Positive Interactions： Crucial Organizers in a Plant Community

For more than a century, ecologists have concentrated on competition as a crucial process for community organization. However, more recent experimental investigations have uncovered the striking Influence of positive Interactions on the organization of plant communities. Complex combinations of competition and positive interactions operating simultaneously among plant species seem to be widespread In nature. In the present paper, we reviewed the mechanism and ecological importance of positive Interactions In plant communities, emphasizing the certainties and uncertainties that have made It an attractive area of research. Positive Interactions, or facilitation, occur when one species enhances the survival, growth, or richness of another. The Importance of facilitation in plant organization increases with ablotlc stress and the relative Importance of competition decreases. Only by combining plant interactions and the many fields of biology can we fully understand how and when the positive Interactions occur.

Positive interactions can produce species-rich communities and increase species turnover through time

Aims The process of facilitation,where a species increases the survival,growth,and fitness of another species,is becoming increasingly recognized as a critical factor in shaping the structure of plant communities.This process is particularly important in stressful environments.Yet few studies have attempted to incorporate positive interactions into community ecological theories such as the neutral theory of biodiversity.Here,we use an equalizing trade-off model as a foundation to study the potential impact of facilitation on species richness and community temporal turnover.Methods Based on a spatially explicit birth–death trade-off model,we assume that the occurrence of facilitation is dependent on the presence of interspecific neighbours.We further propose that the realized birth rate for a given individual subject to facilitation is proportional to the number of interspecific neighbours within its neighbourhood.Thus,in our model,the individuals of rare species will benefit more from the existence of heterospecific individuals than common species.Important Findings As the facilitative coefficient increased,the species richness for simulated communities at the dynamically stochastic equilibrium was also increasing.Simulations also demonstrated that facilitation could increase the replacement of species through time:communities with facilitation become more dissimilar(i.e.have smaller Bray–Curtis similarity values)than communities without or with a lower degree of facilitation after the same time interval.Facilitation from interspecific neighbours on rare species increased their population sizes and consequently made them less prone to extinction,thus enhancing species richness.Meanwhile,in a saturated community,with the increase of species richness,mean population size of entire communities decreased,making species more prone to extinction on average,and thus increased the community temporal turnover.Our results suggest that future experimental work on the effect of facilitation on community-level properties should focus not just on species richness but also on other descriptors of community dynamics such as the temporal species turnover.

Positive interactions by cushion plants in high mountains:fact or artifact?

Positive interactions are defined as non-trophic interactions where at least one of the interacting species is benefited in terms of fitness and the other remains unaffected.Nevertheless,the bidirectional feedbacks between species may be positive,neutral or negative.Thus,if facilitated species induce negative effects on their‘nurses’,the assumed definition of positive interactions could be reconsidered.Methods We assessed if ecological interactions between cushions of Azorella madreporica and their facilitated species are positive.Specifically,we tested if cover of facilitated species has any costs for cushion plants from an ecophysiological perspective,and if these costs increase with the amount of cover of facilitated species.In addition,through pathway analysis and correlations,we assessed if cover and richness of facilitated species have a direct and/or indirect effect on the fitness of cushion plants.Important Findings We found that facilitated plant species induced a significant cost for their nurses(cushion plants),and this cost increases with cover of the facilitated species.Additionally,the facilitated species exert a strong direct negative effect on the cushion’s fitness and a moderate indirect negative cost evident through the nutrient status and physiological performance of cushion plants.We thus contribute evidence that positive interactions between high mountain cushion plants of central Chile and their‘facilitated’species may be an artifact more than a fact,especially when bidirectional effects are considered;contrasting with the majority of studies that document only one side of the interaction.

