A preliminary ecological study of the Alpine flora of Popocatepetl and Iztaccihuatl

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A PRELIMINARY ECOLOGICAL STUDY OF THE ALPINE 1 FLORA OF POPOCATEPETL AND IZTACCIHUATL 2

b)' .f ohn H. Beaman

As part of a program to develop a though taxonomic, phytogeographic, and ccological understanding of the alpine flora of Mexico, an ecological study on Po-pocatepetl and lztaccihuatl was initiated in the summer of 1959. Through this type of investigation it is expected that more precise information can be obtained on specics distribution patterns and community structure than is possible by ordi· nary plant collecting techniques.

In spite of frequent visits by collectors to many of the alpine areas in Mexico, the only serious attention given the alpine flora before the present project was by the early authors Hemsley ( 1887), Heilprin (1892), and Purpus ( 1907). Knowl· edge of the alpine flora has been limited by the scarcity of critica! and comparativc field data. My studies have thus far included about eight months of field work in Mexico. An outline of field work accomplished in 1958 (Beaman 1959) and sorne data on chromosome numbers in the alpine species (Stoutamire and Beaman, 1960) have already been published. Much more study. will be necessary before complete taxonomic and phytogeographic treatments of the alpine flora are possible. This paper is the íirst report on an ecological analysis of the alpine and subalpine meadow vegetation.

It has been convenient for severa] reasons to concentrate the ec;ological studies of the alpine flora on Popocatepetl and Iztaccihuatl. They are the rnost easily ac-cessiblc of ali the high Mexican volcanoes; it is possible to drive to timberline on hoth of them, and a good road extends for a distance of 12 kilometers betwecn them from an altitude of 3680 meters to almost 4000 meters. Excellent topographic maps of the a rea are available (see table 1), and it was therefore possible to geo-¡iraphically locatt' with considerable accuracy the stands where quadrats were ex· amined. Finally, the available geological literature about Popocatepetl and Iztacci-huatl should aid in the interpretation of the ecological data. The glaciers of the two mountains have been examined recently by Lorenzo (1959), and White (1956a. l956h) has been conducting intensive investigations on the Pleistocene geology of thc art'a .

. -\lthrn1gh thc term "atpine" originalty pert:1ined to the E11ropean Alps. it is 11ow widcty 11secl throughout thc wortcl in designatin¡; featurcs o[ high mo11ntains such as the l'cgctation. Especialty in North Amerka it has frequently becn 11secí in rcfcrring to vege-tation abcn·c tree tinc. The term '·p<irarno" has heen 11sed in the same sense in Mexico (Mi -randa, 1902, p. JGG), h11t in the western hemisphcre this appellation of Spanish origin is 1a1hcr finnty attached to thc northern Ancles thc l'Ioristic affinities of Lhc high-111011ntai11 rcgctation o[ í\ícxico with either the Ancles or the Rocky Mo11ntains are nut: strong enoup;h 111 mcrit prefcrence for cithcr tenn. Tn this stucly Lhe flora above trec tinc cm Popocatepetl 'lile! lztacc;ih11ai.l 1s referrecl to as "alpine" rather than "páramo" beca11sc 1he forrner tcrm is liener 11nclerstoocl and more generalty applicahlc than the t:Hter.

2 S11ppor1ed hy a grant frorn the National Scicnce Fu11ndalion of the l'nitecl States of .\1ncrica. This papcr was preparecl for the Primer Congreso Mexicano ele Bot:inica.

--63 -Boletín de la Sociedad Botánica de México 29: 63-75, 1965 DOI: 10.17129/botsci.1089

_______________

Beaman J.H. 1965. A preliminary ecological study of the Alpine flora of Popocatepetl and Iztaccihuatl. Boletín de la Sociedad Botánica de México 29: 63-75.

