Organic Soils: Formation, Classification and Environmental Changes Records in the Highlands of Southeastern Brazil

Abstract

Soils constitute the largest terrestrial carbon (C) pool, representing more than the sum of carbon contained in the atmosphere and vegetation. In this context, organic soils stand out, storing 21% of the global soil organic C stock in only 3% of the Earth’s land surface. Moreover, these soils are a key component in our climate system, biodiversity, water cycle and records of environmental changes. Organic soils require specific attention as they provide a large variety of ecosystem services, but also because of their vulnerability to climate change. In this paper, we present a review of terms and concepts related to organic soils, their formation, pedoenvironments, and taxonomic classification. We also present a synthesis of studies in the highlands of southeastern Brazil using these soils as records of environmental change.

1. Introduction

Organic soils, or soils with a strong or predominantly organic constitution, classified as Histosols [1,2] or Organossolos in the Brazilian Soil Classification System [3], are some of the most important carbon stores globally. They cover only 3% of the terrestrial surface but store a disproportionately high level—about 28%—of the world’s soil organic carbon [4]. These soils are formed of organic materials derived from plants and are commonly referred to as peat, and land covered by Histosols is known as peatland. In Brazil, it is estimated that these soils cover 0.03–0.07% of the land surface area, particularly in the south and southeastern regions [5]. Furthermore, there is evidence suggesting that these values are underestimated [6]. They are found under very local conditions of low temperature or low oxygen availability that prevent organic matter degradation.

Organic soils provide many ecosystem services (Figure 1). In addition to functioning as carbon sinks, they play an important role in maintaining the hydrological cycle since the regions where these soils occur regulate the water flow of watersheds. In addition, the organic soils have great relevance in relation to biodiversity, since they are associated with various phytophysiognomies where the processes of ecological interaction of species of fauna and flora are intrinsically associated with the flow of water table oscillation in organic soils, and this ultimately influences the availability of biodiversity resources and the maintenance of interactions between species [7,8].

2. Peat, Peatland, or Organic Soil?

Organic soils are often associated with peatland ecosystems, which is why some related concepts and definitions may overlap. Although there are different definitions in the literature according to the earth sciences, conventionally there is their basic relation: peat is the material (organic materials, predominantly of plant origin) from which a peatland ecosystem is formed (e.g., marsh, swamp, fen, bog) [9,10,11,12]. The soils found in these areas are generally classified as organic soils.

Peat consists mainly of plant remains that accumulate due to incomplete decomposition under near-saturated conditions with water and/or cold climate with low temperatures. Different plant materials may be involved in the peat-forming process—for example, woody parts, leaves, rhizomes, roots, and mosses (Sphagnum spp.). In the various definitions, depending on the country or even the scientific discipline, the peat layer must show a minimum thickness for an area to be classified as peatland, ranging from 10 cm to 100 cm [10]. There are also a variety of names used for ecosystems with organic soils, including mire, moor, bog, fen, marsh, swamp, and others. These terms apply to specific ecosystems or landscapes characterized by specific organism communities, hydrologic regimes, soil reaction (pH), nutrient status, and landform patterns [13]. Among these, the term “mire” is applied to minerotrophic peatlands, where peat is currently being formed with peat-forming species [12].

Regarding the concept of organic soil, the definitions found in the literature are generally associated with soil classification systems. Each one depends on a set of criteria established by each system and usually involves minimum values for organic carbon content and soil thickness. Thus, peat can be considered as the parent material of organic soils formed in peatland environments (Figure 2). However, the occurrence of organic soils is not restricted to peatland areas. In the Brazilian Soil Classification System and in most classification systems, organic soils are separated into classes according to the formation environments: (1) soils formed from materials deposited under excess water conditions (in wetlands and/or minerotrophic peatland ecosystems). and (2) soils in well-to-moderately-well-drained conditions, in wet, cold, and upper montane vegetation environments and/or ombrotrophic peatland ecosystems [3].

