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The Andes through time: evolution and distribution of Andean floras

2022; Elsevier BV; Volume: 27; Issue: 4 Linguagem: Inglês

10.1016/j.tplants.2021.09.010

1878-4372

Oscar A. Pérez‐Escobar, Alexander Zizka, Mauricio A. Bermúdez, Andrea S. Meseguer, Fabien L. Condamine, Carina Hoorn, H. Hooghiemstra, Yuanshu Pu, Diego Bogarín, Lydian M. Boschman, R. Toby Pennington, Alexandre Antonelli, Guillaume Chomicki,

Botany and Geology in Latin America and Caribbean

We present an evolutionary and floristic synthesis of Andean plant diversity and evolution across time and space.Uplift of the Andes varied across time and space. Particularly, the fast uplift rates between 8 and 5 Ma in the Northern Andes may have favoured plant diversification.Using online specimen databases, we suggest that the Andean flora comprises at least 28 691 species. We identify North Andean montane forests as the potential species richest area.Using a biogeographic analysis on a dataset of 14 501 Neotropical species in 194 clades, we reveal that the Andes are both a key source and sink of Neotropical vascular plant biodiversity. We unveil strong biogeographical links between the Andes, Amazonia, and Central America.We highlight a number of critical research gaps, notably major Andean plant groups are still understudied, and fewer studies exist for the Central and Southern Andes. Filling these gaps will allow a more holistic understanding of Andean floras and provide essential tools for their conservation. The Andes are the world's most biodiverse mountain chain, encompassing a complex array of ecosystems from tropical rainforests to alpine habitats. We provide a synthesis of Andean vascular plant diversity by estimating a list of all species with publicly available records, which we integrate with a phylogenetic dataset of 14 501 Neotropical plant species in 194 clades. We find that (i) the Andean flora comprises at least 28 691 georeferenced species documented to date, (ii) Northern Andean mid-elevation cloud forests are the most species-rich Andean ecosystems, (iii) the Andes are a key source and sink of Neotropical plant diversity, and (iv) the Andes, Amazonia, and other Neotropical biomes have had a considerable amount of biotic interchange through time. The Andes are the world's most biodiverse mountain chain, encompassing a complex array of ecosystems from tropical rainforests to alpine habitats. We provide a synthesis of Andean vascular plant diversity by estimating a list of all species with publicly available records, which we integrate with a phylogenetic dataset of 14 501 Neotropical plant species in 194 clades. We find that (i) the Andean flora comprises at least 28 691 georeferenced species documented to date, (ii) Northern Andean mid-elevation cloud forests are the most species-rich Andean ecosystems, (iii) the Andes are a key source and sink of Neotropical plant diversity, and (iv) the Andes, Amazonia, and other Neotropical biomes have had a considerable amount of biotic interchange through time. The Andes are thought to contain ~10% of the world's vascular plant diversity (30 000 species) in only 0.6% of its land surface [1.Mittermeier R.A. et al.Biodiversity hotspots.in: Zachos F.E. Habel J.C. Global Biodiversity Conservation: The Critical Role of Hotspots. Springer, 2011: 3-22Google Scholar]. With only 25% of the original vegetation remaining, the Andes are the world's most species-rich plant biodiversity conservation hotspot [2.Myers N. et al.Biodiversity hotspots for conservation priorities.Nature. 2000; 403: 853-858Google Scholar]. The Andean mountains played a pivotal role in generating the biodiversity that colonized various regions of the Neotropics across timescales, notably contributing to the rich plant diversity of Amazonia and Central America [3.Antonelli A. et al.Tracing the impact of the Andean uplift on Neotropical plant evolution.Proc. Natl. Acad. Sci. U. S. A. 2009; 106: 9749-9754Google Scholar, 4.Antonelli A. et al.Amazonia is the primary source of Neotropical biodiversity.Proc. Natl. Acad. Sci. U. S. A. 2018; 115: 6034-6039Google Scholar, 5.Hoorn C. et al.Amazonia through time: Andean uplift, climate change, landscape evolution, and biodiversity.Science. 