Wildland Fire and Chaparral Succession Along

Int. J . Wildland Fire 5(1): 13-24,1995
O IAWF. Printed in U.S.A.
Wildland Fire and Chaparral Succession Along the
California-Baja California Boundary
Richard A. Minnichl and Conrad J. BahreZ
'Department of Earth Sciences, University of California,Riverside, California 92521
Tel. 909 787 5515; Fax 909 787 4324; e-mail minnich@ucracl .ucr.edu
2Departmnt of Geography, University of California, Davis, California 9561 6
Tel. 916 752 0798; Fax 916 754 8315
Abstract. Theunited States-Mexicointernationalboundary from El Paso, Texas to the Pacific Coast shows clear
differences in plant communities that were homogeneous
prior to being splitby a continuous fence at the turn of the
century. This study evaluates how disparate fire regimes
in California (fire suppression) and northern Baja California (little or no fire control) have influenced succession in the chamise (Adenostomafasciculatum) chaparral
communities spanning the international boundary between the border towns of JacumC and Tecate. Fire
history was reconstructed using U.S. Forest Service fire
maps and repeat aerial photography. Once plotted onto
topographic maps and dated, the bums were divided into
age-classesand sampledusing 50 mpoint-quarter transects
to develop successional chronosequences. Although
fires are more frequent and smaller on the Mexican side
of the border, our repeat photographs of the boundary
monument markers together with field samples show that
chaparral succession is similar across the international
boundary in species composition, stem densities, and
average mass shrub height. Chamisechaparralappears to
be stable under disparate fire regimes because sprouting
habits and latent seed pools permit efficient stand establishment under either short or long fire intervals. Chaparral recovery during the period examined here is unrelated to fire size because recolonization of bums by longrange seed dispersal is not a trait of the majority of the
local shrub species and the shrubs either resprout or
germinate from soil seed reservoirs.
Introduction
Space imagery and NASA Highflight aerial photography of the United States-Mexico international boundary between El Paso, Texas and the Pacific Coast show
clear differences across the frontier in plant communities that were largely homogeneous prior to completion
of a continuous fence separating the countries at the turn
of the century (Bahre and Bradbury 1978; Humphrey
1987; Bahre 1991). For the most part, these differences
have resulted from contrasting land-use patterns on the
two sides of the border.
The international boundary separating California
and Baja California between the border towns of JacumC
and Tecate, a distance of 30 km, offers an opportunity
to compare the effects of fire suppression (the United
States side) and largely uncontrolled burning (the Mexican side) on chaparral succession (Figure 1). Although
the Mexican government officially advocates fire suppression, there is little or no effective control of chaparral fires in Baja California. Before the initiation of fire
suppression policies in California near the turn of the
century (Lockmann 1981; Pyne 1982) and the clearing
of the international fuel break in the 1930s, fire mosaics
in the chaparral communities spanning the frontier were
similar on both sides of the border (Minnich 1987,
1988).
Minnich (1983; 1989), using Landsat imagery and
aerial photographs to reconstruct fire histories much
farther to the north and south of the study area, found
that because of a long history of uncontrolled burning
the present fire regime in the chaparral of Baja California is one of numerous small bums. He further
maintained that this same pattern occurred in southern
California until 1900 (Minnich 1987, 1988). After
1900, the initiation of federal and state fire suppression
policies reduced the frequency and increased the size of
fires in California (Minnich 1983; Chou et al. 1993).
For example, the 1970 Laguna fire in San Diego County
(70,100 ha) was more than 20 times larger than any bum
in northern Baja California between 1920 and 1972
(Minnich 1989).
Most of the research on fire and chaparral succession indicates that different fire regimes may select
against individual plant taxa depending on their reproductive biology and capacity to reestablish themselves
between fire events (Noble and Slatyer 1980). For
example, fire intervals of less than 10 years can rapidly
eliminate certain chaparral shrubs, seriously affect soil
Minnich, R.A. and Bahre, C.J.
0
2
4
6
8
IOU
Year of Fire
Figure 1. The McCain Plateau, showing chaparral bums and the location of field samples.
seed reservoirs (Sampson 1944; Howe and Carothers
1980; Gautier 1982; Zedler et al. 1983), and change
sprouting success rates, especially in taxa that experience relatively high fire mortality rates such as
Adenostoma fasciculatum (Howe and Carothers 1980;
Zedler 1981; Zedler et al. 1983). Furthermore, large
fires may harm population levels of non-sprouting
shrubs that have suffered heavy soil seed predation
such as Ceanothus greggii and Arctostaphylos glauca
(Keeley 1977).