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BOLETIN DE LA SOCIEDAD BOTANICA DE MEXICO, No. 29

Our knowledge of the climate in the alpine zone is limited, although the clat3 published by Hobles Ramos ( 1944) give sorne idea of the weather conditions thal prcvail. The climatic pattern of the alpine areas is similar to that of most of thc rcst of Mexico in having annual wet and dry seasons. The wet season extends :from May or }une to November or December, and the dry season occupit•s the remaindcr of the year. Night temperatures may drop to near or below freezing, while thr

days are somcwhat warmer, especially when there are no clouds. The snow lint' in

summer usually remaips above 4700 meters, but in winter, snows may somctim1»

covcr the whole alpine area.

The topsoils of tlw alpine rneadows are mostly black loams containing consid<·

ralile organic matter. Below the topsoil are stratified layers of purnice and formrr topsoils. This strati íicalion has resúlted frorn successive volcanic ejections and suh· SL·quent weatlwring.

On

Popocatepetl the soils above 4000 rneters are not rnuch weathered and consist mainly of fine black sand or layers of sand, pumice, and gravel. A series o[ soil samples has been collected in the study area, and an analysis

lor available nutrients in a sample of topsoil from 4.000 meters on lztaccihuatl garc the following results: pH, 6.1; NO~, 16 lbs/acre; P, 28 lbs/acre;

K

,

221 lbs/acre; Ca, 160 lbs/acre; Mg, 16 lbs/acre; Mn, 2 lbs/acre; and Cl, 160 lhs/acre.

PROCEDURE

At every 100-rneter altitudinal level, frorn 3700 meters on the north and south sides of the Paso de Cortés to 4300 meters on Popocatepetl and Iztaccihuatl, speci1·s presence and density were recorded in rneter-square quadrats. Each area sampled is hereafter called a stand. The map locations o[ the stands are indicated in table l. and the relative positions of the same in figure l. The stands were located in thc field with a Thomrnen pocket altimeter (model 3 D 1) in conjunction with thr topographic maps, and are probably within 10 meters of the elevations as shown 011 tlw maps. Five quaclral:s wnc sarnpled in each stand. The :first quadrat iras locatccl l1y throwi11g a pick inlo the air and establishing a corner o[ the quadrat where the pick lancled. The othcr four quadrats were on a straight linc at 25-rnctt•r intervals from thc first ones. .

Ali stands were in open, grass·dorninated meaclow except those at: 3800 and :\900 meters on Popocatepetl whicl1 wcre in a forest of Púws hartwegú. The nor· mal trce line on Popocatepetl and lztaccihuatl is at an altitudt• of ahout

4

00

0

mctcrs, but a large strip oF nearly trecless meadow extends north and south betW(''.'11 tlw two mountains (see Figure 2).

Jn

this paper the meadow vegetation a hove lht· altitude o[ the normal l:ree line is referred to as alpine, and that at lower levt>ls

(clown to thc lowest point in the Paso de Cortés) is callecl subalpine. Althou?:h some alpine specics are found up Lo an altitucle of more than 4500 mett•rs, stands

were not examirwd at altitudes highcr than 4300 rneters becausc this is ahout tire

maximurn altitudt.:

o

I

well-defined alpine meadow.

The deterrninations of specics rccordecl in the quadrats art.: based on herl1ari11111

spC'cimcns collected in Lhe study area. Thcsc specirnens arl' dt'positt·cl in tlie Ht'rli

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-BL\\lA.N,

J.

H., A ECOLOGlC:.-\1. STUDY OF THE ALl'l!\T FLOR.\

rium of Michigan State University, and duplicates will be distributed to other h er-baria. The names of species are provisional, ancl sorne changes un do u btedly will be necessary when the taxonomic studies have Eeen completecl.

