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. References Bahre, C.J. and D.E. Bradbury. 1978. Vegetation change along the Arizona-Sonora boundary. Annals of the Association of American Geographers 68: 145-165. Bahre, C.J. 1991. Legacy of change: historic human impact on vegetation in the Arizona borderlands. University of Arizona Press. Tucson. 23 1 pp. Biswell, H.H. 1974. Effects of fire on chaparral. In: Fire and ecosystems (T.T. Kozlowski andC.E. Ahlgren, eds.), pp. 321-364. Academic Press. New York. 524 pp. Chou, Y.H., R.A. Minnich and R.J. Dezzani. 1993. Do fire sizes differ between southern California and Baja California? Forest Science 39:835-844. Cottarn, G. and J.T. Curtis. 1956. Theuse of distancemeasures in phy tosociological sampling. Ecology 37:45 1-460. Davis, F.W., M.I. Borchert and D.C. Odion. 1989. Establishment of microscalevegetation in maritime chaparral after fire. Vegetatio 8453-67. Freedman, J.R. 1984. Uncontrolled fire and chaparral resilience in the Sierra Juirez, Baja California, MBxico. M.S. thesis. University of Califomia, Riverside. Gastil, R.G., R.P. Phillips and E.C. Allison. 1975. Reconnaissance geology of the state of Baja California. Geological Society of America Memoir 140. Gautier, C.R. 1982. The effects of rye grass on erosion and natural vegetation recovery after fire. In: Dynamics and management of mediterranean-type ecosystems (C.E. Conrad and W.C. Oechel, tech. words.), p. 299. United States Department of Agriculture, Forest Service, GeneralTechnica1Report PSW-58. Pacific Southwest Forest and Range Experiment Station. Berkeley. Gehlbach,R.R. 1981. Mountainislands anddesert seas. Texas A&M Press. College Station. 298 pp. Goodridge, J.D. 1981. Califomia rainfall summary: monthly total precipitation, 1849-1980. State of Califomia, Resources Agency. Department of Water Resources. Sacramento. 43 pp. Gray, J.T. 1983. Nutrient use by evergreen and deciduous shrubs in southern California. I. Community nutrient cycling and nutrient-use efficiency. Journal of Ecology 71:21-41. - Wildland Fire and Chaparral Succession Along the California-Baja California Boundary Green, L.R. 1982. Prescribed burning in the California mediterranean ecosystem. In: Dynamics and managementof mediterranean-typeecosystems (C.E. Conrad and W.E. Oechel, tech. coords.), pp. 464-471. United States Departmentof Agriculture,Forest Service,GeneralTechnical Report PSW-58. Pacific SouthwestForest andRange Experiment Station. Berkeley. Hanes, T.L. 1971. Succession after fire in the chaparral of southern California. Ecological Monographs 41:27-52. Hastings, J.R. and R.M. Turner. 1965. The changing mile: an ecological study of vegetation change with time in the lower mile of an arid and semiarid region. University of Arizona Press. Tucson. 3 17 pp. Hedrick, D.W. 1951. Studies on the succession and manipulation of chamise brushlands in California. Ph.D. dissertation, Texas A&M College, College Station. Horton, J.S. and C.J. Kraebel. 1955. Development of vegetation after fire in the chamise chaparral of southern California. Ecology 36:244-162. Howe, G.G. and L.E. Carothers. 1980. Postfire seedling reproductionofAdenostomafasciculatum.Bulletinof the Southern California Academy of Sciences, 79:5-13. Humphrey, R.R. 1987. 90 years and 535 miles: vegetation changes along the Mexican border. University of New Mexico Press. Albuquerque. 449 pp. International BoundaryCommission (United States andMexico, 1892-1896). 1898. Views of the monuments andcharacteristic scenes along the boundary between the United States and Mexico west of the Rio Grande, 1892-1895. Album in: Report of the Boundary Commission upon the survey andremarking of theboundary betweentheunited States and Mexico west of the Rio Grande, 1891-1896. 2ndSession. GPO. Senate Document247,55thCongress, Washington, D.C. Keeley, J.E. 1977. Seed production, seed populations in soil, and seedling production after fire for two congeneric pairs of sprouting and non-sprouting chaparral shrubs. Ecology 585320-829. Keeley, J.E. 1981. Reproductive cycles and fire regimes. In: Fire regimes and ecosystem properties: proceedings of the conference (H.A. Mooney et al., tech. coords.), pp. 231-277. United States Department of Agriculture, Forest Service, General Technical Report WO-26. Washington, D.C. Keeley, J.E. 1989. Seed germination and life history syndromes inCalifornia chaparral. Botanical Review 57:8 1117. Keeley, S.C., J.E. Keeley, S.M. Hutchinson and A.W. Johnson. 1981. Post-fire succession of the herbaceous flora in southern California chaparral. Ecology 62: 1608-1621. Keeley, J.E. and S.C. Keeley . 