PLANT COLONIZATION OF RECENT LAHAR DEPOSITS ON
POPOCATEPETL VOLCANO, MEXICO
Julio Muñoz-Jiménez
Departamento de Análisis Geográfico Regional y Geografía Física
Facultad de Geografía e Historia
Universidad Complutense de Madrid
Ciudad Universitaria, Madrid, 28040
Spain
Karla Rangel-Ríos and Arturo García-Romero
Departamento de Geografía Física
Instituto de Geografía
Universidad Nacional Autónoma de México
Circuito Exterior s/n, C.U., 04510, Distrito Federal
Mexico
Abstract: This work investigates the initial colonization on recent lahar deposits of the
northeast slope of Popocatepetl volcano, Mexico, where 29 circular sample plots (1.57
m2) were established to determine the development stages of colonization in four types of
habitats: 1997 lahar, 2001 lahar, margins, and terraces at the channel’s bottom. Cluster
analysis and the Sørensen Index were used to determine the floristic affinity of these
lahars. Richness, frequency, percentage of species contribution, plant cover, density, and
stem height were analyzed to determine the composition and structure of plant communities. The distribution of these variables reveals that the communities often have a simple
internal structure; however, a relationship has already been established between changes
in resilience and the age of the four lahars. Thus, floristic composition develops rapidly (1
sp. in terraces, 11 spp. in the margins, 29 spp. in the 2001 lahar, and 34 spp. in the 1997
lahar). Except for the 1997 lahar, however, structural characteristics are poorly developed
in other incipient stages. As colonization advances, the affinity among the various components and sectors of the laharic deposits decreases, thus promoting the gradual incorporation of species found on the adjacent gorge slopes. The Principal Components Analysis
used to identify other explanatory factors shows that of 15 variables studied, those associated with the morpho-sedimentology, the hydrovolcanic dynamics and stability of deposits (microtopography, thickness of the deposit, clast shapes, length of the deposit, depth of
the gorge, and slope processes) explain a large percentage of variance. Only a few species
(Lupinus campestris, Alchemilla procumbens and Penstemon gentianoides), are well
adapted to poor soils and the effects of intense erosion caused by the flows. [Key words:
colonization, lahars, primary vegetation, Popocatepetl volcano, Mexico.]
INTRODUCTION
Research Approach
The biogeography of active volcanic areas is of great interest, particularly in relation to the way in which ecosystems evolve following disturbance by volcanic
192
Physical Geography, 2005, 26, 3, pp. 192–215.
Copyright © 2005 by V. H. Winston & Son, Inc. All rights reserved.
PLANT COLONIZATION ON LAHARS
193
processes with post-impact effects (Lawrence and Ripple, 2000). Resilience,
defined as the rate at which systems recover after a disturbance event (Vitousek et
al., 1981; Pimm, 1999; Kristensen et al., 2003), has been considered as an indicator
of an ecosystem’s capacity to return to the equilibrium state after disturbance
(Kristensen et al., 2003). This capacity to recover has been determined from specific
variables associated with recovery: plant structure and composition, productivity,
plant biomass, nutrient accumulation in soils, and ecological diversity (Vitousek et
al., 1981; Uhl and Jordan, 1984; Pimm, 1999). Unlike other studies, which use the
resilience that results from cultural disturbances, this case evaluates the initial
development stages of temperate forest resilience after a natural disturbance at
small scale.
Prime colonization patterns and processes that take place in relation to volcanoes often differ widely depending on the type of volcanic material involved (del
Moral and Grishin, 1999; Lawrence and Ripple, 2000). Although lahars are volcanic processes that frequently threaten or affect forest ecosystems, the number of
articles published on the prime colonization, succession or revegetation of lahars is
limited (Frenzen et al., 1988; Dale, 1989; García and Muñoz, 2002b). Very little
research of this nature has been conducted in Mexico, and new knowledge currently centers on the differentiation of habitats controlled by lavas and pyroclasts
from Paricutin and Jorullo volcanoes (Eggler, 1959, 1963; Cano and Meave, 1996;
Giménez de Azcárate et al., 1997), lavas from Chichinautzin Sierra in the south of
Mexico City (Espinosa, 1967), and debris flows (Burnham, 1994).
The Popocatepetl volcano (19°03'N, 98°35'W; 5452 m), located 75 km southeast of Mexico City, began a new phase of eruptions in December 1994 marked by
the formation of large water-saturated debris flows known as lahars (Vallance,
2000; Capra et al., 2004). These high-velocity flows contain a mixture of rock
debris, volcanic ash, and abundant water from melting glacier ice and permafrost,
rain, and snowfall. Volcanic activity or meteorological phenomena can trigger the
release of debris and water, which form a mixture with a consistency capable of
traveling downslope as debris flows or hyperconcentrated flows (Pierson and Costa,
1987; Thouret and Lavigne, 2000).
Lahars occurred on the northeast slope of Popocatepetl because of its orientation, exposure, heavy rainfall, and the presence of a glacier on the upper cone, and
originated in a drainage basin whose main axis is Huiloac Gorge (Palacios, 1995;
Palacios et al., 1998; Siebe, 1996). In June/July 1997 and January 2001, large lahars
flowed through the gorge destroying or transforming the preexisting forest at the
volcano’s base. The resulting mosaic of environmental units with their low-density
and poorly developed vegetation cover, disturbed by volcanic activity and exposed
to permanent geomorphic instability, provided the opportunity to study both the
damage and the initial phases of plant colonization in forest ecosystems.
This study focuses on pioneer communities growing on the recent lahar deposits.