RESULTS AND DISCUSSION

From the quadrat data Jt is evident that the stands vary greatly in spL·c1cs composition (see table 2). Variations occur in numbers ancl kincls o[ species prescnt and in numbers of plants present. As the aJtitude increases, the numbn of specif's and the number of individual plants decrease (see t:ahle 1). For example, in Lhe quadrats at 3700 meters on Popocatepetl there were 24 specirs and 1777 individual ¡)lants while at 4300 meters only four species and 84 plants wcre encountered. Thc species cornposition in stands at 3700 meters is almost totally different Irom that at 4.300 meters. On Popocatepetl no one sprcies was common to stands al 3700 and 4·300 meters, and on Iztaccihuatl only two sprcies, Draba jorullensis and Cna-plialiwn vulcanicwn, occurred at both elevations. The distribution and frcquency data of the species included in the stands are summar.izecl in table 2.

The quaclrat data have been employecl in an attempt to evaluate the impor-tance of the species on the basis of their frequency, density, ancl altitudinal rangf'. The method for deterrnining this importance is as follows: The total number of stands from 1 to 10 (no single species occurred in more than 10 stands) in which the specirs

occurrecl is aclded to a number which represents the total numher of quadrats in ali stands in which the species occurred; the sum of these two figures is tlwn adclecl Lo a number representing the total number of plants in ali quaclrats in ali stands in which tlw species was encountered; this final sum is the "irnportance inclex" numlwr

for

the species. The numbers usrd to represent the total number of quadrats in which species occurred were arbitrary and rangrd from one to seven. Likewisc, the numbers n'presenting the total numbers of plants of species in ali quadrats in ali stands were arliitrary ancl rangecl from one to fivr. It was necessary to use thesl' arbitrary numbers to weigh as equitably as possible the contributions to the " irn-portance index" number of each of tlw three sources. The occurrence of the species

by

stand~, quadrats. ancl individual plants ancl 1he arbitrary numbers used in cleLer-mining the "importance inclex" are givrn in table 3.

J

n table 4 the species arr listed, from most important to least important, each wil"h it~ "importanct· indl'x., a11d thc numhers usrd to ohtain the inclex.

The "importance index" nurnbers in general agrer with íield ohserYalio11s oí the alpine specirs. Calamagrostis tolucensis, thr dominan t grass in m uch ·oí tlie alpinr area, has one of the highest numbers. Likewisr, Muhlenbergia quadridentata. which is clorninant in the subalpine rneadow, has a rrlatively high nurnber whil'h results from its high frequency ancl clensity Ht the lower elevations. Although Draba jorullensis ancl Trisetum spicatzun an' not dominant species. their ubiquity 1s n's -llonsihle for thrir high index n urnber.<

Comparisons of the degree of similarity JJJ species composition between sla11ds

were macle with the usr of community coefficients. Thc mrthod of .Taccard as m

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-BOLETIN DE LA SOCIEDAD BOT ANICA DEI MEXICO, No. 29

dified by Gleason (1920) has been employed in determining the community

coe[-ficients. The formula for Gleason's modification 1s as follows: e

Cornmunity coefficient X 100

where a=the surn of the frequencies of species in stand A; b=the sum of the frequencies of species in stand B; and e =

112

the sum of the frequencies of species common to stands A and B. Table 5 surnrnarizes the community coefficients of ad-jacent stands on each of the mountains and of stands at equal altitudes on the two mountains. The community coefficients show that no two stands have the same species structure. The similarity of adjacent stands, however, is reflected by thcir coefficients usually being above 40 per cent. A continuum of similarity from stand to stand is evident, especially on lztaccihuatl where ali the adjacent-stand com -munity coefficients fall within a narrow range between 44 .. 0 and 54.5 per cent. The coefficient is very low only in the comparison of the 3700-3800 and 3900-4000 me

-ter stands on PopocatepetL These coefficients are low because the comparison is between meadow stands and stands in Pirws hartwegi1: forest. The forest stands are similar to ea ch other ( with a coefficient of 73.9 per cent), but contrast sharply with the meadow stands. The stands under Pinus hartwegú differ most conspicuously

from those in the meadows in that the former have fewer species present (see table 2).