2010; 330: 927-931Google Scholar, 6.Pérez-Escobar O.A. et al.Recent origin and rapid speciation of Neotropical orchids in the world's richest plant biodiversity hotspot.New Phytol. 2017; 215: 891-905Google Scholar, 7.Zizka A. Big data suggest migration and bioregion connectivity as crucial for the evolution of Neotropical biodiversity.Front. Biogeogr. 2019; 11e40617Google Scholar]. Andean ecosystems also provide livelihoods and essential ecosystem services, sustaining millions of people [8.Pérez-Escobar O.A. et al.Mining threatens Colombian ecosystems.Science. 2018; 359: 1475Google Scholar]. Despite this, research on the evolution of Andean plants has been sporadic. Three major factors hinder our understanding of the origin and evolution of the Andean flora: (i) insufficient or incomplete knowledge of the Andean orogeny, with sometimes conflicting hypotheses [9.Garzione C.N. et al.Rise of the Andes.Science. 2008; 32: 1304-1307Google Scholar,10.Ehlers T.A. Poulsen C.J. Influence of Andean uplift on climate and paleoaltimetry estimates.Earth Planet. Sci. Lett. 2009; 281: 238-248Google Scholar], (ii) poor understanding of plant species richness and plant distribution patterns across the Andes, largely because of insufficient floristic surveys [11.Antonelli A. et al.Conceptual and empirical advances in Neotropical biodiversity research.PeerJ. 2018; 6e5644Google Scholar], and (iii) the scarcity of genetic data and time-calibrated phylogenies for most Andean lineages [12.Antonelli A. Sanmartín I. Why are there so many plant species in the Neotropics?.Taxon. 2011; 60: 403-414Google Scholar]. Our synthesis has five major aims: (i) to review the geological history of the Andes throughout its entire range to inform biological research, (ii) to estimate plant species diversity across the Andes, (iii) to synthesize our understanding of the species richness and ages of Andean ecosystems, and (iv) to use our new estimate of Andean plant diversity to dissect the migration routes of Andean plants, as well as (v) to identify priority groups for which few sequence data are available. Our work reveals key knowledge gaps which can inform future research and conservation work in the Andes. The Andes extend over 7000 km in South America from ~10°N to 50°S. This mountain range was formed as a result of subduction (see Glossary) of the oceanic Nazca and Caribbean plates under the South American continental plate. The South American subduction zone is one of the oldest in the world, dating back to ~200 million years ago (Ma; Early Jurassic). However, the current Nazca plate subduction is thought to have initiated more recently, at ~80 Ma (Late Cretaceous) [13.Chen Y.W. et al.Southward propagation of Nazca subduction along the Andes.Nature. 2019; 565: 441-447Google Scholar]. Characterized by different geological histories, the Andes can be divided into three sections which broadly coincide with political borders: the Southern Andes (Argentina and Chile), the Central Andes (Peru and Bolivia), and the Northern Andes (Venezuela, Colombia, and Ecuador) (Figure 1A ). The limits of the Northern and Central Andes are mainly shaped by the complex configuration of the Nazca and Caribbean plates, by changes in the slope of subduction, and by interactions with precursor plates (i.e., Farallón and Phoenix) [14.Gianni G.M. et al.Transient plate contraction between two simultaneous slab windows: insights from Paleogene tectonics of the Patagonian Andes.J. Geodyn. 2018; 121: 64-75Google Scholar] (Figure 1A). By contrast, the Southern Andes are delineated by the interaction of the Antarctic, Scotia, and South American plates (e.g., [15.Horton B.K. Tectonic regimes of the central and southern Andes: responses to variations in plate coupling during subduction.Tectonics. 2018; 37: 402-429Google Scholar, 16.Gutscher M.A. et al.The 'lost Inca Plateau': cause of flat subduction beneath Peru? Earth Planet.Sci. Lett. 1999; 171: 335-341Google Scholar, 17.Ramos V.A. Anatomy and global context of the Andes: main geologic features and the Andean orogenic cycle.in: Mahlburg Kay S. Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision. Geological Society of America, 2009: 31-65Google Scholar, 18.Schepers G. et al.South-American plate advance and forced Andean trench retreat as drivers for transient flat subduction episodes.Nat. Commun. 