Abstract
Compared with other hyperdiverse rocky ecosystems of eastern Brazil, canga vegetation, also known as campo rupestre ferruginoso, represents one of the least studied ecosystems but support a high proportion of rare and endemic plants. Large-scale iron mining is the primary cause of the loss and degradation of cangas. Therefore, there is a need to acquire knowledge about campo rupestre ferruginoso, both to support conservation planning and to provide information that can be used in ecological restoration models. In this study, we investigated the structure, diversity, and floristic composition of campo rupestre ferruginoso communities in four canga outcrops in a semiarid region and compared the values with those from existing studies on campo rupestre in eastern Brazil. A total of 5,724 individuals were sampled, and these individuals were distributed among 74 taxa, 54 genera, and 29 botanical families. We found that the plant communities in the cangas of the Vale do Rio Peixe Bravo are characterized by a unique set of functional groups, including a high proportion of succulents and poikilohydric plants, constituting an assemblage of specialized species. Considering the high degree of threat due to large-scale mining projects, our study revealed that the four cangas are in an excellent state of conservation, and we propose that they be considered reference ecosystems for future restoration projects.
Keywords Campo rupestre ; Functional groups ; Phytosociology ; Rarity ; Restoration Planning
Resumo
Comparado com outros ecossistemas rupestres hiperdiversos do leste do Brasil, a vegetação de canga, também conhecida como campo rupestre ferruginoso, representa um dos ecossistemas menos estudados, mas que sustenta uma alta proporção de raridade e endemismo de plantas. Os principais impactos negativos irreversíveis nas cangas são a perda e a degradação de habitat devido a mineração de ferro em larga escala. Então, o desafio é produzir conhecimento sobre os campos rupestres ferruginosos, tanto para contribuir com o planejamento da conservação, quanto para subsidiar informações para os modelos de restauração ecológica. Neste estudo, nós investigamos a estrutura, a diversidade e a composição florística de comunidades de campos rupestres ferruginosos em quatro afloramentos de canga da região do semiárido, e comparamos com outros estudos em campos rupestres do leste do Brasil. Foram amostrados um total de 5.724 indivíduos, distribuídos entre 74 taxons, 54 gêneros e 29 famílias botânicas. Verificamos que as comunidades de plantas rupestres nas cangas do Vale do Rio Peixe Bravo caracterizam-se por um conjunto peculiar de grupos funcionais, principalmente quanto a elevada proporção de suculentas e poiquiloídricas, constituindo uma assembleia única de espécies. Considerando o elevado grau de ameaça devido aos projetos de mineração em larga escala, nosso estudo verificou que as cangas estão em excelente estado de conservação. Portanto, propomos que sejam consideradas como ecossistemas de referência para modelos de planejamento de eventuais projetos de restauração.
Palavras-chave Campo rupestre ; Grupos funcionais ; Fitossociologia ; Raridade ; Planejamento para restauração
Introduction
In Brazil, one of the focal points of the National Biodiversity Policy is the need to generate knowledge about the components of biological diversity, especially in habitats and ecosystems located in priority conservation areas. The first objective of this public policy is to prepare an inventory and characterization of ecosystems, species and populations to generate adequate information for the planning, management, and conservation of Brazilian natural heritage (Brasil 2002).
Among all the ecosystems that have been characterized thus far in Brazil, the hyperdiverse campo rupestre (rocky outcrop vegetation) of eastern Brazil stands out for supporting the highest number of rare, endemic, and endangered plant species. These ecosystems support a relictual flora and are affected by topographic, edaphic, and microclimatic factors (Jacobi et al. 2007, Giulietti et al. 2009, Rapini et al. 2021). Despite its natural heritage, ecological, biogeographic and evolutionary importance, there is still a lack of information about the community structure in these habitats and floristic comparisons with other rocky environments in Brazil (Moura et al. 2011).