When stands at the same altitude on Popocatepetl and Iztaccihuatl are com· pared, the community coefficients are lower than those of adjacent stands. Probably

thf' most important factor in lowering these coefficients is the influence of the sandy soils on Popocatt>prtl which support a poorer flora than the loam soils of

Iztacci-huatl.

Although the comrnunity coefficients revea! a gradual change in species co

m-position in the stands at different altitudinal levels, it is still possible to recognize certain phases of the continuum pattc>rn in thc> alpine meadow vegetation. Four major phases, without sharply defined boundaries, can be distinguished in the study are~. ThesP phases an· designated by the narnes of their dominant species. The Calam

a.-grostis tolucensis-Fesluca tolucensis rneadow is rnost typical at about 4000 meters, hul extends down to about 3800 mett··rs, and on Iztaccihuatl up to the 4300-metcr

kvel. On Popocatepetl, however, it does not reach rnuch higher than 4100 meters. Thcre are va·riations in species pn'sence ancl clensity in this meadow in differenl stands. but the gt·1wral aspect is relatively uniform. A second phase, which has a

low numbcr o( spccies present (sPe table 2). is the Fesluca livida-Arenaria bryoides

nwaclow on Popocatcpetl at the 4200- ancf 4:300-rneter levels. The tbird rneadow

phasP. dorninatcd hy MuJilenbergia quadridentala, occurs in the Paso ele Cortés at

thc 3700-ml'lcr levcL and extends up to slightlv above 3800 rnett··rs where it hlends gradual! y in to t he Calam.agrostis lolu.censis-Feslu.ca tolucensis rnt>adow. :iVIany of the rnernhers o( this pha;::e arr· not alpine spccies. hut are from lower e]e,·ations and

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-HEAMAN,

J

.

H., A ECOLOGICAL STUDY OF THE ALl'INE FLORA

reach their upper limits in this subalpine meadow. The fourth phase is Pinus hartwegii forest with a meadow-like forest floor. Under the pines Festuca tolucensis

is the dominant species. This phase occurs at the 3800- and· · 3900-meter levels on PopocaJepetl. Under the tree canopy Festuca to!ucens1:s develops a very heavy ground cover, but relatively few other species thrive in this habitat (see table 2). As a means for determining the adequacy of sampling, species-area curves for minimal sampling area were constructed. With this method a line tangent to the curve indicates the point at which a 10 per cent increase in the area sampled

re-sulted in a 5 per cent increase in species recorded. An adequate sample for a stand should exceed the number of quadrats where the line

:J

rangent to the curve. fn species-area curves for the alpine meadow stands, the tangent fell between two and

four quadrats, suggesting that a sample of five quadrats per stand was adequate. Although sampling might appear adequate from the species-area curves, the. sampling technique had several imperfections. First, the meter-square quadrats were larger than necessary for efficient sampling of this vegetation. Second, an

estim&te of cover per cent rather than density in the quadrats would have been

easier to obtain and more useful for analyses. Third, only five frequency classes

are possible when five quadrats per stand are sampled. Fourth, the results are not adaptable to refined statistical analyse.s because the quadrals per stand were few in number and not randomly distributed.

In

spite of these objections, however, the data contribute to the understanding o[ species interrelationships in the alpine flora. They clarify distribution patterns

of the alpine species, and indicate the relative importance of s¡:,ecies. Finally, the elata permit a characterization of the species corriposition of thc alpine meadows

ancl comparisons among all phases of the meadows.

A further ecological study, in which the quantitative data can be treated st

:i-tistically, was conducted in the same area in the summer of 1960. This investigation will be publishecl later.

