Some of the Mechanisms for Coexistence of Tree Species Diversity in Tropical Forests: A Review of Effects of Tree Density Dependence

Tree communities contribute to maintenance of species diversity in tropical forests. Coexistence of many tree species is not without competition. Therefore, coexistence of tree species and size diversities occur sequentially or simultaneously in tropical natural forests. Understanding coexistence and competition mechanisms of tree species requires knowledge of interactions within and between species. However, many conservation efforts and strategies failed due to inability to identify and maintain functional coexistence mechanisms among tree species in the forest. Also, most trees died because of pressure on their habitats and not because of limiting growth resources. Hence, species identity, minimum distance and size of the neighbouring trees which are responsible for coexistence of competing trees in most tropical forests have not been explicitly reviewed. Therefore, this review evaluated some of the density dependent mechanisms for coexistence of tree species alpha diversity in tropical forests. Many interactive mechanisms are responsible for coexistence tree species in tropical forests. Inter- and intra-specific competitions are the most significant and both facilitate positive and negative density dependence. Therefore, switching from negative to positive density dependence may occur in some situations. Positive and negative density effects regulate species abundance and coexistence through conspecific and heterospecific structures. Aggregates of conspecific and heterospecific neighbours constitute forest spatial structure. Negative density interactions are mutually exclusive and basically ranged from effect of species identity of neighbours, distance to neighbours and tree size of the neighbours to reference trees in the community structures. Some mechanisms shorten distances for heterospecific than conspecific interactions. Conspecific structures improved survival and growth of rare tree species. Interactive mechanisms in tree community and population structures facilitate species diversity and size inequality, respectively.

Modifying regeneration strategies classification to enhance the understanding of dominant species growth in fire-prone forest in Southwest China

Background:In fire-prone ecosystems,plant species having different post-fire regeneration strategies(PFRS)coexist at the local scale while showing different growth dynamics.To evaluate the effect of PFRS on species regeneration,we investigated the plant communities burned in different years in the central Yunnan Province,Southwest China.Several indicators describing plant growth and population importance were measured to compare the regeneration of four dominant tree species,including one facultative seeders(FS)and three obligate resprouters(OR).Partial correlation and mixed linear effect modeling were applied to disentangle the contribution of intrinsic and environmental factors to the interspecific variation in post-fire regeneration.Results:We identified two subtypes of plant growth strategy in the early post-fire stage for OR species;i.e.,number growth(ORþN)priority and height growth(ORþH)priority for sprouting stems.Generally,the FS and OR species varied in height growth rate with different temporal dynamics.The ORþN species occupied post-fire space horizontally with more resprouts and larger coverage than ORþH and FS species at the earlier stage.In contrast,the ORþH species generally had far less resprouts per clump,more variations in stem height and basal diameter.Factor analysis showed that the variation of post-fire plant regeneration was not effectively explained by environmental factors(R2<20%);however,the linear mixed models with the modified PFRS as a random effect substantially increased the explanation.Conclusions:The differentiated regeneration dynamics and growth priorities in the four dominant tree species indicated a critical dichotomy of habitat occupation strategy for the resprouter species during the early stage of post-fire forest restoration.Our study uncovered a trade-off between height versus number growth priority in the post-fire tree growth strategy and provides a novel perspective in understanding the living space occupying(niche partitioning)process and species coexistence in post-fire forest community assembly.

Fagus sylvatica and Quercus pyrenaica:Two neighbors with few things in common

Background:The Iberian Peninsula comprises one of the largest boundaries between Mediterranean and Eurosiberian vegetation,known as sub-Mediterranean zone.This ecotone hosts many unique plant species and communities and constitutes the low-latitude(warm)margin of numerous central European species which co-occur with Mediterranean vegetation.Two of the main species found in this region are the Eurosiberian European beech(Fagus sylvatica L.)and the Mediterranean Pyrenean oak(Quercus pyrenaica Willd.).It remains unclear how the different physiological and adaptive strategies of these two species reflect their niche partitioning within a subMediterranean community and to what extent phenotypic variation(intraspecific variability)is driving niche partitioning across Eurosiberian and Mediterranean species.Methods:We quantified functional niche partitioning,based on the n-dimensional hypervolume to nine traits related to resource acquisition strategies(leaf,stem and root)plus relative growth rate as an additional wholeplant trait,and the environmental niche similarity between Pyrenean oak and European beech.Further,we analyzed the degree of phenotypic variation of both target species and its relationship with relative growth rates(RGR)and environmental conditions.Plant recruitment was measured for both target species as a proxy for the average fitness.Results:Species’functional space was highly segregated(13.09%overlap),mainly due to differences in niche breadth(59.7%)rather than niche replacement(25.6%),and beech showed higher trait variability,i.e.,had larger functional space.However,both species shared the environmental space,i.e.,environmental niches were overlapped.Most plant traits were not related to abiotic variables or RGR,neither did RGR to plant traits.Conclusions:Both target species share similar environmental space,however,show notably different functional resource-use strategies,promoting a high complementarity that contributes to maintaining a high functionality in sub-Mediterranean ecosystems.Therefore,we propose that conservation efforts be oriented to preserve both species in these habitats to maximize ecosystem functionality and resilience.