Canga vegetation, also known as campo rupestre ferruginoso, sustains highly diverse plant communities, including rare and endemic species, and occurs in several priority areas for biodiversity conservation at State (Minas Gerais 2021) and Federal levels (Brazil 2021a). The main characteristics of the canga habitat are their insular mountaintop distributions and ancient (≈ 50 million years old) lateritic duricrusts—a type of rocky substrate with high concentrations of metallic minerals, predominantly iron—that are home to specialized plant communities (Jacobi et al. 2011, Carmo et al. 2018). Environmental heterogeneity (i.e., topographic and edaphic heterogeneity) results in compositional differences between plant communities that occur on adjacent rocky outcrops (Huggett 1995). For example, studies conducted in the Quadrilátero Ferrífero region in southeastern Brazil, showed that topographic factors, especially slope, influenced the floristic composition of campo rupestre habitats (Messias et al. 2012), and rock surface inclination and topographic heterogeneity were significant predictors of plant functional groups (Carmo & Jacobi 2016).
In Brazil, most of the remains of campo rupestre ferruginoso are located in areas that constitute one of the largest agglomerations of opencast iron mines in the world. In fact, the annual gross Brazilian production of these mines exceeds 551 million tons (ANM 2022). This intense iron mining activity generates large-scale and irreversible environmental impacts, with the environmental degradation of extensive mountain areas, which have been used to store ores, tailings, and residues (Sonter et al. 2018; Carmo et al. 2020). For biota, especially plants, the main irreversible negative impacts are habitat loss and degradation (Carmo et al. 2018, Salles et al. 2019, Carmo & Kamino 2023).
The last canga ecosystems in Brazil that remain unaffected by large-scale iron mining are located in the northern region of Minas Gerais, in the locality known as Vale do Rio Peixe Bravo (VPB) (Pereira et al. 2023). However, very little is known about the biodiversity of these cangas, specifically about the structure and diversity of the plant communities associated with the iron outcrops. The first descriptions of the environmental and geographical aspects of the canga habitat in VPB were published in 2011 in a study of an iron speleological site (Carmo et al. 2011). Some mining megaprojects have already been planned for the large-scale exploration of iron ore in extensive natural areas. One of these projects is projected to produce 27 million tons of iron ore concentrate annually, with the installation of open-pit mines, tailings dams, mineral processing plants, and other industrial structures (Pereira et al. 2023).
Therefore, there is a need to overcome the knowledge gap about canga ecosystems, both to contribute to the planning, management, and conservation of biodiversity and to collect information that can be used for an environmental impact assessments and eventual ecological restoration measures. Studies indicate that the challenges for the restoration of campos rupestres degraded by mining activities are particularly high, partly due to the limited knowledge about these ecosystems, to the point that, in some situations, complete ecological restoration may be unrealistic (Arruda et al. 2023). Yong et al. (2022) established international principles for the ecological restoration of mine sites and specified that ecological restoration requires the identification of suitable reference ecosystems for the development of models that can be used in the planning of efficient restoration actions. A reference ecosystem represents a real and non-degraded community of organisms able to act as a model or benchmark/target for restoration project. Such a reference model needs to reflect the historical ecological trajectory of the impacted ecosystem as much as possible and include critical elements, such as endemic species and characteristic plant species for each stratum or functional group (Clewell 2009; McDonald et al. 2016).
The campo rupestre are among the most threatened ecosystems and should be a high-priority target for the generation of biological knowledge, as established in the National Biodiversity Policy (Brasil 2002). Thus, our study aimed to investigate the structure, diversity, and floristic composition of communities in campo rupestre ferruginoso in the north region of Minas Gerais, the semiarid region of Brazil. Considering the high level of threat due to large-scale mining projects, our study also assessed whether canga vegetation can be characterized as a reference ecosystem for planning models of potential restoration projects located in VPB.
Material and Methods
1.Study area
This study was conducted in four canga outcrops (PB1 to PB4) in the VPB, which is centered at 16°07' S and 42°42' W, between the municipalities of Rio Pardo de Minas, Riacho dos Machados and Grão Mogol in the Jequitinhonha River Basin in northern Minas Gerais (Figures 1 and 2). The VPB is located in the Cerrado biome in the semiarid region of Brazil (Brasil 2021b). The subhumid dry climate type (C1) predominates locally, with an average annual rainfall of less than 800 mm and soil moisture indices between -20 and 0, according to the Thornthwaite Climate Classification System (Marcos-Junior et al. 2018, Brazil 2021b). Furthermore, areas located to the north and east of the VPB have close contact with the Caatinga phytogeographic domain, the largest seasonally dry tropical area in South America, and the Brazilian Atlantic Rainforest, one of the most megadiverse and endangered rainforests in the world (Pereira et al. 2023).