We examined the composition and structure of the plant communities and analyzed
several explanatory variables related to morpho-sedimentologic and bioclimatic
processes and the dynamics of the relief, in order to explain development stages of
colonization and resilience in a temperate forest. Since colonization in the area has
occurred recently, we hypothesized that changes in vegetation would be governed
194
MUÑOZ-JIMÉNEZ ET AL.
by the age, climate, and the morpho-sedimentology of the lahars, rather than by
other environmental variables that would not have allowed clearly defined relationships to appear in such a short period of time. Thus, the aim of this study is to establish the development stages and explanatory factors of plant colonization on recent
lahar deposits differing in age and morpho-sedimentologic characteristics.
Study Area
The upper reaches of Huiloac Gorge are 7 km long and cross two vegetation
belts from 3150 to 3550 m on the east side of Popocatepetl. The vegetation belts
comprise mixed forest of pine and fir, and subalpine peak forest. The mixed forest
of pine (Pinus ayacahuite, P. montezumae, etc.) and fir (Abies religiosa) that covers
most of the middle slopes of the volcano has mean annual temperatures of 8–10°C,
a difference of less than 5 between the mean temperatures of the hottest and coldest
months; mean annual precipitation of 1300–1400 mm, concentrated primarily in
the summer months (30% of the total); humidity with excess moisture throughout
the year although much less in mid-winter. The subalpine peak pine forest (Pinus
hartwegii) grows above 3400 m to the timberline at 4000 m. Above this altitude
pines are replaced by alpine grasslands (Calamagrostis tolucensis) on the upper
cone. The climate is slightly colder with mean annual temperatures of 6–8°C, and
mean annual precipitation of 1200–1300 mm (INEGI, 1986; Rzedowski 1988;
Giménez de Azcárate and Escamilla, 1999; Fig. 1).
On June 20, 1997, a massive fallout of burning pyroclasts blanketed the glacier
on the drainage basin near the crater. The amount of water generated at the start of
the process was three times the volume of solid material, and triggered a large
debris flow that traveled down three gorges above the timberline before evolving to
a hyperconcentrated flow that continued downslope through the entire length of
Huiloac Gorge to the towns of Santiago Xalitzintla and San Nicolás de los Ranchos
on July 1 and 2 (Cenapred, 2002; Capra et al., 2004). The lahar destroyed the vegetation at the lower portion of the gorge walls, and covered the bottom with a
heterometric debris flow measuring 3–6 m thick and 10–15 m wide in the section
observed.
Soil formation and vegetation colonization began on the new floor, although this
was probably interrupted many times by water streams or hyperconcentrated flows
originating near the crater wall and triggered by the interacting forces of eruption,
meltwater from the residual glacier, and heavy rain or snowfall (Cenapred, 2002).
Later lahars traversed and destabilized the sediments deposited in 1997 but were
not powerful enough to either destroy or bury them.
On January 22, 2001, a rapid debris flow reached the gorge floor and eliminated
most of the plant communities that had colonized the surface during the previous
44 months. This lahar was thick (70 cm) but not very long and did not reach the end
of the gorge. It left a layer of medium to small fragments of pumice and andesite in
a sandy matrix, which formed levees directly above the 1997 lahar (González,
2000; Cenapred, 2002). The volume of water in the January lahar, however, did not
exceed 25% of the mixture, so that the flow did not undergo liquefaction processes
PLANT COLONIZATION ON LAHARS
195
Fig. 1. Location and chronological sequence of the 1997 and 2001 lahars, and vegetation belts in
Huiloac Gorge.
and thus maintained the characteristics of a massive debris flow (Capra et al.,
2004).
Despite the short length (7 km) and width (12 m) of the gorge, plant colonization
followed, with varying results. The sparse vegetation growing on the most recent
196
MUÑOZ-JIMÉNEZ ET AL.
Fig. 2. Photo of Lahar 2001. The eruption triggered rapid debris flows accompanied by hypersaturated flows that reached the floor of the gorge.
flow, which covered the first one, is exposed to water streams and hyperconcentrated flows that systematically scour the gorge floor and constantly interfere with
its development (Fig. 2). By February 2002, the bottom of the channel, which had
been modified by infrequent lahars after January 2001, was incised by many channels measuring only a few decimeters deep. It also appeared flat, stepped, and
flanked by small banks or levees.
METHODS
Study Site/Location
The study site was selected on the basis that lahars at the Huiloac Gorge constitute habitats with different characteristics and natural potential for the development
of colonizing vegetation. For this reason, the first study phase consisted in determining the distribution of the different lahar types forming the gorge’s bottom. The
investigation focused on a 7-km reach of the gorge, from 3174 to 3525 masl, where
lahars are distributed as discontinuous strips delimiting the gorge’s bottom.
The distribution of the following four types of lahars of varying ages, morphosedimentology and dynamics was determined from field observations (García and
Muñoz, 2002b): (1) marginal surfaces, free of new deposits, appear on the outcrops
of the 1997 sediments and are covered by vegetation that has developed uninterruptedly over the previous 55 months; (2) levees overlain by deposits from the 2001
flow where colonization occurred for more than one year and was not directly
affected by new lahars; (3) subvertical margins of the channel cut by streamflows
PLANT COLONIZATION ON LAHARS
197
occurring after January 2001 (which are still active) where colonization is very
recent and vegetation development is negligible because of geomorphic instability;
and (4) the bottom of the channel cut by the streamflows is either flat or terraced
and has been filled with fine-textured sediments. This latter zone is repeatedly
affected by intense erosion and deposition.
Rather than choosing our plot locations at random or systematically in each habitat, we had to select them very carefully, given that terraces and levees at lahars are
not continuous but form extremely short and narrow strips (<1.0 m), and only 29
sites were large enough to accommodate the plots. Of the plots, 5 were established
and mapped (GPS technology and SIG ILWIS ver. 3.0) on the surfaces of the outcrops of the 1997 laharic deposits, 17 on the levees buried by the 2001 lahar; 4 on
the margins, and 3 on the terraces at the gorge’s bottom.
