Population structure and dynamics of Cercidiphyllum japonicum in riparian zones of the Shennongjia mountainous region, central China.

Aims Cercidiphyllum japonicum, an endangered tree species confined to East Asia, grows mainly in the riparian zones of the Shennongjia mountainous region of central China. Our objectives were to explore regeneration and maintenance mechanisms of C. japonicum populations in montane riparian zones by examining age structures, life tables and radial growth processes, as well as disturbance history. Methods We surveyed four rivers in the Shennongjia mountainous region and placed 12 quadrats (ranging from 10 m × 20 m to 35 m × 60 m) in riparian zones with sizable C. japonicum populations. In each quadrat, diameter at breast height (DBH) (or basal diameter if tree height 1.3 m) and height of each tree was recorded. For multistemmed trees, the largest stem was treated as the main stem and the rest as sprouts. Scattered C. japonicum occurring along our routes were also recorded. Cores were extracted from 60 large C. japonicum stems at 1.3 m height and basal discs were taken from 10 young stems at slightly 1.3 m for dendrochronological procedures and subsequent DBH-age regression. Increment sequences 40 years were used for detection of release-events based on a percentage growth change filter. Important findings DBH of C. japonicum grew consistently rapidly in the first 30 years. There were two release events before canopy accession at about 60 years. Differentiation in DBH and height took place at 90–120 years. After two mortality peaks in ≤20 and 60–100 years, C. japonicum approached and sustained optimum vitality up to 180 years when senescence occurred. Its survivorship curve was generally between Deevey type II and III. Young individuals were abundant. Regeneration, however, was discrete over longintervals. Cohorts with an age range of about 50 years that established in the 1920s and 1970s and culminated in the 1940s and 1990s were remarkably discernable in most local populations. Regeneration by sprouting was predominant and strongly correlated with stand age and community cover. Dates of heavy disturbances (1920s, 1970s and 1990s) roughly corresponded with dates of population recruitment pulses. We suggest that individual growth and developmental characteristics and moderate-to-long-interval disturbances played important roles in driving C. japonicum population dynamics. For short to moderately long periods, C. japonicum can maintain its population due to adequate young individuals, especially sprouts.