2017; 8: 15249Google Scholar]). The boundary between the Northern and Central Andes is marked by subduction of the Carnegie Ridge, a high on the Nazca Plate, which dives under the South American Plate in Ecuador. This geological phenomenon is geographically expressed by a depression across the Andes, known as the Huancabamba Depression [19.Mitouard P. et al.Post-Oligocene rotations in southern Ecuador and northern Peru and the formation of the Huancabamba deflection in the Andean Cordillera.Earth Planet. Sci. Lett. 1990; 98: 329-339Google Scholar,20.Michaud F. et al.Influence of the subduction of the Carnegie volcanic ridge on Ecuadorian geology: reality and fiction.in: Mahlburg Kay S. Backbone of the Americas: Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision. Geological Society of America, 2009: 217-228Google Scholar]. In addition, the northern Andes have been shaped by interactions with the Caribbean Plate, of which the leading edge collided with the northwestern corner of the South American Plate at ~100 Ma. Collision of the trailing edge of the Caribbean Plate, at ~80–70 Ma, led to initiation of uplift in the Ecuadorian Andes, and since this time the Caribbean plate has been attached to South America, while moving toward its present-day position [21.Kennan L. Pindell J.L. Dextral shear, terrane accretion and basin formation in the Northern Andes: best explained by interaction with a Pacific-derived Caribbean Plate?.in: Pankhurst B. The Origin and Evolution of the Caribbean Plate. Geological Society of London, 2009: 487-531Google Scholar]. The Andean orogeny is the subject of intense study, and some issues remain contentious – including the timing, pace, and sequence of mountain building. Reconstructing mountain building is challenging because the geological record does not provide a direct measure of past elevation, paleo-altimetry methods contain large uncertainties and many caveats [22.Rowley D.B. Garzione C.N. Stable isotope-based paleoaltimetry.Annu. Rev. Earth Planet. Sci. 2007; 35: 463-508Google Scholar], and continental ranges are subject to erosion which results in a highly incomplete rock record [23.Boschman L.M. Andean mountain building since the Late Cretaceous: a paleoelevation reconstruction.Earth-Sci. Rev. 2021; 103640Google Scholar]. Data from the sedimentary basins east of the Andes indicate that uplift in the Southern Andes started at ~100 Ma, in the Northern Andes at ~80 Ma, and in the Central Andes at ~70 Ma [24.Horton B.K. Sedimentary record of Andean mountain building.Earth-Sci. Rev. 2018; 178: 279-309Google Scholar] (Figure 1C–K). These ages are in line with exhumation ages, which are oldest for the Southern Andes (Campanian–Paleocene, 75–55 Ma) and younger for the Central and Northern Andes [5.Hoorn C. et al.Amazonia through time: Andean uplift, climate change, landscape evolution, and biodiversity.Science. 2010; 330: 927-931Google Scholar,23.Boschman L.M. Andean mountain building since the Late Cretaceous: a paleoelevation reconstruction.Earth-Sci. Rev. 2021; 103640Google Scholar]. During those 100 million years of mountain building, however, uplift has not been constant or uniform across time and space, and there is debate in particular about the uplift history of the eastern domains of the Northern and Central Andes (the Eastern Cordillera of Colombia and the Altiplano). Some studies have presented evidence for remarkably rapid uplift during the Miocene [25.Gregory-Wodzicki K.M. Uplift history of the Central and Northern Andes: a review.Geol. Soc. Am. Bull. 2000; 112: 1091-1105Google Scholar,26.Garzione C.N. et al.Rapid late Miocene rise of the Bolivian Altiplano: evidence for removal of mantle lithosphere.Earth Planet. Sci. Lett. 2006; 241: 543-556Google Scholar], which, for the Northern Andes, has been associated with fast species diversification [6.Pérez-Escobar O.A. et al.Recent origin and rapid speciation of Neotropical orchids in the world's richest plant biodiversity hotspot.New Phytol. 2017; 215: 891-905Google Scholar,27.Lagomarsino L.P. et al.The abiotic and biotic drivers of rapid diversification in Andean bellflowers (Campanulaceae).New Phytol. 