Vale do Rio Peixe Bravo region, North Minas Gerais state, southeastern Brazil. Canga (ironstone) outcrops identified as PB1 to PB4. Inset map: South America.
Campo rupestre ferruginoso in Vale do Rio Peixe Bravo, northern Minas Gerais. In the background, the phytophysiognomies of the Cerrado biome.
Structure and diversity of plant communities
We investigated the structures of plant communities in four canga outcrops, which were grouped a priori according to the slope of the relief based on a declivity map of Minas Gerais (Minas Gerais 2024): one group had flat canga areas (PB1 and PB2) and slopes > 10% and were located on plateaus, while the other group had steeper slopes with inclinations between 20 and 45% (PB3 and PB4).
In each canga outcrop, we installed five parallel transects measuring 100 m in length and spaced at least 50 m apart. Along each transect, we demarcated ten 2 × 1 m plots positioned at 10 m intervals, totaling 50 plots in each canga (total area: 100 m2). The transects were positioned to allow sampling of open vegetation, with a predominance of rupicolous or saxicolous habits. Environments with an accumulation of soils, for example, forest capões (associations with termite mounds or large cracks in the rocks) were not sampled. Additionally, environments with shadow or penumbra, such as cliffs and cave entrances, were not sampled. We followed the same sampling design used by Carmo & Jacobi (2016) in a study of saxicolous plant community diversity and structure patterns along edaphic and topographic gradients. Methodological standardization is essential for comparisons between rocky outcrop communities at different sites (Moura et al. 2011).
In each plot, we identified the species of vascular plants, and for each individual with a height greater than 3 cm, the following variables were measured: height, measured from the ground to the top of the plant; the area of vegetation cover above the ground, considered the largest diameter of the plant canopy; and the perpendicular diameter. The ellipse formula was used to calculate the area. For plants that formed colonies (clonal reproduction), we considered only those with a diameter ≥ 3 cm, and each colony was counted as a single individual. The taxonomic nomenclature of the sampled specimens, the geographical distributions of the species and the extent to which each species was threatened were determined by the List of Species of Flora of Brazil (Flora and Funga do Brasil 2023).
The species of vascular plants were classified into nine functional groups based on observable morphological attributes and/or ecophysiological attributes related to adaptations to stressful edaphic conditions, such as water deficit, according to Rizzini (1997) and Mueller-Dombois & Ellenberg (2002) (Table 1).
Functional groups based on observable morphological attributes and/or ecophysiological attributes related to adaptations to stressful edaphic conditions. Adapted from Rizzini (1997) and Mueller-Dombois & Ellenberg (2002).
Data analysis
We verified the height distributions (cm) of the individuals sampled in the four cangas using a box plot with the kernel density function.
We analyzed the similarity between 1) the set of canga outcrops (n = 4) in VPB and 2) the three canga outcrops located in the Quadrilátero Ferrífero, in the central region of Minas Gerais (Carmo & Jacobi 2016). We investigated the similarity between the functional groups in the four cangas of the VPB, considering the sum of the species vegetation cover.
We compared the similarity of the vegetation cover of the functional groups with three more canga outcrops located in the Quadrilátero Ferrífero, a region in the Atlantic Forest phytogeographic domain. For similarity analyses, we used the unweighted pair group method with arithmetic means (UPGMA) and Bray‒Curtis indices. The adequacy analysis of the clusters was tested by estimating the cophenetic correlation coefficient. PAST software (Hammer et al. 2001) was used to perform the analyses.
4.Campo rupestre ferruginoso: reference ecosystem
Absence of threats: Direct degradation threats are absent or minimal. This assessment included verifying whether pollutants occur in the surrounding areas and identifying sources of invasive species, eroding land surfaces, livestock damage, fire damage, large-scale mining operations, and deforestation.
Physical conditions: Environmental conditions required to maintain the ecosystem (for example, heterogeneity and rocky outcrop integrity were considered).
Species composition: Species characteristic of the ecosystem (e.g., finding rare, threatened, or endemic species).
Structural diversity: Appropriate diversity of key structural components (e.g., vegetation strata, functional groups, and vegetation cover).
External exchanges: The ecosystem is appropriately integrated into the larger landscape context through positive abiotic and biotic flows and exchanges (e.g., observe whether the canga is connected to the natural vegetation matrix).
