Table 1 Taxa occurrences in modern relicts and LGM fossil pollen records in relation to proposed connectivity patterns between the Andean and Atlantic domains.Paleoclimatic conditions during the LGM by using modern analogs
PCA of the modern distributions of Araucaria angustifolia, Drimys angustifolia, D. brasiliensis, Hedyosmum brasiliense, Podocarpus lambertii and P. sellowii in Brazil in relation to present-day precipitation regimes reveals five clusters associated with four connectivity patterns during the LGM (Fig. 9), as follows:
The Southeastern Central Brazilian highlands (orange ellipse) (19°–25°S) cluster is associated with the Central South American Connectivity (CSAM) within the Atlantic Forest domain, with prevalence in wet and coastal regions populated by A. angustifolia, D. angustifolia, D. brasiliensis, Hedyosmum brasiliense, P. lambertii and P. sellowii.
The Subtropical Southern Brazil (20°–25°S, red ellipse) and Southern Brazilian lowlands (25°–35°S, green ellipse) clusters are associated with the Southern Atlantic-Continental Shelf Connectivity (SACS) in southeastern Brazilian highlands and lowlands, respectively, with prevalence of Araucaria angustifolia (red stars), D. brasiliensis (orange squares), D. angustifolia (green triangles), Hedyosmum brasiliense, P. lambertii (purple diamonds) and P. sellowii (blue circles).
In addition, PCA revealed two clusters of montane Atlantic Forest enclaves in northeastern Brazil (4°–20°S, blue ellipses), separated mostly by annual accumulated precipitation and prevalence of wide latitudinal distributions of Podocarpus sellowii (blue circles) and associated with the Northeastern Atlantic Connectivity (NATC).
The statistical analysis of annual accumulated precipitation in observed occurrences identified differences in tolerance ranges for each species: A. angustifolia, ca. 1700 mm; D. angustifolia, ca. 1750 mm; D. brasiliensis, ca. 1600 mm; H. brasiliense, ca. 1600 mm; P. lambertii, ca. 1500 mm, and P. sellowii, ca. 1450 mm. Annual accumulated precipitation histograms for each species are presented in Fig. 9b and Supplementary Information Fig. S1.
These results allow us to suggest that paleoprecipitation within the SACS connectivity during the LGM in terms of mean annual accumulated precipitation was on the order of ca. 1700Â mm. Because CSAM and NATC connections lacked Araucaria in their domains, we estimate their mean annual precipitation to be ca. 1500Â mm.
Exposed South American Atlantic continental shelf during the LGM
An important fact in South American paleovegetational studies is the impact on the ecology of coastal forests by the global sea level reduction of ca. 120–150 m34,35,36,37,38 and the emergence of the South American continental shelf. We estimated a total exposed area of 1.94 million km2, equivalent in size to the combined areas of France, Spain, Germany, Italy and the United Kingdom and that in southeastern/southern Brazil/Uruguay and Argentina the past coastline was 200–250 km and 500 km distant from its present location, respectively.
This notable addition to the continental ecosystem, equivalent to the area of a large portion of western Europe, could have possibly played a role in the migration process of cool and montane elements under an ecological scenario of niche expansion. A somewhat similar successional mechanism has been observed in the Atlantic rainforest region of northeastern Brazil during sea level fluctuations of the Mid-Holocene with expansions of rainforest and mangrove elements in synchrony with sea level oscillations108.
This ample niche opening for the Atlantic Rainforest, especially to cold and humid-adapted taxa, possibly favored Araucaria and Drimys in the higher southern latitudes of the shelf. An approximation of the possible architecture of this forest is available for lower latitudes via palynological information from the states of Sao Paulo and Espirito Santo.