Minnich (1983, 1989) has demonstrated a clear
difference in fire regimes across the United States Mexico boundary. Because the changes in fire mosaics
are so dramatic at the border (Figure l), we decided to
examine how disparate fire regimes have influenced
succession in the chaparral spanning the international
boundary between the border towns of JacumC and
Tecate. These chaparral communities were homogeneous before they were separated by the boundary
fence, the international fire break, and different types
of wildfire management.
Study Area
The study area is a narrow zone extending less than
15 km on both sides of the United States-Mexico
boundary between JacumC and Tecate in the middle of
the McCain Plateau, an old, undissected, erosional
surface of the Peninsular Ranges (Figure 1). Elevations
along the boundary in the study area range from 1100
to 1200 m. The Laguna Mountains lie 40 km to the
north and the Sierra Juirez are 50 km to the south.
Substrate is mostly tonalite and undifferentiated granites overlain by shallow acidic lithosols of decomposed
granite (Gastil et al. 1975).
The climate, classified as mediterranean, is characterized by winter precipitation from frontal storms
moving off the Pacific Ocean and protracted summer
drought. Annual precipitation is approximately 40 cm
(Goodridge 1981). Winter temperatures are mild during daylight but because of frequent ground inversions
often drop below freezing at night. Summer afternoon
temperatures generally exceed 30-35" C. Prevailing
winds are from the west (Ryan 1983).
- Wildland Fire and Chaparral Succession Along the California-Baja California Boundary The dominant vegetation is chamise (Adenostoma
fasciculatum) chaparral with patches of red shank (A.
sparsifolium) chaparral along arroyo bottoms and in
swales. Basin floors are covered by meadows, mostly
of exotic annual grasses, and are rimmed by woodlands
of coast live oak (Quercus agrifolia).
Methodology
To compare wildfire occurrence and post-fire chaparral succession across the United States-Mexico international boundary we used U.S. Forest Service (USFS)
fire maps and repeat aerial photography to date and plot
bums that occurred between 1920 and 1990 onto
Mexican DETENAL topographic maps (Mexico 1981).
None of our bum sites is more than 15 km from the
border; in fact, the majority are within 2 km of the
frontier. We stratified the bums into different ageclasses and sampled each age-class to develop successional chronosequences.
The fire history of the southern California sites was
obtained from fire maps compiled for San Diego
County since 1911 largely by the USFS (Cleveland
National Forest) with a scale of 1:125,000, while the
fire history for our Baja California sites was reconstructed by identifying bum scars in aerial photographs
flown in 1938 by Aerofoto Mexicana, and in 1956 and
1972 by INEGI (Mexico 1981). Bums after 1972 in
both countries were plotted from color photomaps
prepared by the U.S. Geological Survey for the Department of Treasury and from Landsat imagery (Minnich
1983). Each of the color photomaps covers an area
extending 7 km on either side of the border.
Each bum site was identified in the aerial photographs using a Bausch and Lamb rollfilm stereoscope
and by scale-matching (registering) sequential aerial
photographs on a Stereo Zoom Transfer Scope (ZTS)
(Minnich 1983, 1984, 1989).
While the year of each bum on the United States
side of the border is noted on the USFS maps, the year
of each bum in Mexico was bracketed from the date
of each set of aerial photographs. Since our field
observations indicate that chaparral species reach full
height but not full cover or density within 20 years, we
classified the burns in each aerial photo set into three
6-year age-classes based upon shrub maturity and
cover (recent bum [less than 6 years before] = shrubs
not evident; moderately recent bum [between 6 and 12
years] = shrubs form open coverJstature partially mature; old bum [between 12 and 18 years] = shrubs
contiguous/stature fully mature). According to this
method, bums in the 1938 aerial photographs may have
15
occurred as early as 1920 (Minnich 1989). Once
collected, fire history data were entered into the ARC/
INFO Geographic Information System (GIs) and fire
references were composited to show the most recent
bums (i.e, earlier bums were deleted in multiple fire
overlap zones).