SUMMARY

At every 100-meter leve! between 3700 and 4300 meters on Popocatepetl anrl lztaccihuatl, five meter-square quadrats were usecl for determining species presenc1·

and frequency. From the resulting elata severa! observations about the alpine flora

were rnade. With an increase in altitude, there was a reduction in nurnber of species and nurnber of individual plants present. The quadrat data clari [ied specÍPS

clistrihution patterns and showed that most species have a restricted altitudinal rangr»

Only two species extended from the 3700- meter level to the 4300-meter leve!. With

the data obtained, it was possible to evaluate the relative importance of each of the

species. Only a few species were extremely important, and a large number of specie~ had little importance. Adjacent stands and stands at equal altitudes were compared by the community coefficient method. No two stands had the same species stru

c-ture, but adjacent stands usually had a community coefficienl of at least 44 per

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-BOLETIN DE LA SOCIEDAD BOTANICA DE, MEXICO, No. 29

cent. Stands in Pinu,s hartwegii forest, however, were distinctly different from those

in meadows, as indicated by the low community coefficients when the two were compared. Four major phases of the alpine meadows were designated and briefly

characterized. Although these phases did not have sharp boundaries, they usually could be readily recognized .in the field.

LIT

E

R

A

T

U

R

E CITE

D

Department of Botany and Plant Pathology

Michigan State University

East Lansing, Michigan

U. S

.

A.

lkalllan.

J.

H. 19:)9. The alpinc llora o[ i\fcxico and Central r\rncric1. Yearbook ;\rner. Phil. Soc. 266-268.

Gleason, H . . -\. 1920. Sorne applications of the quadrat method. Hui!. Torrey Hot. Club

~7: 21-33.

Heilprin, ,\. 1892. ·rhe temperare ami alpine floras o[ thc g·i:int volcanoes of Mexico.. Proc. _\ mer. Ph il. Soc. 30: 4-22.

Hcmsley. \V. 11. 1887. A specirncn o[ the mountain florn of South. Mexico and Central America. Hiologia Centrali-Americ;m;i. J3otany 4: 282-299.

Lorenzo,

J.

L. rnr,9. Los glaciares de i'déxico. Instituto de Geofísica, Universidad Nacional :\utónorna de Mb;ico, México, D. F., J 15 pp.

i\liranda, F. 19:)2. La vegetación de Chiap;1s, Primera Parte. Ediciones del Gobierno del Estado, Sección Autogr;ifica, Departamento de Prensa v Turismo, Tuxtla Gutiérrcz, Chis., M<'xico.

P11rp11s, C. A. 1907. i\fexikanische hochgipfel. Vcgctationsbilder 5: -16-:"íl.

Robles Ramos, R. 1944. _\lgunas ideas sobre la gfaciologia y morfología de lztaccihuatl. Re1·. Geogr. del lnst. Pauarner, de Geogr. e Hist. 4: Ci:i-75.

S1011t;1mire. \•V. !'. & llealllan,

J.

H. 1·960. Chro1110so1nc studies of Mexican alpine pbnts. Brittonia 12: 226-230.

\\'hite. S. E. 19:i6a. Geología glacial del Iztaccih11atl. rn: F. Mooser. S. E. White, ~

J.

L.

Lorenzo. La C11cnca de México: Consideraciones geológicas y arr¡ueológ·icas. 19·2t. Dir. de l'rehist. l'11h. No. 2. México.

- - - 19:i(il>. Probable s11hstages of gfaciation on lztaccihuatl, i\[exico. .Jo11r. of Genl. 64: 389-395.

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-\~'e,\~ \

Location of stands and number of species and individu::il plants encountered in 14 stcinds in the lztaccihuc1tl - Paso de Cortés- Popocatepetl orea.