3. Formation of Organic Soils

Organic soil formation occurs when plant biomass production exceeds decomposition, due to strong limiting conditions on the activity of microorganisms. This includes permanent conditions of saturation with water (low oxygen availability), low temperatures, high rainfall, and various physicochemical properties, such as low nutrient content or low pH [14]. In peatland ecosystems the vegetation is typically composed of mosses and plants that are highly efficient at capturing and preserving nutrients.

There are several classifications for temperate peatlands, based on botanical criteria (forested, herbaceous and mixed peatlands), geographical criteria (parallic and limnic), factors based on the degree of decomposition of the peats generated (non-humified or moss peatlands, and humified or flammable peatlands), etc. [15]. The typologies described here are those appropriate for the discussion in the remainder of the paper. According to Joosten and Clarke [10], historically, peatlands were distinguished based on their status and characteristics after land use, leading to the identification of:

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Bogs: peatlands elevated above the surrounding landscape. After peat extraction, which was usually carried out in dry conditions after drainage, a mineral subsoil suitable for agriculture often remained.

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Fens: peatlands situated in depressions. After peat extraction, carried out by dredging, water is observed to remain.

From a geochemical and hydrological point of view, peatlands are classified as minerotrophic and ombrotrophic (Figure 3) [11,16]:

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Minerotrophic: formed under the influence of water from the outer limits of the accumulation basin and/or groundwater. As they receive solutes from the surrounding area (added ferromagnesian minerals, oxides and/or hydroxides, etc.), the hydrolysis of added minerals will influence the pH.

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Ombrotrophic: not influenced by local groundwater (water table), only by atmospheric sources (rain and/or snow). Living, decomposing vegetation dominates the environment, and the pH is basically determined by the dissociation of carboxylic functional groups in the dead organic matter.

4. Classification of Organic Soils

The definitions and classifications of organic soils vary between classification systems. This item compares the classification of these soils in the Brazilian Soil Classification System (SiBCS) [3], World Reference Base for Soil Resources (WRB) [1], and Keys to Soil Taxonomy (ST) [2]. SiBCS has as its priority the classification of soils in the Brazilian territory and has some criteria, attributes and concepts that originated from correlated definitions in WRB or ST. The WRB and ST systems are recommended by the International Union of Soil Sciences (IUSS) as international for soil correlation activities, reports for technical documents, soil mapping at various scales, and scientific publications [17].

In SiBCS, the organic horizons are called horizontes hísticos. They are defined as dark-colored horizons due to their high levels of organic matter, with organic carbon content ≥ 80 g·kg⁻¹ in which characteristics related to high organic matter content predominate (Figure 4). They result from accumulations of plant residues in varying degrees of decomposition, deposited superficially, although at present it may be covered by mineral horizons or deposits and even more recent organic layers [3]. To be classified as hístico, in addition to organic carbon contents higher than 8%, the horizon must meet one of the following requirements regarding thickness:

• Thickness greater than or equal to 20 cm; or
• Thickness greater than or equal to 40 cm when 75% or more of the volume of the horizon consists of plant tissue in the form of fine branch remains, fine roots, and bark, excluding living parts; or
• Thickness of 10 cm or more when overlying a lithic or fragmentary lithic contact or a horizon and/or layer consisting of 90% or more (by volume) of mineral material greater than 2 mm in diameter (gravels, pebbles, and boulders).

The hístico horizon is formed in two distinct environments, being differentiated in O and H hístico [3]:

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Hístico O horizon—formed from materials deposited in free drainage condition (saturated with water for less than 30 consecutive days in the rainy season), without water stagnation, conditioned mainly by the humid, cold climate and high-montane vegetation. It can sit on lithic contact, fragmentary lithic contact, or any type of horizon (A, B, or C).

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Hístico H horizon—formed from materials deposited under excess water conditions, for long periods or throughout the year, even if, at present, it has been artificially drained. They are usually settled over the C horizon, but also in some cases, due to the influence of artificial drainage, over A and B horizons. They can occur on the surface or be buried by mineral material.

