Spring drought as a possible cause for disappearance of native Metasequoia in Yunnan Province, China: Evidence from seed germination and seedling growth

Metasequoia glyptostroboides is a well known relict conifer species that was widely distributed in the Northern Hemisphere from the Mesozoic (Cretaceous) to early Cenozoic, after which its distributional range was contracted significantly. To explore the possible reasons for the disappearance of Metasequoia in southwest China, controlled experiments were conducted on seed germination and seedling growth and survival over a range of temperatures and at different levels of water stress. Germination was 76%-83.2% at temperatures ranging from 10 degrees C to 30 degrees C and did not differ significantly. However, germination decreased significantly at 5 degrees C and 35 degrees C. Low water stress (-0.2 MPa to -0.4 MPa) promoted germination of Metasequoia seeds, but at -1.5 MPa to -2 MPa normal seedling emergence was only 1.4-2.4%. Seed germination decreased with an increase in storage temperature, and seeds stored at room temperature (23-25 degrees C) for 6 and 12 months germinated to 0-24.3%. As the concentration of polyethylene glycol 6000 increased, elongation of the roots and stems first increased and then decreased, while the number of leaves and percentage of seedling survival decreased. At -0.6 MPa to -0.7 MPa, seedling survival was only 12%-15%. Thus, moisture conditions were the main factor affecting seed germination and seedling growth and survival of Metasequoia. Our results suggest that the intensification of drought in winter and spring due to development of the monsoon climate in Asia was an important factor for the disappearance of Metasequoia in southwest China. (C) 2020 The Authors. Published by Elsevier B.V.