Plots
In February 2002, circular monitoring plots were established in Hiloac Gorge’s
upper sector. The size of the plots was determined by applying the “minimum
sample area” method for floristic inventories. This method takes into account the
variation in the number of species obtained in circles of increasing diameter. The
surface of the smallest sample area is obtained once the number of species stabilizes, even if the surface area of the sample plot continues to expand (Matteucci and
Colma, 1982). The surface of the sample area was 1.57 m2 in all plots. Sample plots
were marked with a metal post painted bright green to facilitate locating them. The
post acted as an axis around which we traced the circumference of the plot.
For each species and community we recorded the number of individuals, the
vegetation cover of the area expressed as a percentage, and the stem height of the
most strongly developed individual. Specimens of all species recorded were collected from field locations and the nomenclature of the plant taxa was determined
by botany experts (Martha Gual and Martha Escamilla W.) at Universidad Nacional
Autónoma de México.
Data Analyses
We used the phytosociologic method of Braun-Blanquet (1979), based on floristic inventories that were added to the vegetation tables, to assess the composition
and structure of the pioneer communities. Cluster analysis was used to quantify the
floristic affinity in the habitats (types of lahars) (StatSoft, 1998) and a dendrogram
was generated for easy data interpretation. The Sørensen Index (Sørensen, 1948)
was calculated and a similarity matrix was developed to corroborate the results
(Spellerberg, 1995). The following expression was obtained from the index:
ISØ = 2A / (B + C)
where A = number of species shared in all samples, B = the number of species
present in sample x, and C = the number of species in sample y (Arozena and
Molina, 2000).
17
50
Umbelliferae
Compositae
Euphorbiaceae
Compositae
Compositae
Compositae
Compositae
Eupatorium glabratum
Euphorbia thymifolia
Geranium potentillaefolium Geraniaceae
Compositae
Eryngium monocephallum
Gnaphalium bourgovii
Gnaphalium leptophyllum
Gnaphalium liebmannii
Gnaphalium nuvicola
Gnaphalium oxyphyllum
13
14
15
16
17
18
19
20
0
17
0
50
33
50
17
17
17
12
9
0
50
Crassulaceae
Conyza schiedeana
8
0
Echeveria secunda
Compositae
Cirsium ehrenbergii
7
17
17
11
Compositae
Castilleja arvensis
6
Compositae
Compositae
Scrophulariaceae
Bidens triplinervia
5
33
Poaceae
Scrophulariaceae
Bacopa chamaedryoides
4
67
Dugesia mexicana
Compositae
Baccharis conferta
3
17
33
Deschampsia pringlei
Caryophyllaceae
Arenaria reptans
%
10
Rosaceae
Alchemilla procumbens
Family
2
Species
1
Number
A
6
19
13
44
31
0
50
0
0
0
0
6
6
6
6
0
44
13
6
0
0
0
0
0
0
0
0
0
0
0
0
0
25
0
0
0
0
0
50
%
C
Frequency
0
%
B
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
%
D
3
14
7
34
28
3
34
10
3
3
3
3
14
7
7
3
31
21
7
14
%
E
0
15
0
84
21
19
14
7
2
9
1
0
23
0
6
4
26
12
1
9
Total
A
0
4
0
20
5
4
3
2
0
2
0
0
5
0
1
1
6
3
0
2
%
2
6
3
64
20
0
33
0
0
0
0
30
20
1
1
0
82
11
1
0
Total
B
0
1
1
11
4
0
6
0
0
0
0
5
4
0
0
0
14
2
0
1
%
0
0
0
0
0
0
0
0
0
0
0
0
0
2
0
0
0
0
0
8
Total
C
0
0
0
0
0
0
0
0
0
0
0
0
0
13
0
0
0
0
0
0
%
D
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Total
Number of individuals
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
%
E
2
21
3
148
41
19
47
7
2
9
1
30
43
3
7
4
108
23
2
17
Total
Table 1. Distribution of Frequency and Number of Individuals According to Morpho-Sedimentologic and Altitude
Variations in (A) 1997 Lahar, (B) 2001 Lahar, (C) Margin of the Channel, (D) Terraces of the Channel, and (E) Total
198
MUÑOZ-JIMÉNEZ ET AL.
Rubiaceae
Compositae
Leguminoseae
Poaceae
Poaceae
Poaceae
Scrophulariaceae
Pinaceae
Rosaceae
Rosaceae
Saxifragaceae
Poligonaceae
Salicaceae
Compositae
Compositae
Compositae
Compositae
Compositae
Compositae
Scrophulariaceae
Compositae
Poaceae
Poaceae
Crassulaceae
Poaceae
Hedyotis pygmaea
Hieracium comatum
Lupinus campestris
Muhlenbergia nigra
Muhlenbergia ramulosa
Nassella mucronata
Penstemon gentianoides
Pinus ayacahuite
Potentilla ranunculoides
Potentilla rubra
Ribes ciliatum
Rumex acetosella
Salix lasiolepis
Senecio angulifolius
Senecio barba-Johannis
Senecio cinerarioides
Senecio procumbens
Senecio roseus
Senecio sanguisorbae
Sibthorpia repens
Stevia salicifolia
Trisetum irazuense
Trisetum kochianum
Villadia batesii
Vulpia myurus
Moss
21
22
23
24
25
26
27
28
29
31
30
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
0
17
17
33
17
0
0
50
17
67
50
0
17
0
33
50
17
17
0
17
67
17
0
0
33
0
0
0
19
0
0
6
63
0
13
87
6
6
0
0
25
0
0
6
6
38
0
13
6
25
0
0
0
0
20
0
0
0
0
25
20
0
25
0
0
0
25
0
25
0
0
25
25
0
25
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
100
0
3
3
3
17
3
3
3
45
3
14
62
7
7
3
7
24
3
7
3
10
34
3
10
17
21
0
12
2
35
1
0
0
17
1
7
12
0
1
0
23
5
1
3
0
2
32
10
0
0
4
0
3
0
8
0
0
0
4
0
2
3
0
0
0
5
1
0
1
0
1
8
2
0
0
1
0
0
0
39
0
0
10
70
0
12
57
1
7
0
0
12
0
0
1
3
16
0
5
1
56
0
0
0
7
0
0
2
12
0
2
10
0
1
0
0
2
0
0
0
0
3
0
1
0
10
1
0
0
0
0
1
0
0
0
0
1
1
0
3
0
0
0
1
0
2
0
0
1
3
0
6
0
0
0
0
6
0
0
0
0
6
6
0
19
0
0
0
6
0
13
0
0
6
19
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
100
0
1
12
2
74
1
1
10
87
1
19
70
2
8
3
23
17
1
4
1
7
48
10
6
8
60
PLANT COLONIZATION ON LAHARS
199
200
MUÑOZ-JIMÉNEZ ET AL.