To develop successional chronosequences, we
sampled 35 bum sites of different age-classes (19 on
the Mexican side and 16 on the United States side)
(Figure 1). Site selection was based on fire history and
access. Fourteen of our bum sites were immediately
adjacent to the border. Every site was in chamise
chaparral on slopes of less than 5". For each sample,
a 50 m transect line was laid out at right angles to local
contours. Transects were inititated by a blind rock toss.
Along each transect, we recorded species composition,
stature, and stem densities by point-quarter and lineintercept methods (Cottam and Curtis 1956; MuellerDombois and Ellenberg 1974). Five pairs of transects
were placed on opposite sides of the boundary monument markers perpendicular to the border (monuments
236, 237, 238, 239, and 240); the remaining transects
were obtained within 15 km of the boundary.
The height of each shrub (0.1 m resolution) was
recorded along each transect. The average mass shrub
height (SH
for)the entire transect was obtained by
multiplying shrub heights by the length of the intercept,
then dividing that figure by the total intercept lengths.
In the point-quarter method, points were taken at 5 m
intervals (10 total) representing the center point of four
90" quarters. In each quarter, the distance to the nearest
shrub root crown was recorded and the shrub was
identified. Using the combined distances, the absolute
density of each species was obtained (Risser and Rice
1971).
Repeat ground photographs of the monument markers have been used along the United States-Mexico
boundary to determine vegetation changes since the
1890s (Hastings and Turner 1965; Bahre and Bradbury
1978; Gehlbach 1981; Humphrey 1987; Bahre 1991).
Between 1892 and 1894 members of the International
Boundary Commission (1898) relocated and built new
monument markers along the international boundary
from El Paso to the Pacific Ocean. D.R. Payne, who
was the official photographer for the Boundary Commission, took photographs of the markers in the study
area in 1893. To gauge long-term vegetational change
in the past 99 years, we took repeat photographs of
Payne's original scenes of markers 236 through 240. In
addition, we examined land-use records and interviewed local people on both sides of the border to
evaluate historical land-use impacts, especially grazing
and wood collecting, on our sites.
16
Minnich, R.A. and Bahre, C.J.
Results
Our repeat photographs of the boundary monument
markers (5 sets) revealed little change in the structure
of chamise chaparral along the border in the past 99
years in spite of evidence of past bums (Figure 2). In
fact, outside of clearing in specific areas, we could not
identify major changes in the physical appearance of
the chaparral near any of the markers.
Our land-use histories and interviews on both sides
of the border indicate that while there was some
grazing on recent bums and in fire breaks, the impact
of livestock grazing on chamise chaparral was minimal
in the study area, largely because the carrying capacity
of chamise chaparral is so low that livestock on both
sides of the border depend almost entirely on irrigated
pasture and/or supplemental feeding (Mexico n.d.).
Chamise chaparral continues to be cleared for settle-
Figure 2. These repeat photographs of Monument 238, which look southeast towards Mexico, show little, if any, change in the
chaparral vegetation since 1893. In the 1893 photograph (A), the chaparral degetation is dominated by Ademstoma fasciculatum,
although scattered Rhus ovata can be seen in the right foreground and inthe front of the workers in the right midground. Dead stems
above the canopy indicate that the site was burned some 10 to 15 years before the photograph was taken. In the 1992 photograph (B),
the vegetation is still dominated by Ademstoma fasciculatum. Scattered Ceanothus greggii grow in the right foreground and a
fewEriogonum fasciculafurn are blooming to the left of the monument. The rounded canopy of a large Arctostaphylospungens can
be seen in the background just to the right of the monument. The view to the southeast in both images shows extensive stands of
chaparral on the Mexican side of the boundary. However, stand patchiness from bums before 1893 cannot be ascertained because
of the poor quality of the earlier photograph.
17
- Wildland Fire and Chaparral Succession Along the California-Baja California Boundary -
scattered annual dicots or exotics like Bromus rubens,
B. tectorum, and Erodium cicutarium.
Bums after 1986 (Table 2) were dominated by
resprouters and seedlings of both chaparral and coastal
sage scrub. The prominent facultative sprouters were
Adenostoma fasciculatum and Arctostaphylos
glandulosa, while the major obligate sprouters were
Rhus ovata, Quercus dumosa, Rhamnus crocea, and
Yucca schidigera. We observed skeletons of
nonsprouting Ceanothus greggii, Arctostaphylos glauca,
and A. pungens in several bums.
