Mingxi Jiang

Contrasting relationships between species diversity and genetic diversity in natural and disturbed forest tree communities

• This study aimed to reveal species-genetic diversity correlations (SGDCs) and their underlying mechanisms in natural and disturbed forests. • A community survey and molecular analyses were carried out to compare species diversity (SD), the genetic diversity of the dominant tree species Euptelea pleiospermum (GD), the altitudinal patterns of SD and GD, SGDC, genetic differentiation (F(ST) ), community divergence (F(ST) -C), effective population size (N(e) ), and recent migration rate between mountain riparian forests along the Yandu (natural) and Nan (disturbed) rivers. • In natural forests, both SD and GD showed a unimodal altitudinal pattern and GD was positively correlated with SD, whereas a unimodal pattern and positive SGDC were not found in the disturbed forests. SD and F(ST) at the natural sites were higher than those at the disturbed sites. However, there were no significant differences in GD, F(ST) -C, N(e) or recent migration rate between the natural and disturbed sites. • A correlation between the patterns of SD and GD along a geographical gradient (e.g. altitude) is an important driver of positive SGDC. The absence of positive SGDC in the disturbed forests may result from reduced SD but unaffected GD, indicating nonparallel changes in SD and GD. This study furthermore cautions against generalizations about changes in SD and GD following disturbance.

Dendroecological study of a subalpine fir (Abies fargesii) forest in the Qinling Mountains, China

Dendroecological techniques were used to investigate the stand dynamics and the disturbance history of the subalpine fir forest in the Qinling Mountains of Shaanxi Province, China. The results indicated that 68% of the fir trees experienced 1–2 release events for a total of 10–29 (an average of 15.8) years, and 1–2 suppression events for a total of 10–27 (an average of 13.4) years before they reached canopy. Large number of Abies fargesii and Betula albo-sinensis recruitment coincided temporally with larger increases in the ring-width index from the 1830s to 1880s, suggesting occurrence of a major stand-wide disturbance during this time period. Few seedlings and saplings were found in the forest, and there was a dramatic decline in recruitment after 1890, probably because of the intensive cover of understory umbrella bamboo (Fargesiaspathacea). Radial growth analyses indicated frequent canopy opening resulting from small-scale disturbances in the forest. Thus, the subalpine fir forest experienced frequent small-scale disturbances and infrequent large-scale disturbances in its developmental history, and these disturbances coupled with the understory umbrella bamboo might have influenced tree growth and species recruitment.

Climate-growth relationships of subalpine fir (Abies fargesii) across the altitudinal range in the Shennongjia Mountains, central China

Dendroecological techniques were employed to explore the growth response of subalpine fir (Abies fargesii) to climatic conditions across its altitudinal range in both the north and south aspects in the Shennongjia Mountains, central China. Correlation function analyses indicated that temperatures in current summer were significantly negatively correlated with fir radial growth at the lower limits, while temperatures in previous autumn and in current spring showed significantly positive correlations with fir radial growth at the mid- and high-elevations in both aspects. Radial growth of the subalpine fir was significantly and positively influenced by precipitation in previous autumn and in current spring at the lower elevations and by precipitation in current spring at the mid-elevations, while precipitation had no significant effects on its radial growth at the upper elevations. Moving correlation functions showed that temperatures in early spring of the current year (i.e., February-April) had a relatively stable effect on tree growth over time at the mid- and upper-elevations in both aspects. Thus, the growth of the subalpine fir responded differently to climatic conditions along the altitudinal gradient, showing that the importance of temperatures for the fir radial growth increased while the importance of precipitation decreased with increasing altitude in both aspects in the Shennongjia Mountains, central China.

