Study Area

The study area includes the municipalities of Tlalpan and Milpa Alta (19°04´-19°19´ N and 98°57´-99°19´ W), located in southern Mexico City (Figure 2). Both municipalities comprise an area of 613 km² and are part of the Trans-Mexican Volcanic Belt Province and the Lakes and Volcanoes of the Anahuac Subprovince (^{Mooser, 1996}; ^{Lugo, 1984}; ^{Bloomfield 1975}). The great variety of volcanic relief forms and steep bioclimatic gradients give rise to a wide environmental and landscape diversity, which increases when considering that the area has been profusely transformed throughout the history of land uses. Today, the pressures and dynamics of land use and occupation threaten the potential of the area as a supplier of water resources, which are essential for the functioning of the Mexico City urban system (^{Ruíz-Gomez, 2006}; ^{SEDEMA, 2016}), and for displaying great forest richness and unique natural characteristics, with important flora and fauna reserves of high landscape value. However, the recent literature does not report any cartography illustrating the diversity of environments with different potential and that limit development.

Stage 1. Environmental analysis and synthesis

It consists of the mapping of the differentiating landscape components (relief, vegetation, and land use) that are used for the characterization and typological and spatial definition of landscapes, as well as of the supplementary components that support the characterization of both the differentiating components and landscapes, at the various taxonomic classification levels (Figure 3).

To avoid topological errors in the spatial delimitation and overlap of units within and between the three landscape levels, the cartographic series is built at an intermediate scale (1:50 000) and with the same degree of resolution (minimum mapping area of 1 hectare) for all topics and classification levels. This requires designing processes for the analysis and synthesis of the information available in literature, statistical, and cartographic sources at different spatial and temporal scales. In other cases, automated procedures based on digital models are used, as well as visual interpretations based on satellite images and thematic cartography. The cartographic results are validated through field verification trips.

Stage 2. Environmental integration and landscape classification

It consists of the integration and reinterpretation of the cartography obtained for the differentiating components (relief, vegetation, and land use), and the spatial and typological classification of landscapes. The use of a single scale (1:50 000) facilitates the organic and spatial integration of landscape units at the three classification levels, so that the boundaries of a natural region coincide with those of the pattern of geosystems within its area, while the boundaries of a geosystem match those of the geofacies within it.

The reinterpretation of thematic information aims to identify environmentally homogeneous units at the three classification levels, in accordance with the resolution adopted by the differentiating components. It is obtained from visual (deductive and associative) interpretation techniques, which require a broad knowledge of the study area and experience by the interpreter, as well as a holistic and inclusive approach for the delimitation of landscape units.

In addition, consideration should be given to the environmental variability of supplementary components, and even of the original sources of information, to avoid topological errors when assigning categories. For instance, natural regions result from the combination of a morphostructure, a pattern of potential vegetation and a pattern of land-use systems; in the case of Tlalpan and Milpa Alta, reconsidering the geological history of the pattern of relief units was useful for setting the spatial boundaries and typology of landscapes (Figure 3).

The overlay of thematic layers is a procedure that produces a high number of residual polygons that do not meet the minimum mapping size required (one hectare); thus, it is necessary to make a mapping adjustment to integrate these polygons into units of greater representativeness or meaning.

Finally, landscape units are named according to descriptive criteria that refer to the abiotic, biotic, and anthropic elements that best represent unique landscape types. For the natural region and geosystem categories, the names assigned refer to the geographic environment of the landscape. This possibility is reduced in the case of geofacies because each category usually occurs in multiple polygons.

Stage 3. Classification validation

It consists of a manual and qualitative revision of the typology, distribution, and spatial boundaries of the landscape units obtained. To this end, a stratified random sampling is followed, which serves to check the quality and consistency of the cartography in relation to the actual characteristics observed in the field, aiming to make any adjustments required. A good level of knowledge of the study area is useful for performing this stage successfully. The procedure is carried out in a similar way for each taxonomic level.

Natural Region of Sierra de Las Cruces and Ajusco Volcano, with temperate forest and forestry and tourism uses

It includes the Ajusco volcano and a southern portion of Sierra de Las Cruces (25.86 km²), which are part of the polygenetic volcanic mountain ranges of the Trans-Mexican Volcanic Belt (^{Mooser, 1996}). These are volcanic morphostructures that formed from late Pliocene to Pleistocene. Orography plays a major role in the landscape, allowing a marked transition of climates and a pattern of potential vegetation characterized by various bioclimatic layers of temperate forest. Deep ravines run along the altitudinal gradient; however, the area is naturally stable, according to a pattern of low-impact land-use systems that include forestry and tourism activities. This landscape unit is organized into two geosystems.