2016; 210: 1430-1442Google Scholar]. By contrast, other researchers regard the rise of the Andes as a gradual process from the Eocene (40 Ma) onwards [10.Ehlers T.A. Poulsen C.J. Influence of Andean uplift on climate and paleoaltimetry estimates.Earth Planet. Sci. Lett. 2009; 281: 238-248Google Scholar,28.Barnes J.B. Ehlers T.A. End member models for Andean Plateau uplift.Earth Sci. Rev. 2009; 97: 105-132Google Scholar, 29.Ramos V.A. et al.The Andean thrust system – latitudinal variations in structural styles and orogenic shortening.in: McClay K.R. Thrust Tectonics and Hydrocarbon Systems. American Association of Petroleum Geologists, 2004: 30-50Google Scholar, 30.Barke R. Lamb S. Late Cenozoic uplift of the Eastern Cordillera, Bolivian Andes.Earth Planet. Sci. Lett. 2006; 249: 350-367Google Scholar, 31.Hartley A.J. et al.A comment on 'Rapid late Miocene rise of the Bolivian Altiplano: evidence for removal of mantle lithosphere' by CN Garzione et al. (Earth Planet. Sci. Lett. 241 (2006) 543-556).Earth Planet. Sci. Lett. 2007; 259: 625-629Google Scholar, 32.Insel N. et al.Response of meteoric δ18O to surface uplift – implications for Cenozoic Andean Plateau growth.Earth Planet. Sci. Lett. 2012; 317: 262-272Google Scholar]. A recent reconstruction of Andean mountain building, integrating paleo-altimetry data from 36 separate geomorphological domains across the Andes, shows that each of these domains has an independent history of surface uplift, and that uplift of the Andes has thus been a highly diachronous process [23.Boschman L.M. Andean mountain building since the Late Cretaceous: a paleoelevation reconstruction.Earth-Sci. Rev. 2021; 103640Google Scholar]. The reconstruction shows that, since the Late Cretaceous, uplift generally migrated from the coastal and western cordilleras eastwards – toward the central and eastern cordilleras and sub-Andean zone (Figure 1C–K). Whereas uplift in the coastal and western cordilleras is generally old, slow, and constant, the central and eastern cordilleras, large parts of the Northern Andes, and the Altiplano all uplifted through young and rapid orogenesis with acceleration phases in the Oligocene and Miocene [9.Garzione C.N. et al.Rise of the Andes.Science. 2008; 32: 1304-1307Google Scholar,25.Gregory-Wodzicki K.M. Uplift history of the Central and Northern Andes: a review.Geol. Soc. Am. Bull. 2000; 112: 1091-1105Google Scholar,33.Leier A. et al.Stable isotope evidence for multiple pulses of rapid surface uplift in the Central Andes, Bolivia.Earth Planet. Sci. Lett. 2013; 371: 49-58Google Scholar,34.Garzione C.N. et al.Tectonic evolution of the Central Andean plateau and implications for the growth of plateaus.Annu. Rev. Earth Planet. Sci. 2017; 45: 529-559Google Scholar]. Most importantly, this reconstruction shows that drawing generalized conclusions about the history of uplift in the Andes as a whole is not warranted. We used this model [23.Boschman L.M. Andean mountain building since the Late Cretaceous: a paleoelevation reconstruction.Earth-Sci. Rev. 2021; 103640Google Scholar] to present the main phases of Andean uplift (Figure 1C–K). In addition, we present a map of apatite fission track (AFT) ages that reveal the cooling ages of Andean rocks across its range (Figure 1B), which may generally be associated with uplift. Young AFT ages can be seen across the Northern Andes, mirroring recent uplift. Nevertheless, the whole range, and the Central and Southern Andes in particular, show interspersions of older and younger age (Figure 1B). This confirms that the timing and rate of Andean uplift have been highly uneven across its range. This new insight conflicts with what is often modeled in macroevolutionary studies attempting to link plant species diversification rate with Andean uplift [6.Pérez-Escobar O.A. et al.Recent origin and rapid speciation of Neotropical orchids in the world's richest plant biodiversity hotspot.New Phytol. 2017; 215: 891-905Google Scholar,27.Lagomarsino L.P. et al.The abiotic and biotic drivers of rapid diversification in Andean bellflowers (Campanulaceae).New Phytol. 2016; 210: 1430-1442Google Scholar,35.Hughes C. Eastwood R. Island radiation on a continental scale: exceptional rates of plant diversification after uplift of the Andes.