

0.4 / ~ - J U D E

02

O. 2

- R_WR QUAR -. QtPIY -- POFR

"'

.i 0

10

20

30

40

0

10

20

30

40

Number of Plots Fig. 6. Sampling effort needed to achieve various levels of precision estimated by recording the proportion of bootstrap estimates within intervals 10, 25, and 50% about the actual overall mean density and total basal area at Madera Canyon. Species codes are: JUDE, Juniperus deppeana; PLWR, Platanus wrightii; QUAR, Quercus arizonica; QUHY, Quercus hypoleucoides; and POFR, PopulusfremontiL

ranking of individual species in any given stand may be more important than the absolute densities. We estimated the average rank on density and its variability across bootstrap replications of up to the six most prevalent species at each site. Sampling effort needed to achieve various levels of precision was estimated by recording the proportion of bootstrap estimates falling within intervals of 10, 25, and 50% about the actual overall mean density and total basal area. Fixed intervals, rather than probability confidence intervals, were chosen as evaluation criteria to remove dependence on the sample size of the actual data.

344

R.C. SZARO AND R.M. KING

Molino Canyon - Basal Area

Molino Canyon - Density

10% Interval

10% Interval 1

•

~

°E ~

W Q.

0.~

0.8

0.6

0.6

0.4

0.4

0 .2

0.2 <

o 0

10

20

30

09

o.6

0

0.4

~

30

40

1 0.8

A r~ r "

~,i

,:~. .,- ,, :,,.,: ~..

O, 4 f

0.2

0.2

O

20

25% interval

25% interval 1

'~ -

0

"10

40

0

10

(~

20

30

40

/

/

0

10

50% Interval

O.

Q.o.6 0.4

rt

O.2

~r

:

,. . .,"',. . . . r.-"

""

:-,'

,"

0.6

)"

0.4

~,~

0.2

' J

j!

0

~

10

20

30

40

50% interval

O. 8

.,;,,~'

20

30

40

0

.....

" .....

/" j ./ ",

..-"-'"-"~ .' - PLWR """ OJOB --JU~ 10

20

30

40

Number of Plots Fig. 7. Sampling effort needed to achieve various levels of precision estimated by recording the proportion of bootstrap estimates within intervals 10, 25, and 50% about the actual overall mean density and total basal area at Molino Canyon. Species codes are: JUDE, Juniperus deppeana; PLWR, Platanus wrightii; QUOB, Quercus oblongifolia; JUMA, Juglans major; and FRPE, Fraxinus pennsylvanica ssp. velutina. RESULTS

There was considerable variability in the number of species sampled within the 40 plots at each site. Cave Creek had the highest species richness, with 19 tree species recorded, while Queen Creek had only three tree species (Table 1 ). No site had more than four tree species present in 31 or more sampling plots. In fact, no tree species was present in more than 30 plots in Molino Canyon. Both Cave Creek and Madera Canyon had four tree species present in more than 31 plots, Bear Canyon had three, and Queen Creek had two (Table 1 ). In order of mean species richness (___SE) per plot, the site, from lowest to highest, were Queen Creek (2.0_+0.05), Molino Canyon

S A M P L I N G A N D SPECIES RICHNESS: D E L I N E A T I N G RIPARIAN PLANT C O M M U N I T I E S

Bear Creek - Density

Bear Creek - Basal Area

10% kCterval

CO

1

0.8

0.8

0.6

0.6

0.4

~'~

0.2

.E_

0.4 e,/~/-J " ,

o

~.--

¢~/~'"

0.2

f

10

CO LU ~.

10% Interval

1

(1)

345

20

30

40

0

10

25% Interval

20

30

40

30

40

25% Interval

1

0.8

CO 0.6 0 0

0.4

r~

0.2

0

0

/

/

O.

0.4

'/"

0.2

if

tO

i 0

10

20

30

40

10

50% Interval

50% Interval

0 Q. 0

20

1 0.8

t,..

0.6

0.6 0.4

LOB

i

0.4 0.2

0.2

10

20

30

40

II / /

- CUAR - PI..WR

10

20

30

40

Number of Plots Fig. 8. Sampling effort needed to achieve various levels of precision estimated by recording the proportion of bootstrap estimates within intervals 10, 25, and 50% about the actual overall mean density and total basal area at Bear Canyon. Species codes are: ALOB,Alnus oblongifolia; CUAR, Cupressus arizonica; FRPE, Fraxinus pennsylvanica ssp. velutina; and PLWR, Platanus

wrightii.