1988. Chaparral. In: North American terrestrial vegetation (M.G. Barbour and W.D. Billings, eds.), pp. 166-207. Cambridgeuniversity Press. New York. 434 pp. Lockmann, R.F. 1981. Guarding the forest of southern California: evolving toward conservation of watershed, woodlands, and wilderness. Arthur H. Clark Company. Glendale, Califomia. 184 pp. 23 McPherson, J.K. and C.H. Muller. 1967. Light competition between Ceanothus and Salvia shrubs. Bulletin of the Torrey Botanical Club 9441-55. Mbxico, Secretaria de Agricultura y Recursos Hidrhlicos. Comisi6n TBcnica Consultiva para la Determinaci6nRegional de 10s Coeficientes de Agostadero (COTECOCA). n.d. Determinaci6n de 10s coeficientes de agostadero del ejidos: Baja California, El Encinal, Jacum6 y Sierra de Juhez. MBxico, Secretaria de Programaci6n y Presupuesto. Instituto Nacional de EstadisticaGeogifica y Informativa(INEGI). 1981. Cartas Topogr@cas (escala: 1:50,000): IllD62 (Tecate). IllD63 (La Rumorosa). Minnich, R.A. 1983. Fire mosaics in southern California and northern Baja California. Science. 219:1287-1294. Minnich, R.A. 1984. Snow drifting and timberline dynamics on Mount San Gorgonio, California. Arctic and Alpine Research 16:395-412. Minnich, R.A. 1987. Fire behavior in southern California chaparral before fire control: the Mount Wilson bums at the turn of the century. Annals of the Association of American Geographers 77:599-618. Minnich, R.A. 1988. The biogeography of fire in the San Bernardino Mountains of Califomia: a historical study. University of California Publications in Geography. Vol. 28. University of California Press. Berkeley. 120 pp. Minnich, R.A. 1989. Chaparral fire history in San Diego County and adjacent northern Baja California: an evaluation of natural fire regimes and the effects of suppression management. In: The chaparral: paradigms reexamined (S.C. Keeley, ed.,) pp. 37-47. Vol. 34, Science Series, Natural History Museum of Los Angeles County. Los Angeles. 171 pp. Moreno, J.M. and W.C. Oechel. 1991a. Fire intensity effects on germinationof shrubs and herbs in southern California chaparral. Ecology 72: 1992-2004. Moreno, J.M. and W.C. Oechel. 1991b. Fire intensity and herbivory effects on postfire resprouting of Adenostoma fasciculcztum in southern Califomia chaparral. Oecologia 85:429-433. Mueller-Dombois, D. and H. Ellenberg. 1974. Aims and methods of vegetation ecology. Wiley. New York. 547 PP. Noble, I.R. and R.O. Slatyer. 1980. The use of vital attributes to predict successional changes in plant communities subject to recurrent disturbances. Vegetatio 43:5-21. Patric, J.H. and T.L. Hanes. 1964. Chaparral succession in a SanGabrielMountain areaof California. Ecology 45:353360. Pyne, S.J. 1982. Firein America: acultural history of wildland and rural fue. Princeton University Press. Princeton, New Jersey. 654 pp. Reid, C. and W.C. Oechel. 1984. Effect of shrublandmanagement on vegetation. In: Shrublands in California: literature review and research needed for management (J.J. DeVries, ed.), pp. 25-41. University of California Water ResourcesCenter Contribution,Number 191.Davis, California. 146 pp. 24 Minnich, R.A. and Bahre, C.J Riggan, P.J., S. Goode,P.M. Jacks andR.N. Lockwood. 1988. Interaction of fire and community development in chaparral of southern Califomia. Ecological Monographs 58:155-176. Risser, P.G. and E.L. Rice. 1971. Phytosociological analysis of Oklahoma upland forest species. Ecology 52:940-945. Rundel, P.W. 1983. Impact of fire on nutrient cycles in mediterranean-typeecosystems with reference to chaparral. In: Mediterranean-typeecosystems: the role of nutrients (F.J. Kruger, D.T. Mitchell, and J.U.M. Jarvis, eds.), pp. 192-207. Springer-Verlag. New York. 552 pp. Ryan, B.C. 1983. Fire potential in California: an atlas for fire managers. In: Proceedings, Eighth Conference on Probability and Statistics in AtmosphericSciences,November 16-18,1983, Hot Springs, Arkansas. AmericanMeteorological Society. Boston. Sampson, A.W. 1944. Plant succession on burned chaparral lands in northern Califomia. Agricultural Experiment Station Bulletin 685. University of California, Berkeley. 144 pp. Vogl, R.J. and P.K. Schorr. 1972. Fire and manzanita chaparral in the San Jacinto Mountains, California. Ecology 53:1179-1188. Wilson, R. and R.J. Vogl. 1965. Manzanita chaparral in the Santa h a Mountains, Califomia. Madroiio 18:46-62. Zedler, P.H. 1981. Vegetation changein chaparral and desert communities in San Diego County, California. Pp. 406424 in D.C. West, H.H. Shugart, and D.B. Botkin (eds.). Forest succession: Concepts and application. SpringerVerlag, New York, N.Y. 517 p. Zedler, P.H., C.R. Gautier and G.S. MacMaster. 1983. Vegetation change inresponse to extreme events: the effect of a short interval between fires in California chaparral and coastal scrub. Ecology 64:809-818.
© Copyright 2026 Paperzz