Materials and methods

Study area. The Flora of Bajío includes that found in the states of Guanajuato and Querétaro in its entirety (46 and 18 municipalities respectively) and the northeastern part of the state of Michoacán (53 municipalities), delimited by the areas located east of the meridian 102° 10 ' W (Figure 1A, ^{Villaseñor & Ortíz 2012}). Xerophilous scrub has been recorded in large areas in the municipalities of San Luis de la Paz, Santa Catarina and Victoria in Guanajuato (Figure 2A, ^{Rzedowski & Galván 1996}). In Querétaro, this vegetation extends into the municipalities of Tolimán, Peñamiller and Cadereyta de Montes (Figures 1B and 2B, ^{Rzedowski et al. 2012}). The xerophilous scrub is found mainly in the Chihuahuan Desert, but also extends into areas within the Trans-Mexican Volcanic Belt in the foothills near Morelia, Michoacán (^{Contreras & Silva 2020}). Deciduous tropical forests are present in Querétaro and Guanajuato; in the first cover the municipalities of Jalpan and Landa, but isolated patches have been also recorded in Tequisquiapan, San Juan del Río and Querétaro (Figures 1B and 2C, ^{Zamudio et al. 1992}). In Guanajuato, this vegetation type has been observed in the ravines of the municipalities of Xichú, Atarjea and Victoria, which correspond to Sierra Gorda, which is part of the Sierra Madre Oriental, as well as some remnants toward the central area of the state (Figures 1B, 3A, and 3B, ^{Rzedowski & Galván 1996}, ^{Zamudio 2012}, ^{Rzedowski & Calderón 2013}). The thorn forests extend into the municipalities of San Juan del Río, Pedro Escobedo and Querétaro. In addition, there is a strip of thorn forests that extends toward the states of Guanajuato and parts of Michoacán, surrounding the Trans-Mexican Volcanic Belt (Figure 1B, ^{Rzedowski 2006}, ^{Zamudio 2012}). The Quercus forests, associated or not associated with conifers, such as Pinus and Abies, are distributed in Guanajuato on the Sierra Gorda, Sierra de Pénjamo and Agustinos; (Figures 1B and 3C, ^{Zamudio 2012}). In Querétaro, this type of vegetation is distributed toward the mountain ranges of Pinal de Amoles, San Joaquín, Arroyo Seco, Jalpan and Landa, extending toward Guanajuato in the municipalities of Xichú and Atarjea. Overall, the floristic composition indicates the influence of the Sierra Madre Oriental (^{Argüelles et al. 1991}, ^{Rzedowski et al. 2012}, ^{Zamudio 2012}). Quercus and Pinus forests have been recorded to a lesser extent toward Amealco, which includes the Sierra del Rincón, whose extension reaches the northern part of Michoacán (^{Cabrera-Luna et al. 2015}). The grassland is located in a small strip within the municipalities of San Juan del Río (Chihuahuan Desert), as well as in Amealco and Huimilpan (Trans-Mexican Volcanic Belt), although in some cases, the grassland was likely induced (Figure 3D, ^{Zamudio et al. 1992}).

Figure 1 A. Flora of Bajío and adjacent regions, Mexico. The gray line indicates the state division, the bold black line indicates the boundaries of the Bajío Region. B. Map of vegetation types. The background shade shows the altitudinal range (light = low, dark = high). The red line indicates the biogeographic provinces. The northeast polygon is the Sierra Madre Oriental, the central polygon is the Chihuahuan Desert, and the southwest polygon is the Mexican Transvolcanic Belt. The blue polygons are water bodies. 

Photo. D. Aquino (A, B), S. Arias (C).

Figure 2 Vegetation types in the Bajío and adjacent regions. A. Xerophilous scrub in Atarjea, Gto., and B. Cadereyta de Montes, Qro. C. Tropical deciduous forest in Jalpan, Qro.  

Photo. D. Aquino.

Figure 3 Vegetation types. A, B. Tropical deciduous forest in Xichú, Gto. C. Secondary grassland with Quercus forest in the background in Santa Catarina, Gto. D. Xerophilous scrub disturbed to induce grasslands in San José Iturbide, Gto.  

