Tropical Rainforests in the Pleistocene. Tropical Rainforests in the Pleistocene. Tropical Rainforests in the Pleistocene

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1 Tropical Rainforests in the Pleistocene tropics stable during Pleistocene? 1 C temperature drop based on 1976 CLIMAP study of warm vs. cold loving forams (vs. 10 C in North Atlantic) Paleothermometers have since pointed to colder and drier tropics vegetation zones in mountains shifted down and up during glacial - interglacial cycles pollen analysis points to 4-6 C cooling in Pleistocene (Hawaii, Bolivia) well characterized events in temperate montane systems Pollen analysis of Mauna Loa cores over the last 40,000 years (Hotchkiss 2001) snow line altitudes in Andean and Hawaiian mountains show lowering during Pleistocene various chemical signatures (CaCO 3, noble gases in water) re-analysis of 1976 CLIMAP data indicates 3-4 C drop Grand Canyon elevation shifts North Great Basin elevation shift

2 palynological data for Central and South America mountains indicate similar shifts (repeatedly?) during Pleistocene and Holocene how zones shifted is still debated repeated shifts probably presented montane species with cycles of sympatry in glacials and allopatry in interglacials speciation pump Lisianthius species complex Central Panama Evidence is pointing to cooler tropics during glacials But what impact on the biogeography of tropical lowland plants and animals? Rainfall levels Jurgen Haffer, geologist and birder, birder, proposed Refugia hypothesis in 1969 (Science,, July 11) observed that present distribution of rainfall over South America gives rise to both forested and non-forested areas Wettest areas the limits of the non-forested habitats closely coincided with annual ppt of mm and that areas > 1500 mm rainfall linked with present day centers of diversity and biogeographical patterns of distribution

3 Rainfall levels Jurgen Haffer, geologist and birder, birder, proposed Refugia hypothesis in 1969 (Science,, July 11) During several dry climatic periods of the Pleistocene and post-pleistocene, the Amazonian forest was divided into a number of smaller forests which were isolated from each other by tracts of open, nonforest vegetation. Wettest areas The remaining forests served as "refuge areas" for numerous populations of forest animals, which deviated from one another during periods of geographic isolation. The isolated forests were again united during humid climatic periods when the intervening open country became once more forest-covered, permitting the refuge- area populations to extend their ranges. This rupturing and rejoining of the various forests in Amazonia probably was repeated several times during the Quaternary and led to a rapid differentiation of the Amazonian forest fauna in geologically very recent times. proposed that climatic fluctuations in Pleistocene forced repeated contraction into refugia in glacials followed by expansion of forest in interglacials allopatric speciation would lead to species pump Refugia recent speciation of birds in the Amazon fits his model species centered over these proposed tropical forest refugia pictured-winged Heliconid butterflies suggested same pattern of correlation of sibling species and tropical forest refugia Toucan sibling species in Amazon basin

4 Sir Ghillean Prance provided many examples of Amazonian plant taxa that supported the tropical forest refugia hypothesis Sir Ghillean Prance provided many examples of Amazonian plant taxa that supported the tropical forest refugia hypothesis Prance proposed 26 refugia based on plants alone Distribution of Chrysobalanaceae in wet tropical forests Plant tropical forest refugia based on centers of plant diversity are correlated with areas of wet conditions during dry periods Tabebuia ochracea (Bignoniaceae) Distribution of this savanna tree Reverse refugia were also found for savanna trees like Tabebuia Savanna refugia were in areas <1500 mm rainfall The elegance of the model was the congruent refugial maps for butterflies, frogs, lizards, and families of plants

5 Pollen evidence for the replacement of rain forests with savannas has been found Hypsithermal aridity? collecting data biased for refugia areas museum model Last interglacial ca. 110kya? Summary pollen diagram from Rondônia, nia, Brazil Collecting intensity of plant taxa used in the construction of refugial arguments ecological reasons for concentration of species in putative refugia soil local climate vegetation type rainfall geology river boundaries ecological reasons for concentration of species in putative refugia soil local climate vegetation type rainfall geology river boundaries Coincidence of range of major zones of habitat uniformity and postulated refugia Coincidence of range of major zones of habitat uniformity and postulated refugia

6 Sediment cores from the Amazon fan provide an insight on the past vegetation of Amazonia at the scale of that landscape and do not support the assumption that large areas of savanna replaced forest Measured variations with depth of the carbon isotope composition (_ 13 C) of soil organic matter (SOM) Detects vegetation changes between communities with varying proportions of C3 vs. C4 photosynthesis OR of forest vs. savanna species Pollen data from modern Amazonian river muds (upper panel) and from ice age deposits of the Amazon Fan The impact of climate changes during the Holocene on vegetation in northern French Guiana Freycon et al. (2010) Quaternary Research 73: Savanna Measured variations with depth of the carbon isotope composition (_ 13 C) of soil organic matter (SOM) Detects vegetation changes between communities with varying proportions of C3 vs. C4 photosynthesis OR of forest vs. savanna species No support for closed forest sites being savanna in the past although one savanna site was more mesic in the past Distribution of intra-specific variation does not fit model of refugia and expected patterns of spatial proximity Forest refugia The impact of climate changes during the Holocene on vegetation in northern French Guiana Freycon et al. (2010) Quaternary Research 73: Licania apetala Chrysobalanaceae

7 Phylogeographic analysis of Amazonian birds based on DNA does not support Refugia model based on either proximity of refugia in area cladogram nor recency of speciation Pre-Pleistocene events! Postulated African refugia Contact points of passerine bird species Assuming refugia, allopatric speciation, and equal rates of expansion from a refugia, present day contact zones of birds are not situated in-between refugia as expected the winner? Gracious losers? Paul Colinvaux Amazon Expeditions: My Quest for the Ice-Age Equator Yale Univ Press, 2008

8 Good read! [see lecture webpage for pdf] But the debate does NOT go away especially in African tropics! sampling across the region including 4 postulated upland forest refugia

9 Bayesian clustering using 10 microsatellite loci identify 4 genetic groups Bayesian clustering using 10 microsatellite loci identify 4 genetic groups Their distributions match that of forest refugia postulated from patterns of species richness and endemism Results cannot be explained on the basis of present-day ecological conditions Similar study done on a wind pollinated and bat dispersed timber tree widespread in Africa Similar study done on a wind pollinated and bat dispersed timber tree widespread in Africa Despite widespread distribution and capability of long distance gene flow, marked geographical genetic structuring Isolated in separate geographical regions probably for several glacial cycles of the Pleistocene

10 Strong patterns of regional differentiation that are consistent with refugial hypotheses for central Africa Similar study done with mtdna and nuclear DNA on 4 subspecies of African gorillas Coalescent theory suggests east-west divergence at 53, ,000 years Geographic distribution of major mitochondrial haplogroups across central Africa