Fossils of Metasequoia, beginning in the Cretaceous Period, have been found throughout the Northern Hemisphere. In North America this genus survived at least through the Pliocene, and in Asia through the early Pleistocene. 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. Metasequoia has completely disappeared in North America. We present the case that the ecological requirement of moist bare soil for seedling establishment constrains Metasequoia to regularly disturbed riparian zones. 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. However, Taxodium, a genus not found in Asia, has the same specialized seedling establishment requirements as Metasequoia. We previously showed that Metasequoia has the superior photosynthetic system for adapting to the weak, continuous light of the high latitudes. But at the lower latitudes of the US Southeast, the photosynthetic system of Taxodium has the competitive edge. We suggest, therefore, that in North America Taxodium preempted the sites that could have been occupied by Metasequoia, eventually leading to its extirpation.
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