Field work. Routes were established in the Flora del Bajío region between 2014 and 2022. Based on the type of vegetation and state of conservation of the locality, surveys were carried out in an area of approximately 500 m² of land and were inspected to locate individuals belonging to the Cactaceae family (^{Aquino & Arias 2010}). For each location, the geographic position was recorded via a Garmin eTrex Vista H WGS84 geopositioner, photographs were taken, one or two samples of the Cactaceae’ species found were collected, and phenological events were recorded. The collected samples were herborized and deposited in MEXU and IBUG (^{Thiers et al. 2023}).

Database. A database was developed from the field collections and the herbarium records from ANSM, ASU, CIMI, ENCB, FEZA, IBUG, IEB, IZTA, MEXU, QMEX and UAMIZ (^{Thiers et al. 2023}). Upon completion of herbarium and field work, three observations corresponding to species not listed in previous works (^{Arias & Aquino 2019}) were added from ^{iNaturalistMX (2023)}. The identification of the samples was carried out using the treatment for the Cactaceae family of the Flora del Bajío (^{Arias & Aquino 2019}, ^{Arias et al. 2024}). The names were updated following the proposal of ^{Korotkova et al. (2021)}. Additionally, recent taxonomic revisions were considered: Coryphantha (Engelm.) Lem. (^{Sánchez et al. 2022}), Echinocactus Link & Otto (^{Vargas-Luna et al. 2018}, ^{Baker 2022}) and Stenocactus (K.Schum.) A.Berger (^{Franco 2020}). Cultivated species were excluded from the analyses.

Analysis of richness and endemism. Cactaceae richness was calculated by municipal political divisions (^{INEGI 2019}), vegetation types (^{Rzedowski 2006}) and biogeographic provinces (^{Morrone et al. 2017}). In the present study, an endemism is a taxon restricted to the Bajío (^{Morrone 2008}). To identify centers of endemism, weighted endemism (WE) was calculated, taking grid cells as the unit of comparison (^{Noguera-Urbano 2017}). Furthermore, CWE seeks to reduce the correlation between species richness and endemism (^{Crisp et al. 2001}, ^{Noguera-Urbano 2017}). Finally, the distribution of endemic taxa in the Flora of Bajío was represented in relation to the richness estimates. The visualization and editing of the maps were performed using QGIS 3.2.2 (^{QGIS Development Team 2009}).

Results

The Cactaceae family was represented in the Flora of Bajío by 125 species of the ca. 700 reported for Mexico (17.8 %, ^{Korotkova et al. 2021}), grouped into 30 genera of the ca. 58 recognized (51.7 %, ^{Korotkova et al. 2021}, Table 1). The most diverse genus was Mammillaria Haw., with 45 species representing 30 % of the ca. 150 species (^{Korotkova et al. 2021}, ^{Aquino et al. 2024}). Next in importance was Opuntia (L.) Mill. with 18 species of the 78 reported for Mexico, representing 23 % (^{Arias et al. 2024}). Only three species were represented with more than 100 records: Cylindropuntia imbricata (Haw.) F.M.Knuth, Myrtillocactus geometrizans (Mart. ex Pfeiff.) Console and Coryphantha cornifera (DC.) Lem.; in contrast, 11 species presented between one and three records: Echinocereus weinbergii Weing. Mammillaria bombycina Quehl; M. monochrysacantha Gonz.-Zam., D.Aquino & Dan.Sánchez, M. multihamata Boed. M. occulta Zamudio & U.Guzmán, M. scheinvariana R.Ortega V. & Glass, M. schwarzii Shurly, M. rzedowskiana Zamudio & U.Guzmán; Selenicereus ocamponis (Salm-Dyck) D.R.Hunt, Stenocactus sulphureus (A.Dietr.) Bravo, and S. wippermannii (Muehlenpf.) A.Berger. We report here four species not listed by ^{Arias & Aquino (2019)}, including Echinocereus weinbergii, which was located via herbarium records (IBUG) and three more records found in the review of ^{iNaturalistMX (2023)}: Mammillaria bombycina, Selenicereus ocamponis and Stenocactus wippermannii (Table 1).

