Vegetation and Palynological Record in Sediments of the Camorim Dam,

Pedra Branca State Park is situated near urban areas of Rio de Janeiro city. Several rivers descend towards the Atlantic Ocean and the damming of some ones, by the end of the 19th century, aimed to provide drinking water to the population. To the present day, Camorim dam is in operation and since its inception in 1908, the surrounding humid tropical forest has recovered, evidenced by the present palynological study of a core withdrawn from the dam’s northwestern part. The six analyzed levels showed pollen grains from botanical taxa characteristic of Rain Forest, Forest Pioneers, Campo vegetation, although less frequent, and Exotic plants. The palynomorphs concentration fluctuated throughout the core, which was attributed to a variation in sedimentation due to large floods that from time to time punished this region, but never decimated the forest around the Camorim dam and its contributing river basin.


Introduction
From the 16th to the 18th century, the Rio de Janeiro city Western Zone was known as the plain of the eleven mills, basically occupied by sugarcane plantations and livestock. There was a brief period in the 17th century of coffee production (Fridman 1999;Montezuma 2012). The forest provided water and energy to the city. In the mid19th century, coal workers, most of them former ex-slaves, worked in the Pedra Branca Geological Complex forests. They transformed the area into a coal production hub, which, along with firewood, was the energy matrix of Rio de Janeiro city. Both were used for domestic consumption, civil construction forges, industrial steam boilers, and railway locomotives. More than 157 old coal mills were found in the forest and responsible for the deforestation of about 80 hectares. A significant loss of plant species due to the exploitation of firewood for coal production may have occurred (Oliveira 2012). The history of the Pedra Branca Massif Atlantic Forest protection was a combination of factors over time (Ribeiro 2012). By the end of the 19th century, water availability was necessary for the survival and development of Rio de Janeiro city and its consequent increasing population. As a solution, the federal government acquired water source areas in the city, including the Rio Grande and the Camorim river basins, aiming to improve the water catchment and distribution system and protect the forest water potential (Magalhães-Corrêa 1936).
Major tragedies related to rainfall and environmental impacts have been recorded in the history of Rio de Janeiro city. In January 1967, an extreme episode became one of the greatest disasters ever. Intense rainfall from the north coast of São Paulo State to Rio de Janeiro State caused significant landslides in the latter. The Serra das Araras disaster in 1967 (Piraí city, Rio de Janeiro State) (https://www.deviante.com.br/noticias/tragedias-climaticas-no-brasil-1967-serra-dasararas) was one of the most striking in terms of rain volume, landslides occurrence and number of victims (1,700). In Rio de Janeiro city, the Tijuca rainfall station recorded about 558 mm of precipitation in 48 hours. A disastrous event in Rio de Janeiro occurred later in 2011 (Dourado, Arraes & Silva 2012). At the beginning of the 20th century, nine major floods were recorded in Rio de Janeiro city, one every five and a half years. However, after 1950, this frequency increased to one every three years on average. The Camorim dam, whose construction was completed in 1908, was violently affected, mainly in 1966, 1967, and 1988 during the summer rainfalls (Dereczynski, Calado & Bodstein-de-Barros 2017;Ferreira 2016;Fialho 2012 Afterward, the landscape was almost entirely recomposed and the forest functionality recovered due to ecological succession processes (Oliveira 2012). Currently, the PBSP holds the title of the largest urban forest in the world (INEA 2022). However, being located inside a densely populated metropolitan region, the PBSP suffers real estate pressure, in addition to illegal hunting and planting activities, which conflict with the environmental preservation purposes of this forest remnant (Magalhães-Corrêa 1933;1936;Oliveira 2005;Solórzano 2006).
Aiming to evaluate the behavior of the vegetation around the Camorim dam in the last hundred years, the present study used palynological analysis of pollen grains and Pteridophytes and Bryophytes spores recovered from a sediment core withdrawn from the dam's deepest area. The research is justified by the scarcity of information about the previous vegetation dynamics in the PBSP.

