Plant megafossils apparently are not well represented in Central America (and Mexico). Graham (1998) and a series of publications (Graham and Dilcher, 1998; Graham 1999, 2010; Burnham and Graham 1999) provided seminal studies and discussions regarding the Neogene pollen and spores from this region. Even though the records reported upon are derived from lowland swamps and comparable settings, the discussions also provide insight on the plant community composition at higher elevations. Central America is presently typified by its tropical rain forest (Fig. 5), but many plant communities of Miocene, Pliocene, and Pleistocene ages have different compositions, as discussed below. A major factor as regards GABI is the virtual absence of any fossil record of early Pleistocene floras. Therefore, the following begins with a Pliocene background for an interpretation of what was a likely floral and environmental scenario during the Pleistocene, discussed subsequently.
Graham and Dilcher (1998) surveyed a number of Neogene plant communities from Central America, and indicated that the region supported a tropical rain forest during the Miocene and into the Pliocene. These conditions began to change about in the Pliocene. The coastal Paraje Solo flora, SE Veracruz, Mexico (Fig. 8) shows evidence of at least local cooling (presence of Picea) and lowering of ecotones, perhaps due to the effects of local upwelling in the adjacent ocean (Graham 1973, 1976; Akers 1979). In any case the rainforest association was disrupted in this area at about 4–5 Ma (the palynoflora is associated with strata that yield Plantkic Foraminiferal Zone N19; chronology follows Lourens et al. 2004). Farther south, in inland Guatemala, the Padre Miguel flora at about 7 Ma contains a sedge marsh association with grasses and composites along with a pike-oak temperate forest (Alfaroa-Oreomunnea, Juglans, Ulmus) and possibly a cool-temperate forest (Picea, Pinus, Quercus) in upland settings. Another Guatemalan plant community in its southeastern coast, the Herrería flora, about 3.1 Ma (Fig. 8), reflects a mangrove setting and surrounding swampy conditions. Individually and collectively these plant communities contrast with the others shown on Fig. 8 in diverging from the otherwise typical lowland rainforest of the region, and suggest a distinct level of diversity. In that the region had achieved nearly its present elevation and topography by about this time, it is plausible that the current eastern moist versus a western drier distinction also was manifested, at least in the Panamanian district.
Another aspect of floral change is shown by the first occurrence of northern temperate elements in these Central American settings. Graham and Dilcher (1998) indicated that the Paraje Solo flora (ca 4–5 Ma) records the appearance of Abies, Picea, Pinus, Alnus, Celtis, Juglans, Liquidambar, Myrica, Populus, Quercus and Ulmus, whereas the Padre Miguel flora, farther south and older (about 7 Ma), contains even earlier records of Picea, Pinus, Juglans, Quercus, and Ulmus. In fact, the approximately coeval Gatun flora of Panama records an even more distant record of Ulmus.
From these examples, it appears that the typical Neogene lowland tropical rainforest of Central America underwent certain degrees of alteration toward the end of the Miocene and possible cooling or drying as suggested by the incursion of temperate elements and potentially compatible with the late Pliocene land mammal dispersals discussed above. A more coherent pattern is shown at the nearly opposite end of the Neogene/Quaternary interval, as summarized by Piperno (2006), with inferences that this pattern also was manifested in earlier parts of the Pleistocene.
Currently, the Central American lowlands are humid areas. Figure 9, however, shows a reconstruction of the vegetation between 20,000 and 10,500 ybp, including the time of the Last Glacial Maximum (LGM; 18,000 ypb). By this time Central America had achieved its full tectonic uplift (Piperno 2006). Note especially on the Caribbean side the increased coastal area afforded by maximum lowering of sea level. Such then-exposed areas mostly lie adjacent to the thorn woodland, low scrub, and wooded savanna vegetation (type 3 on Fig. 9) but, also in Honduras and Nicaragua, adjacent to forests that were significantly drier than at present, but still moist enough to support that type of forest (type 1 on Fig. 9). These floral associations reflected climates that were significantly drier and cooler than at present, with surface temperatures 40C–70C lower than now, and forest canopies possibly more open than at present due to lower levels of atmospheric CO2 (Piperno 2006). Rainfall was less, possibly by 30%–50%.