Species richness. The analysis revealed that the municipalities of Cadereyta de Montes and Peñamiller in Querétaro, as well as San Luis de la Paz in Guanajuato, presented the highest richness of cacti, with 60, 44 and 50 species, respectively (Figure 4A). Other municipalities with a high level of richness were Atarjea, San Miguel de Allende, Victoria and Xichú in Guanajuato, with 34, 32, 40 and 37 species, respectively; and Colón, Pinal de Amoles and Tolimán in Querétaro, with 33, 32, and 39 species, respectively (Figure 4A). Myrtillocactus geometrizans was recorded in 40 municipalities of the study area; followed by Opuntia streptacantha Lem., which was recorded in 31 municipalities; O. lasiacantha Pfeiff. and O. tomentosa Salm-Dyck, which were observed in 30 municipalities.

Figure 4 Richness of cactus species in the Bajío and adjacent regions. A. Municipal political division. 1: Cadereyta de Montes; 2: San Luis de la Paz; 3: Peñamiller; 4: Victoria; 5: Tolimán; 6: Xichú; 7: Atarjea; 8: Colón; 9: Pinal de Amoles; 10: San Miguel de Allende. B. Species richness by cell; the regions delimited within the study area represent the VTB, CD and SMOr. 

The analysis of richness by vegetation type revealed that the xerophytic scrub had the greatest diversity of species, with 112 (Table 2). Cylindropuntia imbricata, Isolatocereus dumortieri (Berge ex K.Schum.) N. P. Taylor, Mammillaria gilensis Boed., M. polythele Mart., Myrtillocactus geometrizans and five species of Opuntia occurred in all types of vegetation in the study area (Table 1), whereas 37 species inhabited only one vegetation type (Table 1). With 97 species the Chihuahuan Desert was the biogeographic province with the greatest number of species (Table 3). Mammillaria rhodantha Link & Otto and five species of Opuntia inhabited the three biogeographic provinces in the study area (Table 1). Cell-based richness analysis revealed two quadrants with the highest cactus richness, i.e., 55 and 54 species, respectively, as well as two secondary cells, with 47 and 41 species (Figure 4B). Species richness was concentrated to the east of the Flora of Bajío area and decreased in the extreme southwest, mainly in the portion of Michoacán included in the Trans-Mexican Volcanic Belt, as well as the southern end of Guanajuato (Figure 4B).

Endemism analysis. The distribution of endemic species was related to the distribution of species richness in the region. These species were concentrated to the northeast of the Flora of Bajío area, on the borders of the Chihuahuan Desert and the Sierra Madre Oriental, and decreased toward the southern region on the borders with the Trans-Mexican Volcanic Belt. The xerophilous scrub was the type of vegetation with the greatest number of endemisms, with 24 species (Tables 1 and 2). On the other hand, the Chihuahuan Desert and the Sierra Madre Oriental included 18 and 16 endemic species, respectively.

At the municipal level, Cadereyta de Montes hosted the greatest number of endemic species, with seven species, followed by Peñamiller, with four species. A second center of endemism was San Luis de la Paz, with four species, while the neighboring municipalities, i.e., Victoria and Xichú, each hosted 2 endemic species (Figure 5). The municipalities of El Marqués and Querétaro hosted 3 and 2 species, respectively. Finally, the municipalities of San Juan del Río, Tequisquiapan, Colón, Pinal de Amoles, San Miguel de Allende, Dolores Hidalgo, Guanajuato and San Felipe, hosted only one endemic species within their perimeters (Figure 5).

Figure 5 Richness of endemic cactus species in the Bajío and adjacent regions by municipal political division: 1) Cadereyta de Montes, Qro.; 2) Peñamiller, Qro.; 3) San Luis de la Paz, Gto.; 4) El Marqués, Qro.; 5) Victoria, Gto.; 6) Xichú, Gto.; 7) Querétaro, Qro.; 8) Ezequiel Montes, Qro.; 9) San Juan del Río, Qro.; 10) Tequisquiapan, Qro.; 11) Colón, Qro.; 12) San Miguel de Allende, Gto.; 13) Dolores Hidalgo, Gto.; 14) Guanajuato, Gto.; 15) San Felipe, Gto.; and 16) Pinal de Amoles, Qro. 