Stand Map* Co-ordinates No. of No. of

species plants

lztacci hu a ti, 4300 íll. lztaccihuatl 14 Q-h (107) 538. l O, 2116.25 10 98

lztaccihuatl, 4200 íll. lztaccihuatl 14 Q-h (107) 538.05' 21 l 6.05 6 94

lztaccihuatl, 4100 m. lztaccihuatl 14 Q-h (107) 537.95, 2115.80 13 221

lztaccihuatl, 4000 m. lztaccihuatl 14 Q-h (107) 536.90, 2114.60 23 479

lztaccihuatl, 3900 m. Popocatepetl 14 Q-h (123) 537.10, 2114.45 25 527

lztaccihuatl, 3800 m. Popocatepetl 14 Q-h ( 123) 537.25, 2112.20 21 720

lztacci h uatl, 3700 m. Popocatepetl 14 Q-h ( 123) 536.70, 2111.10 23 523

Popocatepetl, 3700 m. Popocatepetl 14 Q-h ( 123) 537.15, 2109.60 24 1777

Popocatepetl, 4000 m. Popocatepetl 14 Q-h ( 123) 538.25' 2106.85 14 178

*Maps of the Fotomapas del Valle México series, 14 Q-h (107 and 14 Q-h (123), scale 1:25,000, contour inter-val JO meters, published by the Departamento Cartográfico Miiitar, Secretaría de la Defensa Nacional, Mé-xico, D. F.

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Table 2

.

Distribution and per

cent

frequency of

spec

:

es

in

14 sta

nd

s

on lztaccihuatl and

Popocatepetl.

Stand

Species

lzt

a

ccihuat/

Popocatepet/

4300 4200 4100 4000

3900 3800

3700 3700 3800 3900

4000 4

100 4200

4300

m m m m m m m m m m m m m m

Achillea !anulosa - - -

Carex geophila - - ··- · - --- - --

20

Carex peucophíla - - - -

40

20

- - -

40

Castílleja schaffnerí - - -

60

Cerastíum molle - - - -

60

Cerastíum oríthales

40

60

100

20

100

80

- -

80

Draba jorullensis

10

60

40

- -

40

60

-

80

20

2

0

- - -

20

Festuca amplissima - - -

Geraníum cruceroense - - -

60

20

Gnaphalium sarmentosum - - -

20

Gnaphalium vulcanicum

20

40

-

80

60

100

8

0

- -

80

Halenia nudicaulis - - -

20

Hieracium mexicanum - · - 20 20

Juncus

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Stand

Species lztaccihuatl Popocatepetl

4300 4200 4100 4000 3900 3800 3700 3700 3800 3900 4000 4100 4200 43

0

m m m m m m m m ·m m m m m m

Lithospermum distichum - - - -

40

-

20

Lupinus mexicanus - -

8

0

100

80

60

20

Lupinus montanus -

100

20 --

- - - -

80

40

Luzula racemosa

20

1

00

60

-

40

- - -

20

Muhlenbergia quadridentata - -

8

0

1

00

1

00

100

Oreobroma megarhizum

20

20

- - -

2

0

60

20

Plantago tolucensis - -

-

60

1 0)

8

0

1

00

- -

20

Poa orizabensis

-

- -

40

4

0

40

Ranunculus donianus - -

-

20

100

80

20

Senecio bellidifolius - - -

-

40

100

Senecio calcarius

-

- - - - -- - -

20

Senecio gerberaefolius -

40

60

40

Senecio procumbens

60

-

60

-

- - -

60

80

40

Senecio rose11s -

20

8

0

0

60

100

40

-

40

Trifolium amabile - - - -

-

20

8

0

20

Trisetum rosei - - -

-

8

0

100

Trisetum spicatum

80

20

60

80

1

0

- - - -

20

40

60

20

Vaccinium geminiflorum - - -

-

2

0

-

8

0

60

Verbena teucriifolia - - -

4

0

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BOLETIN DE LA SOCIEDAD BOT ANICA DEI MEXICO, No. 29

TABLE 3. Distribution of the species by stands, quadrats, and individual plants,

and arbitrary numbers for weighting quadrat and individual plants data far the ""importance index".