Species were organized according to type of habitat, and the floristic composition was determined by richness (R; number of species), average richness (AR),
number of individuals (NI), frequency (F), and percentage of species contribution
(PSC). Frequency (% number of samples in which an attribute appears) was
expressed as:
F1 = (m1/M)*100
where (m1) is the number of samples in which the attribute appears and M is the
total number of samples. The percentage of species contribution compared to the
total plant mass was expressed as:
%S = (Ti/TI)*100
where (Ti) is the cumulative total cover of a species and (TI) is the cumulative total
cover of all species in all plots (Arozena and Molina, 2000).
The structure of each group was analyzed using: mean plant cover (MPC; %),
mean density (MD) (individuals/m2), mean stem height (MSH) and maximum stem
height per plot (MSHP).
In order to determine the explanatory factors of the pioneer colonization, we
used a multivariate distribution analysis (Principal Components Analysis; Kendall,
1980) based on data recorded for 15 significant environmental variables: altitude,
exposure, gorge depth and width, slope vegetation, slope, deposit thickness and
length, size and shape of clasts, microtopography, height, and distance from the
bottom of the channel, channel’s type of margin—cumulative or erosive—and number of geomorphologic processes. ORDEN software was used for the analysis
(Ezcurra, 1990).
RESULTS
Floristic Composition
Except for mosses, which have not yet been classified, the vegetation on the
lahars that were generated since 1997 volcanic events is diverse, consisting of 45
species, 33 genera, and 15 families. There is a high concentration of a small number of taxa: 30% of the species and 51% of the individuals belong to just 4 genera—
Senecio, Gnaphalium, Trisetum and Bacopa. Nearly half of the individuals from all
plots (47%) belong to only 6 species: Gnaphalium leptophyllum, Bacopa
chamaedryoides, Senecio cinerarioides, S. sanguisorbae, Trisetum kochianum and
Hieracium comatum. The frequency values reveal the predominance of the genus
Senecio (76% of the sample plots) and Gnaphalium (48%). With regard to species,
Senecio cinerarioides and S. sanguisorbae have the highest frequency (62% and
45%, respectively; Table 1).
The frequency values reveal the predominance of three families: Compositae
represent 48% of the species and 51% of the individuals in 90% of the plots;
Poaceae represent 13% of species and 14% of individuals in 52% of plots; and
PLANT COLONIZATION ON LAHARS
201
Fig. 3. Photos showing the vegetal covering in two morpho-sedimentologic units. (A) The June 1997
lahar formed a plain between the levee of the 2001 lahar and the wall of the gorge at 3174 m. The
deposit is colonized by grass and Baccharis conferta, among others. (B) The surface of a levee formed
by the debris flow of January 2001 at 3190 m, with incipient colonization of Senecio and Gnaphalium
genera, among others.
Scrophulariaceae, with an average of 8% of species and 9% of individuals, are
present in 48% of plots (Table 1).
On the 1997 lahar, richness is 34 species and an average 10.3 species per plot.
Additionally, 53% of the number of individuals (NI) is represented by only 6 species
(Table 1, Fig. 3A), but only for 3 of them is frequency (F) equal to or higher than
50%: Nasella mucronata, Gnaphalium leptophyllum, and Conyza schiedeana. Values of the percentage of species contribution (PSC) to the total plant mass were
202
MUÑOZ-JIMÉNEZ ET AL.
Table 2. Percentage of Species Contribution Indicating the Contribution of a
Species Compared to the Total Plant Massa
Number
Species
Lahar 1997
Lahar 2001
Margin
of channel
Bottom
of channel
1
Baccharis conferta
17.22
1.51
0.00
0.00
2
Nassella mucronata
8.10
2.26
0.00
0.00
3
Senecio procumbens
1.27
3.02
0.00
0.00
4
Rumex acetosella
2.03
0.00
0.00
0.00
5
Trisetum irazuense
0.25
0.00
0.00
0.00
6
Potentilla ranunculoides
0.51
0.00
0.84
0.00
7
Conyza schiedeana
5.82
1.89
0.00
0.00
8
Eupatorium glabratum
1.77
0.00
0.00
0.00
9
Vulpia myurus
1.27
0.00
0.00
0.00
10
Muhlenbergia ramulosa
1.27
0.00
0.00
0.00
11
Hieracium comatum
0.51
5.66
0.00
0.00
12
Bacopa chamaedryoides
1.77
6.42
0.00
0.00
13
Euphorbia thymifolia
1.01
5.28
0.00
0.00
14
Ribes ciliatum
0.76
1.89
0.00
0.00
15
Lupinus campestris
0.00
0.38
2.52
100.00
16
Gnaphalium bourgovii
1.77
2.64
0.00
0.00
17
Senecio cineraroides
1.01
11.32
0.84
0.00
18
Senecio sanguisorbae
3.79
8.30
0.00
0.00
19
Senecio angulifolius
0.25
0.38
0.00
0.00
20
Gnaphalium leptophyllum
8.35
9.06
0.00
0.00
21
Gnaphalium nuvicola
22
Moss
23
Pinus sp.