The general conditions shown on Fig. 9 persisted until about 10,500 yrs ago, which indicates that the modern lowland tropical forest of Central America did not develop until after that time (also Leyden 1984). Piperno (2006) noted that even today, the flora of western (Pacific) coastal Panama is a deciduous tropical forest living under fairly dry conditions, which, in the above late Pleistocene reconstruction, was the thorn woodland, low scrub, and wooded savanna association (type 3, Fig. 1), and consisted of many grasses, sedges, and herbaceous plants, along with the archetypal savanna indicator shrub or small tree, Curatella americana. The grasses (short and tall) used the C4 photosynthetic pathway, indicative of a dry and open landscape. Palms adapted to dry land habitats also were present. Modern deciduous trees were present only sparingly, likely along watercourses. Rainfall was reduced by 35%. Piperno (2006) suggested that much of the Panamanian land bridge at this time likely was a dry open habitat, but perhaps not exactly a savanna.
Middle-elevation Panamanian late Pleistocene sites (such as La Yeguada, El Valle, Fig. 9) then as now occur at 500–650 m. The Pleistocene vegetation was composed of a forest with trees that nowadays live at 1500 m, but at that time had descended downslope by 800–1200 m, compatible with the dry climate indicated by enclosing sediments (illite clays), as compared to the present 3–4 m annual precipitation.
Piperno (2006) characterized the Pleistocene lowland Neotropical climate as being substantially drier and cooler than at present, as a result of a peculiar combination of low temperature, low precipitation, and low atmospheric CO2. Areas having what is now arboreal forest vegetation supported herbaceous types instead. Some upland areas still would have been moist enough to support C3 plants, but in general drier-adapted C4 grasses prevailed. In situations where the present forest exists in high rainfall zones, the late Pleistocene flora was a savanna/thorny scrub association. The modern flora thus is not analogous to that of the late Pleistocene, and the latter flora contained associations of taxa not presently found together. Forest elements now found at elevations at or above 1500 m descended to much lower elevations (ca 800–1200 m) in the late Pleistocene.
Piperno (2006) noted that, on average, glacial periods last four to five times longer than interglacials, so during the past ca 2 m.y., plants and animals of Central America spent over 80% of their time in, and must have been adapted to, the above-summarized climatic conditions. The modern closed-canopy forests apparently represent those typical of the short-term interglacial floral associations, whereas those of the more prevalent glacial conditions are not like modern floras, and contain combinations of species not presently found together. In that sense, the glacial floras are not analogous to those of the present. As indicated by Bush et al. (2004) for Brazil, the pre-modern Pleistocene floras there contain associations of elements not found together today, and also represent ‘non-analog’ forests. This appears to be comparable to the situation in Central America and, overall, the general patterns are similar. In this context it seems reasonable to project deeper into the early Pleistocene the environmental setting and composition of the late Pleistocene floras of the region, with implications for similar climatic conditions during those older times.
It thus appears that, at the very least, conditions amenable to the dispersal of non tropical-adapted land mammals must have predominated in the Pleistocene of Central America, as well as Amazonia (i.e., Andreissen et al. 1993; below). The increased area of at least dry and open, if not completely savanna-type, environments along coastal Central America during times of decreased sea level must have played a part in facilitating GABI dispersals across the region. Whereas the early Pleistocene record of the strong onset of cooler, glacial conditions seen at Bogotá, Colombia (Andreissen et al. 1993), is not yet recorded in Central America, the Colombian record validates the above proposed projection of late Pleistocene conditions into the earlier part of the epoch, which must have affected Central America, as well.
