热带次生林刀耕火种过程中土壤节肢动物群落结构及多样性的变化

通过模拟刀耕火种过程,对热带次生林土壤节肢运动群薄结构及多样性在刀耕火种过程中变化进行了初步研究,结果表明,火烧1周后,土壤节肢运行类群数和个体数从次生林地的14类,564头降为7类,164头,并且在群落组成上发生了较大变化,膜翅目的烧后1天成为突出优势类群,蜱蟥目在烧后1周成为唯一的优势类群,而在多样性方面,土壤节肢运动群落类群数（丰度）,个体数（多度）和DG多样性指数在刀耕火种过程中均降低,土壤节肢运动的垂直分布因刀耕火种干扰发生了很大变化,尚存土壤节肢运动随干扰强度的加剧,出现逐步由土壤表层向深层迁移的现象,一些土壤节肢运动在土层间做垂直迁移活动,说明土壤层在一定程度上减轻了刀耕火种对它们的破坏作用,维持了某些类群数量及群落多样性。     

Habitat fragmentation influences genetic diversity and differentiation: Fine‐scale population structure of Cercis canadensis (eastern redbud)

Forest fragmentation may negatively affect plants through reduced genetic diversity and increased population structure due to habitat isolation, decreased population size, and disturbance of pollen‐seed dispersal mechanisms. However, in the case of tree species, effective pollen‐seed dispersal, mating system, and ecological dynamics may help the species overcome the negative effect of forest fragmentation. A fine‐scale population genetics study can shed light on the postfragmentation genetic diversity and structure of a species. Here, we present the genetic diversity and population structure of Cercis canadensis L. (eastern redbud) wild populations on a fine scale within fragmented areas centered around the borders of Georgia–Tennessee, USA. We hypothesized high genetic diversity among the collections of C. canadensis distributed across smaller geographical ranges. Fifteen microsatellite loci were used to genotype 172 individuals from 18 unmanaged and naturally occurring collection sites. Our results indicated presence of population structure, overall high genetic diversity (H_E = 0.63, H_O = 0.34), and moderate genetic differentiation (F_ST = 0.14) among the collection sites. Two major genetic clusters within the smaller geographical distribution were revealed by STRUCTURE. Our data suggest that native C. canadensis populations in the fragmented area around the Georgia–Tennessee border were able to maintain high levels of genetic diversity, despite the presence of considerable spatial genetic structure. As habitat isolation may negatively affect gene flow of outcrossing species across time, consequences of habitat fragmentation should be regularly monitored for this and other forest species. This study also has important implications for habitat management efforts and future breeding programs.     

Rapid recovery of tropical forest diversity and structure after shifting cultivation in the Philippines uplands

Shifting cultivation is a widespread land‐use in the tropics that is considered a major threat to rainforest diversity and structure. In the Philippines, a country with rich biodiversity and high rates of species endemism, shifting cultivation, locally termed as kaingin, is a major land‐use and has been for centuries. Despite the potential impact of shifting cultivation on forests and its importance to many people, it is not clear how biodiversity and forest structure recover after kaingin abandonment in the country, and how well these post‐kaingin secondary forests can complement the old‐growth forests. We investigated parameters of forest diversity and structure along a fallow age gradient in secondary forests regenerating after kaingin abandonment in Leyte Island, the Philippines (elevation range: 445–650 m asl). We first measured the tree diversity and forest structure indices in regenerating secondary forests and old‐growth forest. We then measured the recovery of tree diversity and forest structure parameters in relation to the old‐growth forest. Finally, using linear mixed effect models (LMM), we assessed the effect of different environmental variables on the recovery of forest diversity and structure. We found significantly higher species density in the oldest fallow sites, while Shannon’s index, species evenness, stem number, basal area, and leaf area index were higher in the old‐growth forest. A homogeneous species composition was found across the sites of older fallow age. Multivariate analysis revealed patch size as a strong predictor of tree diversity and forest structure recovery after shifting cultivation. Our study suggests that, secondary forests regenerating after shifting cultivation abandonment can recover rapidly. Although recovery of forest structure was not as rapid as the tree diversity, our older fallow sites contained a similar number of species as the old‐growth forest. Many of these species are also endemic to the Philippines. Novel and emerging ecosystems like tropical secondary forests are of high conservation importance and can act as a refuge for dwindling tropical forest biodiversity.     