Study Area
Pedra Branca mountainous massif, delimited between 23°52' to 23°04' S and 43°23' to 43°32' W ( Figure 1), presents granitic, crystalline, and crystallophilian rocks, mainly facoidal gneiss, interspersed with basic rocks, such as diabase (Galvão 1957). It presents a much-diversified topography. All the Pedra Branca massif hills and slopes are located inside the PBSP, extending over 125 km 2 and bordering several neighborhoods in the West Zone and the Jacarepaguá Plain of Rio de Janeiro city. Most of the mountains are arranged in a west-east direction, separated by deep valleys. The PBSP comprises the highest altitude point of Rio de Janeiro city, the Pedra Branca Peak (1024m), and it is the largest water-dispersing center in the municipality, with three hydrographic basins flowing into Guanabara Bay, Jacarepaguá Plain, and Sepetiba Bay. Jacarepaguá Plain is delimited by the Pedra Branca and Tijuca massifs, and was developed by intense erosion processes of these slopes after the last marine regression (approximately 3,500 years BP). It currently presents accentuated growth in urban occupation and it is subjected to frequent flooding (Maia et al. 1984;Togashi, Montezuma & Leite 2012).
Northern slopes face Realengo, Bangu, and Campo Grande neighborhoods, such as Bangu Mountain Range Anu. Inst. Geociênc., 2023;46:54699 and Viegas Mountain Range, are quite degraded, due to irregular human occupation and the replacement of the original vegetation by grasses.
The Camorim dam is located in the PBSP, on the southeastern slope of the Pedra Branca massif, at altitude of 436m, and occupies an area of 350 ha. Its construction was completed in 1908 aiming to supply the Jacarepaguá region with drinking water. With a volume of 210,000 m³, 18 meters of depth, and a water storage capacity of 2.4 million m³, the Camorim reservoir represents the largest water source of the Jacarepaguá group and is surrounded by the Atlantic Forest in several successional stages (Figures 2 and 3).
The vegetation in the study area and its surroundings is dominated by a humid tropical forest, inserted in the Atlantic Rain Forest domain. According to IBGE (1992), the area is characterized as a Submontane Dense Ombrophilous Forest, with a subhumid climatic typology, with little or no water deficit. It is an exuberant secondary forest directed towards the PBSP Atlantic slope and it is the largest remaining fragment of the Atlantic Rain Forest in Rio de Janeiro city with 12,500 ha, occupying about 10% of the city's total area. Botanical studies and surveys have been carried out over the years, including the vegetation of surroundings river basins and isolated hills adjacent to the PBSP, as in the Capoeira Grande Mountain Range State Environmental Protection Area (Bruni 2002;Freire 2010;Freire et al. 2009;INEA 2022;Oliveira 2007;Oliveira et al. 1995;Peixoto et al. 2004;2005;Santos et al. 2006).
The PBSP climate is characterized by an average annual temperature of 23.6 ºC; February is the hottest month (mean temperature of 26.7ºC) and June the coldest (average of 21.0ºC). The median annual total precipitation is 1,027.2 mm. Rainfall is abundant in the summer and scarce in the winter, with a water deficit from July to October (Peixoto et al. 2004). According to the Köppen system, the region's climate is classified as Aw -Tropical Rainy Climate. In addition to the N-NE winds coming from the regional atmospheric circulation, the Pedra Branca massif harbors a very dynamic local wind system, determined by its mountains and valleys location. It is intensely subject to polar fronts influence.

