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pentachlorophenol or methyl parathion, except between the tip and the first emerging root hairs, where luteolin concentration is highest6,10. This effect is similar to that shown in Fig. 1b, where inhibition is overcome by increasing concentrations of luteolin. Nitrogen fixation is controlled by both agonistic and antagonistic phytochemical signalling to Rhizobium5,13. As with the vertebrate endocrine system, plant–bacterial symbiosis relies on recognition of specific agonists and antagonists for gene regulation2,10,13. We have shown that some pesticides and planar phenolic EDCs can interfere with plant–Rhizobium signalling and nitrogen-fixing symbiosis, thereby exposing a previously unrecognized similarity to the effects of EDCs on vertebrate endocrine signalling. Jennifer E. Fox*†, Marta Starcevic*, Kelvin Y. Kow*, Matthew E. Burow*‡, John A. McLachlan*‡ *Environmental Endocrinology Laboratory, Center for Bioenvironmental Research at Tulane and Xavier Universities, †Molecular and Cellular Biology Program, Tulane University, and ‡Department of Pharmacology, Tulane University Medical School, New Orleans, Louisiana 70112, USA e-mail:
[email protected] 1. Tham, D. M., Gardner, C. D. & Haskell, W. L. J. Clin. Endocrinol. Metab. 83, 2223–2235 (1998). 2. Collins-Burow, B. M. et al. Nutr. Cancer 38, 229–244 (2000). 3. McLachlan, J. A. Endocrinol. Rev. 22, 319–341 (2001). 4. Schultze, M. & Kondorosi, A. Annu. Rev. Genet. 32, 33–57 (1998). 5. Peters, N. K., Frost, J. W. & Long, S. R. Science 233,
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977–980 (1986). 6. Djordjevic, M. A. et al. EMBO J. 6, 1173–1179 (1987). 7. Firmin, J. L. et al. Nature 324, 90–93 (1986). 8. Gyorgypal, Z. & Kondorosi, A. Mol. Gen. Genet. 226, 337–340 (1991). 9. Mulligan, J. T. & Long, S. R. Proc. Natl Acad. Sci. USA 82, 6609–6613 (1985). 10. Peters, N. K. & Long, S. R. Plant Physiol. 88, 396–400 (1988). 11. Redmond, J. W. et al. Nature 323, 632–635 (1986). 12. Gage, D. J., Bobo, T. & Long, S. R. J. Bacteriol. 178, 7159–7166 (1996). 13. Baker, M. E. Adv. Exp. Med. Biol. 439, 249–276 (1998).
COMMUNICATIONS ARISING
Palaeovegetation
Diversity of temperate plants in east Asia he exceptionally broad species diversity of vascular plant genera in east Asian temperate forests, compared with their sister taxa in North America, has been attributed to the greater climatic diversity of east Asia, combined with opportunities for allopatric speciation afforded by repeated fragmentation and coalescence of populations through Late Cenozoic ice-age cycles1. According to Qian and Ricklefs1, these opportunities occurred in east Asia because temperate forests extended across the continental shelf to link populations in China, Korea and
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Japan during glacial periods, whereas higher sea levels during interglacial periods isolated these regions and warmer temperatures restricted temperate taxa to disjunct refuges. However, palaeovegetation data from east Asia2–6 show that temperate forests were considerably less extensive than today during the Last Glacial Maximum, calling into question the coalescence of tree populations required by the hypothesis of Qian and Ricklefs1. Pollen data2–6 from 14C-dated sediment cores, processed using a standard interpretive technique7, record the changes in vegetation between the Last Glacial Maximum (Fig. 1a) and the present (Fig. 1b). These data are derived from work carried out by Chinese and Japanese palynologists during the two decades since the reconstruction by CLIMAP cited in ref. 1. The data indicate that temperate forests were restricted in distribution during the Last Glacial Maximum, a finding that is contrary to the inference by Qian and Ricklefs1. The forests were displaced either by boreal and mixed forests lacking most of the temperate forest taxa, or by treeless vegetation (steppe and desert, with pollen assemblages dominated by Artemisia, Chenopodiaceae and grasses), over much of their present range. Low levels of precipitation, compounded by the low