Size-class structure and variation in seed and seedling traits in relation to population size of an endangered species Craigia yunnanensis (Tiliaceae)

Craigia yunnanensis W. W. Sm. & W. E. Evans is an endangered canopy tree species distributed in southern China and northern Vietnam. We located and surveyed six remnant populations, all from subtropical areas of Yunnan, China. These six populations contained between 6 and 167 adult trees in 2007, plus larger numbers of seedlings and resprouts from cut trunks. Bulk seed samples were collected from these populations and examined for 10 fitness traits (e.g. seed number, seed length, width and 1000-seed weight, germination in the controlled light and darkness conditions and nursery, seedling height, root collar diameter and dry weight). All traits differed significantly among populations, but only number of seeds per capsule was significantly correlated with population size. For some populations, germination capacity in light was significantly higher than that in darkness, indicating that canopy removal might promote regeneration from seed. Examination of population size-class structures in four C. yunnanesis populations determined that small seedlings and resprouts are abundant, although very few of these reach even the sapling stage. Hence, the remaining populations could be expanded and conserved by allowing a proportion of seedlings and resprouts to reach maturity. However, ex situ conservation, informed by data from the present study, is also advisable in case the remaining wild populations are lost.