Material and Methods
A sediment core was withdrawn from the Camorim dam's northwestern side (Figures 4 and 5). This area was well preserved and was invaded by semi-submerged and rooted vegetation. The core upper part was unconsolidated and discarded so a 30 cm long column was retrieved (Figures 6 and 7). Eight cm 3 sediment samples were taken from six levels each, corresponding to 0, 10, 15, 20, 25, and 30 cm depth from top to bottom. The 5 cm level was not appropriate for sampling.  The sample preparation followed the standard methodology for the Quaternary proposed by Ybert et al. (1992). The chemical treatment included the addition of Lycopodium clavatum spores, 40% HF, 10% HCl, 10% KOH, glacial acetic acid, and classical acetolysis successively. Three microscope slides were prepared for each sample using glycerin-gelatin and sealed with paraffin.
Palynomorph counting and photomicrographs were carried out using 400x and 1000x magnifications. Specialized literature like Barth et al. (1962-2014; Lorente et al. (2017); Luz, Barros and Barth (2018); Roubik and Moreno (1991), and Ybert et al. (2016-2018, and the pollen slides collection of the Palynology Laboratory at Federal University of Rio de Janeiro/IGEO were used for palynomorphs identification. The collected data evaluation was performed using Tilia and Tilia Graph softwares (Grimm 1992) yielding palynomorphs relative percentages and concentrations diagrams.
It is noteworthy that pollen grains of several tropical botanical families do not resist the described chemical treatment due to their delicate exine. Therefore, many pollen grains from plants like hydrophilic monocots and Lauraceae and Musaceae species that occur nowadays in the Atlantic Forest could be present in the analyzed sediments but were not found.

Results
The Camorim dam core (Figure 7) consisted mainly of dark-brown amorphous compacted decomposed organic material, streaky with roots and plant fragments. Sandy sediments were absent at the top, present only below the 35 cm level. Several types of vegetation cover the study area actually which composition, according to the families and botanical species considered in the present study, is shown in Table 1.
The recovered finely granulated sediment, pollen grains and spores were well dispersed inside the slides. The palynological analysis of the six samples showed well preserved to corroded, oxidized, and mechanically damaged pollen grains and spores. Pollen taxa were identified, corresponding to the Atlantic Rain Forest, Pioneers of Forest, Campo (grassland), Wide distribution and Exotic ecological groups, as well as fern and mosses (Tables 1  and 2, Figures 8,9,10 and 11).
The 0 cm and 10 cm levels contained few pollen grains, most belonging to pioneer plants. In terms of frequency, Alchornea predominated, followed by Myrtaceae (Myrcia type), Araceae, and Melastomataceae/Combretum.
Further down, at the 15 cm level, the diversity of pollen grain taxa exploded. Among the forest elements, Allophylus, Araceae, Cupania, Malpighiaceae, Myrtaceae, Myrsine, and Sapotaceae stood out. Rare pollen grains of Moraceae occurred. Amid the forest pioneers, in addition to those already mentioned in the two upper levels, Cecropia, several species of Fabaceae-Mimosoideae and Piper were more frequent. Plants characteristic of campo vegetation were mainly represented by Asteraceae, Cyperaceae, Gomphrena, and Poaceae. Fern and mosses spores were also quite significant. The next level below, the 20 cm, is poorer in pollen grains, presenting reduced forest and forest pioneer elements, but a greater relative frequency of campo taxa, such as Asteraceae, Gomphrena, and Poaceae. Pteridophyte and Bryophyte spores continued to be frequent. At the 25 cm level, there was once more an increase in pollen grains diversity. The forest species continued to be the same, with a significant increase in representatives of forest pioneers. Campo species were still present and Pteridophyte spores were much more frequent than those of Bryophytes.
The base level, 30 cm, presented lower pollen grains diversity than the one above, maintaining forest and forest pioneer elements, and Campo species represented only by Asteraceae and Poaceae. Pteridophytes and Bryophytes spores were not observed.
Typha pollen was found at 15 cm and 25 cm levels at very low relative frequency. Eucalyptus pollen was not frequent but was detected at 15, 25, and 30 cm levels.

Discussion
The palynological analysis of the Camorim dam core documented the preservation of the secondary Atlantic Rain Forest, the largest plant community within the Pedra Branca State Park (Oliveira et al. 1995;Solórzano, Guedes-Bruni & Oliveira 2012) throughout the study. The six sampled levels were similar in their palynological composition.
The greatest pollen types richness stood out in the 15 cm sample, followed by the 25 cm sample. These two were interspersed by the 20 cm sample, poor on palynomorphs, which was evidenced in Table 2 and Figures 10 and 11. The lowest number of botanical taxa corresponded to the 0 cm and 10 cm samples in the core upper part, indicating bad preservation conditions. Barth et al. Anu. Inst. Geociênc., 2023;46:54699 Table 2 Botanical taxa represented by the respective pollen grains identified in the six analyzed levels of the Camorim dam core, PBSP, Rio de Janeiro, with more than 10% frequency.