Niche suitability analysis, represented by Potential distribution Modelling, during the LGM, of pollen records of Brejo do Louro60,61 (site 30) and Ilha do Cardoso51,52 (site 17), in areas presently at sea level and covered by mangrove vegetation, suggests the presence of a vertically stratified subtropical to temperate forest with distinct non-analogous, under cold and humid conditions. The data suggest a forest physiognomy composed of 5 tree layers, based on the average height of the tallest taxon within each genera or family: an emergent layer of 45–50 m composed very likely of Araucaria angustifolia and Virola, a novel combination of taxa, above a 30-m main canopy of Alchornea, Cordia, Didymopanax and Urticaceae/Moraceae, over a 20-m subcanopy of Croton, Eriotheca, Euplassa, Ilex, Melastomataceae, Myrsine, Myrtaceae, Podocarpus, Protium, Sloanea and Sapium, followed by a 10-m understory composed of small trees and shrubs of Drimys, Ericaceae, Hedyosmum, Symplocos, Tapirira and Weinmannia. The forest floor was likely covered by Cyathea tree ferns, Polypodiaceae ground ferns, and other herbs.
We argue that such forest was likely the result of the ability of cold-adapted taxa to migrate downward from the southeastern Brazilian highlands, covering considerable distances on the exposed continental shelf. Such migration capacity could have been enhanced by advantageous anemophilous and zoophilous pollen and zoochorous seed dispersal syndromes5 during periods of favorable climatic change with cooling and sufficient precipitation during the LGM. We believe that the combined forces of these syndromes appear to have been the motor that drove the emergence of novel plant communities with no modern analogs and that other morphological characters and physiological aparatusses must have played an important role in their ability to survive in the new connectivities’ territories. A summary of reproductive strategies for each individual pollen taxa is presented on Table 2.
Table 2 Plant reproductive strategies in each connectivity pattern.
This same pattern of forest formation can be applied to the exposed Amazonian portion of the South American continental shelf based on LGM pollen spectra present in offshore marine sediments (site 53) indicating the presence of a cool and moist tropical forest. Its floristic composition was dominated by the montane taxa Podocarpus, Weinmannia, Myrica, Symplocos, Hedyosmum and Alnus, also suggestive of a downslope migration into the lowlands. A recent analysis of pollen signatures transported by Amazonian rivers to its delta109 suggests nearby terrigenous sources as the main component in marine sediments, which implies that cold and humid forests were thriving in fluvial valleys of eastern Amazonia and by consequence on the exposed coastal shelf during the LGM. A similar scenario of large scale regional vegetation coverage was reported for the LGM of the exposed Southeast Asia-Australasia continental shelf with the predominance of rainforest in wetter areas, resulting in a greater area of rainforest than nowadays110.
Non-analogous plant assemblages
The combined LGM MaxEnt niche suitability maps for all fossil taxa indicate that the proposed connectivity patterns were characterized by expanded ranges of coldâ€tolerant forest taxa, leading to the establishment of a series of plant assemblages without modern analogs. In the SACS and CSAM connections, for instance, cold-adapted montane taxa appear more prevalent at the expense of Cerrado elements as depicted in the Lagoa Bonita (site 39), Chapada dos Veadeiros (site 41), Lagoa dos Olhos (sites 28 and 29), and Serra do Salitre (site 30) records. During the LGM these taxa, currently restricted to cool and humid montane ecosystems, thrived sympatrically along with Cerrado elements such as Antonia ovata Pohl, Byrsonima L., Caryocar A. St.-Hill, Emmotum Benth., Kielmeyera (Spr.) Mart., Mauritia L., Neea Ruiz & Pav., Ouratea Aublet, Qualea Mart., Stryphnodendron Mart. and Vochysia Aublet, among others. Another significant example is given by Araucaria and Podocarpus thriving sympatrically during the LGM with Cerrado elements at Lagoa Santa in south Central Brazil18 and during most of the last glacial cycle (ca. 90–23 cal ka) of the Serra Negra record3. This wide scenario of multiple migrations of subtropical taxa during the final stages of the last glacial cycle offers a plausible explanation for the modern disjunct occurrences of Podocarpus in the semi-deciduous tropical rainforest region of São Paulo111,112,113 and in the semi-arid caatinga domain of Bahia114,115.