Encroachment drives facilitation at alpine shrublines

Ongoing encroachment is driving recent alpine shrubline dynamics globally,but the role of shrub-shrub interactions in shaping shrublines and their relationships with stem density changes remain poorly understood.Here,the size and age of shrubs from 26 Salix shrubline populations along a 900-km latitudinal gradient(30°-38°N)were measured and mapped across the eastern Tibetan Plateau.Point pattern analyses were used to quantify the spatial distribution patterns of juveniles and adults,and to assess spatial associations between them.Mean intensity of univariate and bivariate spatial patterns was related to biotic and abiotic variables.Bivariate mark correlation functions with a quantitative mark(shrub height,basal stem diameter,crown width)were also employed to investigate the spatial relationships between shrub traits of juveniles and adults.Structural equation models were used to explore the relationships among conspecific interactions,patterns,shrub traits and recruitment dynamics under climate change.Most shrublines showed clustered patterns,suggesting the existence of conspecific facilitation.Clustered patterns of juveniles and conspecific interactions(potentially facilitation)tended to intensify with increasing soil moisture stress.Summer warming before 2010 triggered positive effects on population interactions and spatial patterns via increased shrub recruitment.However,summer warming after2010 triggered negative effects on interactions through reduced shrub recruitment.Therefore,shrub recruitment shifts under rapid climate change could impact spatial patterns,alter conspecific interactions and modify the direction and degree of shrublines responses to climate.These changes would have profound implications for the stability of alpine woody ecosystems.

Annual-perennial plant relationships and species selection for desert restoration

Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems. By creating areas of ameliorated microclimates and fertile soil below their canopies, perennial plants might influence exotic annual plant invasions. We conducted a quantitative literature review of studies that compared exotic annual plant abundance among native perennial plant species and interspace （open areas） microsites in North America＇s Mojave Desert, where exotic plant invasion has corresponded with increasing extent of wildfire and broad-scale ecosystem transformation. Ten studies compared exotic annual plant abundance between inter- spaces and below a total of 36 native perennial species. These studies revealed that： （1） With few exceptions, most native perennial species supported a greater abundance of exotic annuals than interspaces, indicating overall facilitation of exotic species by native perennials. （2） Exotic species abundance varied by orders of magnitude among native perennial species, with some perennial species harboring amounts of exotics similar to interspaces. （3） Distributions of dominant exotic species varied, where Bromus rubens displayed a greater affinity for below-perennial microsites than did Schismus spp. and Erodium cicutarium that often were most abundant in interspaces. Results suggest that the degree of facilitation of exotic plants warrants consideration when selecting native perennial species for revegetation and restoration projects.

Exploring Some Tactics on How to Make English Lessons More Effective

This paper explores some effective tactics on the teaching of English. It starts with the introduction of the importance of learning English, then it reveals the qualities and useful strategies that a teacher of English should master if he or she wants to make his or her lessons successful. It also conducts the analysis of how these tactics interact in the classroom teaching. The paper concludes that examining effective ways of teaching is useful in helping teachers either in their academic field or in their professional development.

Effects of traits,species identity and local environmental conditions on the assessment of interactions:insights from an alpine meadow community

Aims The prediction that facilitation is the dominant interaction in physically stressful conditions has been supported by many but not all field studies.In the present paper,we tested the effects of the identity of species,the local environmental conditions and the currencies of performance measurement on such variation.Methods Using contrasting two plots,six species,and up to five multiple traits,we comprehensively explored the effects of the above factors on the assessment of plant interactions in an alpine meadow of the QingHai Tibetan Plateau.Additionally,we attempted to figure out the possible mechanisms underlying the responses observed.The data were analysed by both standard ANOVAs and multivariate statistics.Important findings Our results demonstrated that the response to the removal of neighbours was both species and trait specific,and the effect of the ocal environmental conditions was dependent on the species involved.The contrast between plots had crucial influence on the net interactions of Kobresia macrantha,but little effect on Elymus nutans.Regarding the abiotic conditions,neighbours had significant impact on soil temperature,moist and solar radiation.The results contribute to advance our knowledge on the potential underlying factors influencing the assessment of facilitation.