water-use efficiency of C3 photosynthesis at atmospheric CO2 concentrations during the Last Glacial Maximum (less than 200 p.p.m.; refs 7, 8), confined temperate forests to medium elevations in northern China, whereas low temperatures caused mixed forests (mixtures of boreal conifers with only the most cold-tolerant temperate taxa) to dominate in much of Japan and southern China2–6,8. To provide a spatially continuous vegetation map (Fig. 1c) that is physically consistent with climate forcing for the Last Glacial Maximum, we used a model of climate in the last glacial maximum (based on known changes in insolation, atmospheric composition and physiography)9 as input to an equilibrium biogeography model, BIOME4 (Fig. 1d)10. The large-scale features of this simulation are supported by pollen data (Fig. 1a). Temperate forests are shown as confined to a relatively narrow belt at low elevations, whereas non-forest vegetation is shown to be greatly extended, as the pollen data indicate. On the other hand, a band of temperate deciduous forest is shown extending across the East China Sea, consistent both with Qian and Ricklefs’ hypothesis1 and with pollen evidence for the existence of an offshore glacial refuge for temperate deciduous trees. Quaternary palaeodata have repeatedly overturned historical biogeographical hypotheses that were based on presentday taxon distributions. The palaeodata indicate a more complex history in which 129
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Figure 1 Observed and simulated vegetation of east Asia during the Last Glacial Maximum and today. a, Pollen data for the Last Glacial Maximum, 18,0001,000 14C yr BP, classified to top-level vegetation units (biomes) using a standard method7. b, Subrecent pollen data analysed in the same way, reflecting present vegetation patterns. c, Simulated vegetation during the Last Glacial Maximum, 21,000 cal yr 14 9 BP (~18,000 C yr BP). Climate anomalies from the NCAR CCM1, with computed (mixed-layer ocean model) sea-surface temperatures ; vegetation simulation from BIOME4 (ref. 10). d, Simulated present potential natural vegetation, on the basis of twentieth-century climatology (CLIMATE 2.2). Colour scale: dark green, tropical forest, savanna and woodland; light green, temperate deciduous forest; light blue, temperate coniferous forest; dark blue, warm temperate evergreen forest; olive green, mixed (temperate and boreal) forest; brown, boreal forest; orange, non-forest.
habitat reductions in some regions occurred simultaneously with increases in others. Palaeodata from east Asia may still be broadly consistent with the allopatricspeciation hypothesis for species diversity put forward by Qian and Ricklefs1, but only if it is recognized that population fragmentation occurred during glacial as well as interglacial periods. Although eastern North America also shows evidence of relative aridity during the Last Glacial Maximum, temperate forests there seem to have been more extensive and continuous7. S. P. Harrison*†, G. Yu*‡, H. Takahara§, I. C. Prentice* *Max Planck Institute for Biogeochemistry, PO Box 100164, 07701 Jena, Germany e-mail:
[email protected] †Dynamic Palaeoclimatology, Lund University, 22300 Lund, Sweden ‡Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210093, China §University Forest, Kyoto Prefectural University, Kyoto 606, Japan 1. 2. 3. 4.
Qian, H. & Ricklefs, R. E. Nature 407, 180–182 (2000). Yu, G. et al. J. Biogeogr. 27, 635–664 (2000). Takahara, H. et al. J. Biogeogr. 27, 665–683 (2000). Xu, J. S. Selected Papers from the First Symposium of the Palynological Society of China (Scenic, Beijing, 1982) (in Chinese). 5. Han, Y. S. & Meng, G. L. Oceanogr. Limnol. China 17, 196–205 (1986) (in Chinese). 6. Meng, G. L. & Wang, S. Q. Oceanogr. Limnol. China 18, 253–263 (1987) (in Chinese). 7. Prentice, I. C., Jolly, D. & BIOME 6000 Participants. J. Biogeogr. 27, 507–519 (2000). 8. Farrera, I. et al. Clim. Dyn. 15, 823–856 (1999). 9. Kutzbach, J. et al. Q. Sci. Rev. 17, 473–506 (1998). 10. Kaplan, J. O. Geophysical Applications of Vegetation Modeling. Thesis, Univ. Lund (2001).
