We previously showed that Metasequoia has the superior photosynthetic system for adapting to the weak, continuous light of the high latitudes. However, Taxodium, a genus not found in Asia, has the same specialized seedling establishment requirements as Metasequoia. The river systems of the western United States could have provided migratory routes for Metasequoia to the Southeast as climate cooled and land masses rose. We present the case that the ecological requirement of moist bare soil for seedling establishment constrains Metasequoia to regularly disturbed riparian zones. Metasequoia has completely disappeared in North America. Fossils from the Holocene are missing on both continents, yet Metasequoia survived, presumably in wet-site refugia along the Yangtze River, and survives today as relic populations in central China. In North America this genus survived at least through the Pliocene, and in Asia through the early Pleistocene. This difference in carbon allocation may have provided Metasequoia with a two fold advantage in the paleoarctic by minimizing depression of photosynthetic capacity and increasing photosynthetic surface.įossils of Metasequoia, beginning in the Cretaceous Period, have been found throughout the Northern Hemisphere. In contrast, Metasequoia allocated photosynthate to continuous production of new foliar biomass. In midsummer, foliar starch concentration substantially increased in both Taxodium and Larix saplings grown in continuous light, which may have contributed to end-product down-regulation of photosynthetic capacity. To provide insight into attributes that favor competitive success in a continuous-light environment, we subjected saplings of these genera to a natural photoperiod or a 24-h photoperiod and measured gas exchange, chlorophyll fluorescence, non-structural carbohydrate concentrations, biomass production and carbon allocation.Įxposure to continuous light significantly decreased photosynthetic capacity and quantum efficiency of photosystem II in Taxodium and Larix, but had minimal influence in Metasequoia. However, the contemporary relative abundance of these genera differs greatly from that in the paleoarctic. Taxodium, Larix and Metasequoia, three genera of deciduous conifers that occurred in paleoarctic wet forests, have extant, closely related descendents. Although maximum irradiances were much lower than at mid-latitudes, the 24-h photoperiod provided similar integrated light flux. With a winter of total or near darkness and a summer of continuous, low-angle illumination, these temperate, high-latitude forests were characterized by a light regime without a contemporary counterpart. Polar regions were covered with extensive forests during the Cretaceous and early Tertiary, and supported trees comparable in size and productivity to those of present-day temperate forests. Finally, although Larix Miller demonstrated improved water-use-efficiency under continuous light, it had a less-competitive photobiology than Metasequoia, and sequestered more resources into stem biomass. Glyptostrobus Endlicher would have been restricted to minor component status by its slow growth rate and short stature. Sequoia Endlicher should have been highly competitive, but was likely limited by its evergreen habit. Based on these physiological comparisons we conclude that Taxodium Richard would have been limited primarily by a significant decrease in photosynthetic efficiency when growing under continuous light. Don) Endlicher, Taxodium distichum (L.) Richard, Glyptostrobus pensilis (Staunton ex D. To estimate the relative competitiveness of the fossil conifers, mechanical/hydraulic stem attributes, photosynthetic water-use-efficiency and photobiology were compared among extant nearest-living-relatives: Metasequoia glyptostroboides Hu et Cheng, Sequoia sempervirens (D. Although the fossil remains of several conifer species have been found from numerous lowland Eocene sites throughout the very high latitudes (above 75 N), many were rarities, and only Metasequoia Miki appeared to have thrived under the warm and unique lighting conditions of the High Arctic regions of the Northern Hemisphere.
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