Level 0 cm 10 cm 15 cm 20 cm 25 cm 30 cm
Taxa Common pollen taxa presenting more than 10% relative frequency were recognized in all six samples. The forest taxa belonged to Araceae and Myrtaceae (Myrcia type) families. Fabaceae-Mimosoideae (Anadenanthera and Piptadenia) also occurred at all analyzed levels, but at the 10 cm level, this pollen taxon remained below 10% relative frequency. In addition to these forest taxa, several others could be recognized, at low frequency, standing out respectively of Allophylus, Cupania and Serjania (Sapindaceae), Myrsine (Primulaceae), Annonaceae, Malpighiaceae, Moraceae, and Sapotaceae pollen types.
Along the core, there was a predominance of pollen grains from pioneer forest plants, commonly represented in all samples by Alchornea and Melastomataceae/Combretum pollen types. Important taxa like Mimosa caesalpiniifolia (Mimosaceae) and Piper (Piperaceae) also occurred but at a lower frequency. Interestingly, Cecropia (Urticaceae) pollen, an indicator of secondary forest, was detected at the 15 cm and 30 cm levels, more significantly at the former, indicating a possible good recovery of the forest.
Representatives of Campo vegetation belonged to anemophilous plants, such as Asteraceae, Poaceae, and Cyperaceae. These pollen grains possibly came from more distant areas by wind and water inflow, as no Campo vegetation occurred in the vicinity of the dam.
Pollen grains of Eucalyptus, which is an exotic genus, also showed low representation, but its presence demonstrated the human influence on the region. Plants characteristic of the rain forest, such as Araceae species from a high humid vegetation, were less frequent than pioneer ones.
Regarding the palynomorphs concentration, a significant decrease in palynological deposition occurred at the 20 cm level. Considering the dam's age (built in 1908), this deposition gap could be attributed to torrential rain events with landslides that occurred in Rio de Janeiro city in 1966 and 1967 (Braga 2012  Furthermore, after a large deposition and good preservation of palynomorphs at the 15 cm level, the palynological concentration and relative frequency decreased at 0 cm and 10 cm levels reflecting a long period of few catastrophic events since 1967. After 1988 and several frequent rainfall peaks at shorter intervals, like the 1996 event, (Dereczynski, Calado & Bodstein-de-Barros 2017;Ferreira 2016;Fialho 2012) that may have mobilized the dam bottom sediments, the Park vegetation got better protected under the 1974 legislation.

Figure 10
Relative frequency (%) diagram of palynomorphs according to the established groups for the Camorim dam core.

Figure 11
Concentration diagram of palynomorphs/cm 3 according to the established groups for the Camorim dam core.

Conclusions
Despite the Pedra Branca massif diverse environmental history, including being a major coal production hub until the beginning of the 20th century, the Pedra Branca State Park forest recovered well after the end of coal production and the acquisition by the federal government in 1974. However, the predominance of pioneer plant pollen grains and spores deposited in the Camorim dam highlighted the low representation in the ecological succession of climax species, demonstrating the marks in the forest's structural attributes throughout the secular history of human interventions. The palynological analysis data validated the predominance of a secondary forest, with exotic plant species, and the local absence of open areas.
The vegetation maintained a relatively constant composition, so the lower richness of botanical taxa observed in two levels could be explained by the occurrence of large catastrophic events as in 1966/1967 and 1988. It is possible to affirm that, during the last hundred years, there was no significant change in the vegetation composition of the Camorim dam region, but the sediments and pollen deposition varied due to torrential and prolonged rainfalls that punished the Atlantic slope of the Pedra Branca State Park.