Stands*

number of number of

species stands

14 l

6 2

12 3

5 4

5 5

6 6

3 7

2 8

9 10

Individual Plants

*

number of number of

5pecies plants

10 1-5

11 6-20

14 21-50

12 51-200

8 over200

Quadrats*

number of number of

species quadrats

21 1-5

12 6-10

10 11-15

5 16-20

4 21-25

2 26-30

31-35

arbitrary number

for weighting

1 2 3 4

5

arbitrary number for weighting

2 3

4

5 6

7

*

Under Stands read "number of species which occurred in how many stands", e. g., 14 species occurred in one stand only; 6 species ocurred in 2 stands, etc. Under Quadrats read "number of species which occurred in how

many quadrats", e. g., 21 species occurred in one to five quadrats, etc. Under

Individual Plants read "number of species which were represented by how

many individual plants", e.g., 10 species 'A/ere represented by one to five plants,

etc ..

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-BEAMAN,

J.

H., .-\ ECOLOGIC.-\L STUD\ OF THE .\1.1'1'\E FI.OR.\

Table 4. lmportance Indexes

Draba jorullensis Calamagrostis tolucensis Trisetum sp·icatum Gnaphalium vulcanicum Lupin11s mexicanus Conyza schiedeana Plantago tolucensis Arenaria reptans Cerastium vulcanicum Festuca tolucensis Stachys eriantha Cirsium nivale Luzula racemosa

Muhlenbergia quadridentata Alchemilla vulcanice

Cerastium orithales Arenaria bryoide·s Phacelia platycarpa Ranunculus donianus Senecio bellidifolius Senecio procumbens Eryngium protaeflorum Festuca livida

Lupinus montanus Oxalis alpina Senecio roseus Agrostis tolucensis

Drymaria effusa var. depressa Trifolium amabile

Vaccinium geminiflorum Penstemon gentianoides Poa orizabensis

Senecio gerberaefolius

Trisetum rosei

Carex peucophila Cerastium molle Eryngium carlinae Geranium cruceroense lithospermum distichum

stand weighted value quadrat

value 9 8

10

8 7 6 6 7 5 6 6 7 6 4 5 5 4 6 4

3

5 5 3 4 3 4 3 3 3

3

3 3 2 3 l l 2 2

7 3 -6

7

5

6

index

20

19 19 18 17 16 16 14 14 14 14 13 13 13 12 12 11

11

11 11 11

10

8 8 8 8 7 7 7

7

6 5 5 5

5

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BOLETIN DE LA SOCIEDAD BOTANICA DE1 MEXICO, No. 29

Oreomyrrhis orizabae Sisyrinchium bra'Cteatum Viola humilis

Carex geophila Halenia nudicaulis Hieracium mexlcanum

Juncus balticus var. mexicanus Sisyrinchium quadrangulatum Verbena teucriifolia

Achillea !anulosa Castilleja schaffneri Festuca amplisslma Gnaphalium sarmentosum Oreobroma megarhizum Senecio calcarius Solanum demrssum

stand weighted value quadrat

value

l

3

2 l l 2 l 2 l l 1 l l l l l l l l l l l l l l l l l l l l l weighted density

value

3

l 2

2

2 l 2 l 2 l l l l l l l importance index

5

4 4 4 4 4 4

3

3 3 3

TABLE 5. Community coefficients, in per cent, of ad¡acent stands and of stands of equal altitudes on Popocatepetl and lztaccihuatl.

lztaccihuatl altitudes

4300 m.

4200 m.

4100 m.

4000 m.

3900 m.

3800 m.

3700 m.

lztaccihuatl equal-ad¡acent- altitude-community community

coefficients coefficients

27.6 54.5

14.9 44.0

34.7 47.0

38.1 54.5

11.l 46.6

4.1 53.3

6,8.0

- 74

.

13.l

(13)

ESPECIES

"

m

Figure 1. ing relative

..

'"

•ooo M

_"ª

3900 M

"'

3001} M

'"

3700 M

"

1177 POSO Df CORTEZ

Profile of the lztaccihuatl-Paso de Cortés-Popocatepetl mea, show -positions of stands studied and total null!ber of species and

indi-vidual plants which occurred in each stand.

Figure 2. Photograph from 3750 meters on

lztaccihuatl, looking south through the Paso de

Cortés to Popocatepetl.