Results
1.Structure and diversity of plant communities
In the four cangas located in the VPB, a total of 5,724 individuals were observed. These individuals were distributed among 74 taxa (54 identified to the species level), 54 genera, and 29 botanical families (Table S1). The families with the greatest number of taxa were Cyperaceae and Poaceae (seven each); Fabaceae (six); Bromeliaceae and Rubiaceae (five each); and Apocynaceae, Asteraceae, Cactaceae, and Euphorbiaceae (four each). The genera with the greatest richness were Mimosa (four species), Croton, Ditassa, and Rhynchospora (three each).
Six species were sampled from the four cangas: Croton campestris A.St.-Hil. (Euphorbiaceae); Discocactus piscibarbarus N.P. Taylor & Olsthoorn (Cactaceae); Mesosetum sp. (Poaceae); Microtea tenuifolia Moq. (Microteaceae); Pfaffia siqueiriana Marchior. & Miotto (Amaranthaceae); and Portulaca hirsutissima Cambess. (Portulacaceae). At the other extreme, 46 species (62%) were sampled from only one canga. Most of the species in the cangas are infrequent, with 85% being inventoried in fewer than 20 plots out of a total of 200 plots distributed in the four cangas.
Eight families represented approximately 80% of the total vegetation cover area, mainly Amaranthaceae (two species) and Poaceae (seven species). Nine genera accounted for approximately 80% of the total vegetation cover area, with Pfaffia (one species) and Mesosetum (one species) being the dominant genera (Figure 3).
Families and genera that accounted for 80% of the total vegetation cover (cm²) in 200 plots 2x1 m sampled in four cangas located in the Vale do Rio Peixe Bravo, northern Minas Gerais.
Ten species dominated the community structure, representing 82% of the measured vegetation cover and 69% of all individuals sampled in the four cangas (Table S2). Among these 10 species, four have not yet been officially evaluated for the threat of extinction, three are threatened with extinction, and one is considered data deficient. Considering all the plants sampled in the four cangas, five endangered species were observed, with three in the endangered category and two in the vulnerable category. Two species were considered data deficient, and two were considered near threatened.
Sixty-four percent of the total area of vegetation cover in the four cangas included plants with distributions restricted to Minas Gerais and/or Bahia that were found in the transition areas between the phytogeographic domains of the Cerrado and Caatinga. These species are often found in nonforest environments, such as campos rupestres, campos cerrados, and carrascos.
The data for each canga outcrop were analyzed, and the results showed that the most frequently observed species in PB1, which occurred in at least half of the plots, were Mesosetum sp. (90% of the plots), Vellozia hirsuta Goethart & Henrard (Velloziaceae) (90%), P. hirsutissima (68%), and D. piscibarbarus (50%). The grass Mesosetum sp. and the poikilohydric species V. hirsuta were the dominant species, with the largest areas of vegetation cover. In PB2, the most frequently observed species were D. piscibarbarus (90% of the plots), Mesosetum sp. (84%), P. siqueiriana (82%), P. hirsutissima (68%), and Rhynchospora setigera (Kunth) Griseb. (Cyperaceae) (62%). The sclerophyte P. siqueiriana and the grass Mesosetum sp. were the dominant species.
The sclerophyte P. siqueiriana and the succulent Encholirium reflexum Forzza & Wand. (Bromeliaceae) were the dominant species in PB3, with the largest areas of vegetation cover. The most frequently observed species were P. siqueiriana (86% of the plots), E. reflexum (78%), Mesosetum sp. (68%) and D. piscibarbarus (54%). In PB4, the most frequently observed species were P. siqueiriana (90%), Mesosetum sp. (76%), D. piscibarbarus (74%), Euphorbia attastoma Rizzini (Euphorbiaceae) (66%) and E. reflexum (52%). The dominant species in PB4 were E. attastoma and P. siqueiriana.
The largest numbers of individuals were observed on the flat cangas (PB1 and PB2), which are located on the plateaus of the chapadas; however, these species had the lowest average heights. The lowest richness and smallest vegetation cover were observed in these cangas. In cangas PB3 and PB4, which are located on gentle slopes, the highest richness, areas of vegetation cover and average heights were recorded. However, the lowest numbers of individuals were observed at these sites (Table 2). The vegetation on the flat cangas was smaller and generally consisted of a single herbaceous-subshrubby stratum with a height of approximately 20 cm. On the other hand, on the hillside cangas, two strata were observed: a herbaceous-subshrub stratum and a shrub stratum with a height of approximately 75 cm (Figure 4).