In the WRB, the organic constitution horizons can be classified as folic or histic. Both must be ≥10 cm thick and are differentiated by the water regime and aeration of soil [1]:

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Folic horizon (from Latin folium, “leaf”) consists of well-aerated organic material. Occur predominantly in cold climates or at higher altitudes. Saturated with water for <30 consecutive days/year, without artificial drainage.

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Histic horizon (from Greek histos, “fabric”) consists of poorly-aerated organic material. They occur predominantly in wetlands or peatlands. Saturated with water for ≥30 consecutive days/year or drained artificially.

The composition of the histic horizon is generally different from the composition of the folic horizon, because the vegetation cover is often different. The lower limit of 20% soil organic carbon differentiates the folic horizon from the chernic, mollic, or umbric horizons, which have this content as upper limits. The folic horizon can also have either andic or vitric properties, depending on the system definitions [1].

In Soil Taxonomy [2] there are two organic epipedons—folistic and histic:

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Folistic is a layer saturated with water for < 30 days (cumulative) in normal years, not artificially drained, that meets one of the following criteria:

• It consists of organic soil material that:
  • is 20 cm or more thick and contains 75% or more (by volume) Sphagnum fibers or has a bulk density, wet, of less than 0.1 g·cm⁻³; or
  • is 15 cm or more in thickness; or
• Is an Ap horizon that, when mixed to a depth of 25 cm, has an organic carbon content (by weight) of:
  • CO ≥ 16% if the mineral fraction contains 60% or more clay; or
  • CO ≥ 8% if the mineral fraction contains no clay; or
  • CO ≥ 8% + (percent clay divided by 7.5) if the mineral fraction contains less than 60% clay.

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Histic is a layer saturated with water for 30 days or more (cumulative) in normal years, or drained artificially, that meets one of the following criteria:

• It consists of organic soil material that:
  • is 20 to 60 cm thick and contains 75% or more (by volume) Sphagnum fibers or has a bulk density, wet, of less than 0.1 g·cm⁻³; or
  • is 20 to 40 cm thick; or
• Is an Ap horizon that, when mixed to a depth of 25 cm, has an organic carbon content (by weight) of:
  • CO ≥ 16% if the mineral fraction contains 60% or more clay; or
  • CO ≥ 8% if the mineral fraction contains no clay; or
  • CO ≥ 8% + (percent clay divided by 7.5) if the mineral fraction contains less than 60% clay.

Most folistic and histic epipedons consist of organic soil material. However, item 2 in each definition provides an epipedon that consists of soil mineral material (Ap horizon). Moreover, a histic epipedon that consists of soil mineral material can also be part of a mollic or umbric epipedon [2].

5. Organic Soils and Records of Environmental Changes: A Summary of Studies in the Atlantic Forest of Southeastern Brazil Highlands

The Atlantic Forest represents one of the most important biodiversity hotspots in the world [17] with a high level of endemism [18,19], even though it is one of the most threatened and fragmented biomes [20,21]. Over the past century, in part related to Brazil’s economic development, the Atlantic Forest has been extensively deforested for coffee and sugarcane monocultures [22]. The lack of a precise definition of the Atlantic Forest resulted in delayed environmental conservation policies and research funding [23]. It was not until the 1980s that Atlantic Forests (plural) were first defined to include separate components linked to different geographic factors [24]. Today, some of the largest remaining fragments of Atlantic Forest are found in the Serra do Mar and Serra da Mantiqueira in the states of Rio de Janeiro, Minas Gerais, and São Paulo [20].

Extremely heterogeneous in composition, the Atlantic Forest encompasses a wide range of vegetation formations and climate, from tropical to subtropical [25]. Altitude ranges from sea level to 2900 m, with abrupt changes in soil type and depth [26], average air temperature [27], and average annual precipitation [23]. Detailed phytosociological analyses have revealed that a number of plant formations, which include mangroves, restingas, semideciduous forest, Araucaria forest, rupestrian fields, high-altitude fields (campos de altitude), and brejos-de-altitude (wet forests resulting from orographic rainfall in northeastern Brazil) are actually the expression of a single forest domain and are closely linked to each other [23].