24
1.27
1.13
0.00
0.00
13.92
17.36
0.00
0.00
0.00
0.38
0.00
0.00
Trisetum kochianum
10.13
13.20
0.00
0.00
25
Cirsium ehrenbergii
0.00
0.38
8.40
0.00
26
Senecio barba-Johannis
0.00
0.38
0.84
0.00
27
Villadia batesii
0.25
0.00
0.00
0.00
28
Echeveria secunda
1.01
0.00
0.00
0.00
29
Potentilla rubra
0.25
0.00
0.00
0.00
30
Geranium potentillaefolium
1.52
0.00
0.00
0.00
31
Bidens triplinervia
5.06
0.00
0.00
0.00
32
Castilleja arvensis
3.80
0.38
0.00
0.00
33
Eryngium monocephallum
0.51
0.00
0.00
0.00
34
Alchemilla procumbens
0.76
0.00
50.42
0.00
35
Salix lasiolepis
0.00
0.00
16.81
0.00
36
Stevia salicifolia
0.00
0.00
0.84
0.00
37
Gnaphalium oxyphyllum
0.00
0.38
0.00
0.00
38
Gnaphalium liebmannii
0.00
0.75
0.00
0.00
(table continues)
203
PLANT COLONIZATION ON LAHARS
Table 1 (Continued)
Number
Species
Lahar 1997
Lahar 2001
Margin
of channel
Bottom
of channel
39
Deschampsia pringlei
0.00
0.75
0.00
0.00
40
Sibthorpia repens
0.00
3.02
0.00
0.00
41
Senecio roseus
0.25
0.00
0.00
0.00
42
Arenaria reptans
0.25
0.38
0.00
0.00
43
Dugesia mexicana
0.25
0.00
0.00
0.00
44
Penstemon gentianoides
1.77
1.13
0.84
0.00
45
Muhlenbergia macroura
0.00
1.13
0.84
0.00
46
Hedyotis pygmaea
0.00
0.00
16.81
0.00
Total
100.00
100.00
100.00
100.00
Cumulative total cover (TI)
395
265
119
3
a
Expressed as %S = (Ti/TI)*100, where (Ti) is the cumulative total cover of one species and (TI) is
the cumulative total cover of all species in all of the plots.
more specific, with Baccharis conferta and Trisetum kochianum representing
17.2% and 10.1% of total plant mass, respectively (Table 2).
There are 14 exclusive species in the area. With the exception of Eupatorium
glabratum, these species are characterized by their scarcity and low frequency:
Senecio roseus, Bidens triplinervia, Dugesia mexicana, Trisetum irazuense, Vulpia
myuros, Muhlenbergia ramulosa, Potentilla ranunculoides, P. rubra, Villadia batesii,
Echeveria secunda, Rumex acetosella, Geranium potentillaefolium, and Eryngium
monocephalum (Table 1, Fig. 3A).
On the 2001 lahar, the richness consists of 29 species and an average of 5.9 species per plot. The 5 most abundant species are Bacopa chamaedryoides, Senecio
cinerarioides, S. sangisorbae, Gnaphalium leptophyllum, and Hieracium comatum,
and make up 57% of NI in this habitat (Table 1, Fig. 3B). There are 9 exclusive species, which are characterized by their low frequency, and only one, Deschampsia
pringlei, is more important because of a greater number of individuals.
The species Senecio cinerarioides appears in 87%, and Senecio sanguisorbae in
63% of the plots on the 2001 lahar. The PSC of the total plant mass indicates that
the mosses, Trisetum kochianum and Senecio cinerarioides have the greatest contribution with 17.3%, 13.2%, and 11.3%, respectively (Table 2). Among the 19 common species on the 1997 and 2001 lahars, Gnaphalium leptophillum, Bacopa
chamaedryoides, Senecio cinerarioides, S. sanguisorbae, Hieracium comatum,
Trisetum kochianum, or Nasella mucronata, are the most abundant and frequent.
Vegetation is scarce on the margins and terraces of the hyperconcentrated
streamflow channel. It comprises a few individuals of about a dozen species, the
most representative of which are either non-existent or scarce in other morphosedimentologic units where colonization is more advanced. On the margin of the
channel the floristic stock (i.e., richness) consists of 11 species. The vegetation is
sparse and limited to a few small sites of newly established plants such as
Alchemilla procumbens (50.4% of the total plant mass), that are capable of
204
MUÑOZ-JIMÉNEZ ET AL.
Fig. 4. Mean density (MD), mean plant covering (MPC), maximum stem height of each plot (MSHP),
and mean stem height (MSH) of vegetation for morpho-sedimentologic units.
surviving in unstable environments. This species is sometimes accompanied by Cirsium ehrenbergii, Lupinus campestris, and by individuals of the genera Trisetum,
Muhlenbergia, and the species Senecio cinerarioides and Penstemon gentianoides.
The bottom of the gorge forms a band of bare clay-sand deposits, completely
devoid of any plant communities. In very localized terraces unaffected by recent
flows, there are occasional isolated individuals or groups of 2–3 individuals of a single species, Lupinus campestris. This leguminous species appears capable of rooting quickly and developing in the mud and clay deposited by the hypersaturated
flows (Tables 1 and 2).
Structure of Plant Communities
The vegetation on the recent lahar deposits on the bottom of Huiloac Gorge has
low cover and density. The mean plant cover (MPC) for the entire group of plots is
25.8% and the mean density (MD) is 14.1 individuals/m2. These values differ
widely, however, depending on which of the four lahar habitats is involved.