It is worth mentioning that SDM and modern occurrences especially of Podocarpus and Hedyosmum in the tepui region of southern Venezuela and northern Brazilian Amazonia (sites 51–52, Lagoa da Pata) depict possible former connections between these areas under a scenario of 1100 m downslope migration23,116 of cold-adapted taxa, forming plant assemblages of warm- and cold-adapted taxa living sympatrically23,106,117 and without modern counterparts.
In summary, we envision these multiple phases of mixed temperate plant assemblages of glacial age in South America as a parallel to the well-established phases of generalized migration of temperate forest taxa in North America118,119 adjusting to the rearrangement of their fundamental niches driven by climatic changes related to Marine Isotope Stage 2 (MIS 2; ~ 29–14 ka).
Former climatic mechanisms and northward displacement of cold air masses
Our overall data confirm that during the Last Glacial Maximum, cold fronts were intensified and could have reached farther north than at present, thus affecting northeastern Brazil. Because the resulting equator-pole temperature gradient was therefore larger, transient systems had probably higher intensity thus causing convergence of humidity precisely where the mean humidity zonal stream flow was high13,15,120,121.
In this period, in the regions near the equator, the changes in humidity and temperature were smaller and, at high latitudes, larger, with lower temperatures and less humidity. Circulations such as Rossby waves were likely to be intensified and amplified, which resulted in the propagation of cold fronts and associated cold air masses to the equatorial region13,97,98,122. This scenario is corroborated by recent cases of the incursion of cold waves from the high latitudes of the southern hemisphere that even cross the equator because of the break in the geostrophic balance caused by the Andes123,124,125,126,127,128. The pressure gradient is maintained (high temperature contrast) with decreasing Coriolis force (decreasing wind)13, thus enhancing the cold air mass acceleration northwards. These cold outbreak events, denominated in Brazil as “friagensâ€, could have been intensified during glacial periods of the South American continent.
Genetic and additional palynological evidence for a late Quaternary Andean-Atlantic connection
A wide array of evidence for historical connections between Andean and Atlantic ecosystems has been provided by genetic analyses in certain groups of animals, such as birds101,129,130,131,132,133, frogs134 and rodents135. Among these studies, there is significant support for a major disjunction between the Andes and the Atlantic Forest between 1Â Ma and 0.15Â Ma, with multiple cycles of connectivity through the Chaco-Southern Brazil and Cerrado, in Central Brazil101,136,137. Other analyses also based on genetic flow between closely related bird species with Andean-Atlantic disjunct distributions focused on the LGM as a more recent decisive moment of connectivity, under humid phases and expansion of forests in both the Cerrado and Chaco routes129.
Genetic analyses of two bird species (Syndactyla rufosuperciliata and S. dimidiata) occurring in both regions suggest genetic flow through two main migration corridors between the Andes and coastal Brazil129: one connecting the southern Andes to the Atlantic via southern Brazil, known as the Chaco Connection, along the main rivers in that region, i.e., the Bermejo and Pilcomayo Rivers. A second route occurred in central Brazil. The authors propose the hypothesis that the connection between the Andean and the Atlantic regions occurred through past forest expansions (i.e., gallery and or semideciduous) in the Cerrado region, and call this the Cerrado Connection101,137. Additional analysis of past distributions of Podocarpus, Ilex, Hedyosmum and Myrsine138, plant genera also occurring in the Restinga vegetation, a sub-set ecosystem of the Atlantic Rainforest, has shown that Central Brazil functioned as a node of migration between the Amazon and Atlantic rainforests, linking the Andes to the central and coastal mountains of Brazil, during glacial periods.
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Publish date : 2024-01-24 03:00:00
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