Plant community structure parameters in four cangas located in the Vale do Rio Peixe Bravo, MG. FG: functional group; AD: absolute density; CobT: total vegetation cover.
Distribution (boxplot with the kernel density function) of the heights (cm) of individuals sampled in the four cangas located in the Vale do Rio Peixe Bravo, northern Minas Gerais. The circles and asterisks correspond to observations with at least 1.5 times and 3 times the interquartile range (Q3-Q1), respectively.
The functional groups that occurred in all the cangas of VPB were sclerophytes, succulents, poikilohydrics and graminoids. The sclerophyte group had the largest total area of vegetation cover (42%), considering the sum of the four cangas, followed by the succulent group (30%) and the graminoid group (20%). The inconspicuous functional groups were the geophytes, therophytes, infundibuliform rosettes and hemiparasites, with values of < 2% each. The classification analysis based on vegetation cover by functional group identified two clusters with 45% similarity (Figure 5). In one of the groups, formed by the cangas on the steeper slopes (PB3 and PB4), the sclerophytes represented between 41% and 47% of the total vegetation cover, and the succulents represented between 30% and 46%. The other group, which included the cangas on the flat areas (PB1 and PB2), had the highest proportions of graminoids (33% and 41%, respectively) and poikiohydric plants (24%), in addition to sclerophytes (between 28% and 48%).
Similarity among the campos rupestres ferruginosos (cangas PB1 to PB4) located in the Vale do Rio Peixe Bravo, based on the vegetation cover area (cm²) of each functional group. Dendrogram generated from the Bray‒Curtis index (UPGMA); cophenetic correlation coefficient = 0.88).
Based on the 43 species with the largest coverage areas in the three cangas located in the Quadrilátero Ferrífero (Carmo & Jacobi 2016), measured with the same sampling design as in the present study, it was observed that there was practically no similarity between the cangas located in the VPB and those of the Quadrilátero Ferrífero (Figure 6).
Similarity between campos rupestres ferruginosos (cangas PB1 to PB4, 43 spp.) located in the Vale do Rio Peixe Bravo and in the Quadrilátero Ferrífero (cangas C1 to C3, 43 spp.), based on the occurrences of the 86 species of vascular plants with the largest areas of vegetation cover (cm²). The dendrogram was generated from the Bray‒Curtis index (UPGMA); cophenetic correlation coefficient = 0.99).
Campo rupestre ferruginoso: reference ecosystem
In the four cangas investigated, we found adequate physical conditions of the rocky outcrops; presence of indicator, rare, and endemic species; diversity of functional types; and a landscape matrix with minimal anthropic interference. No degradation factors were observed (Table 3). These key attributes also indicate that the campo rupestre ferruginoso areas of the VPB are in an excellent state of conservation and can be characterized as reference ecosystems.
Key attributes observed (☺) to characterize the reference ecosystem in four campos rupestres ferruginosos in the Vale do Rio Peixe Bravo, northern M, Brazil. Adapted from McDonald et al. (2016) and Yong et al. (2022).
Discussion
The community of rupestrian plants in the cangas of the VPB constitutes a unique assemblage of species. Considering the 10 species with the highest levels of dominance observed in the cangas of the VPB (see Table 1), only one was also reported in the community structure studies of other cangas of eastern Brazil, specifically in cangas in the Quadrilátero Ferrífero region (Vincent & Meguro 2008). The common species is C. campestris, the only species that has a wide distribution in Brazil, occurring in campo rupestre, cerrado and caatinga (Cordeiro 2004). None of the other nine dominant species in the cangas of the VPB were recorded in phytosociological studies conducted in canga ecosystems. Further expanding the geographical range of comparison, those species were also not observed in the quartzite campos rupestres of Serra do Espinhaço (MG) or Chapada Diamantina (BA), according to data published in Conceição & Giulietti (2002), Conceição & Pirani (2005, 2007), Le Stradic et al. (2015) and Mota et al. (2018).