Among the Atlantic Forest ecosystems, the upper montane forests and high-altitude fields stand out due to their high biodiversity and number of endemic species [28]. These ecosystems are found in the higher regions of the Serra da Mantiqueira and Serra do Mar in southeastern Brazil, with forests often occupying concave slopes and fields covering mountain tops and convex slopes [19]. In recent decades, several paleoecological studies have been carried out on organic soils in these “high montane environments” of the Atlantic Forest in southeast Brazil to better understand past, current and future trends in vegetation and climate. Many of them use paleoenvironmental proxies (e.g., pollen and spore grains, soil phytoliths, coals, stable carbon isotopes (δ¹³C) and ¹⁴C dating) found in soils with high organic matter contents. We present a synthesis of some of these studies is presented in Figure 5 and a summary of the results from each of the selected studies below.

There are an increasing number of studies on environmental changes and the history of vegetation and climate in the highlands of southeastern Brazil, in areas of the Atlantic Rainforest, but also in the Cerrado biome. Based on palynological analysis of a core material collected at Serra Negra (1160 m, MG), De Oliveira [29] assumed that between ~40,000 and ~14,000 years B.P., climatic conditions were more humid and cooler than current conditions in southeast Brazil. Before ~40,000 years B.P., an Araucaria forest would have extensively dominated the landscape of western Minas Gerais, and possibly most areas above 800 m in southeast Brazil. In Salitre (1050 m, MG), Ledru [30] observed the presence of Araucaria pollen grains and other elements associated with the Mixed Ombrophylous Forest between ~15,500 and 10,350 years B.P., suggesting a decrease in temperature in the region during the late Pleistocene. This time interval would also have been marked by a decrease in the concentration of arboreal elements and a predominance of herbaceous elements.

Behling [31], studying palynological records in a peat bog at Morro de Itapeva (1850 m, SP), identified variations in vegetation composition in the past, possibly associated with climatic changes. During the last glacial period (35,000–17,000 years B.P.), an expansion of high-altitude grasslands and absence of forest formations would be indications of a markedly cooler and drier climate than the present. The development of a Sphagnum peat bog, the rare presence of Araucaria and the existence of a narrow high-altitude forest and rainforest belt at lower elevations during the late glacial period (17,000–10,000 years B.P.) were associated with a shift to cooler and somewhat more moist climates. During the early Holocene, the development of a high montane forest near the study site would be related to a warmer and more humid climate on the slopes, and the rare presence of Araucaria and Podocarpus in the higher parts of the relief would indicate a drier climate. Only during the late Holocene would the humidity have increased in the region, a condition inferred by the higher frequency of Araucaria and Podocarpus.

In another study in the highland region of Catas Altas (755 m, MG), Behling [32] studied evidence of dry and cold climatic conditions during glacial periods. His results indicated that the last landscape during the glacial period was covered by extensive areas of subtropical grasslands and small areas of riparian forests along the rivers, where today semideciduous forests exist. The climate would have been dry and cold with strong frosts during the winter months. The subtropical gallery forests would have been composed of forest species such as Araucaria angustifolia, Podocarpus, Drimys, Ilex and Symplocos. The records also indicated that the subtropical grassland vegetation, which is now found in patches in the southern Brazilian highlands, expanded from southern to southeastern Brazil over more than 750 km.

In a study carried out in Botucatu (770 m, SP), Behling et al. [33] analyzed pollen and charcoal records in sediments rich in organic matter that indicated environmental changes in the Late Quaternary. A vegetation cover of grasslands with small clusters of subtropical forest, associated with cooler and drier climatic conditions, was documented during the recorded glacial period (30,000 and 18,000 years B.P.), with fires occurring in the grassland vegetation. Between 18,000 and 6000 years B.P., a change in climate would have caused decomposition of the ice-age deposits and a sedimentation gap. Pollen assemblages also indicated land use by indigenous peoples in the last 2900 years B.P., with deforestation and planting of Zea mays and Manihot.