For the 1997 lahar, the MPC is 57.5% and MD is 29.2 individuals/m2; while for
the 2001 lahar, the values only reach 16.4 and 3.9, and for the terraces of the
channel, 1.0 and 0.4, respectively. MPC is relatively high for the plots at the margins
of the channel (34.7 and 2.1), because the few plants growing there are creepers,
and these spread easily across the narrow surface (Fig. 4).
The cover values are taken from sample plots with wide variation intervals (i.e.,
16% to 85% on the 1997 deposit). The only homogeneous set of percentages is
associated with values for channel terraces, which are always very low (1.0%). A
wide range of densities appear on outcrops of the 1997 lahar (11 to 52 individuals/
m2) and 2001 lahar (2 to 31), while a much narrower range (0.4 to 4.0) is associated
with the two channel habitats.
PLANT COLONIZATION ON LAHARS
205
Fig. 5. Dendrogram (Euclidean distances) of 29 plots with three levels of integration.
The maximum stem height per plot (MSH) of the most developed individuals of
each species is only 10.26 cm and even when the MSHP are taken into account, the
mean height does not exceed 22.36 cm. The herbs and low shrubs in this narrow
range grow appreciably taller on the surfaces of the 1997 lahar (MSH 14.58 cm and
MSHP 36.08 cm) compared to the 2001 lahar (7.7 and 18.31, respectively) (Fig. 4).
The margins of the channel are populated by fast-growing plants, and some shoots
have sprouted from the roots of the shrubs on the slopes (MSH 15.77 cm and MSHP
28.75 cm). Only one species of medium height is present on the terraces of the
channel and is affected by frequent stream and hypersaturated flows. The values for
MSH and MSHP are both around 8.0 cm.
Development Stages of Colonization
The affinity analysis conducted for 29 plots displays marked differences in the
pioneer colonization associated with the four habitats: (1) 1997 lahar, (2) 2001
lahar, (3) terraces of the present channel, and (4) the margin of the present channel.
Thus, each one corresponds to a different development stage of colonization.
Figure 5 shows the dendrogram of the cluster analysis for the 29 plots grouped
according to eight levels of integration (StatSoft, 1988). There is a clear tendency for
plots from the same habitat (i.e., the same type of lahar) to cluster. Except for plot
10 (1997 lahar), which includes six exceptional species (i.e., Villadia batesi,
Echeveria secunda, Potentilla rubra, and Eryngium monocephallum), the first level
clusters the remaining sites into the same branch. The second level is subdivided
into two branches, one of which clusters 15 of the 18 plots associated with the 2001
lahar. The other branch is subdivided into several cluster levels. Three inventories of
the 2001 lahar, which are located on the highest point in the area (>3400 m), are
206
MUÑOZ-JIMÉNEZ ET AL.
Fig. 6. The Sorensen Index shows the affinities between a pair of plots according to presence/absence
of species, with (1) indicating the maximum affinity and (0) the minimum affinity. Plots in segmented
squares showing the highest values (>3.0).
less rich and include Muhlenbergia nigra, Penstemon gentianoides, Arenaria
reptans, and Conyza schiedeana, which are not present in other plots of the 2001
lahar but are also associated with this group.
Figure 6 shows the results of the Sørensen Floristic Affinity Index (Sørensen,
1948). The overall results reveal a mean affinity of 0.3, while specific values range
from 0.000 to 1.000. Nevertheless, values that are equal to or exceed 0.3 were interpreted as high affinity indicators, since the correlation between plots and habitats is
clearly defined within this range (Fig. 6). A greater level of affinity is associated to the
plots on terraces of the hyperconcentrated flow channel where only one species,
Lupinus campestris (Sørensen Index of 1.000), was observed. The mean affinity on
the margins of the channel was much lower (0.440). This is explained by the recent
establishment of plants that sprout from the roots of shrubs on the slopes.
Although these results confirm a high affinity within each type of habitat, the
average affinity for the plots located on the 1997 lahar (0.509) is similar to the value
obtained for the plots associated with the 2001 lahar (0.440), and is a result of the
large number of plots that share a high floristic affinity between both habitats.
Explanatory Factors of the Pioneer Colonization
The Principal Components Analysis (PCA; Kendall, 1980) revealed a low variance percentage (<7%) for both axes in the group of 15 variables analyzed. As a
result, the plots belonging to diverse habitats are not clustered and, in fact, are very
PLANT COLONIZATION ON LAHARS
207
Fig. 7. Principal Component Analysis. (A) Distribution of the sample plots. (B) Distribution of the
variables.
scattered (Fig. 7A). This suggests that the influence of environmental variables
changes at a local level in each case. The variance percentages, however, reveal
that variables related to morpho-sedimentology and stability exert a more profound
control on colonization. For example, slope processes, clast shape, deposit length,
and the depth of the gorge explained the 54% variance observed in axis 1, while
the number of slope processes, microtopography, clast shape, deposit thickness,
and the depth of the gorge explained the 53% variance in axis 2.
Figure 7B reveals that some environmental variables tend to cluster, which suggests a relationship between these and the possible consequences of colonization.
As expected, the thickness of the deposit and microtopography are grouped, thus
suggesting the importance of morpho-sedimentology on colonization. The height
and distance from the bottom of the channel and the type of channel margin are
208
MUÑOZ-JIMÉNEZ ET AL.
Fig. 8. Distribution of richness, richness/plot, frequency, number of individuals/plot, mean density,
and mean plant cover for altitude units.
also grouped together, and can be interpreted as a consequence of the influence of
the present flows on plant establishment and permanence. The slope, altitude,
slope vegetation and the width of the gorge are also grouped, which suggests that
the influence of slope vegetation depends on temperature and sunlight as energy
sources.
