The present article addresses the deterioration of 12 forest geosystems located in a mountain range to the west of Mexico City. On the basis of identification of plant species that are indicators of environmental deterioration, as well as application of a Deterioration Index that considers these species in relation to the richness, vertical structure, and total cover of the climax forest facies—representing the most developed and stable evolutionary stage of each geosystem—forest geosystems are classified according to the present stage of deterioration that affects the best preserved forests in each case.
The accelerated growth that characterizes several cities in the countries with transition economies leads to a rapid invasion of the agroforest fringes that limit them. Severe ecological degradation (including deterioration of the natural environment, air, water, and soil pollution, and hydrological and geomorphic imbalances) is reflected not only in the physical deterioration of forests but also in their capacity to mitigate future impacts (Berry et al 1994; Riebsame et al 1996). Mexico City's metropolitan area—comprising 22 million inhabitants—is a good example of this type of environmental dynamics. Its western border is located on the eastern slope of the Las Cruces mountain range (Figure 1). This slope, with an area of 832.8 km2 ranging up to 1500 m in altitude, features the ecological heterogeneity of temperate mountains (Ferreras 1987; Buzhuo et al 1997; Price and Thompson 1997; Forsyth 1998) as well as the high occupation indexes and changes in land use, which result in contrasting rural–urban landscapes and imbalances that affect forest systems.
Twelve forest geosystems
Figures 2 and 3 show an overview of the environmental aspects of the slope, allowing classification into the 12 geosystems described below. The climate and water availability are subordinated to the slope's morphological features. In the mountain area (MA), between 2800 and 3580 m (Lugo 1990; García-Romero 1998), precipitation exceeds 1300 mm/y, and monthly average temperatures are below 10°C, favoring a maximum water surplus of 800 mm/y. Below 2800 m, on the connecting ramp (CR) that links the MA with the bottom of the Mexico basin, the lack of large topographic features mitigates orographic rainfall and promotes warming, which increases toward the ramp's base, where the mean annual temperature exceeds 18°C and precipitation is below 800 mm/y. The low moisture retention capacity in some soils and fractured pyroclastic materials reduces water availability, so that the annual average does not exceed 200 mm.
As shown in Figure 3, the distribution of forests reflects an altitudinal pattern resembling “steps” or floors on a series of soils that are similar to the parental materials both in structure and chemical features (Wright 1972; FAO-UNESCO 1981, 1991):
Peak floor (>3400 m). Dominated by peak pine forest, situated near the timberline located at 4000 m (Troll 1971). These forests grow above intercalated humic Andosols and lithic Leptosols.
Montane floor (3200–3400 m). Dominated by fir and fir–pine forests that grow on sequences of humic Andosols and lithic Leptosols that are frequent in rocky scarps at the mountain border.
Transition floor (2700–3200 m). Dominated by mixed pine–oak forests growing on umbric Andosols, chromic Cambisols, and eutric Regosols, which evolve on compact lava and cemented pyroclastic flows.
Piedmont floor (<2700 m). Dominated by oak forests growing on sequences of umbric Andosols and eutric Regosols. Chromic Luvisols and eutric Vertisols coincide with loose and altered pyroclast deposits under seasonal but abundant pluviometric conditions that favor percolations and wash-away processes.
During the mid-19th century Mexico City's western border reached the base of the Las Cruces mountain range. The population increased from 70,000 inhabitants in 1940 to 1,700,000 in 1980 and 3,000,000 in 1990 (García-Romero 1998). Although the urban area had not reached the 2300 m altitudinal zone by 1960, it had invaded the inner portion of ravines by 1980 without a single efficient control measure. Since then, changes in land use have mostly consisted in the replacement of agricultural and forest uses by several kinds of residential uses—dispersed settlement, concentrated (village) settlement, and urban settlement—as well as commercial establishments and services, offices, and transport infrastructure.
The forest geosystems classification presented in previous studies (García-Romero 1998, 2001) is based on the analysis and cartographical synthesis (1:50,000) of a series of natural and cultural variables covering several aspects: morphostructures, climate, runoff, landform dynamics, soil types, plant formations, and land use (Steedman and Haider 1993). This classification allows the identification of 12 forest geosystems in the study area (Figures 1–3).
I. Cold and humid geosystem located on the Catedral and San Miguel mountain peaks, covered by mountain pine forests (Pinus hartwegii); land use: silviculture and herding.
II. Cold and very humid geosystem on the Catedral, Las Cruces, and San Miguel upper slopes, covered by dense Abies religiosa and P. hartwegii forests; land use: silviculture.
III. Cold and very humid geosystem on the Catedral, Las Cruces, and San Miguel middle slopes, covered by A. religiosa forests; land use: silviculture.
IV. Cold and very humid geosystem on the Las Cruces mountain peaks and the Catedral and San Miguel lower slopes, covered by A. religiosa and Garrya laurifolia mixed forests; land use: silviculture and herding.
V. Cold and very humid geosystem on the San Miguel lower slopes, covered by G. laurifolia forest; land use: agriculture, recreation, and dispersed and concentrated settlements.
VI. Cold and very humid geosystem on the Las Cruces lower slopes and the La Magdalena river valley, covered by Quercus rugosa and Q. crassipes forests; land use: agriculture and concentrated settlements.
V11. Moderately cold and humid geosystem on the ramp's northern sector, covered by P. leiophylla and Q. crassipes mixed forests; land use: agriculture, silviculture, herding, and concentrated residential land.
VIII. Cool and semihumid geosystem on the ramp's central sector upper slopes, covered by Q. crassipes forests; land use: agriculture, silviculture, and concentrated residential land.
IX. Cool and semihumid geosystem on the ramp's northern sector upper slopes, covered by Q. crassipes and P. leiophylla forests; land use: agriculture, silviculture, herding, and concentrated residential and urban land.
X. Cool and semihumid geosystem on the ramp's central and northern sector lower slopes, covered by Q. centralis and Q. castanea forests, where agriculture, silviculture, and herding have been abandoned in favor of dispersed residential land use.
XI. Temperate and subhumid geosystem on the ramp's lower slopes in the central and northern sectors, covered by low Q. hartwegii and Q. centralis forests, where agriculture, silviculture, and herding have been abandoned in favor of dispersed and concentrated settlements.
XII. Temperate and subhumid geosystem on the ramp's southern sector, covered by Q. centralis and Stevia salicifolia forests, where agriculture has been abandoned in favor of dispersed and concentrated settlements.
Lack of research
Despite the problems associated with reduction of the forest area to one third of its original surface (5% in the last decade) and forest fragmentation into disconnected and exposed patches prone to physical damage and pollution, published studies on environmental effects are rare. Existing studies include the work of Pezzoli (1998), who warns against the severe environmental degradation associated with the urban dynamics of southern and western Mexico City, and that of García-Romero (1998), who offers the classification system of mountain range forest geosystems mentioned above.
Although it is of considerable geographical interest, the ecological meaning of flora—especially herbs and shrubs—in forest communities is poorly understood because it is often barely obvious in the landscape. The present study assesses the extent of deterioration currently a