The 43 species with the largest coverage areas in the four cangas of the VPB and the three cangas located in the Quadrilátero Ferrífero, which were all measured using the same sampling design, were compared, and the results indicated that there was practically no floristic similarity between these locations. Only one species occurred in the cangas of both locations: Pleroma heteromallum (D. Don) (Melastomataceae), a subshrubby sclerophyte that is common in rocky outcrops of southeastern and northeastern Brazil. In the study conducted in the Quadrilátero Ferrífero (Carmo & Jacobi 2016), for the three cangas sampled, P. heteromallum had a total coverage of 145,300 cm², while the total coverage area of the four cangas of VPB was only 17,841 cm².
Among the genera with greater vegetation cover found in the cangas of the VPB (see Figure 3), only Vellozia (6th), Croton (7th) and Pilosocereus (8th) were also among the most important species in the community structure studies conducted in other locations with cangas in eastern Brazil. The Vellozia genus has a center of endemism in Serra do Espinhaço (Mello-Silva 1995), and 98% of the species cited for Brazil occur in Minas Gerais and Bahia. Vellozia is also one of the most important genera in cangas located in the Quadrilátero Ferrífero in the central region of Minas Gerais (Jacobi et al. 2008, Messias et al. 2012, Carmo & Jacobi, 2016, Caminha-Paiva et al. 2021) and in the cangas of the Conceição do Mato Dentro region in the southern Serra do Espinhaço (MG) (Oliveira et al. 2018). However, the species differed among the studied locations.
Croton was one of the most important genera in some cangas located in the Quadrilátero Ferrífero (Messias et al. 2012) and in the Conceição do Mato Dentro region (MG); both localities were represented by the other species: Croton erythroxyloides Baill. (Euphorbiaceae). Pilosocereus was also observed in the canga of the Conceição do Mato Dentro region (MG), but with a different species from that found in the VPB, namely, Pilosocereus aurisetus (Werderm.) Byles & G.D. Rowley (Cactaceae).
The genus Mesosetum had the 2nd highest vegetation cover and was one of the most common in the cangas of the VPB, a unique position compared to the Serra do Espinhaço and Quadrilátero Ferrífero regions. Viana & Filgueiras (2008) investigated the geographic distribution of grasses (Poaceae) in the Serra do Espinhaço region, and Mesosetum was not among the genera with the greatest richness; these genera included Paspalum, Panicum, Eragrostis, Axonopus, Andropogon, and Dichanthelium. Some of these genera, namely, Andropogon, Axonopus, Panicum, and Paspalum, were among the most common in the plant communities in cangas of the Quadrilátero Ferrífero (Jacobi et al. 2008, Messias et al. 2012, Carmo & Jacobi 2016).
The pattern of dissimilarity (beta diversity) observed between the cangas of the VPB and other rupestrian communities in iron outcrops is probably related to climatic and geomorphological/geological factors. While the cangas located in the eastern part of the country are outcrops resulting from millions of years of weathering of Paleoproterozoic rocks, known as banded iron formations, the cangas of the VPB originated from the weathering of Neoproterozoic rocks known as iron diamictites and metadiamictites (Carmo & Kamino 2017). Geological differences are reflected in different geomorphological characteristics; for example, the cangas of the VPB are located at elevations between 800 and 900 m and are usually associated with plateaus, while the other cangas of eastern Brazil integrate residual mountainous reliefs, with elevations often higher than 1,200 m and reaching 1,850 m (Carmo & Kamino 2015).
In addition, the VPB region is located in the semiarid region of Brazil, i.e., in an area with intense water deficit, due to interactions between factors such as average annual rainfall lower than 800 mm, average annual temperatures between 23 and 27 °C, average insolation of 2,800 hours/year and average evaporation of 2,000 mm/year (Brasil 2021b). On the other hand, the other cangas of eastern Brazil are located in the phytogeographic domain of the Atlantic Rainforest (Carmo & Jacobi 2013) and are influenced by a humid climate (B2 Thornthwaite), with average annual rainfall between 1,500 and 2,000 mm and average annual temperatures between 17 and 22 °C (Martins et al. 2018). Thus, the restricted geographical distributions of most of the 10 species with the largest areas of vegetation cover observed in the cangas of the VPB fall within the Brazilian semiarid region, with four species having known occurrences in only a few locations in Minas Gerais and Bahia in transition areas between the Cerrado and Caatinga phytogeographic domains. Two of those species are endemic to the northern region of Serra do Espinhaço (Minas Gerais), and two species have not yet been identified to the species level: the grass of the genus Mesosetum and the poikilohydric Vellozia sp. 2. Only one species occurs in locations along the Serra do Espinhaço in Minas Gerais; therefore, it is distributed beyond the semiarid region.
