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The fossil record of Miki is extensive and demonstrates that the genus was widely distributed throughout North America and Eurasia from the early Late Cretaceous to the Plio-Pleistocene. The genus first appears in Cenomanian age deposits from western Canada, Alaska and the Arkagala and Koylma River basins in Russia and indicates that had achieved a wide distribution early in its evolutionary history. Exchange of between Asia and North America probably occurred across Beringia, which had become functional at the Albian-Cenomanian boundary (ca. 100 million years ago). However, if the inter-continental exchange of the early representatives of this genus occurred prior to the establishment of Beringia, migration would have still been possible across the Spitsbergen Corridor, which was functional during the Early Cretaceous. By the early Tertiary, the distribution patterns do not appear to have changed considerably from that seen during the Late Cretaceous, except that became a dominant constituent of the polar Broad-leaved Deciduous Forests. More importantly, the distribution of indicates that the genus grew and reproduced under a diverse range of climatic and environmental conditions throughout geologic time, including the cold and unique lighting conditions of the polar latitudes. Of particular interest is the apparent lack of fossils in Europe despite the presence of two land bridges linking North America and Europe throughout the early Tertiary and the drying of the Turgai Straits that separated eastern and western Asia up until Oligocene time. persisted in western Siberia and the Canadian Arctic until late Pliocene time, and in western Georgia and Japan until the late Pliocene-early Pleistocene. Following the apparent early Pleistocene extinction, re-appeared in southeastern China. The pronounced reduction in distribution during the Miocene appears to be coupled with increasing global aridity and cooling and increased competition for resources and habitat from representatives of the Pinaceae. With few exceptions, the bulk of the fossils described in the literature indicate that the fossils assigned to are indistinguishable from the living species. The remarkable morphological stasis observed in demonstrates that the genus has remained unchanged, at least morphologically, since the early Late Cretaceous.

The occurrence of fossil Miki in and around Japan was reviewed based on recent stratigraphic and paleoenvironmental data to reconstruct the regional ecology and discuss the processes that led to the extinction of in Japan during the late Miocene and its survival in eastern Asia. The frequent and abundant occurrence of fossils with wetland plants during the Eocene indicates that was one of the dominant components of the flood plain wetland forests of Japan. In the Paleogene, was widely distributed in the vegetation zones located between the Paratropical Rain Forest and polar Mixed Coniferous Forest zones. populations in East Asia were limited by a mild maritime climate and were absent from the subtropical regions where fossil assemblages were represented by semi-arid sclerophyllous forests. Climatic cooling may have exterminated populations in northeastern China and the Russian Far East during the late Miocene. However, survived up until the latest early Pleistocene in central and southwestern Japan. was less common in the small subsiding basins than in the wide sedimentary basins that were associated with contiguous fluvial flood plain ecosystems. Thus, orogenic and volcanic induced changes in topography in the flood plains were also assumed to have influenced the distribution of since the late Miocene. These topographic changes culminated during the late early Pleistocene and were accompanied with eustatic events and likely exterminated populations in the alluvial lowlands in southwestern Japan and hindered the post-glacial migration of the genus.

A high-resolution sampling protocol was used to study the microflora from a two-meter thick siltstone unit located between two prominent coals representing -dominated swamp forests from the middle Eocene (ca. 45 million years old) Buchanan Lake Formation at Napartulik, Axel Heiberg Island, Nunavut, Canada. This detailed analysis facilitated the reconstruction of the local vegetation history and provided possible explanations for the changes seen in the local shifting vegetation patterns. These changes are likely due to two phenomena: environmental disturbances, such as flooding and/or climate change and floral succession. Members of the Pinaceae dominated the local flora at times when the area was relatively dry, whereas Miki was predominant when local environmental conditions were more mesic. The pollen data provide evidence of successional processes and suggest that the local vegetation responded to climatic and environmental changes. More importantly, the pollen data indicate that the local floodplain vegetation was part of a larger, dynamic floral mosaic within a regional polar broad-leaved deciduous forest community and that periods of prolonged environmental stasis were generally limited to the swamp forest communities. Two major fern spikes were identified in the sequence and large-scale flooding was identified as being the likely disturbance factor responsible for landscape level reorganization.