Ledru et al. [34] studied a 7.8 m core site with records of forest expansion variations during the last 100,000 years at Colônia, in the Serra do Mar (900 m, SP). The results were compared with other cores and it was confirmed the Atlantic Forest experienced phases of climatic change during the Quaternary. Comparisons with ice cores from Antarctica and Greenland suggested that temperature changes characterized by stable isotope ratios were related to changes in humidity rates in the tropics. Pollen records in the same testimony indicated changes in tropical forest vegetation during expansion phases, during interglacial and glacial episodes. Araucaria angustifolia had high expression until about 50,000 years B.P. [35]. Lower rates of biodiversity were recorded between 23,000 and 12,000 years B.P. and 40,000 and 30,000 years B.P., indicating marked phases of stress for the rainforest.

Behling et al. [36] studied the pollen and charcoal records in two testimonies (buried soil and peat bog) at Serra da Bocaina (1500 and 1650 m, RJ). According to the authors, during the period between 18,570 and 14,570 B.P., extensive grassy fields with frequent burning existed in the higher part of the Serra da Bocaina which, together with other evidence, indicate relatively dry and cold climatic conditions for this period. A high montane forest vegetation would have been restricted to sheltered valleys or slopes at lower altitudes. The presence of the conifer Araucaria angustifolia has been observed since the Late Pleistocene. Reworked deposits without pollen grains resulting from erosion in the glacial period, were related to increased rainfall during the Younger Dryas period. The authors report that the climate was relatively drier and warmer during the Lower Holocene, with a longer annual dry season than current conditions, and that high montane forest taxa increased during the Holocene (especially after 7260 years B.P.).

Saia et al. [37] studied the records of vegetation change during the Last Glacial Maximum on a soil transect in São Paulo state (100–847 m). Using δ¹³C and ¹⁴C dating of buried charcoal fragments and the humin fraction of organic matter, the authors found evidence of a drier climate with more open vegetation than current conditions (between 20,000 and 16,000 years B.P.). From 16,000/14,000 years B.P. the predominance of C3 plants increases, indicating an expansion of the forest, probably associated with the presence of a more humid climate than in the previous period. Their results also indicated the presence of open vegetation during the late glaciation, probably associated with a drier period, also observed in other regions of Brazil.

Pessenda et al. [38] analyzed carbon isotope and pollen records to assess the impact of glaciation on native vegetation in the tropical rainforest of Serra do Mar (SP). The results indicated that between 28,000 and 22,000 years B.P., a subtropical forest with conifers developed under cooler and wetter conditions. Between 28,000 and 19,000 years B.P., the vegetation was composed mainly of plants of the C3 photosynthetic cycle. Between 19,450 and 19,000 years B.P., a significant increase in sedimentation and spore rate indicated increased moisture, associated with an increase in erosional processes (between 19,000 and 15,600 years B.P.). From 15,600 years B.P. onwards, a substantial increase in tree and grass elements was observed, indicating a more humid and warmer climate.

Behling and Safford [39] studied pollen and charcoal records in an altitude field peatland in Serra dos Órgãos (2130 m, RJ). According to the authors, the highest region was naturally covered by altitude fields during the entire recorded period (12,380 years B.P.). A diverse high montane forest would have occurred near the studied peat bog at the end of the late glacial period. Evidence of small populations of Araucaria angustifolia were found in the study area until the early Holocene, after which the species apparently became locally extinct. Between 10,380 and 10,170 years B.P., there was a reduction in high-altitude grasslands parallel to an expansion of upper montane forest to higher altitudes, reflecting a wetter and warmer period (higher temperatures than currently observed) at the end of the Younger Dryas. During the Lower Holocene, the climate would have been drier, probably with a relatively long dry season. The high-altitude grasslands expanded, and the high montane forest retreated, expanding again until it reached configurations similar to current conditions, around 5640 years B.P., when the climate became more humid, with shorter dry seasons. The authors also indicate that fire frequency was high during the Lower Holocene, but decreased sharply from 7850 years B.P. onward.