A) Geosystem of Sierra de Las Cruces and Ajusco Volcano Slopes, with conifer forest and grassland, and forestry and tourism uses (22.34 km²). The relief unit comprises complex slopes (lava and pyroclastic material of andesitic-dacitic composition), modeled by fluvial erosion. Its altitudinal location (3200 to 3600 meters a.s.l.) corresponds to a semi-cold subhumid mesoclimate, and a potential vegetation of conifer forest, with pines and fir mixed and pure communities. Notwithstanding the limitations imposed by topography, the land-use system includes agriculture, logging and forestry, and tourism around the main accesses to the Ajusco volcano.

Anthropic geofacies are concentrated in small alluvial plains (3.3% of the area), being one of the best-conserved geosystems, with a high plant regeneration capacity that gives rise to a moderately diverse landscape pattern. It comprises 16 types of geofacies distributed in 35 polygons that include complex slopes with conifer forest (80% of the area) and domes and ravines with a diverse forest cover (Figure 5 and Table 2).

Source: Own elaboration.

Figure 5 Map of landscape geofacies of the Tlalpan and Milpa Alta municipalities. Boxes illustrate details of the pattern of landscapes in various gesystems.  

B) Geosystem of Ajusco Volcano Peak, with highland pine forest and alpine grassland, and tourism use (3.52 km²). The relief unit consists of an extensive area of peaks that range between 3600 and 4000 m a.s.l. (Cruz del Marqués). It was formed in the Pleistocene by the accumulation of andesitic and dacitic lavas and comprises a series of rocky ridges that are emblematic of the Ajusco volcano. The cold and subhumid mesoclimate corresponds to a potential vegetation of highland pine (Pinus hartwegii) open forest. Thermal limitations and poor accessibility restrain the land-use system, characterized by low-scale conservation and tourism activities.

The landscape pattern is defined by the simplicity of its internal organization, with only 4 types of geofacies distributed in seven polygons, mainly highland natural landscapes that include open grasslands growing on complex slopes, domes, and ravines (99% of the area) and scarce pine forest patches on complex slopes (Figure 5 and Table 2).

Natural Region of Chichinautzin Volcanic Field, with temperate forest, agriculture and forestry uses, and human settlements

This natural region (568.72 km²) is located on the morphostructure of the Chichinautzin Volcanic Field, which developed during the Quaternary from strombolian vulcanism, leading to one of the highest concentrations of monogenetic volcanoes in central Mexico. Its morphology is characterized by an extensive area of mountain peaks and slopes that reach a level of ~500 m relative to the Mexico’s Basin lowlands. It comprises 73 volcanic cones separated by lava spills and depressions covered with pyroclastic material and alluvial deposits, with gentle to moderate slopes (3º-15°). The local orography favors a temperate subhumid climate at the base of the mountain range and semi-humid at the summits, as well as a pattern of potential vegetation characterized by different bioclimatic layers of temperate forest.

The pattern of land-use systems reflects the influence of the distribution of lavas and alluvial deposits on the develop of forests, agriculture, and urban areas. In this region, the landscape is organized into three geosystems:

A) Geosystem of Peak of the Chichinautzin Volcanic Field, with conifer forest and agriculture, forestry, and tourism uses (282.01 km²). It is distributed in two extensive mountain peak areas separated by the vast Acopiaxco volcano agricultural area. The relief unit consists of the most extensive tefra and shield volcanic cones of the mountain range, including the Pelado volcano in Tlalpan, and the Chichinautzin, Tlaloc, and Cuatzin volcanoes in Milpa Alta. The altitudinal range of 3100 to 3600 m a.s.l. gives rise to a cold and subhumid mesoclimate, with a potential vegetation of conifer forest growing on poorly developed soils. The land-use system is forestry, with agricultural activities on floodplains with suitable soil.

The landscape pattern is diverse and fragmented, with 74 geofacies types distributed in 617 polygons. It is the best-conserved geosystem of the mountain range, with abundant forest geofacies, mostly pine forest patches growing over steep lava spills (23.8% of the area) and shield volcanoes (13.2%). However, the constraints imposed by the lava environment, forestry, and fires promote geofacies of open pine forest, alnus forest, grassland, shrubland, and bare soil. Agriculture is restricted to the interior of craters and small alluvial plains, while human settlements are located in geofacies of ancient lava spills (Figure 5 and Table 2).