The plant community in the cangas of the VPB is also characterized by a unique set of functional groups compared to those in other stress-prone rock ecosystems in eastern Brazil. The communities of plants growing in iron outcrops are exposed to numerous stressors, such as intense solar radiation; high concentrations of iron and other metallic minerals, acidic and oligotrophic soils; mechanical impediments to root development; and water restrictions (Jacobi et al. 2007, Caminha-Paiva et al. 2021, Rios et al. 2022). Therefore, limited water availability likely represents the main environmental filter in the cangas of the VPB, and this finding is consistent with the high proportions of succulents and poikilohydric species within those communities. These functional groups share attributes related to ecophysiological strategies for saving water resources and/or avoiding drought periods (Lambers et al. 2008, Griffiths & Males 2017).
Succulents are generally characterized by the CAM (Crassulacean Acid Metabolism) photosynthetic pathway, requiring photosynthetic tissues to have cells with large vacuoles for osmotic storage of liquids, together with the predominant opening of stomata at night, which drastically reduces the loss of water through transpiration. Therefore, succulents have high water use efficiency and are often found in semiarid regions where the water deficit is high (Griffiths & Males 2017). In the cangas of the VPB, four of the 10 species with the largest total areas of vegetation cover were succulents. In addition, the functional groups represented by the succulents were diverse, with cactiform species (Cactaceae and Euphorbiaceae), herbaceous species (Bromeliaceae, Orchidaceae, Portulacaceae), and Clusia obdeltifolia Bittrich. (Clusiaceae), which is representative of the only tree genus with CAM metabolism (Lüttge 2007).
Another functional group that has drought avoidance strategies is represented by the poikilohydric organisms, whose unique ability to resist the loss of more than 95% of cellular water is rare among angiosperms. Approximately 300 species of poikilohydric angiosperms are known, most of which occur in rocky outcrops in Africa, America and Australia (Lüttge et al. 2011). In the cangas of the VPB, the poikilohydric plants were represented by species of Vellozia.
Intense exposure to solar radiation is another stressor in rock outcrop ecosystems since excess light energy can generate high levels of reactive oxygen species, causing damage to the photosynthetic complex, and this process is amplified when water deficit also occurs (Lemos-Filho 2000, Rios et al. 2022). Therefore, under this constant selective pressure, species with specialized traits related to the dissipation of excess light/heat energy and atmospheric water uptake, e.g., scleromorphic leaf structures such as thick waxy cuticles, multistratified epidermal and hypodermal layers; lower specific leaf areas; trichomes and trichome-like emergences; and woolly stems, are expected to occur (Negreiros et al. 2014, Vitarelli et al. 2016). The greatest proportion of sclerophytes was found in the cangas of the VPB; this functional group was the most abundant in the VPB, reaching 42% of the vegetation cover area. Three of the 10 species with greater dominance were also observed in this area (see Table S1).
The structure of the rupestrian communities of the VPB indicates that cangas constitute harsh environments whose environmental filters and edaphic heterogeneity affect the determination of functional diversity, resulting in high proportions of functional types with traits related to water saving and drought avoidance and the ability to dissipate excess light energy. Rios et al. (2022), studying a canga in the Quadrilátero Ferrífero, observed a high diversity of photochemical strategies to prevent oxidative damage. The authors found that most species (75%) are characterized by “stress-tolerant” strategies, sensu Grime (1977).
Small-scale differences in rock habitats, such as surface heterogeneity and/or microtopographic profiles, also generate changes in the structure and diversity of rupestrian communities (Kruckeberg 2004). Carmo et al. (2016) performed the first study on campo rupestre and demonstrated that fine-scale (centimetric) variations in the rock surface promote alterations in plant community composition. The authors found that less diverse plant communities, with monodominance of desiccation-tolerant clonal plants, predominated in softer, smooth, and flat rocky outcrops. On the other hand, more irregular outcrops with a high frequency of microforms (cracks, depressions, pores, and rocky blocks) and greater slopes result in plant communities that are more diverse and have a wider range of functional groups with a better distribution. Therefore, the differences observed in the VPB between the plant communities associated with flat and sloping cangas (see Figure 5) may be linked to variations in topographic heterogeneity.