The early Oligocene Bridge Creek flora of the John Day Formation in Oregon, USA, is an example of the broad-leaved deciduous vegetation that became widespread in the Northern Hemisphere following the Eocene-Oligocene climatic cooling, and demonstrates the significant participation of Miki in newly-developing communities analogous to the modern Mixed Mesophytic Forest of China. Before the discovery of living Hu Cheng in China, the Bridge Creek fossils were identified as Endlicher, which led to erroneous ecological and community composition interpretations of the Bridge Creek flora based upon comparisons with the modern redwood forest of California. Soon after the discovery of living , paleobotanist Ralph W. Chaney visited the modern forest in China in 1948, and this had a major influence on his concepts about the composition and biogeographic history of North American fossil forests. Many of the North American fossils that had previously been assigned to or Richard were subsequently reassigned by Chaney to . Chaney postulated that the Oligocene deciduous forests such as Bridge Creek had reached the middle latitudes by the “migration” of an intact Arcto-Tertiary geoflora from the higher latitudes, but this concept is no longer well supported. Instead, the Mixed Mesophytic Forest apparently originated through various responses of individual species during the Eocene-Oligocene transition, including 1) extinction or extirpation, 2) pre-adaptation or evolution in place, 3) dispersal from higher elevations, or 4) dispersal from higher latitudes. Extant is endemic to a very restricted area in central China, and survival of the few remaining natural stands is doubtful unless new measures are taken to conserve the Mixed Mesophytic Forest as an entire community.

The story of the establishment of the fossil genus Miki in 1941 by the Japanese paleobotanist S. Miki and the discovery and classification of the living Hu Cheng (Dawn Redwood) in 1946–1948 by Professors H. H. Hu and W.C. Cheng are well known. In addition, the efforts of Elmer D. Merrill of the Arnold Arboretum of Harvard University and paleontologist Ralph Chaney of the University of California, Berkeley to collect and disperse seeds throughout the world have been carefully chronicled. But what is not well-known is the story of how Wilhelm Gunther (1893–1983), a German national who lived and worked in China between 1914 and 1941, acquired seeds in the late 1920s that decades later would be identified as those of . Drawing from historical and contemporary photographs as well as interviews with Gunther’s daughters, this essay unravels the odyssey of these seeds that possibly predate the discovery of the native population in China.

The recent discovery of two distinct cuticle types, Uneven Type and Even Type, within the native population of Hu Cheng has prompted re-evaluation of the taxonomic utility of cuticle characters in both living and fossil Miki. The result is a comprehensive review of the existing data and methods used in the past to analyze living and fossil cuticle, as well as the impact of cuticle analyses for interpreting the evolutionary history of the genus. Previous studies on the cuticle micromorphology of living and fossil are discussed. The application of scanning electron microscopy (SEM) methodology is deemed critical for re-evaluating the results previously obtained using light microscopy (LM) techniques and allows reinterpretation of some of the cuticle characters that were either poorly understood or misinterpreted. Except for the possible presence in immature seedlings, the Even Type cuticle appears to be restricted to a tree in Paomu Village in Hunan Province, China; whereas the Uneven Type cuticle is widely distributed in the main population. So far, all of the fossil materials examined for cuticle micromorphology possess the Even Type, suggesting that the Even Type cuticle is the ancestral character state for the genus. This finding bears significant implications for the evolution and paleogeography of this genus, as well as the conservation of this endangered species. Further research perspectives using cuticle analyses should provide us with a better understanding of the origin and distribution of these different cuticle types in both living and fossil material.

Mummified leaflets of middle Eocene Miki sampled at 5 mm vertical intervals from a section of lignite extracted from the Upper Coal member of the Buchanan Lake Formation at Napartulik on Axel Heiberg Island were examined via transmission electron microscopy (TEM) to quantitatively and qualitatively assess the extent of sub-cellular preservation. The presence of chloroplasts and chloroplast constituents were tallied on a per cross-section basis, with equal examination time allotted to each cross-section. Features of note were stacked membranes, appressed membranes (grana), associated membranes, disassociated membranes, vesicular membranes, lipid droplets, starch grains and amorphous breakdown lipids. The size and shape of each organelle was measured, as well as the orientation. Intact chloroplasts were observed in 41.6% of the fossil leaves examined, and in 66.6% of the 5 mm intervals. In 27.7% of the leaves, the chloroplasts were so well preserved that features related to thylakoid geometry (degree of grana stacking, number of thylakoids per granum, stack width) were measurable. By performing a nested multivariate analysis of variance, significant differences in preservation were observed between individual leaflets as well as between vertical intervals. An apparent oscillation in preservation quality with an estimated period of 62.5 years was distinguished using the chloroplast features found to best discriminate between layers of lignite. The predominantly observed elliptical shape of the chloroplasts was deemed in general not to be the result of compression, but rather representative of the original geometry upon abscission, based upon the standard deviation of the orientation of the chloroplasts within each leaflet cross section. Where discernible, thylakoid features were used to classify the fossil leaflets as originating from high or low-light environments. Between 67 and 78% of the leaflets were classified as ‘shade-types.’