Contrary to our expectations, affinity values were largely unaffected by altitude
as indicated by the consistently moderate level for all three elevation sectors in the
gorge. We noted, however, that the middle sector at 3250 to 3450 masl, had a
mean value of 0.202, slightly higher than the value for the lower sector (0.176), and
clearly above the average for the upper sector (0.148). Furthermore, other analyses
show logical and well-defined changes in the relationship between vegetation and
altitude. For example, Figure 8 shows that the variations in MPC, MD, and R/per
plot have an inverse relationship with altitude. The MD value is 15.6 individuals/m2
in the lower sector of the study site, 11.4 individuals/m2 in the middle sector, and
8.9 individuals/m2 in the upper sector. The segment at 3250 to 3450 m is the rich-
PLANT COLONIZATION ON LAHARS
209
est, with 33 species identified (6.5 species/plot); the next lower segment starting at
3250 m has 26 species (5.8 species/plot); and the highest segment above 3450 m
has only 16 species (2 species/plot).
Plant affinity follows an opposite tendency that favors the lower sectors. Only a
few of all inventoried species were common to all three segments: Senecio
sanguisorbae, Baccharis conferta, and Ribes ciliatum for the 1997 lahar deposit,
and only one, Senecio cinerarioides, for the 2001 deposit. We also found an appreciable affinity in the floristic content of the mid and lower segments. Also, 35% of
the colonizing species of the first lahar and 39% of those on the second lahar,
including the most abundant and frequent species, are common to these two segments located below 3450 m. However, there are fewer plants in common in the
mid to upper sectors (15% and 4%, respectively) and these belong to species that
are much less abundant.
The inventory of the flora in the mid and lower segments of the 1997 lahar
reveals 17 species (49% of the total) associated with the belt of mixed pine and fir
forest at 2900 to 3250 m. Of the species in the upper segment of the gorge, 60% are
associated with subalpine pine forest. Similarly, the mid segment of the 2001 lahar
has 16 species (52% of the total) associated with mixed-conifer forests, and in the
upper segment, approximately 65% of the species correspond to the understory of
the pine forest belt and are rarely found below this altitude.
DISCUSSION
As explained above, our sample size is not sufficiently large (only 29 sites were
large enough to accommodate the plots) and this situation affects the statistic analyses. However, we consider our results as significant, mainly because they reveal
trends of pioneer colonization on lahar deposits. We observed that a limited number of species dominate the first phases of colonization, and include Compositae
(Gnaphalium and Senecio), and to a lesser degree, Poaceae and Scrophulariaceae
that appear on newly created deposits. These plants not only act as pioneer colonizers, but also establish a core floristic composition for communities that develop
during incipient colonization in more stable sectors. Gnaphalium leptophillum,
Senecio cinerarioides, S. sanguisorbae, Bacopa chamaedryoides, or Trisetum
kochianum are as abundant in areas where vegetation was established one year ago
as in sectors where plants had developed over a span of five years. As time passes,
density increases more rapidly than floristic richness (Fig. 7). Unlike the situation on
Mount St. Helens in the northwest United States where, according to Halpern and
Harmon (1983), there was a wide variety of plants (with rhizomes and tubers) growing on the mudflows near the active channels (Carex, Juncus, Lupinus, etc.), at our
sites there was very little plant colonization in the terraces near the active channel.
Although some of these species (Alchemilla procumbens, Cirsium ehrenbergii,
and Lupinus montanus) manage to establish, grow, and spread rapidly, they are
grouped into thin patches on the margins of the gorge and are eventually destroyed
by mass movements triggered by hypersaturated flows. If this type of erosion activity
continues, these small communities will not develop beyond the initial colonization
stage (Fig. 7). The early appearance of Lupinus montanus in the fine sediments of the
210
MUÑOZ-JIMÉNEZ ET AL.
terraces near the present channel confirms the importance of this leguminous species in maintaining a symbiosis with nitrogen-fixing microorganisms that promote
plant growth (Halpern and Harmon, 1983; del Moral et al., 1995; Cano and Meave,
1996; Wood and del Moral, 2000).
On the more geomorphologically stable 1997 and 2001 lahar deposits, we
found a group of a few species that played a key role in the initial colonization
phases. This pioneer group grows all along the bottom of the gorge and combines
with complementary groups composed of forest plants that populate the slopes in
each sector. We noted that although the pioneer group remains stable, the complementary groups increase in importance as colonization progresses. As a result,
while the floristic affinity among the communities established on the floor of the
gorge decreases, the opposite is true for the herbs and shrubs of the surrounding
conifer forest.
As regards environment factors, del Moral and Jones (2002), explained that colonization is contingent upon stability associated with protected areas, while Cano
and Meave (1996) maintained that this is related to resource availability. Our results
also reveal that the stability and availability of resources for plant growth depend on
the age, composition, and thickness of the deposits, as acknowledged by other
authors (Eggler, 1963; Espinosa, 1967; Halpern and Harmon, 1983; Vargas, 1985;
Halpern et al., 1990; Harrington et al., 1998; del Moral and Grishin, 1999;
Lawrence and Ripple, 2000).
Lithology is a fundamental variable because it is linked to the extent of rock
weathering, which affects water retention and soil formation, especially where
there are pumice or fine clastic deposits rather than lava and coarse pyroclastics
(Eggler, 1963; Espinoza, 1967; Halpern and Harmon, 1983; del Moral et al., 1995).