Verissimo et al. [40] performed a reconstitution of the Holocene vegetation and fire history of Serra do Caparaó (2150 m, ES) based on pollen and charcoal analysis. The authors report that high-altitude grasslands have been the dominant vegetation during the entire recorded period (from 11,400 years B.P.). In the Lower Holocene (11,400 to 9000 years B.P.), a wet phase was followed by a drier one. Between 9000 and 2700 years B.P., a gradual increase in diversity and abundance of Atlantic Forest taxa indicated an increase in humidity and/or precipitation. Between 2700 and 1200 years B.P., forest expansion stopped and apparently reversed after 1200 years B.P., possibly due to human activities. Simultaneously, the high-altitude grasslands would have expanded—abrupt variations in pollen assemblages suggest environmental instability. The authors concluded that the altitude fields are a natural vegetation of Serra do Caparaó, but their current extent was probably influenced by anthropogenic activities, as the results found suggest an increase in humidity after 1200 years B.P., which should have caused forest expansion. Reduced human disturbance at higher altitudes would probably result in forest succession in some of the lower grasslands.

Calegari et al. [41] used phytoliths and δ¹³C to study Holocene environmental changes in a Latosol with humic A horizon at Machado, southern Minas Gerais (1155 m, MG). Three phytolithic zones (sections) were identified, indicating vegetation changes: Zone I (~12,131 to ~6103 years B.P.), open vegetation with C3 grasses and arboreal elements, associated with a drier climate than today, with Araucaria phytoliths recorded at the top of the zone (~6000 years B.P.) indicating increased arboreal elements in the vegetation cover; Zone II (~6000 years to ~180 years B.P.), increased tree cover, including Araucariaceae and Arecaceae (palms), related to wetter climatic conditions; and Zone III (~180 years B.P.), representing the current vegetation, an ecotone of Subperenifolia Tropical Forest and Cerrado, with fire occurrence indicated by charcoal fragments and charred phytoliths.

Augustin et al. [42] found evidence of paleoenvironmental changes by studying geomorphic processes in the Serra do Espinhaço (1149 m, MG). Using phytoliths and carbon isotopes (δ¹³C and ¹⁴C), the authors found evidence of wetter environmental conditions, with less open vegetation and a predominance of C₃ plants between 10,506–10,230 years B.P. and 5919–5152 years B.P. in the deeper horizons of the studied soils. In addition, the large amount of broken/intemperate phytoliths allowed us to infer that in this period the erosive processes were also very pronounced, due to the higher energy levels arising from the presence of more superficial running water in the environment. In a similar study in the Serra do Espinhaço, using phytoliths and carbon isotopes (δ¹³C and ¹⁴C), Chueng et al. [43] report the predominance of Cerrado vegetation since about 6000 years B.P., with some variations in the presence of woody species in the studied areas.

Horák-Terra et al. [44] investigated a peat deposit in the Serra do Espinhaço Meridional, Cerrado biome (1400 m, MG), using a multiproxy approach to identify Holocene environmental changes. The authors described six distinct phases: Phase I (~10,000 to 7360 years B.P.), wetter and cooler climate, with major local erosion; Phase II (~7360 to 4200 years B.P.), warm and humid climate and less soil erosion; Phase III (~4200 to 2200 years B.P.), dry and warm climate, with further increase in soil erosion; Phase IV (~2200 to 1160 years B.P.), drier climate; Phase V (~1160 to 400 years B.P.), climate-like conditions with increased peat accumulation; and Phase VI (last 400 years B.P.), same climate conditions, but with increased local and regional erosion. In another study in the Serra do Espinhaço, Hórak-Terra et al. [45] studied the factors that influenced the expansion and contraction of Cerrado and Capões (small islands of semideciduous forest vegetation) since the Late Quaternary. In this study, five environmental changes were identified: Phase I (~35,000 to ~29,600 years B.P.), dry and warm climate with cooling events and some landscape instability; Phase II (~29,600 to ~16,900 years B.P.), cooler and wetter climate, with reduced landscape instability; Phase III (~16,900 to ~6100 years B.P.), similar conditions to the previous phase, but with increased humidity; Phase IV (~6100 to ~3100 years B.P.), drier and warmer climate; and Phase V (since ~3100 years B.P.), dry and warm to subhumid climate.