Adenostoma fasciculatum made up most of the
seedlings in recent bums with lower numbers of
Ceanothus greggii and Arctostaphylos glandulosa
being locally abundant. No recruitment was observed
among the fleshy-fruited obligate sprouters Quercus
dumosa, Rhus ovata, and Rhamnus crocea. Seedlings
of the subshrubs Eriogonum fasciculatum and Lotus
scoparius were also abundant. Except for the domi-
ment on both sides of the border and some chaparral
shrubs are used for fuel and construction in Mexico
(Freedman 1984), but none of our sites showed any
obvious evidence of being cleared or harvested in the
recent past. Mechanized removal of chaparral in some
areas, however, has destroyed resprouting shrubs and
soil seed pools, making it difficult for chaparral to
recolonize (Freedman 1984).
The dominant species in all of our sample sites was
Adenostoma fizsciculatum (Table 1). Less significant
sclerophyllous shrubs were Ceanothus greggii,
Arctostaphylospungens, A. glauca,Rhus ovata, Quercus
dumosa, and Adenostoma sparsifolium. Also, noted at
the sites were scattered leaf and stem succulents such
as Yucca schidigera, Y. whipplei, and Opuntia parryi,
as well as certain subligneous subshrubs common to
coastal sage scrub - Eriogonum fasciculatum, Lotus
scoparius, and Keckiella antirrhinoides. For the most
part, herbaceous cover was absent or consisted of thinly
Table 1. Chaparral cover by species (percent ground cover).
Sample Year of
Bum
No.
Mexican Side
29
1990
25
1988
20
1987
Species1
Af
As
20
18
15
Cg
Ap
Ag
A1
Ro
Qd
Rc
Ys
Yw
Ef
Ls
Ka
El
Op
OT TOT
1
United States Side
08
1989
Species: Af, Ademstoma fasciculatum; As, Adenostoma sparsifolium; Cg, Ceanothus greggii; Ap, Arctostaphylos pungem; Ag, A. glauca; Al, A.
glandulosa; Ro, Rhus ovata; Qd, Quercus dumosa; Rc, Rhamnus crocea; Ys, Yucca schidigera; Yw, Y . whipplei; Ef, Eriogonum fasciculatwn; Ls,
Lotus scoparius; Ks, Keckiella antirrhinoides; El, Eriodictyon lanuturn; Op, Opuntia parryi; OT (other), Ceanothus leucodermis, C. oliganthus,
Prunus ilicifolia, Salvia clevelandii, Opuntia littoralis, Xylococcus bicolor, Cercocarpus betuloides.
* = Estimated bum date (mid-point of 6 yr = age class) based on aerial photo date and chaparral successional status on the photo (see text).
Minnich, R.A. and Bahre, C.J.
18
Table 2. Frequenc of resprouts and seedlings in bums from
1987-1990 (100 m-Y).
Sample Year of Species1
No.
Bum
Af Cg A1 Ro Qd Rc Ys Ef Ls Ka OT TOT
Mexican Side
Resprouts
29
1990
25
1988
20
1987
Seedlings
29
1990
25
1988
20
1987
50
9
21
1 7 9
8 5
17 11
United States Side
Resprouts
08
1989 34
10
1987 74
1
1
17
3
4
1
4
2
1 38
78
Species: Af, Adenostoma fasciculatwn; Cg, Ceanothus greggii; Al,
A. glandulosa; Ro, Rhus ovata; Qd, Quercus dumosa; Rc, Rhamnus
crocea; Ys, Yucca schidigera; Ef, Eriogonumfasciculatum; Ls, Lotus
scolparius; Ks, Keckiella antirrhinoides; OT (other), Ceanothus
leucodermis, Prunw ilicifolia.
nance of seedlings of Eriogonum fasciculatum at site
20 (192 per 100m2), resprouting stems were more
frequent than seedlings in the other burns. Adenostoma
fasciculatum dominated total stem frequency in recent
bums.
Although all of our stands in the chronosequences
were dominated by Adenostoma fasciculatum with
Ceanothus greggii being locally abundant, some small
shifts in species composition occurred from the young1). Several stands burned
est to oldest stands
after 1979 contained coastal sage scrub species such as
Eriogonumfasciculatum, Keckiella antirrhinoides,Lotus
a able
scoparius and the ruderal Eriodictyon lanatum. Of
these, only Eriogonum fasciculatum occurred in old
bums, usually in large gaps or cleared areas.