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Qian and Ricklefs reply — The point we wished to make was simply that the more complex geography and topography of eastern Asia compared with eastern North America, in conjunction with climate change and sea-level fluctuations, have provided greater opportunity for allopatric speciation. This explanation of the greater diversity of vascular plants in temperate regions of eastern Asia cannot yet be tested using any particular biome reconstruction, all of which are poorly resolved. The estimated distribution of vegetation during the Last Glacial Maximum (18,000 yr BP) and at 6,000 yr BP, on the basis of fossil-pollen data1 and climate, shows that mesic vegetation types were shifted southwards during the Last Glacial Maximum relative to their modern distributions in both eastern Asia and eastern North America2,3. However, the temperate deciduous forest biome, which currently harbours many of the eastern Asian/eastern North American disjunct genera we discussed4, is poorly represented as a vegetation type in fossil-pollen deposits from the Last Glacial Maximum, casting doubt on the accuracy of the reconstruction by Harrison et al. For example, the palaeovegetation of the Last Glacial Maximum for the vast area between 20 and 30 N and 105 and 120 E was reconstructed from only six pollen
localities, including only one from the interior of eastern Asia, which currently harbours the greatest plant diversity in the region. It is not possible to determine whether temperate forests coalesced or fragmented during the Last Glacial Maximum without more detailed information. Nonetheless, the reconstruction by Harrison et al. confirms that the currently isolated temperate forests of China, Japan, and probably the Korean peninsula, were connected during the Last Glacial Maximum. Speciation and extinction are processes of populations, not biomes. Climate change causes restructuring of forest communities, probably including the dispersion of temperate deciduous forest taxa among other vegetation types5. Vegetation maps cannot provide a detailed picture of past fragmentation and coalescence of particular species populations. Furthermore, because most eastern Asian/eastern North American disjunctions pre-date the onset of major glacial climate cycles in the Northern Hemisphere6, diversification of disjunct lineages may have began under prePleistocene climate conditions that were quite different from those in the present or in the Last Glacial Maximum. In these prePleistocene conditions, eustatic sea-level changes may have been important in isolating and rejoining populations. Reconstruction of vegetation history, combined with physiographical and climatic heterogeneity, conveys a general impression of the capacity of a region to promote or retard diversification through allopatric speciation7. However, the biome reconstruction by Harrison et al., including their interpretation of restricted deciduous forest vegetation during glacial maxima, does not contradict the idea that the more complex landforms and climates of eastern Asia provided greater opportunities for allopatric formation of new species compared with eastern North America. Hong Qian*, Robert E. Ricklefs† *Research and Collections Center, Illinois State Museum, Springfield, Illinois 62703, USA e-mail:
[email protected] †Department of Biology, University of Missouri-St Louis, St Louis, Missouri 63121, USA 1. Prentice, I. C. et al. J. Biogeogr. 19, 117–134 (1992). 2. Yu, G. et al. J. Biogeogr. 27, 635–664 (2000). 3. Williams, J. W., Webb, T., Richard, P. H. & Newby, P. J. Biogeogr. 27, 585–607 (2000). 4. Qian, H. & Ricklefs, R. E. Nature 407, 180–182 (2000). 5. Webb, T. in Global Warming and Biological Diversity (eds Peters, R. L. & Lovejoy, T. E.) 59–75 (Yale Univ. Press, New Haven, Connecticut, 1992). 6. Wen, J. Annu. Rev. Ecol. Syst. 30, 421–455 (1999). 7. Dynesius, M. & Jansson, R. Proc. Natl Acad. Sci. USA 97, 9115–9120 (2000).
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