Beta diversity determinants in Badagongshan, a subtropical forest in central China

Niche and neutral theories emphasize different processes contributing to the maintenance of species diversity. In this study, we calculated the local contribution to beta diversity (LCBD) of every cell, using variation partitioning in combination with spatial distance and environmental variables of the 25-ha Badagongshan plot (BDGS), to determine the contribution of environmentally-related variation versus pure spatial variation. We used topography and soil characteristics as environmental variables, distance-based Moran’s eigenvectors maps (dbMEM) to describe spatial relationships among cells and redundancy analysis (RDA) to apportion the variation in beta diversity into three components: pure environmental, spatially-structured environmental, and pure spatial. Results showed LCBD values were negatively related to number of common species and positively related to number of rare species. Environment and space jointly explained ~60% of the variation in species composition; soil variables alone explained 21.6%, slightly more than the topographic variables that explained 15.7%; topography and soil together explained 27%, slightly inferior to spatial variables that explained 34%. The BDGS forest was controlled both by the spatial and environmental variables, and the results were consistent across different life forms and life stages.

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.

Leaf traits of natural populations of Adiantum reniforme var. sinensis, endemic to the Three Gorges region in China

Leaf mass per unit area (LMA), carbon and nitrogen contents, leaf construction cost, and photosynthetic capacity (Pmax) of Adiantum reniforme var. sinensis, an endangered fern endemic to the Three Gorges region in southwest China, were compared in five populations differing in habitat such as soil moisture and irradiance. The low soil moisture and high irradiance habitat population exhibited significantly higher LMA, area-based leaf construction (CCA), and carbon content (CA), but lower leaf nitrogen content per unit dry mass (NM) than the other habitat populations. The high soil moisture and low irradiance habitat populations had the lowest CCA, but their cost/benefic ratios of CCA/Pmax were similar to the medium soil moisture and irradiance habitat population due to their lower leaf Pmax. Hence A. reniforme var. sinensis prefers partially shaded, moist but well-drained, slope habitats. Due to human activities, however, its main habitats now are cliffs or steeply sloped bare rocks with poor and thin soil. The relatively high energy requirements and low photosynthetic capacity in these habitats could limit the capability of the species in extending population or interspecific competition and hence increase its endangerment.

Limited genetic impacts of habitat fragmentation in an “old rare” relict tree, Euptelea pleiospermum (Eupteleaceae)

Euptelea pleiospermum is an “old rare” tree species distributed along the high-elevation streamsides in Burma, China, and India. Deforestation and construction of roads for timber transport have highly fragmented the natural habitats of this species in the Shennongjia Forestry District. In this study, we used this fragmentation to test the hypothesis that “old rare” tree species are insusceptible to the genetic consequences of habitat fragmentation. Using eight microsatellite loci, we estimated allelic richness (AR), observed heterozygosity (HO), expected heterozygosity (HE), Wright’s inbreeding coefficient (FIS), and genetic differentiation (FST and DEST) between pre- and post-fragmentation cohorts. We found no significant differences in either genetic diversity or genetic differentiation between the two cohorts. The limited genetic effects of fragmentation may result from too few fragmented generations, because the time between the start of fragmentation (year 1970) and our study (year 2008) was less than one generation of this tree species. It should be mentioned that clonal reproduction by sprouting, a common phenomenon in many “old rare” tree species, can help E. pleiospermum buffer the genetic impacts of fragmentation by delaying the time between generations. Therefore, we conclude that this “old rare” tree species show limited genetic impacts from recent habitat fragmentation. However, the elimination of rare alleles and increase of inbreeding coefficient in the post-fragmentation cohort are early warnings of deleterious genetic consequences of fragmentation. Our results provide valuable information to formulate conservation and restoration guidelines for E. pleiospermum.

Improving allometry models to estimate the above‐ and belowground biomass of subtropical forest, China

Subtropical forests are important carbon sinks and have a huge potential for mitigating climate change. However, few studies have developed biomass models to give robust estimates of subtropical forest aboveground and belowground biomass. Although wood density (WD) can greatly reduce the uncertainty in aboveground biomass (AGB) estimates in tropical forest, it has never been applied in other ecosystems. In addition, crowns hold a large component of tree biomass and vary among forest types, so crown dimensions as new variables have been recommended for AGB models. To test the role of wood density and crown dimensions and to select the best AGB model in subtropical forest, we harvested and weighted dry mass of 147 trees from 41 dominant species in subtropical forest. In order to account the belowground biomass (BGB) of these forests, 23 roots systems were excavated following aboveground harvest. Models with wood density performed better than all those without wood density, and models with height performed b...

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.