Ceanothus greggii was occasional to common in
bums after 1950. An inventory of dead shrubs (Table
3) shows that most of the dieback consisted of C.
greggii and Adenostoma fasciculatum. The highest
mortalities were in pre-1958 bums. However, recent
mortality could account for the paucity of Ceanothus
greggii in certain old stands because almost 20% of the
stems in some of the old burn sites were dead. The
greatest mortality rates in old bums occurred in
Adenostoma fasciculaturn. Nevertheless, this is unlikely the result of recent dieback because mortality
represents only about 1% of the stems. Sample 18,
which has numerous dead Ceanothus oliganthus, was
the only recent burn site with evidence of dieback. No
difference was found in shrub mortality across the
international boundary. Old growth in bums before
1945 consisted almost entirely of Adenostoma
fasciculatum and a few scattered Quercus dumosa,
Rhus ovata, Arctostaphylos spp., and Eriogonum
fasciculatum.
Stem densities in our samples varied from 3 1 to 244
per 100mZ (Table 4). Most of the stems were
Adenostoma fasciculatum. Other species had 10 or
fewer stems per 100mZ. There appears to be no
relationship between stem density and stand age on
either side of the border or across it (southern California, r = 0.14, f- value = 0.264; Baja California, r = 0.17,
f-value = 0.557).
Our repeat aerial photography shows that after a
fire, chamise chaparral takes a few decades to mature
Table 3. Cover of dead shrubs (percent)'.
Sample
No.
Sample Year of
No.
Bum
Mexican Side
' Shrubs retain fine stems indicative of mortality within the past 1-2years.
Year of
Bum
TOT
United States Side
Af, Adenostoma~asciculatum;Cg, Ceanothusgreggii;Ap, Arctostaphylospungens; Ef, Erigonwn fasciculatum; OT (Other), Ceanothus oliganthus,
Xylococcus bicolor.
* = Estimated bum date (mid-point of 6 yr = age class) based on aerial photo date and chaparral successional status on the photo (see text).
- Wildland Fire and Chaparral Succession Along the California-Baja Califomia Boundary Table 4. Frequency of Chaparral Stems (100 rn-'-).
Sample Year of
No.
Bum
Species
Af
As
Cg
Ap
Ag
A1
OT
TOT
Mexican Side
29
90
67
25
88
17
20
87
38
30
85
10
16
74
37
22
73
95
19
69
26
03
68
85
24
58*
78
23
52*
22
05
50*
46
28
45*
114
12
45*
109
27
41*
64
26
35*
60
14
35*
37
01
30*
78
32
30*
93
21
15^*
46
United States Side
08
89
48
10
87
78
07
79
40
18
75
31
11
72
51
33
58
55
09
58
80
34
53
38
02
45
38
Species: Af, Adenostoma fasciculatum; As, Adenostoma sparsijolium; Cg, Ceanothus greggii; Ap, Arctostaphylos pungens; Ag, A. glauca; Al, A.
glandulosa; Ro, Rhus ovata; Qd, Quercus dumosa; Rc, Rhamnus crocea; Ys, Yucca schidigera; Yw, Y . whipplei; Ef, Eriogonum fasciculatum; Ls,
Lotus scoparius; Ks, Keckiella antirrhinoides; El, Eriodictyon lanatum; Op, Opuntia parryi; OT, (other), Ceanothus leucodermis, C . oliganthus,
Prunus ilicifolia, Salvia clevelandii, Opuntia littoralis, Xylococcus bicolor, Cercocarpw betuloides.
2.00 -
from open to contiguous cover on both sides of the
frontier. In our field samples, burns of less than 5 years
have an open cover of non-contiguous shrubs. Stand
cover appears to increase hyperbolically to 85% after
ca. 10 years and then more slowly with increasing stand
age (Figure 3). The F-ratio for comparison of similarity of regressions of the two groups (= 2.06, P-value
= 0.14) shows that the slopes or intercepts do not differ
beyond chance between Baja California and southern
California. SH also increases hyperbolically with stand
age (Figure 4). SH reaches about 1.25 m after 10 years,
then increases slowly. Most stands have uniform
stature and variability is largely a result of differences
in species composition. For example, tall stands have
more arboreal species such as Quercus dumosa and
Rhus ovata. The F-ratio for comparison of Baja
California and southern California differences are not
significant (= 1.16, p-value, 0.3).
