Differences Between Quaternary And Tertiary Megathermal Forests

Using Quaternary analogs to interpret ecological and climatic successions from the Tertiary raises two main issues; (1) was there a fundamental difference between Quaternary and Tertiary rainforests and (2) were Tertiary species compositions so different from the Quaternary as to make comparison fruitless?

With respect to the first of these issues, it is now clear that there was one major difference between Quaternary rainforests, and those from the Miocene and Early Pliocene. Over the last 2.8 Myr, equatorial climates were, at least intermittently, significantly cooler than at any time since the Oligocene, as indicated by the sudden dispersal of numerous microthermal taxa into equatorial montane forests of each rainforest block in the mid-Pliocene (Morley, 2000a, 2003; Van der Hammen and Hooghiemstra, 2000 and Figure 1.1). This also implies that pre Late Pliocene equatorial climates were a few degrees warmer than today. Quaternary rainforests in the equatorial zone are therefore likely to have exhibited greater altitudinal stratification than in the Neogene since lowland rainforests, which exhibit little internal altitudinal stratification, would have extended to higher altitudes, giving less room for montane forests. During the Early Pliocene, and most of the Miocene, microthermal taxa were essentially missing at equatorial latitudes, or so poorly represented as to go virtually unrecorded in palynological analyses. During the Oligocene, cooler climates resulted in the intermittent expansion of frost-tolerant vegetation into the equatorial zone in a manner not even seen in the Quaternary, clearly shown for the Southeast Asian region (Morley et al., 2003).

Refuge Hypothesis

Figure 1.1. Ice-age distributions of closed canopy megathermal rainforests has been a subject of debate. The refugial hypothesis depicted substantial replacement of forest by savanna during ice ages (e.g., Whitmore and Prance, 1987) (shaded areas) and for Amazonia (Van der Hammen and Hooghiemstra, 2000) (circled by gray line; figure after Morley, 2000a). While historically important, this view of forest fragmentation has now been replaced by paleoecologically-based reconstructions that show much less change in forest cover. Also shown are noteworthy instances of Pliocene and Pleistocene dispersal directions of microthermal taxa into low latitudes.

Figure 1.1. Ice-age distributions of closed canopy megathermal rainforests has been a subject of debate. The refugial hypothesis depicted substantial replacement of forest by savanna during ice ages (e.g., Whitmore and Prance, 1987) (shaded areas) and for Amazonia (Van der Hammen and Hooghiemstra, 2000) (circled by gray line; figure after Morley, 2000a). While historically important, this view of forest fragmentation has now been replaced by paleoecologically-based reconstructions that show much less change in forest cover. Also shown are noteworthy instances of Pliocene and Pleistocene dispersal directions of microthermal taxa into low latitudes.

Figure 1.2. Stratigraphie range of angiosperm and pteridophyte megathermal species, or species pairs, which can be identified on the basis of pollen and spores, and have a well-defined Tertiary fossil record: (1) from Rull (1999); (2) Morley (unpublished); (3) Morley (1991); others from Morley (2000a). Age shown in Myr.

Figure 1.2. Stratigraphie range of angiosperm and pteridophyte megathermal species, or species pairs, which can be identified on the basis of pollen and spores, and have a well-defined Tertiary fossil record: (1) from Rull (1999); (2) Morley (unpublished); (3) Morley (1991); others from Morley (2000a). Age shown in Myr.

The second issue, regarding species composition, can be approached from two angles: rates of speciation, and comparison of Quaternary and Tertiary ecological successions. During the heyday of the "glacial refuge" hypothesis, the suggestion was frequently made that most of the diversity of present day rainforests was essentially a Quaternary phenomenon, with new species being generated by successive isolation and subsequent expansion of populations (Haffer, 1969; Prance, 1982). This scenario always seemed at odds with the pollen record of those megathermal species that can be differentiated on the basis of pollen (Figure 1.2), most of which show very long histories. It is thus comforting that this theory is now discredited on paleoecological grounds (Colinvaux et al., 2000; Bennett, 2004); also, current molecular studies of rainforest trees demonstrate species longevity of the same order as the pollen record

(e.g., Dick et al, 2003), emphasizing that rainforests contain many species of great antiquity; Kutschera and Niklas (2004) estimate that shrubs and hardwoods have mean species durations of 27-34 Myr. With respect to comparing Quaternary and Tertiary ecological successions, the classic study of a Middle Miocene coal from Brunei (Anderson and Muller, 1975), that showed a peat swamp succession with phasic communities with close similarities to those seen in present day peat swamps (Morley, in press), demonstrates close ecological parallels between Neogene and Quaternary vegetation. There is therefore a just case for using Quaternary analogs to interpret Tertiary vegetational scenarios, particularly back as far as the Oligocene.

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