Forests of the Past: a Window to Future Changes
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Forests in Flux the last glaciation involved species reshuffling, PERSPECTIVE changes in relative abundances, and local and permanent extinctions (5, 6). Forests in high- latitude regions became established within the Forests of the Past: Holocene (past 11,600 years) as a result of pop- ulation invasions from southern glacial refugia A Window to Future Changes into previously ice-covered terrains and local expansions of small tree populations that sur- Rémy J. Petit,1,2* Feng Sheng Hu,3 Christopher W. Dick4,5 vived the Last Glacial Maximum (~20,000 years ago) in “cryptic refugia” (7, 8). The study of past forest change provides a necessary historical context for evaluating the outcome A key recent development in studies of past of human-induced climate change and biological invasions. Retrospective analyses based on vegetation is the fusion of genetic and fossil data. fossil and genetic data greatly advance our understanding of tree colonization, adaptation, and Fossil records are indispensable for vegetation extinction in response to past climatic change. For instance, these analyses reveal cryptic refugia reconstruction and can help constrain inferences near or north of continental ice sheets, leading to reevaluation of postglacial tree migration rates. of historic events from genetic data. But fossil Species extinctions appear to have occurred primarily during periods of high climatic variability. records rarely offer any information on population- Transoceanic dispersal and colonization in the tropics were widespread at geological time scales, level processes (e.g., founder events, migration inconsistent with the idea that tropical forests are particularly resistant to biological invasions. of specific lineages). Geographic patterns of DNA polymorphisms are traces, albeit often some- or Heraclitus, a 6th-century BCE Greek changes, with large-scale species’ range shifts, what fuzzy, of such processes. When fossil and philosopher and naturalist, “change is the population contractions and extinctions, as well genetic data are combined, much information can Fonly constant in nature.” Trees are orga- as aggregation and disassociation of forest com- be acquired about the whereabouts of small pop- nisms of exceptional size and longevity, and are munities. At low latitudes, forests experienced al- ulations during the last glaciation and the trajec- symbols of stability and resilience in the living titudinal shifts and range fragmentation, although tories of postglacial population spread (Fig. 1). on July 25, 2008 world. Yet, trees are no exception to Heraclitus’ many tropical and subtropical tree species we Recent studies that applied this integrative ap- rule. Their ranges have been and continue to be see today persisted in the same region through proach have offered new insights. For example, extremely dynamic. In some parts of the world, glacial and interglacial intervals (4). At mid- some temperate and boreal trees apparently sur- tree species have started to shift their distributions latitudes, forest development after the end of vived the Last Glacial Maximum in periglacial in response to anthropogenic climatic warming (1, 2). Given the long generation time of trees, 10W 70N 5W 35E and possible migrational lags, these shifts fore- 0 30E 70N 5E 10E 15E 20E 25E shadow the more pronounced future changes. Knowledge of past forest change can inform www.sciencemag.org current predictions and conservation options. 65N 65N Paleo-studies offer abundant evidence for cli- 8000 matic changes and vegetation shifts at various spatial and temporal scales. During the Quater- 60N nary period (roughly 1.8 million years ago to the 60N present), glacial-interglacial climatic variations 9000 occurred as a result of changes in controlling 9000 8000 ’ 10000 factors such as Earth s orbital parameters, con- Downloaded from 55N 0 0 9000 0 55N tinental ice sheets, sea-surface temperature, and 0 1 9000 atmospheric CO2 concentration. Abrupt climatic 0 0 0 000 events at much shorter time scales have also 11 0 10 0 0 0 3 1 occurred ( ), some of which have pronounced 50N 1 1000 50N magnitudes and rates (e.g., temperature shifts of 9000 11000 up to ~10°C within a few decades during the 110 0 0 last glacial-interglacial transition in some areas). 1000 0 11000 11000 Networks of fossil pollen and plant macrofossils 45N 10000 9000 11000 45N show that in response to these climatic fluctua- 13000 00 12000 12000 0 tions, the biosphere has experienced dramatic 12 10000 13000 14000 1400 12000 1 1 0 30 INRA, UMR1202 Biodiversity, Genes and Communities, 69 40N 00 1500 40N Route d’Arcachon, F-33612 Cestas, France. 