.
-
1.75
A
1SO
A
A
-
A
A
A
*
'
A 4 - . - - ~ - ~ - -
Baja California
5
>
.
1.00-
oh
0
I
1
I
I
I
I
I
I
10
20
30
40
50
60
70
80
Time (yr)
Figure 3. Stand shrub wver (percent) versus stand age since
last fire for southern California (dots) and Baja California
(triangles). Southern Califomia: multiple R = 0.83, R2 = 0.69,
p-value = 0.0003. Polynomial regression, (l/CVi) = 0.013 +
0.092 (ITYEAR ). Baja California: multiple R+ 0.74, R2 =
0.55, p-value 0.d001. Polynomial regression, (l/CVi) = 0.015
+ 0.052 (INEAR,). F-ratio for wmparison of similarity of
regression of two groups = 2.062, p-value = 0.144.
Minnich, R.A. and Bahre, C.J.
20
Southern California
.
. .
A
Y
A
A
A
-
0
10
20
30
40
50
60
70
80
Time (yr)
Figure 4. Shrub height (m) versus stand age since last fire for
southern California (dots) and Baja California (triangles).
Southern California: multiple R = 0.84, R~ = 0.71, p-value <
0.0001. Polynomial regression, (l/SH,) = 0.657 + 1.272 (11
YEAR.). BajaCaliiomia: multipleR=0.81, R2=0.66, p-value
< 0.0061. Polynomial regression, (1/SH ) = 0.683 + 1.688 (11
YEAR,). F-ratio for comparison of sirni/arity of regression of
two groups - 1.263, p-value = 0.297.
Discussion
In general, fires are smaller and more frequent in
Baja California than in southern California. A 1938
aerial photograph mosaic of the study area (Figure 5)
shows fine-grainedpatchiness in chaparral stands south
of the international boundary and coarse, large patches
of homogeneous mature chaparral to the north. In our
study area (Figure 1) we counted 204 stands in an area
of 445 km2 south of the boundary and 67 stands in an
area of 773 km2 north of the boundary. Similar
differences in chaparral stand densities have been
identified by Minnich (1989) for a much larger area
spanning the California-BajaCalifornia boundary. Most
important, the densities shift immediately at the border.
In Baja the fragmentation of charnise chaparral stands
precludes large fires because the fires occur primarily
in old growth stands and stop at, or bum around,
younger stands that lack fuel. On the other hand, in
southern California, the infrequency of bums, because
of fire suppression and the similarity of the stand-age
structure, leads to less frequent, larger fires (Minnich
1983,1989).
The difference in fire regimes across the border in
the study area is clearly not the result of contrasting
climatic or other physical environmental factors because the McCain Plateau is too small and too homogeneous for the development of distinctive surface
wind patterns on opposite sides of the border and the
discontinuity in the fire pattern across the border is
clearly delineated by the international fire break that
parallels the border on the United States side. The
difference is simply the result of effective fire suppression (California) versus little or no fire suppression
(Baja California).
Few fires now spread across the internationalboundary because the prevailing winds during the fire season
come from the west-southwest and the fires, which
usually spread parallel to the border, are suppressed
near or at the international fuel break.
Despite differences in the present fire regimes, our
repeat photographs of the monument markers, field
samples, repeat aerial photography, and fire history
maps show that chaparral succession is similar across
the international boundary between JacumC and Tecate
and that chaparral succession along the border is
similar to that found elsewhere in Califomia (Sampson
1944; Horton and Kraebel 1955; Patric and Hanes
1964; Wilson and Vogl 1965; Hanes 1971; Vogl and
Schorr 1972; Keeley et al. 1981).
Our aerial photography reveals that post-1972 burns
have less cover than pre-1972 bums, and that burns
tend to blend in with surrounding mature stands after
30 or 40 years. For example, the burns recorded in the
1938 aerial photography (Figure 5) could not be distinguished from surrounding older stands in the 1972
aerial photography (Minnich 1989).