On the other hand, the analysis of endemism by cell revealed that four of the cells presented high WE values (Figure 6A), which were located in the same region observed in the richness analysis. Moreover, the CWE analysis revealed only two cells with high values (Figure 6B) at the northeast and northwest ends of the Bajío Flora area. A cell in the far north is surrounded by others with a WE value of >1.86. It is likely that there are no endemic taxa for that cell. The CWE corroborates this assertion because, despite being a species-rich area (Figure 4B), there are no restricted species for that cell.

Figure 6 Analysis of endemic cacti in Bajío and adjacent regions. A. Weighted Endemism (WE). B. Corrected weighted endemism (CWE). The inland boundaries correspond to the biogeographic provinces VTB, CD and SMOr. 

Discussion

This work recognizes the Flora of Bajío area as a region with an important diversity of cacti, with 30 genera and 125 species distributed over an area of 60,171.42 km² (^{Suárez-Mota et al. 2015}, Figures 6 and 7). This value is higher than that of other regions with high diversity, such as the Tehuacán-Cuicatlán Valley, which has 28 genera and 87 species distributed over an area of 10,000 km² (^{Valiente-Banuet et al. 2000}, ^{Arias et al. 2012}, ^{Tristan et al. 2018}). In turn, it is comparable to the richness of a State such as Tamaulipas, where 33 genera and 157 species are currently known, with an area of 80,249 km² (^{García-Morales et al. 2022}).

Photo D. Aquino (A-H, J-O), D. Sánchez (I).

Figure 7 Some Cactaceae representative in the Bajío and adjacent regions. A. Kadenicarpus pseudomacrochele (D. Aquino & M. Vázquez-Sánchez 572 (MEXU)). B. Kroenleinia grusonii (R. Ortega s. n. (MEXU)). C. Mammillaria monochrysacantha (P. González-Zamora et al. 84 (IBUG, IEB)). D. M. ariasii (D. Aquino et al. 510 (MEXU, IBUG)). E. Strombocactus disciformis (D. Aquino & P. González-Zamora 547 (MEXU)). F. Thelocactus hastifer (S. Arias & D. Aquino 2263 (MEXU)). G. Echinocereus schmollii (D. Aquino & P. González-Zamora 550 (MEXU)). H. Lophophora diffusa (S. Arias & T. Terrazas 1698 (MEXU)). I. M. scheinvariana (D. Sánchez & A. Martínez-Poiré 884 (IBUG)). J. Ferocactus mathssonii (S. Arias & T. Terrazas 1688 (MEXU)). K. M. albiflora (D. Aquino et al. 506 (MEXU)). L. Strombocactus disciformis subsp. esperanzae (D. Aquino et al. 524 (MEXU)). M. F. macrodiscus subsp. septentrionalis (D. Aquino et al. 364 (MEXU)). N. M. mathildae (D. Aquino & P. González-Zamora 553 (MEXU)). Ñ. M. hahniana (D. Aquino et al. 522 (MEXU)). O. Astrophytum ornatum (D. Aquino et al. 517 (MEXU)).  

The number of genera of Cactoideae recorded by ^{Arias & Aquino (2019)} for the Flora of Bajío differs from the present work, since recent phylogenetic analysis allows the recognition of Kadenicarpus Doweld (^{Vázquez-Sánchez et al. 2019}) and Kroenleinia Lodé (^{Vargas-Luna et al. 2018}) as independent genera, which are included here (27 correspond to Cactoideae and three to Opuntioideae), thus adding up to 30 genera. On the other hand, studies focused on the delimitation of species in Stenocactus allow us to confirm that the individuals previously identified as Stenocactus obvallatus (DC.) A.Berger ex A.W.Hill corresponds to S. pentacanthus (Lem.) A.Berger ex A.W.Hill and S. dichroacanthus (Mart. ex Pfeiff.) A.Berger ex Backeb. & F.M.Knuth (^{Franco 2020}). Likewise, field observations allow us to rule out the presence of Mammillaria fittkaui Glass & R.A.Foster for the study area and that, such records correspond to M. mathildae Kraehenb. & Krainz, so its distribution previously known only for Querétaro now includes Guanajuato. ^{iNaturalistMX (2023)} allow us to corroborate the presence in Guanajuato of Disocactus speciosus (Cav.) Barthlott and Opuntia pachyrrhiza H.M.Hern., C. Gómez-Hinostrosa & Bárcenas, previously known for the northern part of Michoacán and northeast of Querétaro, respectively. On the other hand, we confirm the presence of Echinocereus weinbergii, Mammillaria bombycina, Selenicereus ocamponis and Stenocactus wippermannii as new records for the study area (Table 1). Additionally, owing to recent exploration work, three new taxa in the study area are described: Stenocactus × irregularis Gonz.-Zam., D.Aquino & Dan.Sánchez (^{González-Zamora et al. 2023a}), Mammillaria monochrysacantha (^{González-Zamora et al. 2023b}) and M. ariasii U.Guzmán & D.Aquino (^{Aquino et al. 2024}), so new discoveries in the future should not be ruled out.