Andreissen et al. (1993) summarized the palynology and climatic implications of extensive cores drilled in the vicinity of Bogotá, Colombia (Fig. 10) at an elevation of about 2500 m. In contrast to the record in much of Amazonia, as well as Central America, the sequence in the cores, Funza I and Funza II, extends nearly continuously from the Pliocene through the Pleistocene, and records the climatic and tectonic conditions that were in effect during that interval. Funza I core is 357 m deep, Funza II, 586 m deep.
The interval between 541 and 586 m in Funza II has few pollen grains, but in the interval between 465 and 540 m palynofloras show high arboreal percentages and a warmer climate than subsequent parts of the core. The open character of the Andean forest is indicated by high percentages of arboreal taxa (Ilex, Rapanea, Myrica, Eugenia, plus the shrub Borreria, and high maxima of Graminae).
At 465 m in Funza II (and 357 m in Funza I) pollen indicate that conditions became much colder, and the Bogotá basin became alternately covered with Andean forest and open paramo (basically high country tundra) vegetation. The age of this level is estimated at 2.7 Ma, near an ash at horizon 506 m dated at 2.7 ± 0.63 Ma. The climatic conditions recorded here represent the beginning of global glacial influence in this part of Colombia, and was accompanied by the onset of mountain glaciation around Bogotá. Interestingly, the onset of glaciation here fits well with the later part of NHG and onset of GABI I, as discussed above.
The interval between 320 and 357 m (Funza I) or 320–465 m (Funza II) yields alternate indications of cold and warm climates considered to reflect major pulses of glaciation in the vicinity of Bogotá. At 233 m in both Funza cores, the palynofloral record indicates that climatic oscillations became longer and of greater amplitude. The upper 233 m in these cores is interpreted as reflecting ten major climatic cycles, apparently beginning with MIS 23 at about 0.9 Ma (Lisecki and Raymo 2005) and extending to MIS 3 at about 0.05 Ma (50 ka). During these cold episodes, the forest line shifted between about 3400 and 1800 m (arrows on Fig. 10), to follow a decrease of mean annual temperature from 150C to about 60C.
Based on the taxonomy of the units, Andreissen et al. (1993) arranged the succession into biozones. In Biozone I, which ranges in age from about 5.0 Ma to about 4.2 Ma, the plant community consists of lower elevation tropical vegetation. Hedyosmum, a northern taxon, dispersed into the region at this time.
Biozone II ranges in age from about 4.2 Ma to 3.2 Ma, and reflects the presence of tropical to sub-Andean vegetation at relatively low elevations. The floras previously characteristic of the lowlands were replaced by those from intermediate altitudes. Floras indicate a lower tropical to lower sub-Andean forest belt, between ca 1,000 and 1,500 m, as at present (Fig. 10a). Hedyosmum is present, but Myrica still absent.
Biozone III, of 3.2–2.5 Ma is represented by an upper sub-Andean vegetation, and continues to reflect communities characteristic of intermediate altitudes. Myrica disperses into the region. Note that, as discussed above, the Paraje Solo flora (ca 4–5 Ma) of Mexico contains the first record of Myrica in Central America (also as summarized in Graham 1999).
Biozone IV ranges from about 2.5 Ma to 1.2 Ma. In the lower part, plants of Andean type predominate, but in the upper part of the section these alternate with paramo vegetation, and show a consistently high representation of Borreria. The Biozone IV flora is of high-altitude type, and its upper part begins to show the alternating vegetation types reflective of cool and warmer climates and Pleistocene glaciations.
Biozones V-VII range from 1.2 Ma to 0.2 Ma, and contain vegetation that alternates between cool and warm climates considered to be related to major Quaternary climatic changes. Alnus disperses at beginning of biozone VI, Quercus at beginning of biozone VII. Biozone V begins at about 1.2 Ma, Biozone VI at ca 0.8 Ma, and Biozone VII at about 0.2 Ma. Note that the Padre Miguel flora of Guatemala contains the first Central American record of Quercus at about 7 Ma (discussed above).
Tectonic activity in the region is related to Andean uplift (Andreissen et al. 1993). The sediments associated with Biozone I reflect ongoing uplift, and the presence of tropical lowland forests with extensive stands of Mauritia. Biozone II reflects the final Andean uplift in this region.