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 10334-10339Google Scholar]. Thus, future diversification models implementing Andean elevation as a time-dependent variable should avoid relying on a single uplift curve produced for an entire Cordillera, and should instead consider uplift heterogeneity as a function of species occurrences, whenever biological resolution allows [36.Boschman L.M. Condamine F.L. Mountain radiations are not only rapid and recent: ancient diversification of South American frog and lizard families related to Paleogene Andean orogeny and Cenozoic climate variations.BioRxiv. 2021; (Published online April 26, 2021)https://doi.org/10.1101/2021.04.24.441240Google Scholar]. The Andean orogeny has affected regional climate, hydrological conditions, nutrient cycling, landscape development, and thus potential plant evolution mechanisms at the continental scale. In the Northern and Central Andes, uplift increased rainfall east of the mountain range (and established a rain shadow with dry conditions in the west) and sediment flux into Amazonia [37.Hoorn C. et al.Andean tectonics as a cause for changing drainage patterns in Miocene northern South America.Geology. 1995; 23: 237-240Google Scholar, 38.Armijo R. et al.Coupled tectonic evolution of Andean orogeny and global climate.Earth-Sci. Rev. 2015; 143: 1-35Google Scholar, 39.Flantua S.G. et al.The flickering connectivity system of the north Andean páramos.J. Biogeogr. 2019; 46: 1808-1825Google Scholar]. This resulted in the current configuration of the Amazon drainage basin with precursors such as the Pebas and Acre depositional systems [5.Hoorn C. et al.Amazonia through time: Andean uplift, climate change, landscape evolution, and biodiversity.Science. 2010; 330: 927-931Google Scholar,40.Hoorn C. et al.The Amazon at sea: onset and stages of the Amazon River from a marine record, with special reference to Neogene plant turnover in the drainage basin.Glob. Planet Change. 2017; 153: 51-65Google Scholar] and in the establishment of the 'South American Dry Diagonal' consisting of the Caatinga, the Cerrado, and the Chaco biomes (e.g., [41.Werneck F.P. et al.Deep diversification and long-term persistence in the South American 'dry diagonal': integrating continent-wide phylogeography and distribution modeling of geckos.Evolution. 2012; 66: 3014-3034Google Scholar,42.Azevedo J.A. et al.On the young savannas in the land of ancient forests.in: Rull V. Carnaval A.C. Neotropical Diversification: Patterns and Processes. Springer, 2020: 271-298Google Scholar]). It also led to the formation of an orographic rain shadow on the foothills of the Central and Southern Andes [43.Blisniuk P.M. et al.Climatic and ecologic changes during Miocene surface uplift in the Southern Patagonian Andes.Earth Planet. Sci. Lett. 2005; 230: 125-142Google Scholar,44.Rohrmann A. et al.Reconstructing the Mio-Pliocene South American monsoon and orographic barrier evolution (Angastaco Basin, NW Argentina).in: Proceedings of the American Geophysical Union Fall Meeting 2013, PP43B-2093. AGU, 2013Google Scholar] from late Miocene (~11 Ma) onwards. As for the latter phenomenon, AFT data have revealed swift mountain uplift in the past 8–5 Ma in the Northern Andes (the Cocuy area of the Eastern Cordillera in particular [45.Bermúdez M.A. et al.Exhumation-denudation history of the Maracaibo Block, Northwestern South America: insights from thermochronology.in: Cediel F. Shaw R.P. Geology and Tectonics of Northwestern South America. Springer, 2019: 879-898Google Scholar, 46.Bermúdez M.A. et al.Exhumation of the southern transpressive Bucaramanga fault, Eastern Cordillera of Colombia: insights from detrital, quantitative thermochronology and geomorphology.J. S. Am. Earth Sci. 2020; 103057Google Scholar, 47.Siravo G. et al.Constraints on the Cenozoic deformation of the northern Eastern Cordillera, Colombia.Tectonics. 2018; 37: 4311-4337Google Scholar, 48.Siravo G. et al.Slab flattening and the rise of the Eastern Cordillera, Colombia.Earth Planet. Sci. Lett. 2019; 512: 100-110Google Scholar]), but less so in the Central and Southern Andes. This scenario is supported by dated phylogenies from various plant groups showing young ages and rapid diversifications in the Northern Andes, but older ages in Central and Southern Andes [4.Antonelli A. et al.Amazonia is the primary source of Neotropical biodiversity.Proc. Natl. Acad. Sci. U. S. A. 2018; 115: 6034-6039Google Scholar,5.Hoorn C. et al.Amazonia through time: Andean uplift, climate change, landscape evolution, and biodiversity.Science. 