Species richness. The sites that host the greatest number of species occur in the eastern region of the Flora of Bajío area, with Cadereyta de Montes in Querétaro being the municipality with the highest concentration of species. In contrast, toward the south, in the municipalities of Guanajuato that border Michoacán, the richness decreases noticeably. In the case of Asteraceae of Flora del Bajío, ^{Villaseñor & Ortiz (2012)} reported that the greatest number of species is concentrated at the southern end of the study area, while the eastern region occupies second place in terms of species richness. On the other hand, our results for the Flora of Bajío Cactaceae agree with those reported by ^{Hernández-Oria et al. (2007)} for central Querétaro, since quadrants 1 and 2 of our results (Figure 4B) coincide with the “Tolimán” quadrant, where the areas corresponding to Cadereyta de Montes and a sector bordering Hidalgo are recognized as areas of high species richness. The high incidence of species can be explained by the fact that it is a transition zone between the canyons of the Estórax and Moctezuma River basins and the mountainous areas of the El Doctor massif (^{Hernández-Magaña et al. 2012}). ^{Bárcenas (1999)} reported a high number of species in the central-eastern and northeastern regions of Guanajuato. This area corresponds to the municipalities of San Luis de la Paz, Victoria, Xichú, Atarjea and San Miguel de Allende (Figure 1A) in quadrants 3 and 4 (Figure 4B). This richness is probably related to its condition as a climatic refuge during the Pleistocene and to the wide diversity of soil types of presents (^{Hernández & Bárcenas 1995}). Additionally, Sierra Gorda (including the Querétaro sector) presents a diverse relief that supports different types of vegetation such as the Quercus Forest and xerophilous scrubs (^{Rzedowski et al. 2012}), so lineages were isolated and diversified in this mountain range, which is reflected in the WE and CWE values which are significant (Figure 6). There are no previous investigations of the northeastern part of Michoacán; however, based on our results, the low level of taxon richness is relevant, and only widely distributed species such as Disocactus speciosus and Opuntia streptacantha are recorded in the area. Based on the presence of members of Asteraceae, ^{Villaseñor & Ortiz (2012)} consider the southern sector of the Flora of Bajío to be relevant, as it hosts a significant number of species belonging to this family. However, many species of Asteraceae tend to be synanthropogenic, which is why they withstand severe disturbance events (^{Ruiz-Acevedo et al. 2024}). In contrast, members of the Cactaceae are affected by severe disturbances (Figure 3D). With the notable exception of Cylindropuntia (Engelm.) F.M.Knuth, Opuntia, and Pereskiopsis Britton & Rose, they can be synanthropic, having the ability to establish themselves on roadsides or persist through in situ management (^{Bravo-Hollis & Sánchez-Mejorada 1991}, ^{Arias et al. 2024}).