Facilitation by nurse plants contributes to vegetation recovery in human-disturbed desert ecosystems

Aims Facilitation by nurse plants is a common interaction in harsh environments and this positive plant-plant interaction may promote vegetation recovery in ecosystems affected by human activities.Determining the relevance of this process,however,requires assessing how nurse plants influence the establishment of other species,as well as the proportion of species in the regional species pool that would benefit from the presence of nurse plants in human-disturbed areas.Further,since vegetation recovery is a time-dependent process,the community-level consequences of facilitation are likely to vary among landscapes with different disturbance history.Thus,an integrative perspective of the relevance of nurse plants for vegetation recovery could be obtained by measuring their effects across different human-disturbed landscapes of the target region.This study focuses on these issues and uses a regional-scale approach to assess the community-level effects of a widespread nurse plant of American deserts,the creosotebush(Larrea tridentata).Methods This study was conducted in the southernmost portion of Chihuahuan Desert because most floodplain valleys of this region have been affected by human activities during the past centuries.For this study,we selected 10 floodplain valleys differing in their age(i.e.the time elapsed after human activities were ceased).At each landscape,we measured the cover of creosotebushes and the proportion of plant species positively associated with them,as well as the density of seeds in the soil beneath creosotebush canopies.All these data were regressed against the age of the landscapes.Further,to assess whether positive association patterns were due to facilitation or other processes,we conducted field experiments and measured the ecophysiological performance of plant species established beneath and outside creosotebush canopies.Important Findings Most plant species from the target region were positively associated to creosotebushes,and our field experiments and ecophysiological measures indicated that these distribution patterns can be attributed to facilitative interactions.In most landscapes,the density of seeds was higher beneath creosotebushes than in the surrounding habitats,suggesting that these shrubs may also act as seed traps.The community-level effects of creosotebushes increased with landscape age and creosotebush cover,indicating that magnitude of these effects depends on the disturbance history of each site.These results highlight the relevance of performing large-scale assessments for identifying the consequences of facilitation on vegetation recovery across space and time.We then propose that this kind of large-scale approach should be taken into account in the development of conservation programs aimed at the recovery and preservation of plant biodiversity in harsh environments.

A study on volunteer augmentation navigation technology

Navigation augmentation technology is one of the most common methods to increase the continuity,reliability and integrity of the global satellite navigation system.The concept of volunteer augmentation navigation (VNA) is proposed and the elements and topological structure of VNA are also analyzed in this paper.The study focuses on the neural network model that volunteers and ordinary users use modern communication information network to exchange self-organizing information.The neural cell model of Volunteer Augmentation Navigation using shared information is built.Thus interactive general relative positioning is realized.Then basic theories and methods of volunteer augmentation navigation are formed on the basis of the above-mentioned study.This study of realization mechanism of volunteer augmentation technology helps to form a relatively integral architecture of volunteer augmentation navigation.A user self-service satellite navigation augmentation which combines information exchange and navigation services may strengthen the continuity,reliability and integrity of the navigation system.The volunteer augmentation navigation theory proposed in this paper improves the traditional satellite navigation application model and expands the connotation and denotation of satellite navigation augmentation methods.

Influence of bifurcation position and length of side chains on the structure of isoindigo-based conjugated polymer thin films

We chose a series of isoindigo-based conjugated polymer（ⅡDDT,ⅡDDT-C3 and ⅡDDT-C4） with different length of side chains and bifurcation positions to investigate the relationship between the degree of alignment and the length of side chains and bifurcation positions.We found that the dichroic ratio was increased from 2.37 to 5.23 when the side chain was longer and the bifurcation position was away from the backbone.The π-π stacking distance was decreased from 3.67 A to 3.61 A when the bifurcation position was away from the backbone because of its smaller hindrance and the d-spacing of the（100）was increased from 20.06 A to 25.21 A when the side chain was longer.All the polymers were adopted an edge-on orientation with the backbone paralleled with the long axis of fibers.The weak interaction of side-chain in ⅡDDT-C4 was beneficial for the molecules being rearranged in parallel during the contact line receding and the strong n-n interaction could accelerate the interchain assembly of the parallel molecules through π-π interaction to form aligned fibers.