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Nearest Living Relatives |
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Fossil floras are used to infer paleoclimate. One approach is based on climatic needs of the living forms, often called the "nearest living relatives" (NLR's). Another is based on an analysis of climate-related features, particularly leaf morphology. NLR's have been applied widely to interpret Tertiary environments. (e.g., MacGinitie 1941; Hickey 1977). In using this approach, we assume that the physiological requirements and climatic tolerances of the fossil representatives did not change appreciably through geologic time, though there is little theoretical or empirical support for this. Reliability of NLR use is increased when a) there is a close relationship between a fossil species and its NLR; b) there are a large number of NLR's representing members of a fossil flora which have similar climatic affinities; c) the living representatives belong to widespread and diverse groups; and d) the plant groups used possess anatomical and morphological features linked to their climatic tolerances (Wing and Greenwood 1993). The presence of taxa such as Ginkgo, Metasequoia, or Glyptostrobus in the Axel Heiberg fossil floras are taken to indicate temperate to warm-temperate climates with cold month means (CMMs) of > 0-2°C (e.g., Schweitzer 1980; McIver and Basinger 1993; Basinger et al. 1994). However, while the distribution of these taxa presently coincides with the temperate and warm temperate regions of southeast China, their present ranges are very restricted and may not accurately reflect their actual range of physiological tolerances (Wolfe 1971, 1985; Hickey 1977). For example, Metasequoia grows (but probably does not reproduce) in arboreta in cities as far north as Montreal, and grows and reproduces in St. Louis where the mean minimum winter temperatures range from -4 to -8°C, with extreme cold temperatures reaching -25°C (temperature data from Ruffner and Bair 1984). The presence of Picea, Tsuga, Abies, and Larix in the Axel Heiberg fossil floras is problematic for inferring relatively warm winters. First, living species of these genera tend to occur primarily in the boreal and montane regions where climate is cool to cryic. Second, Tsuga, Picea, and Abies are evergreen and elicit the question of the effect of respiration demands on survival given dark, but relatively warm high latitude winters. It is important in this regard, to determine if the remains of the evergreen conifers are in place, or if they were growing in the cooler climate of higher elevations and transported to the meanderplains by flooding. Alligator sp. are used as an indicator for warm temperate climates with a CMM of ca. 4°C in the fossil record (e.g., Estes and Hutchison 1980; McKenna 1980; Hutchison 1982; Wing and Greenwood 1993; Basinger et al. 1994). The present northern limit of Alligator reportedly corresponds to a CMM of 4.4°C (Hutchison 1982). However, the historical northern limit of Alligator extends to regions where CMM temperatures range from ca. -3° to 1°C, and extreme cold temperatures -25°C have been recorded (Ruffner and Bair 1984). Paleotemperature estimates based on foliar physiognomy provide another perspective (Wolfe 1993; Wilf 1997). While NLR analyses of the Axel Heiberg flora provide an estimate of mean annual temperature (MAT) of 12-15°C, warm month mean (WMM) of > 25°C, and a CMM of 0-4°C (Basinger et al. 1994). This is based on the assumption that frost sensitive species such as Metasequoia and Glyptostrobus, and the presence of crocodillians (Alligator) can be taken to imply frost-free conditions with a minimum CMM of 5-7°C. However, the foliar physiognomic signatures of two Arctic sites indicate a much cooler MAT of 8.2-9.3°C, a mean annual range of temperature (MART) of 13.8-14°C, and a CMM of -0.8 to -2°C (Greenwood and Wing 1995). Basinger et al. (1994) suggested that the temperature discrepancies between the two methods were a result of deciduousness induced by low winter light which produced physiognomy-based estimates that were too low compared to the NLR estimates. In sum, there is enough uncertainty in temperature estimates that new, independent paleotemperature estimators will be valuable.
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