As indicated above, Myrica dispersed into the region in Biozone III, prior to Pleistocene glaciations, but Alnus still is absent. The associated plants reflect an upper sub-Andean forest, which would have a modern elevation of about 2,200 m (Fig. 10a). By the early part of Biozone IV (ca 2.5–1.8 Ma), the Bogotá area (black dot in valley, Fig. 10a, b) supported an Andean forest belt that would occur at a modern elevation of about 2,600 m (Fig. 10a). Beginning with the later part of Biozone IV and on into biozones V–VII, the Pleistocene changes in climatic conditions resulted in downward altitudinal shifts of vegetation belts along the Cordillera. Paramo vegetation dominated during glacial conditions in the Bogotá district. As at present (Fig. 10a), Andean forests returned to this area during interglacial intervals. Mountain glaciers were present in the glacial times. Quercus entered the record in Biozone VII, likely at 200,000 yrs.
In summary, the Colombian succession records the presence of an open Andean forest that lived under a warm climate at the elevation of Bogotá in the late Pliocene. In the Pleistocene the situation changed markedly in which plants of high-altitude character descended several hundred kilometers to co-exist with lowland elements under much cooler conditions, with mean annual temperatures near 60C in contrast to the current 140C. The cool versus warmer pattern is manifested throughout the Pleistocene in these cores and provides empirical demonstration of the older, as well as late, Pleistocene age of such changes. The Colombian record gives credence to the above supposition that late Pleistocene floral configurations in Central America may be projected into earlier times.
Bush et al. (2004) reported on a pollen succession derived from cores drilled in the Hill of Six Lakes district at an elevation of about 300 m in the Amazon Plain of northwestern Brazil (H6 on inset map of Fig. 10). The site thus represents conditions that occurred at lowland settings in this part of Amazonia. A record developed from three drilled cores extends to about 140,000 yrs and records at least two major glacial cycles.
The results indicate the Hill of Six Lakes region supported a mesic lowland forest that became infiltrated with montane elements during the last two glacial cycles, as a result of major climatic cooling. The arrival and departure of these montane elements occurred in the context of a stable backdrop of a lowland forest. The pollen in the cores indicate that upland elements descended into the lowland environment, with the amount of descent being 800–900 m relative to the modern upland setting. The development of these glacially-timed non-analog mixed lowland and montane communities was in response to the periodic cooling of mean annual temperatures by 5–6 oC. It appears that the indigenous lowland flora was able to withstand the temperature changes that drove the montane taxa to lower elevations.
From the examples cited here, it appears that Pleistocene cooling from Central America to Amazonia was on the order of 9oC to 6oC, respectively, and was the greatest cause of environmental and floral change, regardless of the potential effects of diminished levels of CO2 or rainfall. The effect in Brazil was to periodically lower the elevation at which montane forests existed into those areas where lowland mesic forests already were present, and which remained under the new circumstances. This pattern, over the past 140,000 yrs in the Hill of Six Lakes, Brazil, is comparable to that seen in the temporally more extensive record in the Bogotá plain of Colombia (Andreissen et al. 1993). If these comparisons are used as a model, it suggests that the late Pleistocene floral configurations reconstructed in Central America by Piperno (2006) also can be extended to glacial conditions earlier in the Pleistocene of that region.
An interesting aspect of Fig. 10 is the location of savanna floras in the low, coastal regions of Colombia, comparable to their setting in Central America (Fig. 9). Floral changes in Central America include the emergence of a broader coastal region at times of lowered sea level that nominally would be expected to support savanna-like ecologies, based on those that are reconstructed as lying adjacent to the coast at that time. Graham and Dilcher (1998) consider it unlikely that any given floral association formed a through-going avenue across the whole of Central America during the Pleistocene. Still, it appears that an expanded coastal environment during the 80% of the Pleistocene occupied by glacial conditions would have supplied at least one of the avenues in support of GABI dispersals.