Endemism and conservation areas. The distribution of endemic species occurs toward the east of the study area, whereas toward the south, the level of endemism is very low. The strictly endemic species for the Flora of Bajío are concentrated between Cadereyta de Montes, Peñamiller and Ezequiel Montes (Figure 2). Some of the endemism presents are Mammillaria herrerae Werderm., M. scheinvariana, M. occulta, Strombocactus corregidorae S.Arias & E.Sánchez and Stenocactus sulphureus, among others. It has been proposed that Cadereyta de Montes, Peñamiller and Tolimán be declared protected natural areas, since anthropogenic factors have been detected that negatively affect the diversity of the flora in general and the Cactaceae in particular (^{Sánchez-Martínez et al. 2006}, ^{Suárez-Mota et al. 2015}). The second most important center of endemism is the municipality of San Luis de la Paz, with four species, followed by Victoria and Xichú, with two species each (Figure 3A). Some endemic species are Ferocactus mathssonii (Berge ex K.Schum.) N.P.Taylor, Mammillaria albiflora (Werderm.) Backeb., M. ariasii, M. duwei Rogoz. & P.J.Braun, M. monochrysacantha, M. multihamata and Turbinicarpus alonsoi Glass & S.Arias.

The quadrant endemism analyses do not include information on the distribution of the taxon outside the study area, examples of which are Kroenleinia grusonii (Hildm.) Lodé, Strombocactus disciformis (DC.) Britton & Rose and Thelocactus hastifer (Werderm. & Boed.) F.M.Knuth, which are distributed in Cadereyta de Montes and in localities in the state of Hidalgo (unpublished data). The limits of the Chihuahuan Desert and its continuation to the north with the Sierra Madre Oriental are recognized as centers of endemism for Cactaceae (Figure 3A). The WE analysis does not consider subspecies; however, at the recovered center of endemism is distributed, Strombocactus disciformis subsp. esperanzae Glass & S.Arias, a very particular morphotype of the Sierra Gorda. Additionally, some species endemic to the Flora of Bajío (Mammillaria schwarzii, M. zeilmanniana Boed. and Opuntia elizondoana E.Sánchez & Villaseñor, among others) are distributed in areas with a low level of endemism. Similarly, Mammillaria microhelia Werderm. is distributed in the Sierra de Zamorano (municipality of Colón), which is also an area with a low level of endemism for Cactaceae (Figure 2); however, this region is relevant for other plant families (^{Suárez-Mota et al. 2015}). Although the portion belonging to Guanajuato is recognized as a protected natural area (^{SMAOT 2024}), it is necessary to include the portion of the Sierra de Zamorano corresponding to Querétaro, since it is a mountainous massif covered with temperate climate vegetation (Abies forest) surrounded by arid climate vegetation (xerophilous scrub), forming an ecological island, which favors the isolation of species (^{Rzedowski et al. 2012}). A similar case occurs in the municipality of San Felipe, where Mammillaria schwarzii is found, although it is recognized as a relevant area for conservation (^{Suárez-Mota et al. 2015}). The WE and CWE values indicate that they are low, however, in this quadrant the presence of endemism has been reported for other families such as Crassulaceae (^{Pérez-Calix 1997}), so a floristic study is required to integrate other families to evaluate the presence of endemism in San Felipe.

The CWE shows cells with high values at the northern and southern ends of the study area because it recovers species that are found only in that cell and not in others; however, these taxa are common in regions outside the study polygon; for example, Rhipsalis baccifera (Sol.) Stearn and Selenicereus spinulosus (DC.) Britton & Rose at the northern end and S. ocamponis were distributed in a single quadrant at the southern end (Figure 3B).

Considering that species such as Mammillaria monochrysacantha, M. microhelia or Lophophora diffusa cover an area of less than 150 km² (^{Hernández et al. 2010}, ^{González-Zamora et al. 2023a}), and that a set of species that are listed under the protection of national and international laws are distributed in Sierra Gorda (^{Santa-Anna-del-Conde et al. 2009}), including species that are not endemic to the study area but are in vulnerable conditions, it is necessary to evaluate the direct and indirect impacts on these populations to propose in situ preservation strategies (^{Hernández-Oria et al. 2006}, ^{Záhora et al. 2017}). Notably, the work carried out at the Regional Botanical Garden of Cadereyta’s "Ing. Manuel González de Cosío", where propagation exercises have been carried out of an initial number of nine to 19 priority species (^{Sánchez-Martínez et al. 2006}, ^{Chávez et al. 2006}). In addition, their research has contributed to the understanding of the biology of threatened species, such as Echinocereus schmollii (Weing.) N.P.Taylor (^{Hernández-Oria et al. 2006}), as well as the discovery of a new species, namely, Strombocactus corregidorae (^{Arias & Sánchez-Martínez 2006}).