Floristic composition, structure, and diversity of four forest plots in the Sierra Maigualida, Venezuelan Guayana

A phytosociological study was carried out in four 1-ha forest plots in the Sierra Maigualida region, Venezuelan Guayana. Density, dominance, and frequency values were calculated as well as the species and family importance values (FIVs) in each plot. A total of 65 families, 232 genera and 533 species were represented in the 4ha of forest. The number of stems 10cmdbh varied from 355 to 563, the number of species from 133 to 191, the genera from 76 to 120, the families from 38 to 51, and the basal area from 20.56 to 40.83m² per hectare. Overall the most species rich family was Fabaceae (38 species), the most numerous in total stems was Burseraceae (165 individuals), and the most dominant in terms of basal area was Sapotaceae (1367.13cm²). The most conspicuous families, as represented among the top five FIV, include Burseraceae in all four plots, Moraceae in three of the plots, and Sapotaceae in two plots. Less than 20% of the total inventory of species are represented in two or more plots. The percentages of species which are unique to a single plot range from 56 to 67%. The within-plot diversities recorded in the present study indicate that the Sierra Maiguailida harbors the richest forests thus far documented in Venezuelan Guayana. Meanwhile, the low between-plot similarity indices reinforce the notion that the species richness of this region is unusually high and suggest the need to extend collections and inventory studies.     

Flooding and soil composition determine beta diversity of lowland forests in Northern South America

Beta diversity may be determined by dispersal limitation, environment, and phylogeographic history. Our objective was to advance the understanding of plant species turnover in rain forests in northern South America and determine which factors are affecting species beta diversity. We evaluated the relative effect of environmental variables (i.e., soil, climate, fragmentation, and flooding frequency) and dispersal limitation (i.e., geographical distance and resistance distance due mountain barriers) on tree beta diversity in 32 1‐ha lowland forest plots. We found that tree species turnover was better explained by environmental distance than by geographical distance. Although soil conditions and flooding regime were good predictors of tree species composition, almost half of the variance remained unexplained. In our study system, the eastern Andean ridge had no significant effect on plant beta diversity, probably because of its young age in relation to the phylogeny. Our results provide support for the importance of environmental factors and suggest a more restricted role of dispersal limitation. Therefore, we advise that conservation strategies of lowland trees should consider specific forest types (e.g., seasonally flooded vs. terra firme, as well as piedmont vs. central Amazonian forests).     

Exploring the role of asexual multiplication in poplar rust epidemics: impact on diversity and genetic structure

Fungal plant pathogens, especially rust fungi (Pucciniales), are well known for their complex life cycles, which include phases of sexual and asexual reproduction. The effect of asexual multiplication on population genetic diversity has been investigated in the poplar rust fungus Melampsora larici‐populina using a nested hierarchical sampling scheme. Four hierarchical levels were considered: leaf, twig, tree and site. Both cultivated and wild poplar stands were sampled at two time points at the start and end of rust epidemics. A total of 641 fungal isolates was analysed using nine microsatellite markers. This study revealed that the genetic signature of asexual multiplication in the wild poplar stand was seen only at lower hierarchical levels (leaf and twig). Moreover, we observed an erosion of clonal structure through time, with an increase in both gene and genotypic diversity. New genotypes contributed to host infection over time, which demonstrates the importance of allo‐infection in the epidemic process in this host‐pathogen system. Compared with the wild stands, the nearly lack of detection of clonal structure in the cultivated stands reflects the higher infection level on cultivated poplars. More generally, this genetic analysis illustrates the utility of population genetics approach for elucidating the proportion of asexual reproduction in the multiplication of isolates during an epidemic, and for proper quantification of asexual dispersal in plant pathogens.     