Root phosphatase activity is a competitive trait affiliated with the conservation gradient in root economic space

Background:The diversity of resource acquisition strategies of plant roots determines the species coexistence patterns to a certain extent.However,few root physiological traits have been investigated,such as root phosphatase activity(PA)that affects plant phosphorus(P)uptake.Methods:Root PA and classical root functional traits were investigated for 21 coexisting species in a deciduous broad-leaved forest in warm temperate-subtropical transition zone,China.We analyzed the root order variation of absorptive fine root PA,clarified the attribution of root PA in root economic space(RES)and the different P acquisition strategies of co-occurring species based on the multidimensional RES theory,and determined the dominant factors affecting interspecific variation in root PA.Results:There was no distinct pattern of PA variation with root order in the first three root orders of absorptive fine roots,and root PA was constrained by phylogeny.Root PA is a competitive trait affiliated with the conservation gradient in RES.The tight linkages among root PA,mycorrhizal colonization,diameter,specific root length,and nitrogen concentration suggested trade-offs among P acquisition strategies of co-occurring species,i.e.species with long and fine roots acquire inorganic P by actively exploring the soil and secreting phosphatase to mineralize and hydrolyze organic P,while species with short and thick roots obtain P mainly by investing C in mycorrhizal partners.Conclusions:Collectively,our study provides an insight into the forest species coexistence in climatic transition zones,i.e.species coexistence mechanisms based on diverse phosphorus acquisition strategies.

Habitat heterogeneity and biotic interactions mediate climate influences on seedling survival in a temperate forest

Seedling stage has long been recognized as the bottleneck of forest regeneration,and the biotic and abiotic processes that dominate at seedling stage largely affect the dynamics of forest.Seedlings might be particularly vulnerable to climate stress,so elucidating the role of interannual climate variation in fostering community dynamics is crucial to understanding the response of forest to climate change.Using seedling survival data of 69 woody species collected for five consecutive years from a 25-ha permanent plot in a temperate deciduous forest,we identified the effects of biotic interactions and habitat factors on seedling survival,and examined how those effects changed over time.We found that interannual climate variations,followed by biotic interactions and habitat conditions,were the most significant predictors of seedling survival.Understory light showed a positive impact on seedling mortality,and seedling survival responded differently to soil and air temperature.Effects of conspecific neighbor density were significantly strengthened with the increase of maximum air temperature and vapor pressure deficits in the growing season,but were weakened by increased maximum soil temperature and precipitation in the non-growing season.Surprisingly,seedling survival was strongly correlated with interannual climate variability at all life stages,and the strength of the correlation increased with seedling age.In addition,the importance of biotic and abiotic factors on seedling survival differed significantly among species-trait groups.Thus,the neighborhood-mediated effects on mortality might be significantly contributing or even inverting the direct effects of varying abiotic conditions on seedling survival,and density-dependent effects could not be the only important factor influencing seedling survival at an early stage.

Context-dependent responses of food-hoarding to competitors in Apodemus peninsulae:implications for coexistence among asymmetrical species

Superior species may have distinct advantages over subordinates within asymmetrical interactions among sympatric animals.However,exactly how the subordinate species coexists with superior species is unknown.In the forests west of Beijing City,intense asymmetrical interactions of food competition exist among granivorous rodents(e.g.Apodemus peninsulae,Niviventer confucianus,Sciurotamias davidianus and Tscherskia triton)that have broadly overlapping habitats and diets but have varied body size(range 15-300 g),hoarding habits(scatter vs larder)and/or daily rhythm(diurnal vs nocturnal).The smallest rodent,A.peninsulae,which typically faces high competitive pressure from larger rodents,is an ideal model to explore how subordinate species coexist with superior species.Under semi-natural enclosure conditions,we tested responses of seed-hoarding behavior in A.peninsulae to intraspecific and interspecific competitors in the situations of pre-competition(without competitor),competition(with competitor)and post-competition(competitor removed).The results showed that for A.peninsulae,the intensity of larder-hoarding increased and the intensity of scatter-hoarding declined in the presence of intraspecifics and S.davidianus,whereas A.peninsulae ceased foraging and hoarding in the presence of N.confucianus and T.triton.A.peninsulae reduced intensity of hoarding outside the nest and moved more seeds into the nest for larder-hoarding under competition from intraspecific individuals and S.davidianus.In most cases,the experimental animals could recover to their original state of pre-competition when competitors were removed.These results suggest that subordinate species contextually regulate their food-hoarding strategies according to different competitors,promoting species coexistence among sympatric animals that have asymmetrical food competition.