( 3 . 3 + 0 . 1 5 ) , Madera Canyon ( 4 . 6 + 0 . 1 4 ) , Bear Canyon (5.8_+0.24), and Cave Creek (7.6_+0.24) (Fig. 1 ). Variability of average species richness stabilized at 15-20 plots on all five sites (Fig. 2 ). However, the variability in total species richness remained much higher than that of average richness except on the least diverse site, Queen Creek. At Molino Canyon, because no species was present on more than 30 of the sample plots, the bootstrap standard error of estimated total speciesrichness remained high even with 40 samples. This was also somewhat true of total species richness at Madera Canyon, although several species had much greater frequency than those at Molino Canyon. This indicates that total spe-

346

R.C.

E

1

1

0.8

0.8

0.6

0.6

0.4

0.4

0,2

o

09

LU 1

t~ I,~

0.8

r~

0 .6

, , , ,

,

........

0

~.j~

O,

10

20

30

10

40

0.4 2

10

.o_ I.--

0

30

40

0

10

20

40

50% Interval

50% Interval 1

1

O . 0.8

0.8

,--'/'~

0

0_

20

30

0.6

0,4

o

40

0.8

0.2

E

30

25% Interval

0

NO

20

1

rl~

0

KING

0.2 / ....

25% Interval

~..

R.M.

10% Interval

10%Interval

-I~

AND

Cave Creek - Basal Area

Cave Creek - Density

(D

SZARO

/" f~

O.6

O. @

,'J

0.4

O. 4

'/

%, /

Pt.WR - QUAR QUHY - CUAR

0.2

0.2

0 0

10

20

30

40

10

20

30

40

Number of Plots Fig. 9. Sampling effort needed to achieve various levels of precision estimated by recording the proportion of bootstrap estimates within intervals 10, 25, and 50% about the actual overall mean density and total basal area at Cave Creek. Species codes are: PLWR, Platanus wrightii; QUAR, Quercus arizonica; QUHY, Quercus hypoleucoides; and CUAR, Cupressus arizonica.

cies richness estimates are subject to high variability even on sites with relatively low average species richness (Fig. 1 ). The variability is more a function of tree species frequency than it is of tree species richness because the variability in total species richness was much lower on the more diverse sites such as Cave Creek and Bear Canyon. Variability in species density ranks for the most frequent species stabilized with approximately 10-20 plots (Figs. 3, 4 ). As species richness increased so did the variability associated with estimated ranks. Depending on the desired level of precision there were definite patterns in density and basal area for the more common species at each site (Figs. 5-9 ). A 10% interval proved to be a stringent criterion for estimating these parameters at all sites. Only at Queen Creek did the proportion of bootstrap esti-

SAMPLING AND SPECIES RICHNESS: DELINEATING RIPARIAN PLANT COMMUNITIES

347

mates reach the 80% level, and that was for the less abundant but more evenly dispersed Goodding willow (Salix gooddingii) (Fig. 5 ). This level of precision required a sample of 40 plots. For the more prevalent species, the maximum proportion of bootstrap estimates within the 10% interval varied between 40 and 70% at a sample of 40 plots. Again, as frequency of occurrence increased so did the precision. The proportion of bootstrap estimates falling within the selected precision interval increased at the 25% level for the more prevalent species (Figs. 59). A 90% level was common with 40 plots. With the exception of Molino Canyon, an 80% level was attained with approximately 20 plots. Tree species frequency appears again to be having an effect on estimating these parameters at this site. Density and basal area estimates for less-frequent species such as Fremont cottonwood (Populusfremontii) at Madera Canyon never reached the 80% level. At the 50% interval, the proportion of bootstrap estimates within the interval approached 100% for many of the more common species. The required sample size increased with decreasing species frequency (Figs. 5-9). Rarer species had much lower precision even at this more relaxed level. For example, the effect was most pronounced on basal-area estimates for species occurring with low numbers of large individuals on a few plots, e.g., alligator juniper (Juniperus deppeana), at Molino Canyon (Fig. 7 ). DISCUSSION

Replicate samples from within a given area generally do not have constant species richness, making it difficult to elucidate the structure of a given plant community (Palmer, 1987). One would expect some variability in species richness because of chance alone, even if the environment was spatially homogeneous and species were independently distributed (Kwiatkowska and Symonides, 1986; Palmer, 1987). The number of plots necessary to accurately classify plant communities is dependent on the spatial pattern of point diversity within them. Even within communities considered relatively homogeneous, most classification systems are highly dependent on the relative rank of species. Our analyses show that at least ten to twenty 5 X 25-m plots are necessary to stabilize the standard error in species rankings in southwestern riparian communities. The level of precision necessary to delineate riparian plant communities is dependent on the variability and complexity of the area studied. The less distinct the boundaries between communities, the greater the sampling effort required to reach a desired level of precision. Precision levels should also be determined by the scale or desired resolution of the resulting classification. For example, regionwide classifications obviously require less definition than does an intense study of a single riparian system. However, we can say with