The greatest number of species and the greatest diversity of functional groups were observed in the sloping cangas, in addition to the greatest heights and greatest plant coverage (see Table 2). This fact may be related to the greater availability of microenvironments formed by rocky blocks, soil accumulations and cracks, since on inclined surfaces, fluids (including rain and runoff) reach greater speeds, generating greater intensity of weathering processes. On the other hand, in flat cangas, there are practically no blocks or cracks, and a more compact rocky surface predominates, with little soil accumulation compared to that on sloping cangas. The greatest proportions of poikilohydric plants and graminoids were found in the flat cangas. Both of these functional groups are characterized by clonal plants that are capable of adapting to unfavorable conditions, such as the difficulty of anchoring roots on the bedrock.
The plant communities of the canga in the VPB largely include rare and endangered plants, and one-third of the total area of vegetation cover in the four inventoried cangas was occupied by species formally evaluated as threatened with extinction (three in the endangered category and two in the vulnerable category), in addition to data deficient and near threatened species. To date, restricted endemism has been observed in four species recorded only in the cangas of the VPB, two of which were recently described: the bromeliad Orthophytum minimum Leme & OBC Ribeiro (Bromeliaceae) (Leme et al. 2020) and the cactus D. piscibarbarus (Taylor et al. 2023). In addition, another two new species has not yet been described: Orthophytum sp. nv. (Rafael B. Louzada, personal communication) and Vellozia sp. 2 (Mota et al. 2017). It is also noteworthy that among the four cangas, there were no nonnative or invasive species. Therefore, the species composition and community structure show that the cangas investigated have not undergone alterations or observable anthropic disturbances and that the plant communities reached their maximum local expression or edaphic climax (sensu Rizzini 1997).
The four cangas showed an excellent state of conservation and can be considered pristine areas; these may be the last areas with these conditions in Brazil. Nevertheless, in a recent study, Pereira et al. (2023) estimated that there is a high degree of direct threat to iron ecosystems since 99.6% of canga areas overlap federal authorizations of availability, application, and mining concessions. The challenge to conserve these areas will be enormous given that only a small global fraction of ecosystems degraded by mining have been properly recovered, even considering technological and operational potential (Young et al. 2022). Thus, our study indicates that cangas in the VPB can be considered a “reference ecosystem” for planning restoration projects and, subsequently, for their evaluation based on assumptions, goals, and metrics.
Supplementary Material
The following online material is available for this article:
Table S1 – Phytosociological parameters of four cangas (PB1-PB4), Vale do Rio Peixe Bravo, MG, in decreasing IVI order. p: number of occurences registered in the sampling units (2 × 1 m); n: number of individuals; Cob.: plant cover; FA: absolute frequency; FR: relative frequency; DA: absolute density; DR: relative density; DoA: absolute dominance; DoR: relative dominance; IVC: coverage value index; IVI: importance value index.
Table S2 – Dominant species according to the number of individuals and the total area of vegetation cover in 200 plots 2 × 1 m each sampled in four cangas located in the Vale do Rio Peixe Bravo, northern Minas Gerais.
Acknowledgments
We thank Nova Aurora Community, Peixe Bravo Quilombola Community, Nilson Ferreira, José Eugênio do Carmo and Prof. Filipe Vieira Santos de Abreu for their support during fieldwork. We dedicate our work to the eternal advisor Professor Claudia Maria Jacobi for her tireless encouragement and pioneering spirit in research on the iron ecosystem of Minas Gerais. The authors FFC, ICC and FFC thanks the following the Graduate Program in Ecology, Conservation and Wildlife Monitoring at the Federal University of Minas Gerais for the opportunity to carry out this research; This study was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq for the Edital Universal (award number # 014/2010).
Data Availability
The datasets generated during and/or analyzed during the current study are available at material supplementary in this article.
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Publication Dates
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Publication in this collection
13 Dec 2024 -
Date of issue
2024
History
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Received
08 June 2024 -
Accepted
11 Nov 2024