Quantifying variation in forest disturbance,and its effects on aboveground biomass dynamics,across the eastern United States

The role of tree mortality in the global carbon balance is complicated by strong spatial and temporal heterogeneity that arises from the stochastic nature of carbon loss through disturbance. Characterizing spatio‐temporal variation in mortality (including disturbance) and its effects on forest and carbon dynamics is thus essential to understanding the current global forest carbon sink, and to predicting how it will change in future. We analyzed forest inventory data from the eastern United States to estimate plot‐level variation in mortality (relative to a long‐term background rate for individual trees) for nine distinct forest regions. Disturbances that produced at least a fourfold increase in tree mortality over an approximately 5 year interval were observed in 1–5% of plots in each forest region. The frequency of disturbance was lowest in the northeast, and increased southwards along the Atlantic and Gulf coasts as fire and hurricane disturbances became progressively more common. Across the central and northern parts of the region, natural disturbances appeared to reflect a diffuse combination of wind, insects, disease, and ice storms. By linking estimated covariation in tree growth and mortality over time with a data‐constrained forest dynamics model, we simulated the implications of stochastic variation in mortality for long‐term aboveground biomass changes across the eastern United States. A geographic gradient in disturbance frequency induced notable differences in biomass dynamics between the least‐ and most‐disturbed regions, with variation in mortality causing the latter to undergo considerably stronger fluctuations in aboveground stand biomass over time. Moreover, regional simulations showed that a given long‐term increase in mean mortality rates would support greater aboveground biomass when expressed through disturbance effects compared with background mortality, particularly for early‐successional species. The effects of increased tree mortality on carbon stocks and forest composition may thus depend partly on whether future mortality increases are chronic or episodic in nature.     

Genetic diversity and differentiation in narrow versus widespread taxa of Helianthemum (Cistaceae) in a hotspot: The role of geographic range,habitat, and reproductive traits

Unraveling the relationships between ecological, functional traits and genetic diversity of narrow endemic plants provide opportunities for understanding how evolutionary processes operate over local spatial scales and ultimately how diversity is created and maintained. To explore these aspects in Sierra Nevada, the core of the Mediterranean Betic‐Rifean hotspot, we have analyzed nuclear DNA microsatellite diversity and a set of biological and environmental factors (physicochemical soil parameters, floral traits, and community composition) in two strictly endemic taxa from dolomite outcrops of Sierra Nevada (Helianthemum pannosum and H. apenninum subsp. estevei) and two congeneric widespread taxa (H. cinereum subsp. rotundifolium and H. apenninum subsp. apenninum) that further belong to two different lineages (subgenera) of Helianthemum. We obtained rather unexpected results contrasting with the theory: (a) The narrow endemic taxa showed higher values of genetic diversity as well as higher average values of pollen production per flower and pollen‐to‐ovule ratio than their widespread relatives; and (b) the two taxa of subg. Helianthemum, with larger corollas, approach herkogamy and higher pollen production than the two taxa of subg. Plectolobum, displayed lower genetic diversity and higher values of inbreeding. Altogether, these results disclose how genetic diversity may be affected simultaneously by a large number of intrinsic and extrinsic factors, especially in Pleistocene glacial refugia in mountains where the spatial context harbors a great ecological heterogeneity. On the other hand, differences in mating system and the significant effect of the substrate profile, both being highly diverse in the genus Helianthemum, in the genetic variability illustrate about the importance of these two factors in the diversification and species differentiation of this paradigmatic genus in the Mediterranean and open the field to formulate and test new hypotheses of local adaptation, trait evolution, and habitat diversification.     