348

R.C. SZARO AND R.M. KING

some assurance that the m i n i m u m sampling intensity should be in the 10-20plot range, even for less-complex communities. Our findings consistently showed that species ranks, tree density, and basal area estimates required a m i n i m u m o f 10 observations to stabilize standard error and suggests that future studies of southwestern riparian systems need to be based on larger numbers o f observations. In as much as we did not look at the possible effects of plot size on these estimates, we can not make any statements about the effects of larger plots on the n u m b e r of observations needed, but we suggest this m a y be an area for future work. Total species richness may not be a reliable parameter on which to base c o m m u n i t y classification and comparisons. Its variability was much greater than average species richness and did not stabilize on sites such as Madera and Molino Canyons with low species frequency (Figs. 3, 4). In conclusion, studies based on a few specifically chosen sample plots m a y not produce reliable results. Selecting plots on the basis o f preconceived homogeneity and internal biases can, and do, predetermine the o u t c o m e o f an analysis. An objective o f classification studies should be to characterize the natural range o f variation within communities as a means of defining what constitutes a c o m m u n i t y and also developing reliable estimates of richness, density, and basal area. Based on our results, small numbers of plots do not achieve this goal. ACKNOWLEDGEMENTS We thank J. Kevin Aitkin for his help in collecting the field data. Our sincere thanks to Leonard Debano, Michael Ryan, and Gary Whysong for their help in reviewing, polishing, and refining the scope of the final manuscript.

REFERENCES Andersson, P.-A., 1988. Ordination and classification of operational geographic units in southwest Sweden. Vegetatio, 74: 95-106. Brown, D.E. (Editor), 1982. Biotic communities of the American Southwest - United States and Mexico. Desert Plants, 4: 1-342. Crovello, T.J., 1981. Quantitative biogeography: an overview. Taxon, 30: 563-575. Dargie, T.C.D., 1986. Species richness and distortion in reciprocal averaging and detrended correspondence analysis. Vegetatio, 65: 95-98. Daubenmire, R., 1952. Forest vegetation of northern Idaho and adjacent Washington, and its bearing on concepts of vegetation classification. Ecol. Monogr., 22:301-330. Daubenmire, R. and Daubenmire, J.B., 1968. Forest vegetation of eastern Washington and northern Idaho. Washington Agric. Exp. Stn. Tech. Bull., 60, 104 pp. Efron, B., 1982. The Jacknife, the Bootstrap and Other Resampling Plans. Society for Industrial and Applied Mathematics, Philadelphia, PA, 92 pp. Gauch, H.G. Jr., 1982. Multivariate Analysis in Community Ecology. Cambridge Studies in Ecology. Cambridge University Press, Cambridge, 298 pp.

SAMPLING AND SPECIES RICHNESS: DELINEATING RIPARIAN PLANT COMMUNITIES

349

Gauch, H.G. Jr. and Whittaker, R.H., 1981. Hierarchical classification of community data. J. Ecol., 69: 537-557. Gibson, D.J. and Greg-Smith, P., 1986. Community pattern analysis: a method for quantifying community mosaic structure. Vegetation, 66:41-47. Kwiatkowska, A.J. and Symonides, E., 1986. Community pattern analysis: a method for quantifying community mosaic structure. Vegetatio, 66:41-47. Kwiatkowska, A.J. and Symonides, E., 1986. Spatial distribution of species diversity indices and their correlation with plot size. Vegetatio, 68: 99-102. Lehr, J.H., 1978. A Catalogue of the Flora of Arizona. Desert Botanical Garden, Phoenix, AZ, 203 pp. Minchin, P.R., 1987. An evaluation of the relative robustness oftechniques for ecological ordination. Vegetatio, 69: 89-107. Mohler, C.L., 1983. Effect of sampling pattern on estimation of species distributions along gradients. Vegetatio, 54: 97-102. Mueller-Dombois, D. and Ellenberg, H., 1974. Aims and Methods of Vegetation Ecology. John Wiley, New York, 547 pp. Palmer, M.W., 1987. Variability in species richness within Minnesota oldfields: a use of the variance test. Vegetatio, 70:61-64. Szaro, R.C., 1989. Riparian forest and scrubland community types of Arizona and New Mexico. Desert Plants, 9:69-138. Szaro, R.C. and Patton, D.R., 1986. Riparian habitat classification in the southwestern United States. Trans. N. Am. Wildl. Nat. Resour. Conf., 5 l: 215-221.