2University of 10 0 000 Bordeaux I, UMR1202 Biodiversity, Genes and Communities, 12 11000 ’ 3 000 69 Route d Arcachon, F-33612 Cestas, France. Departments of 1 10003 2 0 14000 Plant Biology and Geology, and Program in Ecology, Evolution, 0 0 35N 15000 and Conservation Biology, University of Illinois, Urbana, IL 35N 4 10 61801, USA. Department of Ecology and Evolutionary Biology W 35E 5W and Herbarium, University of Michigan, 830 North University 30E 5 0 25E Avenue, Ann Arbor, MI 48103, USA. Smithsonian Tropical Re- 5E 10E 15E 20E search Institute, Post Office Box 0843-03092, Balboa, Ancón, Republic of Panama. Fig. 1. Migration timing (isochrone curves) and routes (arrows) of Pinus in Europe inferred from *To whom correspondence should be addressed. E-mail: paleobotanical and modern genetic data. The red, light green, and dark green arrows correspond [email protected] to the migration routes for different maternal lineages of Pinus sylvestris (8). 1450 13 JUNE 2008 VOL 320 SCIENCE www.sciencemag.org SPECIALSECTION stakes dispersal,” the dispersal of species across wide physical barriers such as oceans (20). For instance, sweepstake dispersal of the wind- dispersed rainforest kapok tree, Ceiba pentandra, was inferred on the basis of DNA-sequence phylo- geography and molecular clock methods: African populations established through oceanic dispersal from Neotropical sources at least 13,000 years before the present (21). Many other tropical tree taxa are shared across the Atlantic. Although these forest similarities were previously attributed to Gondwana vicariance, molecular phylogenet- ic studies point to a predominant role of oceanic dispersal in establishing this range disjunction. In an Amazon forest inventory plot in Ecuador, at least 21% of the tree species (232 out of 1104) were derived from clades that had arrived in South America via long-distance dispersal (20). Thus, natural invasions of trees explain some of the similarity between tropical forests across con- Fig. 2. Sparsely distributed spruce considered to be analogous to glacial-refuge populations. DNA and tinents, contradicting the idea that diversity could fossil pollen data suggest that such small populations survived the Last Glacial Maximum in Alaska and provide sufficient protection against invasions. that they were important for boreal-forest development after the end of the last glaciation (7). Evaluating the frequency and consequences of sweepstakes dispersal into different communities on July 25, 2008 environments probably tens of kilometers from ice in the absolute climate tolerances of species (14). could help predict the consequences of modern sheets (8, 9) and even in ice-free areas north of ice Thus, future extinctions of tree species in re- invasions into more or less “naïve” floras charac- sheets (7). Thus, it appears that small populations sponse to climate change are probable, especially terized by different levels of endemism. of trees can endure extreme climatic conditions for if their geographic distribution or climatic range Many lessons can be drawn from the study tens of thousands of years (Fig. 2). is already highly restricted. Here again, the retro- of past forest change, and our ability to acquire However, these studies also imply that the ca- spective approach could be illuminating. Europe such knowledge is improving thanks in partic- pacity of trees to keep up with the rate of future lost at least 89 tree genera during the climatic ular to the interactions between paleoecologists warming is probably more limited than suggested transitions of the Late Tertiary to the Quaternary and geneticists. The integration of paleoclimate by previous estimates from fossil data. Extremely (15). A key question is whether past extinctions data, fossil records, and genealogical analyses www.sciencemag.org fast tree migrations during the early Holocene were took place during glacial or interglacial periods. within a hypothesis-testing modeling framework inferred from the fossil records of certain tree spe- If extinctions had taken place mostly during represents a particularly fruitful direction (8, 22). cies, on the assumption that northern refugia did interglacial periods, this would support pessi- However, novel climates will appear, leading to not exist during the Last Glacial Maximum (5, 10). mistic views of the consequences of future global “no-analog” communities (23), which may limit On the basis of more recent fossil and DNA warming on population and species survival. the retrospective approach discussed here. Indeed, studies, it appears that the actual rates may be an However, extant Asian and American congeners while no-analog communities are well documented order of magnitude lower, e.g., <100 m/year for of extinct European tree species are less cold- in the paleorecord (4), they cannot offer a direct 11 two North American deciduous tree species ( ). tolerant than