The QQS orphan gene of Arabidopsis modulates carbon and nitrogen allocation in soybean

The genome of each species contains as high as 8% of genes that are uniquely present in that species. Little is known about the functional significance of these so‐called species specific or orphan genes. The Arabidopsis thaliana gene Qua‐Quine Starch (QQS) is species specific. Here, we show that altering QQS expression in Arabidopsis affects carbon partitioning to both starch and protein. We hypothesized QQS may be conserved in a feature other than primary sequence, and as such could function to impact composition in another species. To test the potential of QQS in affecting composition in an ectopic species, we introduced QQS into soybean. Soybean T1 lines expressing QQS have up to 80% decreased leaf starch and up to 60% increased leaf protein; T4 generation seeds from field‐grown plants contain up to 13% less oil, while protein is increased by up to 18%. These data broaden the concept of QQS as a modulator of carbon and nitrogen allocation, and demonstrate that this species‐specific gene can affect the seed composition of an agronomic species thought to have diverged from Arabidopsis 100 million years ago.     

Influence of northern limit range on genetic diversity and structure in a widespread North American tree,sugar maple (Acer saccharum Marshall)

Due to climate change, the ranges of many North American tree species are expected to shift northward. Sugar maple (Acer saccharum Marshall) reaches its northern continuous distributional limit in northeastern North America at the transition between boreal mixed‐wood and temperate deciduous forests. We hypothesized that marginal fragmented northern populations from the boreal mixed wood would have a distinct pattern of genetic structure and diversity. We analyzed variation at 18 microsatellite loci from 23 populations distributed along three latitudinal transects (west, central, and east) that encompass the continuous–discontinuous species range. Each transect was divided into two zones, continuous (temperate deciduous) and discontinuous (boreal mixed wood), based on sugar maple stand abundance. Respective positive and negative relationships were found between the distance of each population to the northern limit (D_north), and allelic richness (A_R) and population differentiation (F_ST). These relations were tested for each transect separately; the pattern (discontinuous–continuous) remained significant only for the western transect. structure analysis revealed the presence of four clusters. The most northern populations of each transect were assigned to a distinct group. Asymmetrical gene flow occurred from the southern into the four northernmost populations. Southern populations in Québec may have originated from two different postglacial migration routes. No evidence was found to validate the hypothesis that northern populations were remnants of a larger population that had migrated further north of the species range after the retreat of the ice sheet. The northernmost sugar maple populations possibly originated from long‐distance dispersal.     

Exotic Spartina alterniflora invasion alters ecosystem–atmosphere exchange of CH4 and N2O and carbon sequestration in a coastal salt marsh in China

Coastal salt marshes are sensitive to global climate change and may play an important role in mitigating global warming. To evaluate the impacts of Spartina alterniflora invasion on global warming potential (GWP) in Chinese coastal areas, we measured CH₄ and N₂O fluxes and soil organic carbon sequestration rates along a transect of coastal wetlands in Jiangsu province, China, including open water; bare tidal flat; and invasive S. alterniflora, native Suaeda salsa, and Phragmites australis marshes. Annual CH₄ emissions were estimated as 2.81, 4.16, 4.88, 10.79, and 16.98 kg CH₄ ha^?1 for open water, bare tidal flat, and P. australis, S. salsa, and S. alterniflora marshes, respectively, indicating that S. alterniflora invasion increased CH₄ emissions by 57–505%. In contrast, negative N₂O fluxes were found to be significantly and negatively correlated (P < 0.001) with net ecosystem CO₂ exchange during the growing season in S. alterniflora and P. australis marshes. Annual N₂O emissions were 0.24, 0.38, and 0.56 kg N₂O ha^?1 in open water, bare tidal flat and S. salsa marsh, respectively, compared with ‐0.51 kg N₂O ha^?1 for S. alterniflora marsh and ?0.25 kg N₂O ha^?1 for P. australis marsh. The carbon sequestration rate of S. alterniflora marsh amounted to 3.16 Mg C ha^?1 yr^?1 in the top 100 cm soil profile, a value that was 2.63‐ to 8.78‐fold higher than in native plant marshes. The estimated GWP was 1.78, ?0.60, ?4.09, and ?1.14 Mg CO₂eq ha^?1 yr^?1 in open water, bare tidal flat, P. australis marsh and S. salsa marsh, respectively, but dropped to ?11.30 Mg CO₂eq ha^?1 yr^?1 in S. alterniflora marsh. Our results indicate that although S. alterniflora invasion stimulates CH₄ emissions, it can efficiently mitigate increases in atmospheric CO₂ and N₂O along the coast of China.     