Kirchner et al. [46] investigated Middle to Upper Holocene fluvial sediments in the Guapi-Macacu River basin (RJ). Using stratigraphic, sedimentological and geochronological methods, the authors found that between 6600 and 4700 years B.P., reduced fluvial dynamics and geomorphic stability prevailed in this region, followed by a period of geomorphic activity, indicated by increased fluvial dynamics from this date. This environmental change was related to the onset of near-modern humid climatic conditions in southeastern Brazil during the Middle/Upper Holocene transition. A new period of increased activity was recorded from 290 years B.P. onward, linked to the beginning of European colonization, when large areas of Atlantic Forest were converted into agricultural land and pastures, causing strong soil erosion and destabilization of slopes. The authors also point out that, under certain climatic and environmental conditions, even a closed forest cover does not necessarily result in geomorphic stability.

Portes et al. [47] analyzed the pollen and charcoal records in a testimony collected in a marshy area in the Serra da Bocaina (1539 m, RJ) with the aim of assessing the human (Amerindian/Pre-Columbian and European/Post-Columbian civilizations) and climate influence on vegetation dynamics. The results of the study highlighted the interrelation of regional climate change and local human actions “shaping” the Upper Holocene forest-field vegetation mosaic in the Serra da Bocaina. Human activities by Amerindians had important impacts on the mountain vegetation, maintaining a more open habitat probably through slash-and-burn agriculture. They settled originally along the coast, but migrations and cultural transitions led to a greatly-increased human footprint in inland forests maintaining more open habitats in the highlands, probably through slash-and-burn agriculture. Depopulation of the study region after the arrival of Europeans in 1500 and increased precipitation would have led to a marked and rapid recovery of forest cover. After 1720, the establishment of permanent European communities and agriculture in the study area would have led to forest loss and a renewed expansion of grasslands.

Silva Neto et al. [48,49] studied paleoenvironmental records in soil profiles in the mountainous region of Espírito Santo (1059–1227 m, ES). Through a multiproxy analysis, the authors concluded that the studied soils experienced different environmental conditions associated with distinct climatic moments during the Upper Holocene: Phase I (before −2330 years B.P.), warmer and drier climate than the present; Phase II (between 2330 and 2063 years B.P.), cooler and wetter climate than the present period; Phase III (from 2063 years B.P.), transition to the present climatic conditions. The authors concluded that the studied soils went through different phases of pedogenesis during the Upper Holocene, therefore being considered polygenetic soils.

Silva et al. [50], studying peatlands at different altitudes in the Serra do Espinhaço (1320 to 1593 m, MG), found differences in current vegetation cover, soil organic matter composition, and δ¹³C values. The chronological succession of fields and forest formations in tropical mountain peatlands was influenced by altitude and was related to paleoclimatic changes. In another study in the same region, Silva et al. (2020) [48] determined the age of the basal organic matter of eighteen testimonies collected in peatlands (1244 to 2014 m, MG), seeking to correlate the dates (¹⁴C) with altitude and to identify the environmental factors that acted on the formation of the peatlands. The results showed that the peatlands situated below 1370 m would have started to form in the upper Pleistocene, while those situated between 1580 and 1610 m would have started to form in the lower and middle Holocene, and those situated between 1760 and 2014 m would have started their formation in the upper Holocene. In the southern hemisphere, average temperatures would have been 9 °C lower than current average temperatures between 22,000 and 18,000 years B.P., 2 to 5 °C lower between 18,000 and 12,000 years B.P., while between 12,000 and 8800 years B.P., average temperatures ranged from +2 to −1 °C relative to current average temperatures.