2010; 330: 927-931Google Scholar,35.Hughes C. Eastwood R. Island radiation on a continental scale: exceptional rates of plant diversification after uplift of the Andes.Proc. Natl. Acad. Sci. U. S. A. 2006; 103: 10334-10339Google Scholar,49.Madriñán S. et al.Páramo is the world's fastest evolving and coolest biodiversity hotspot.Front. Genet. 2013; 4: 192Google Scholar,50.Luebert F. Weigend M. Phylogenetic insights into Andean plant diversification.Front. Ecol. Evol. 2014; 2: 27Google Scholar]. Another possible explanation for this pattern is that erosion in the tropical Andes could have been substantially higher than in the Southern Andes (Figure 1A), where more extensive ice caps would have slowed erosion [38.Armijo R. et al.Coupled tectonic evolution of Andean orogeny and global climate.Earth-Sci. Rev. 2015; 143: 1-35Google Scholar,51.Vuille M. et al.Rapid decline of snow and ice in the tropical Andes – impacts, uncertainties and challenges ahead.Earth Sci. Rev. 2018; 176: 195-213Google Scholar,52.Anderson E.P. et al.Consequences of climate change for ecosystems and ecosystem services in the tropical Andes.in: Herzog S.K. Climate Change and Biodiversity in the Tropical Andes. Inter-American Institute for Global Change Research (IAI) and Scientific Committee on Problems of the Environment (SCOPE), 2011: 1-18Google Scholar]. Thus, three take-home messages on Andean orogeny should be carefully considered in future studies of plant diversification and biogeography in the Andes: (i) Andean uplift was highly diachronous, starting in the Southern Andes at ~100 Ma, in the Northern Andes at ~80 Ma, and subsequently in the Central Andes at ~70 Ma. (ii) Uplift in the coastal and western cordilleras was generally old, slow, and constant, but the central and eastern cordilleras, large parts of the Northern Andes, and the Altiplano uplifted through young and rapid orogenesis with acceleration phases in the Oligocene and Miocene. (iii) This argues against using a single uplift curve in a diversification or biogeographic context. To gain insights into the biotic assembly, diversity, and distribution of Andean floras, we investigated Andean plant species diversity using global distribution databases, and generated a working list of Andean vascular plants (see Materials and Methods in the supplemental information online) based on the list of Neotropical plants of Ulloa et al. [53.Ulloa C. et al.An integrated assessment of the vascular plant species of the Americas.Science. 2017; 358: 1614-1617Google Scholar], GBIF global distribution databasesi and taxonomic expertise. We identify 28 691 tentative Andean vascular plant species, defined as species currently occurring in the Andean cordillera at an elevational range between 100 and 6086 m. We suggest that this may be an underestimate; even if some species are lumped taxonomically in future, the Andes may house other species that have not yet been digitized and georeferenced, and others remain to be scientifically described. The elevational delimitation of the Andes is contentious [54.Cuatrecasas J. Aspectos de la vegetación natural de Colombia.Rev. Acad. Colomb. Cienc. Exact. Fis. Nat. 1958; 10: 221-264Google Scholar], and a multitude of studies rely on different elevational ranges starting at 100, 500, and 1000 m [55.Stadel C. Altitudinal belts in the tropical Andes: their ecology and human utilization.in: Yearbook – Conference of Latin Americanist Geographers. Vol. 17/18. University of Texas Press, 1991: 45-60Google Scholar, 56.Bernal M.H. Lynch J.D. Review and analysis of altitudinal distribution of the Andean anurans in Colombia.Zootaxa. 2008; 1826: 1-25Google Scholar, 57.Dussaillant I. et al.Two decades of glacier mass loss along the Andes.Nat. Geosci. 2019; 12: 802-808Google Scholar]. To assess the robustness of our elevational delimitation, we compiled additional lists of Andean species with elevation ranges starting at 500 and 1000 m (instead of 100 m) to 6086 m and found a difference of 3–20%, respectively, between the species richness reported when using a lower altitudinal bound of 100 m. This shows that the 'lowland' (100–500 m) and the 'premontane' (500–1000 m) intervals share many species, and that there is more floristic difference at elevations greater than 1000 m, consistent with previous biome reconstructions using pollen fossil data [58.Wille M. et al.Environmental change in the Colombian subandean forest belt from 8 pollen records: the last 50 kyr.Veg. Hist. Archaeobotany. 