As indicated in our chronosequences of cover and
young stands have an
average mass shrub height
open cover of resprouting stems and a closed canopy
does not form for 10 to 20 years after a fire when
sprouts and seedlings mature at heights of 1.5-1.7 m
(Biswell 1974). Mature stands in the study area have
SHs of 1.3-1.7 m and covers of 40 to 80%. Summary
data on biomass, cover, and %Is (Green 1982; Rundel
1983) indicate that above-ground biomass of chamise
chaparral along the border ranges from about 20-50
tons ha-'. Stem densities in mature stands are comparable to stands in the rest of California (Keeley and
Keeley 1988). Seedlings of coastal shrub species,
which are common in recent bums, decline in stands
that are 20 or more years old, apparently because they
are shaded out by the chaparral overstory (McPherson
and Muller 1967; Gray 1983). Some coastal sage
shrubs, however, such as Eriogonum fasciculatum,
persist in large gaps in the canopy or in once-cleared
areas such as the international fire break.
We hypothesize that chamise chaparral is stable
under different fire regimes across the international
boundary because sprouting habits and latent seed
pools permit efficient stand establishment under short
and long fire intervals (Keeley 1981; Reid and Oechel
1984; Keeley 1989). Chamise chaparral can withstand
fire intervals as short as 10 years because of high
survivorship of sprouters and development of the replacement seedbank in the soil (Reid and Oechel1984).
(r~,
- Wildland Fire and Chaparral Succession Along the California-Baja California Boundary
-
21
Figure 5. 1938 aerial photograph mosaic of the border in the vicinity of monuments 238 and 239 (north to top right), showing the
international fuel break (light narrow strip, middle left), a road paralleling the border to the east, numerous bums (light patches) in
Mexico, and homogeneous mature chaparral in the United States. Only one burn crosses the international boundary near the photo
center. The mosaic covers an area of ca. 10 km x 7 km. Source: Aerofoto Mdxicana, Mdxico D.F.
Minnich, R.A. and Bahre, C.J.
Our field sampling and interpretations from aerial
photographs provide no evidence that old growth
chamise chaparral, extending as far back as 1915, is
degrading or converting to some other vegetation type
(cf. Hedrick 1951). Chaparral recovery from fire is
unrelated to fire size because recolonization of bums
by long range seed dispersal is a trait of only one shrub
species in the study area Cercocarpus betuloides - but
it also re-establishes itself by sprouting. Hence, all of
the other shrub species either sprout or have seed
available in the burn site.
The homogeneity in chamise chaparral composition
in our samples precludes assessment of changes in
shrub flora through fire sequences. It is possible that
variability of Ceanothus greggii is because of local
fluctuations in fire intensity (Riggan et al. 1988; Moreno
and Oechel 1991a). Likewise, the large range in
Adenostoma fasciculatum stem densities may reflect
rates of seedling recruitment after fires (Moreno and
Oechel 1991b). Whether mortality rates cause longstanding changes in chamise chaparral is unclear because fire intensities, being a function partially of local
weather, are extremely variable at local scales (Davis
et al. 1989). However, our recent aerial photographs
and field transects show no permanent decline after
1938 in chaparral cover in Baja California, despite the
high number of bums and the potential for shortinterval overlap zones.
Average fire return intervals in chaparral in Baja
California and southern California are nearly identical
(ca. 50-80 years) (Minnich 1983, 1989). Although the
size distribution of fire patches in chaparral in Baja is
different from that of California, the proportion of
young to old growth patches adds up to the same
amount of area in both states (Minnich 1989).
The incidence of anthropogenic fires does not
necessarily affect long-term fire regimes in chaparral.
Increases in the number of fires reduce fire sizes but
do not necessarily affect regional burning rates (Minnich
1983, 1989). Zedler et al. (1983) show that the
establishment of exotic annuals, combined with aerial
seeding of Lolium multflorum, encouraged a 2-year
fire recurrence and destruction of shrub cover in coastal
San Diego County. However, flashy herbaceous cover
is too sparse in the chaparral along the boundary
between JacumC and Tecate to affect the fire regime.
Without fire control in southern California, fire intervals of less than 20 years occur in only 3% of stands
and recurrences of less than 6 years are rare (Minnich
1989).
Conclusions
Chaparral structure and succession are little affected by differences in fire sizes or fire return intervals
across the international boundary. Usually only old
growth stands burn and potentially destructive short
fire intervals are precluded by the the scarcity of
herbaceous cover. What is clear from our study is that
different fire regimes in Mexico and the United States
have had no effect on succession in the chamise
chaparral communities spanning the border between
JacumC and Tecate.
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