In March 2024, the Critically Endangered ironwood tree Ostrya rehderiana (family Betulaceae) blossomed for the first time in Kunming Botanical Garden, China. This tree was originally transplanted from the Hangzhou Botanical Garden in 1990. Its natural habitat is Tianmu Mountain, Hangzhou City, Zhejiang Province. Since its description in 1927, only a single remaining wild population of five mature individuals is known, a consequence of extensive and long-term anthropogenic disturbance. It is designated as a Class I Protected Wild Plant Species in China and is included in the national conservation initiative for Plant Species with Extremely Small Populations.
Comparative research of O. rehderiana and its more widely distributed relative, Ostrya chinensis, indicates that the effective population size of O. rehderiana has declined over the past 10,000 years, with an accumulation of deleterious mutations. On the brink of extinction, the remaining wild population is safeguarded within a nature reserve. Conservation efforts, including pollination management, seed collection, germination, ex situ conservation and in vitro cultivation, have been implemented for over 4 decades. More than 3,000 seedlings have been propagated, and eight ex situ conservation sites have been established across China.
In Kunming Botanical Garden, O. rehderiana, influenced by Kunming's cold climate and high altitude, has a slow growth rate. The tree is 8.42 m tall and has a diameter at breast height of 9.8 cm. Its crown measures 7.5 × 4.8 m. Despite the tree taking approximately 30 years to bloom—a significantly delayed development—the event is unprecedented and is significant for botanical records.