The lahars present in the Huiloac Gorge have been classified as debris flows (Capra
et al., 2004) that consist of a mixture of soil and rocks which provides heterogeneity
and potentially protected sites (del Moral and Grishin, 1999). However, the clasts
of the 2001 lahar are firmly consolidated, and this impedes colonization. In contrast, the 1997 lahar absorbed more water during its formation and transformed into
a hyperconcentrated flow that left deposits with finer and less cohesive materials
(sand matrix). Thus, the evolution of the vegetal cover on the 1997 lahar may not
only be the result of age, but also of a greater capacity of soil development
(Harrington et al., 1998).
Contrary to our expectations, the results of the PCA analysis confirm the findings
from several studies indicating that other environmental factors have very little
influence on pioneer colonization (Halpern et al., 1990; del Moral, 1999).
Although soil is usually a key factor in plant colonization (Espinosa, 1967;
Harrington, 1998), it is interpreted here as a highly dependent factor (García and
Muñoz, 2002a). This is explained by the young materials, the limited soil development and the wash-outs caused by flows and precipitation that produce soils that
are thin (<5cm), discontinuous, and similar in chemical composition and structure
to the parent material. For these reasons, we did not conduct a detailed soil analysis
and chose other variables to explain the distribution of habitats.
Since the study site is oriented along a topographic gradient, we assumed that
temperature would be a stress factor that may limit productivity (Cano and Meave,
PLANT COLONIZATION ON LAHARS
211
1996; del Moral and Grishin, 1999). However, the PCA analysis shows that the
differences in altitude had little influence on colonization. Of the 15 variables analyzed, only a few of them related to the morpho-sedimentology and the dynamics
of the present flows (microtopography, deposit thickness, clast shape, deposit
length, gorge depth, and slope processes) explain a greater percentage of the
variance (PCA analysis) and therefore have a greater impact compared to other
environmental characteristics.
It is widely recognized that nearby forest vegetation may strongly affect pioneer
colonization (Beard, 1945; Eggler, 1959; Halpern and Harmon, 1983; Vargas,
1985; Frenzen et al., 1988; Wood and del Moral, 2000) because of seed dissemination, which plays a major role in the distribution of species (Eggler, 1963; Cano and
Meave, 1996; Titus and del Moral, 1998; Lawrence and Ripple, 2000). According
to Frehner (1957), seed dissemination from nearby forests is responsible for 90% of
the propagation in the first 100 m, while Larsen and Bliss (1998) maintained that it
may be effective as far away as 400 m from the border of the forest. This is also the
case of the colonizing flora growing at Huiloac Gorge, which is derived almost
entirely from the vegetation covering the channel slopes. Although the lahars are
recent, there is a floristic affinity between the lahars and the slope vegetation, and
this is directly related to age. As one would expect, the species capable of rooting
in barren and loose unstable sediments become the predominant species, particularly along the margins and terraces.
A small group of these plants, mainly Compositae and Poaceae, is responsible
for establishing pioneer communities. These species display significant structural
and floristic composition differences depending on the morpho-sedimentologic
environment in which they are located on the bottom of the gorge. In all cases, progressive development is evident in plant growth and in the density of microsites that
support development.
As reported in other studies, trees that were toppled by lahars play a key role in
colonization by providing organic matter and nutrients for the soil, preventing
wash-outs, stabilizing the surface and promoting soil development (Halpern and
Harmon, 1983; Frenzen et al., 1988). According to Halpern et al. (1990), a high
density of fallen trees can contribute to the growth of vegetation with high survival
rates, but this was not observed at Huiloac Gorge. Alnus, Populus, and Salix are
some of the genera found in these habitats (Halpern and Harmon, 1983; Harrington
et al., 1998; Wood and del Moral, 2000), but in Huiloac, Salix lasiolepis is the only
plant that regenerates from trees that have been uprooted and buried under a thin
layer of sediment.
CONCLUSIONS
In the upland forests of intertropical volcanic mountains (3000 to 4000 m) colonization occurs rapidly on recent lahars but 4–5 years after deposition plant cover
remains limited to short herbs or sparse shrubs, although these display a relatively
high species richness. In the absence of disturbance, the incipient plant communities
become denser and stem length increases in the vegetation growing on recent lahars.
As colonization progresses, diversity increases and the affinity among the various
212
MUÑOZ-JIMÉNEZ ET AL.
components and sectors of the lahars decreases, thus promoting the gradual incorporation of species found on the adjacent gorge slopes.
Only a few plants belonging to a small number of species (Lupinus campestris,
Alchemilla procumbens, and Penstemon gentianoides) are scattered among sectors
affected by frequent lahars. These species are well adapted to poor soils and the
effects of intense erosion caused by the flows. Consequently, their presence is
reduced as geomorphic stability increases. The stock in areas adjacent to the lahars
quickly forms plant groups composed primarily by Senecio procumbens, Baccharis
conferta, and Nasella mucronata, which are pioneers since they develop rapidly on
the lahars and form the nucleus for future viable communities.
A short time has elapsed since the last lahars occurred, but a relationship has
already been established between the pioneer group and the species common to
the adjacent woodland undergrowth. Specifically, the pioneer group dominates the
initial phases of colonization, while the woodland plants gradually gain prominence later. At present, the pioneer group is still the active colonizer on the gorge
floor even though five years have elapsed since the last lahar event. The communities that develop quickly on the recent lahars often have such a simple internal
structure that they cannot be considered colonies nor can their phytosociology be
defined. The only formations with well-organized vegetation covering, species
richness and decreased floristic affinity are those found in places sheltered from
ongoing lahar activity.
Acknowledgments: This work was supported by Spain's Ministry of Science and Technology
(Research Project REN2000-0742RIES “Los lahares del volcán Popocatépetl. México. Control y
previsión de riesgos”). We would like to thank CENAPRED and Instituto de Geografía of Universidad
Nacional Autónoma de México (Mexico City) for their collaboration and help in facilitating field work.
We thanks Roger del Moral and two anonymous referees for their valuable suggestions, and Alice Ferrero
for her translation of the text to English.
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