Biomolecules from living and fossil Miki provide insight into the biological and geological history of the genus, as well as its phylogeny and evolution, population structure and molecular taphonomy. Chemically labile biomolecules such as protein and nucleotide sequences from Hu Cheng have reconfirmed the systematic position of as the sister taxon to Endlicher and J. Buchholz. These three genera form the subfamily Sequoioideae, which is one of the basal groups of the Taxodiaceae-Cupressaceae complex. Relative rate testing based on different gene sequences suggests an evolutionary rate slow down in the Sequoioideae lineage. Non-sequence based molecular studies indicate a low overall genetic diversity and a lack of spatial genetic structure in its native population in southern China. An isolated tree in Paomu, Hunan Province was found to contain both molecular and morphological plesiomorphic characteristics, providing important clues to its evolutionary history. Moreover, given the importance of these data, critical conservation issues are raised regarding the management and preservation of the genus. Molecular and biochemical investigations of ancient biomolecules from well-preserved Tertiary fossil remains reveal the preservation of labile biomolecules such as carbohydrates that are normally absent from the Cenozoic fossil record. Comparative geochemical and SEM analyses of well preserved fossil specimens revealed the sources of these chemically unstable biomolecules and suggests that they may carry important structural functions to support the integrity of plant fossil morphology. These ancient biomolecules from fossil also offer valuable information regarding the environmental factors controlling the preservation of labile biomolecules and possible origins of homologous series of -alk-1-ene/-alkane pairs preserved in the geological record.

Miki is a deciduous conifer with a highly restricted natural distribution in central China. It is a riparian species that thrives under conditions of abundant growing season moisture availability, although as is shown by its distribution in cultivation, it is capable of sustained growth under drier conditions as well. Whereas the variability of climate across its natural range is minute, under cultivation it grows across a gradient of 16.3°C of mean annual temperature and 2360 mm of mean annual precipitation. No cultivated Hu Cheng are known to exist without supplemental water in areas that receive less than ca. 500 mm of mean annual precipitation. grows tall quickly and is capable of obtaining stem lengths in excess of 30 m in less than 50 years. Based on measurements of the trees from its natural range, maximum height may be just over 50 m. Analysis of growth performance of through time indicates that trees from different seedlots grown in common garden experiments remain static in their relative growth rates through time; trees that grew quickly from the outset continued to outpace slower growing trees eight years later. A mixture of environmental factors may alter growth form. Most notably, shrubby varieties are known from a mixture of cold climates, but are not always found in extremely cold areas.

Morphologically identical representatives of the modern taxon Hu Cheng have been found as fossils in many Northern Hemisphere locations, indicating that since its origin in the Late Cretaceous, the genus has migrated throughout the Northern Hemisphere, and at times had a range extending over some 40 degrees of latitude. Today, the taxon is found only in a restricted area in southern China; however, it has been disseminated widely as a horticultural curiosity, and grows successfully in a variety of modern habitats. This paper examines the modern species’ autecology, focusing on gas exchange relations, to address the role of physiological constraints in range contraction over time. Examination of CO uptake and transpiration in response to light, temperature, humidity and CO concentration indicate that is drought-intolerant, adapted to low light levels and relatively cool climates, as might be expected in foggy coastal or riparian habitats. The gas-exchange relations of the modern species are sufficiently flexible to explain the presence of fossil Miki at all known sites, based on paleoclimatic reconstructions, implying that the species has not undergone any major adaptive changes in its core physiological processes. In spite of its horticultural popularity, there has been relatively little quantitative physiological examination of this species. This paper attempts to summarize what is available in the literature, providing an additional perspective on the species autecology in the context of species migration and habitat selection. is shade-intolerant, germinates best on open soils, intolerant of high light levels and resists pests and disease well due to the accumulation of toxic secondary compounds. Finally, the role of deciduous habit is discussed with reference to adaptation to high-latitude photoseasonality.