2001; 10: 61-77Google Scholar]. The Andean flora is a highly uneven assemblage of the plant tree of life. Only 10 plant families (Orchidaceae, Asteraceae, Leguminosae, Rubiaceae, Melastomataceae, Bromeliaceae, Piperaceae, Solanaceae, Araceae, and Poaceae) make up about half of all Andean plant species, while 226 plant families account for the remaining Andean plant diversity (Figure 2A and see Dataset S1 in the supplemental information onlineii). The top 10 families in numbers of species are the same across the Andean elevation gradient up to >2000 m, but show turnover at >3000 m and >4000m, where 30% and 50% of the families change, respectively, and four of the top 10 families are exclusive to the high elevation flora above 4000 m (Figure 2A). A suggested hyper-dominance of a reduced number of families on the diversity of Andean plants was first noted by Cuatrecasas [54.Cuatrecasas J. Aspectos de la vegetación natural de Colombia.Rev. Acad. Colomb. Cienc. Exact. Fis. Nat. 1958; 10: 221-264Google Scholar], and later by Gentry [59.Gentry A.H. Neotropical floristic diversity: phytogeographical connections between Central and South America, Pleistocene climatic fluctuations, or an accident of the Andean orogeny?.Ann. Missouri Bot. Gard. 1982; 69: 557-593Google Scholar], but a comparison of the 10 most species-rich families of the Neotropical plant list [53.Ulloa C. et al.An integrated assessment of the vascular plant species of the Americas.Science. 2017; 358: 1614-1617Google Scholar] and Neotropical dry forests [60.DRYFLOR – Latin American and Caribbean Seasonally Dry Tropical Forest Floristic Network Plant diversity patterns in neotropical dry forests and their conservation implications.Science. 2016; 353: 1383-1387Google Scholar] show a similar pattern where 10 dominant families account for half of the diversity, suggesting that this pattern is not specific to the Andean flora. The classification, distribution, and diversity of such a rich array of Andean ecosystems have been investigated for decades. Numerous systems have been proposed mostly based on their altitudinal position, climatic characteristics, and floristic associations [54.Cuatrecasas J. Aspectos de la vegetación natural de Colombia.Rev. Acad. Colomb. Cienc. Exact. Fis. Nat. 1958; 10: 221-264Google Scholar,61.Cleef A.M. The phytogeographical position of the Neotropical vascular paramo flora.in: Larsen K. Holm-Nielsen L.B. Tropical Botany. Academic Press, 1979: 175-184Google Scholar,62.Rangel-Chui J.O. Diversidad Biótica IV: El Chocó Biogeografico/Costa Pacífica. Universidad Nacional de Colombia, Instituto de Ciencias Naturales, and Conservación Internacional, 2011Google Scholar]. Nevertheless, which Andean ecosystems are the most species-rich and the similarities of the diversity they share remain open questions (see Outstanding questions). Gentry [59.Gentry A.H. Neotropical floristic diversity: phytogeographical connections between Central and South America, Pleistocene climatic fluctuations, or an accident of the Andean orogeny?.Ann. Missouri Bot. Gard. 1982; 69: 557-593Google Scholar] suggested that Andean plant diversity is mostly concentrated in the Northern Andes, a geologically discrete section of the cordillera that hosts a wide diversity of vegetation types [62.Rangel-Chui J.O. Diversidad Biótica IV: El Chocó Biogeografico/Costa Pacífica. Universidad Nacional de Colombia, Instituto de Ciencias Naturales, and Conservación Internacional, 2011Google Scholar, 63.Olson D.M. et al.Terrestrial ecoregions of the world: a new map of life on Earth: a new global map of terrestrial ecoregions provides an innovative tool for conserving biodiversity.BioScience. 2001; 51: 933-938Google Scholar, 64.Cleef A.M. The vegetation of the páramos of the Colombian Cordillera Oriental.Meded. Bot. Mus. Herb. Rijksuniv. Utrecht. 1981; 481: 1-320Google Scholar]. Our review is in line with these results, pointing to the hotspots of Andean vascular plants in the Northern Andes (Figure 2B). However, this pattern correlates with the number of collections and thus sampling effort, which likely bias the real floristic contribution of other regions (