Genetic diversity and differentiation of Daphnia galeata in the middle and lower reaches of the Yangtze River,China

Mitochondrial 16S rDNA and CO I gene were used as molecular markers for the analysis of the genetic diversity and differentiation of Daphnia galeata populations in nine water bodies in the middle and lower reaches of the Yangtze River. In the combined 16S rDNA and CO I gene sequences, 54 variable sites and 44 haplotypes were observed among 219 individuals belonging to nine D. galeata populations. Average haplotype diversity and nucleotide diversity were, respectively, 0.72% and 0.56%. The F‐statistics (F_ST) value of the D. galeata populations was 0.149. According to the results of the neutral test, D. galeata in the middle and lower reaches of the Yangtze River had experienced a bottleneck effect in the history. Molecular variance analysis indicated that the genetic differentiation of the D. galeata populations mainly occurred within populations (85.09%). Greater genetic differentiations of D. galeata among individuals within populations appeared in the populations from the Huaihe River basin, whereas smaller genetic differentiations occurred in the populations from the middle reaches of the Yangtze River. Strong gene flows were all observed between Group I (four populations from the middle reaches of the Yangtze River) and Group ΙΙ (three populations from the middle and lower reaches of the Yangtze River), and Group ΙΙΙ (two populations from the Huaihe River basin). The effective migration rates (M) were 851.49 from Group I to Group ΙΙ and 685.96 from Group I to Group ΙΙΙ, respectively. However, no significant relationship was observed between the genetic differentiation and geographical distance of the nine populations (r = .137, p > .05). Results suggested that the genetic differentiation of D. galeata in the water bodies in the middle and lower reaches of the Yangtze River resulted mainly from geographical isolation.     

Disentangling the effects of acidic air pollution,atmospheric CO2, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75‐year tree ring chronologies of basal tree growth, carbon isotope discrimination (?¹³C, a proxy for leaf gas exchange), and δ¹⁵N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO₂, while reductions in pollutant emissions of NO_x and warmer springs played smaller, but significant roles. Tree ring ?¹³C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ¹⁵N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75‐year chronology, and was driven by changes in atmospheric CO₂ and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.     

Characteristics of snags and forest structure in southern mistbelt forests of the Amatole region,South Africa

Standing dead trees (or snags) are an important component of forest ecosystems, especially for tree cavity‐nesting vertebrate species, but their prevalence in South African forests remains under studied. Consequently, we investigated forest structure, and the presence and abundance of snags in six southern mistbelt forests in the Eastern Cape, South Africa. These forests have had varying levels of timber extraction over the past 150 years or more. We found snags were relatively rare in all six forests (<4.3% of trees sampled). Mean diameter at breast height (dbh) of snags ranged from 52 to 82 cm across the forests, with smaller snags in Kologha Forest and larger snags in Tyume Forest. A bimodal distribution of snag successional stages was found, with frequencies peaking at early and late stages, and few in the intermediate stages. Tree species diversity in the forests was relatively low (twelve–nineteen species across forests; only 28 species in total). There was no significant difference in dbh of trees between forests, with most occurring in the 20–29‐cm dbh size class. Future studies are required to identify trees that most likely support suitable cavities for tree cavity‐nesting bird species, and to determine cavity‐nester assemblage requirements in southern African forests.