B) Geosystem of High Slope of the Chichinautzin Volcanic Field, with mixed forest, agriculture and forestry uses, and human settlements (173.20 km²). It is distributed around mountain peaks, where the relief unit consists of gentle to steep lava slopes alternating with scattered shield volcanoes, domes, and volcanic cones. The altitude ranges from 2700 to 3100 m a.s.l. and corresponds to a semi-cold and subhumid mesoclimate with mixed pine-oak forest as potential vegetation. This context is characterized by plains of pyroclastic material and alluvial sediments, which were favorable for the historical development of small agricultural villages. In the second half of the 20th century, the growth of Mexico City and the construction of railroads and motorways that run across the study area favored the transformation of the land-use system as a result of population growth and urbanization of the old rural villages in both municipalities.

The diversity of environments and the anthropization of rural areas have led to a complex landscape pattern, with 76 geofacies types distributed in 512 polygons. The dominant geofacies comprise an array of crops and human settlements over lava spills (33.0% and 11.3%, respectively), while native pine-oak forest geofacies have been fragmented and transformed into open pine and alnus forests and shrubland due to fires and forestry (Figure 5 and Table 2).

C) Geosystem of Low Slope of the Chichinautzin Volcanic Field, with oak forest, agriculture and forestry uses, and urban development (113.50 km²). It comprises the base of the northern slope of the Chichinautzin mountain range, where the relief unit is defined by the lava spills located in the most remote areas within the mountain range, with few volcanic cones and remains of the ancient Xochitepec mountain range located in the Tlalpan municipality. The altitude of 2300 to 2700 meters allows a temperate and subhumid mesoclimate and a potential vegetation of oak forest. While in Milpa Alta the predominant land-use system is agriculture, along with larger human settlements concentrated in villages and high deforestation levels, the expansion of the Mexico City urban areas in Tlalpan completely displaced agricultural land uses; consequently, only a few natural geofacies have managed to survive in protected zones.

Heavy deforestation and urbanization processes in Tlalpan translate into a relative homogeneous landscape pattern, with only 27 geofacies types distributed in 114 polygons. The largest geofacies are human settlements on recent lava spills (35.8% of the area, concentrated in Tlalpan) and a mosaic of crops on recent lava spills (24.5% in Milpa Alta). In contrast, forest geofacies are restricted to a few fragments of oak and alnus forests, shrubland and grassland on recent lava spills, volcanic cones, craters, and the Teuthli shield volcano (Figure 5 and Table 2).

Natural Region of the Fluvial-Lacustrine Plateau of Mexico’s Basin Lowlands, with urban use

Mexico’s Basin is an extensive morphostructure (32.02 km²) of flat and multi-layer morphology that was formed in the Plio-Quaternary from the accumulation of volcanic, alluvial, and lacustrine sediments carried from the volcanic mountain ranges that border it (^{Mooser, 1996}). The subhumid and temperate climate, combined with the endorreic basin hydrology, favor a pattern of potential vegetation represented by different types of temperate forest. The large extension and heterogeneity of the basin translate into a complex pattern of land-use systems, which, however, is not apparent in the small area of the basin represented in the study area, where only a single geosystem is recorded.

a) Geosystem of Marginal Fluvial-lacustrine Plateau, with urban development (17.66 km²). The edges of the plateau in the Mexico’s Basin penetrate the territory studied in two areas: one north of Tlalpan (14.42 km²) and another north of Milpa Alta (3.24 km²). The relief unit consists of sloped plains formed in the Holocene from the accumulation of volcanic material and alluvial sediments from the Las Cruces (Tlalpan) and Chichinautzin mountain ranges. Its altitudinal range, between 2200 and 2300 m a.s.l., is suitable for a potential vegetation of oak forest, currently inexistent. The availability of stable land, soil, and water resources allowed the development of agriculture until the second half of the 20th century, when the expansion of Mexico City urban area transformed the land-use system, currently dominated by urban uses.

The small surface area and environmental homogeneity translates into the simplest landscape pattern of the whole study area, with seven geofacies types distributed in 14 polygons. While in Tlalpan the largest geofacies corresponds to the Mexico City urban area on the alluvial plain (77.4% of the area), a rural environment predominates in Milpa Alta, with a mosaic of crops (7.4%) and human settlements (5.3%).

The map of landscape types in Figure 5 shows 113 types of geofacies distributed in the six geosystems that make up the municipalities of Tlalpan and Milpa Alta. While geofacies is a landscape resolution level that integrates three components (relief form, vegetation type, and land use), to facilitate reading the map, the 113 geofacies types were pooled into five groups and 28 classes based on the most distinctive differentiating component (relief form, vegetation type, or particular land use).