Hongjie Meng

Landscape Genetic Structure of a Streamside Tree Species Euptelea pleiospermum (Eupteleaceae): Contrasting Roles of River Valley and Mountain Ridge

We used landscape genetics and statistical models to test how landscape features influence connectivity or create barriers to dispersal for a mountain riparian tree species, Euptelea pleiospermum. Young leaves from 1078 individuals belonging to 36 populations at elevations of 900–2000 m along upper reaches of four rivers were genotyped using eight nuclear microsatellite markers. We found no evidence for the unidirectional dispersal hypothesis in E. pleiospermum within each river. The linear dispersal pattern along each river valley is mostly consistent with the “classical metapopulaton” model. Mountain ridges separating rivers were genetic barriers for this wind-pollinated tree species with anemochorous seeds, whereas river valleys provided important corridors for dispersal. Gene flow among populations along elevational gradients within each river prevails over gene flow among populations at similar elevations but from different rivers. This pattern of gene flow is likely to promote elevational range shifts of plant populations and to hinder local adaptation along elevational gradients. This study provides a paradigm to determine which of the two strategies (migration or adaptation) will be adopted by mountain riparian plants under climate warming.

Community Composition and Structure of Badagongshan (BDGS) Forest Dynamic Plot in a Mid-subtropical Mountain Evergreen and Deciduous Broad-leaved Mixed Forest, Central China

Mountain evergreen and deciduous broadleaved mixed forests in China are mainly distributed in the mid-subtropical mid-mountain zones.Mount Badagongshan(BDGS) is located on the northern end of the Wuling Mountains at the north rim of the mid-subtropical zone,and has a typical mountain evergreen and deciduous broadleaved mixed forest.To investigate dynamics of this forest,biodiversity origin and maintenance mechanisms in East Sichuan and West Hubei in 2010-2011,the Wuhan Botanical Garden,Chinese Academy of Sciences established one 25 hm2(500 m× 500 m) forest dynamic plot of subtropical mountain evergreen and deciduous broadleaved mixed forest in the Badagongshan(BDGS) National Nature Reserve according to the standards of the Center for Tropical Forest Science(CTFS).Here,we present the flora,community composition,structure,size class and spatial distribution based data collected from all woody(bamboo and liana not included) plants with DBH≥1 cm.Our results demonstrated that BDGS was a typical mountain evergreen and deciduous broadleaved mixed forest,dominated by pantropic distribution families(24.50%) and northern temperate distribution genera(24.56%).There were nine rare and endangered wood plants.We classified 186556 individuals into 53 families,114 genera and 238 species(94 evergreen and 144 deciduous).Dominant species were from the families of Fagaceae,Ericaceae,Lauraceae and Theaceae.There were 38 species with 1000 individuals(63.16% evergreen).In total,103 rare species( 25 individuals) accounted for 43.28% of all the woody plants.The forest strata was divided into a canopy layer dominated by Cyclobalanopsis multinervis and Fagus lucida,an understory layer dominated by Rhododendron stamineum and Litsea elongata,and a shrub layer dominated by Eurya brevistyla and Symplocos anomala.Abundant(1000 individuals) and rare species were mainly from the shrub layer.Limited by growth form,the plot had a fairly small mean DBH(5.41 cm) and was dominated by small sized individuals(68.40%).The size class of five dominant trees was reverse J.

Species associations in a species-rich subtropical forest were not well-explained by stochastic geometry of biodiversity.

The stochastic dilution hypothesis has been proposed to explain species coexistence in species-rich communities. The relative importance of the stochastic dilution effects with respect to other effects such as competition and habitat filtering required to be tested. In this study, using data from a 25-ha species-rich subtropical forest plot with a strong topographic structure at Badagongshan in central China, we analyzed overall species associations and fine-scale species interactions between 2,550 species pairs. The result showed that: (1) the proportion of segregation in overall species association analysis at 2 m neighborhood in this plot followed the prediction of the stochastic dilution hypothesis that segregations should decrease with species richness but that at 10 m neighborhood was higher than the prediction. (2) The proportion of no association type was lower than the expectation of stochastic dilution hypothesis. (3) Fine-scale species interaction analyses using Heterogeneous Poisson processes as null models revealed a high proportion (47%) of significant species effects. However, the assumption of separation of scale of this method was not fully met in this plot with a strong fine-scale topographic structure. We also found that for species within the same families, fine-scale positive species interactions occurred more frequently and negative ones occurred less frequently than expected by chance. These results suggested effects of environmental filtering other than species interaction in this forest. (4) We also found that arbor species showed a much higher proportion of significant fine-scale species interactions (66%) than shrub species (18%). We concluded that the stochastic dilution hypothesis only be partly supported and environmental filtering left discernible spatial signals in the spatial associations between species in this species-rich subtropical forest with a strong topographic structure.

Gene flow and genetic structure of a mountain riparian tree species, Euptelea pleiospermum (Eupteleaceae): how important is the stream dendritic network?

Riparian landscapes are dendritic in nature. However, much attention on genetic structure of riparian plants has been paid to linear models of connectivity while studies that investigate the influence of dendritic landscape are scarce. In this study, we used nuclear microsatellite markers to investigate genetic diversity, gene flow, and genetic structure of a streamside tree species (Euptelea pleiospermum) in a natural stream dendritic network in the Shennongjia Mountains, central China. We tested the following hypotheses: (1) genetic diversity is higher at confluence than that at headwater populations and (2) genetic structure within the stream dendritic network was determined by in-stream dispersal or out-of-stream dispersal. Contrary to our prediction, we found that both genetic diversity and effective population size are congruent at headwater and confluence populations. We found symmetrical gene flow in most (four out of six) headwater–confluence pairs and asymmetrically downstream gene flow in the other two headwater–confluence pairs. Analysis of molecular variance (AMOVA) detected significant differentiation at two scales (among streams within catchments, among populations within stream) and did not reveal significant structure among catchments. STRUCTURE analysis clustered individuals from different catchments into the same genetically homogeneous group. There was no significant isolation by distance (IBD) with Euclidean, stream, or overland distance. Our results suggest that E. pleiospermum populations within the stream dendritic network did not present a hierarchical genetic structure probably because of extensive out-of-stream dispersal.

Root-sprouting ability in an evergreen and deciduous broad-leaved mixed forest

As an important way for forest regeneration, root-sprouting plays an important role in maintaining population and species diversity. Using the initial census data of Badagongshan 25-hm2 forest plot, we analyzed the relationship between root-sprouting ability and topographic factors, and tested the phylogenetic signal of root-sprouting ability. We found that there are 33039 individuals with root-sprouts which account for 17.70% of all the individuals and the total number of root-sprouts was 54184. Results of Spearmans correlation analyses with torus-translation tests showed that the root-sprouting ability significantly negatively correlated with elevation, convex, and vertical distance from channel network. However it significantly positively correlated with topographic wetness index. These may be due to that the low valley areas were disturbed by the stream flow, thus accelerating the root-sprouting ability. The root-sprouting ability of 126 common species showed significant phylogenetic signal. It showed significant phylogenetic signal for shrub species and deciduous species but not for canopy species, sub-canopy species and evergreen species in different life forms. These suggested that root-sprouting ability was a very unstable functional trait in the evolutionary history. In conclusion, the root-sprouting ability of ligneous plants at Badagongshan plot was influenced by topographic factors and phylogenetic history.