Portes et al. [51] studied a testimony collected in a peat bog in an area of altitude fields in Serra dos Órgãos National Park (2003 m, RJ). The authors found that from 9840 to 4480 years B.P., altitude fields were the dominant vegetation at the site, indicating that the climate was relatively cold and dry. However, pollen data documented that an upland forest developed near the site throughout the Holocene. Relatively frequent high-magnitude fires would have occurred during the Lower Holocene and become rarer in the Middle Holocene after 4480 years B.P., when the climate became more humid. In the Middle Holocene, forest and fern taxa became somewhat more frequent at the site, but upland field vegetation continued to dominate most of the upper montane landscape. A climatic shift to wetter and warmer conditions during the last 1350 years B.P. was evidenced by an increase in upland forest and even floodplain forest taxa, as well as the almost complete absence of fire after this date.

Behling et al. [52] studied the pollen and charcoal records in sedimentary testimony collected near Agulhas Negras in Itatiaia National Park (2140 m, RJ). The authors reported that high altitude grasslands with small areas of montane forest and periodic fires dominated the landscape during the period of the last 7430 years B.P., with individual specimens of Araucaria angustifolia only present after 4200 years B.P. in the Serra do Itatiaia. The forest areas would have expanded after 4870 cal yr B.P., reflecting a transition to the wetter climatic conditions of the upper Holocene. Between 4450 and 4000 years B.P., fires may have markedly reduced forest cover. During the period between 1960 and 530 years B.P., no records of local fires were found, and the forest area expanded continuously, being interrupted after 530 years B.P., when local fires became more common. The authors suggest that this increase in fires may have limited forest expansion under the humid climatic conditions of the past 600 years, and that during this period, Araucaria angustifolia became more frequent in high-altitude forests.

Gonçalves [7] studied the evolutionary dynamics of Capões (small islands of semideciduous forest vegetation) associated with peatland ecosystems, in the Serra do Espinhaço Meridional (1240 to 1600 m, MG). Dating (¹⁴C) of soil organic matter showed that these forest formations probably began to emerge from the Pleistocene (33,265 years B.P.) and became more frequently established throughout the Holocene. Changes in the isotopic signal of δ¹³C indicated occasional increases in C4 and CAM plants, probably associated with ecological succession and cycles of climate oscillations in the region throughout the Quaternary. The study concluded that the Capões may not be a relict vegetation as has been proposed by other studies but represent the existence of periods with wetter conditions and deeper soil formation, at least in the drainage lines of the watercourses along the lithic contacts between the relief blocks.

In a review on the past dynamics of Atlantic Forest vegetation, Pieruschka and Ledru [56] presented a synthesis of changes in water balance and temperature shown in paleoecological studies conducted at different sites in southeastern Brazil (Figure 6). A trend towards wetter and warmer conditions is noted between the end of the glacial period and the beginning of the Holocene (~11,500 years B.P.). During the Holocene, i.e., the present interglacial period, the stability of forest ecosystems has been affected by sudden changes in seasonality. Charcoal interpretations from different paleoenvironmental records indicate anthropogenic alteration, in the Atlantic Forest biome, during the last millennia [56].

Considering a future scenario of global warming, Atlantic Forest species may likely reduce their range and shift to more southern regions of the country [62]. In the highlands of southeastern Brazil, where high-altitude fields currently prevail, open vegetation may be suppressed by the migration and expansion of the forest toward higher elevations [39]. Thus, it can be concluded that the organic soils of these regions will play a crucial role in the dynamics of the ecosystems in southeastern Brazil.

6. Conclusions

Organic soils are important natural archives for understanding the past through paleoenvironmental reconstruction, providing data for predictions of future climate and vegetation in the face of climate change that is already happening. They also provide data on the contribution of carbon to the lithosphere, and have great relevance in climate regulation, water storage, maintenance of biodiversity and various other information that research has brought to light about the importance of these soils. The preservation of ecosystems with organic soils implies the preservation of countless possibilities for life, water, and food security. The challenge regarding knowledge about these soils is to transform theoretical and scientific knowledge into tools for decision-making and information for land use planning and public policies. One of the greatest threats is the use and occupation of the land that unbalances the natural dynamics of these soils that took thousands of years to form and can be destroyed in a few decades. The singularity of the ecosystems where organic soils occur provides a rich biodiversity, with the coexistence of species that go beyond the classification of phytogeography and ecology.