Early Eocene Metasequoia and Taxodium (Cupressaceae) from Lingbao, Henan Province and their implication for a perhumid climate in central China

In this study, fossil leaves of Metasequoia Hu and Cheng, along with leaves and cones of Taxodium Richard, from the lower Eocene of the Xiangcheng Group in Lingbao, Henan Province, central China, were examined. These fossils have been taxonomically assigned to Metasequoia sp. (cf. M. glyptostroboides Hu and Cheng) and Taxodium dubium (Sternberg) Heer. The closest modern relatives are Metasequoia glyptostroboides and Taxodium distichum (Linnaeus) Richard, which typically thrive in humid environments, indicating that similar conditions may have prevailed in early Eocene. Through the Species Distribution Model (SDM) experiment, it was suggested that T. distichum is significantly influenced by precipitation levels, highlighting the importance of moisture in their historical distribution. Moreover, the analysis of the survival environments of native Metasequoia and Taxodium indicates that the annual precipitation in Lingbao during the early Eocene exceeded 1000 mm, which is roughly double the current annual precipitation in the area. This precipitation level is comparable to the present-day subtropical climate zone. During the late Eocene and early Oligocene, the seasonal variation in precipitation was intensified due to the strengthening of the East Asian monsoon, which constrained the seed germination and seedling survival. This may have contributed to the decline in the populations of Metasequoia and Taxodium in East Asia during that period.