Aggregated spatial distributions of species in a subtropical karst forest,southwestern China

Aims spatial distribution patterns of species reflect not only the ecological processes but also the habitat features that are related to species distribution.in karst topography,species distribution patterns provide more specific information about their environments.The objectives of this study are as follows:(i)to analyse and explain the spatial distribution patterns of conspecific trees in an old-growth subtropical karst forest;(ii)to investigate pattern changes at different spatial scales;(iii)to test the spatial pattern similarity(or dissimilarity)between trees at different abundances,diameter at breast height classes,canopy layers and different functional groups(shade tolerance and seed dispersal mode);(iv)to examine whether habitat heterogeneity has an important effect on the species spatial distribution.Methods The spatial distributions of woody species with≥20 individuals in a 1-ha subtropical karst forest plot at maolan in southwestern China were quantified using the relative neighbourhood densityΩbased on the average density of conspecific species in a circular neigh-bourhood around each species.Important Findingsaggregated distribution is the dominant pattern in the karst forest,but the ratio of aggregated species in total species number decreases with an increase in spatial scale.less abundant species are more aggregated than most abundant species.aggregation is weaker in larger diameter classes,which is consistent with the prediction of self-thinning.seed dispersal mode influences spatial patterns,with species dispersed by animals being less aggregated than those dispersed by wind and gravity.other species functional traits(e.g.shade tolerance)also influence the species spatial distributions.moreover,differences among species habitat associations,e.g.with rocky outcrops,play a significant role in species spatial distributions.These results indicate that habitat heterogeneity,seed dispersal limitation and self-thinning primarily contribute to the species spatial distributions in this subtropical karst forest.

Demographic trade-offs determine species abundance and diversity

Aims Much recent theory has focused on the role of neutral processes in assembling communities,but the basic assumption that all species are demographically identical has found little empirical support.Here,we show that the framework of the current neutral theory can easily be generalized to incorporate species differences so long as fitness equivalence among individuals is maintained through trade-offs between birth and death.Methods Our theory development is based on a careful reformulation of the Moran model of metacommunity dynamics in terms of a non-linear one-step stochastic process,which is described by a master equation.Important Findings We demonstrate how fitness equalization through demographic trade-offs can generate significant macroecological diversity patterns,leading to a very different interpretation of the relation between Fisher’s a and Hubbell’s fundamental biodiversity number.Our model shows that equal fitness(not equal demographics)significantly promotes species diversity through strong selective sieving of community membership against high-mortality species,resulting in a positive association between species abundance and per capita death rate.An important implication of demographic trade-off is that it can partly explain the excessively high speciation rates predicted by the neutral theory of the stronger symmetry.Fitness equalization through demographic trade-offs generalizes neutral theory by considering heterospecific demographic difference,thus representing a significant step toward integrating the neutral and niche paradigms of biodiversity.

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.

Distribution patterns of tree species in an evergreen broadleaved forest in eastern China

Ecological assembly rules in evergreen broad-leaved forest are far from clear understanding.Spatial dispersion of individuals in a species is central in ecological theory.We analyzed the spatial patterns as well as associations between adult and juvenile of each tree species in a 5-ha subtropical evergreen broad-leaved forest plot in eastern China.Out of the 74 species occurring with more than 10 individuals,88.4%of these species are aggregated.Most of them are aggregated from small to large scales.Spatial distributions of some species corre-spond with topography.Many bad dispersed species in the Baishanzu exhibit a highly aggregated distribution at small scales.These suggest that environmental heterogeneity and/or dispersal limitation may be the most important mechanisms that control the distribution patterns of these species.Our observations of the aggregations of abundant species basically support the hypothesis that dispersal limitation decreases as the number of reproductive trees increases.The rest species are randomly distributed,with less than 10 individuals.For most common species,spatial aggregation is weaker in larger diameter classes,and the distance between adults is larger than that between juveniles and adults,suggesting that density-dependence works on loosing aggregation and excluding conspecific juveniles away from adult trees.However,the density-dependent effect is not strong enough to eliminate all seedlings near adult trees and to result in a regular distribution of trees;thus the density dependence is usually masked by the refuge effect.

Asymmetric competition for seeds between two sympatric food hoarding rodents:implications for coexistence

Asymmetric competition occurs when some species have distinct advantages over their competitors and is common in animals with overlapping habitats and diet.However,the mechanism allowing coexistence between asymmetric competitors is not fully clear.Chinese white-bellied rats(Niviventer confucianus,CWR)and Korean field mice(Apodemus peninsulae,KFM)are common asymmetric competitors in shrublands and forests west of Beijing city.They share similar diet(e.g.plant seeds)and activity(nocturnal),but differ in body size(CWR are bigger than KFM),food hoarding habit(CWR:mainly larder hoarding;KFM:both larder and scatter hoarding),and ability to protect cached food(CWR are more aggressive than KFM).Here,we tested seed competition in 15 CWR-KFM pairs over a 10-day period under semi-natural enclosure conditions to uncover the differences in food hoarding,cache pilferage,and food protection between the 2 rodents,and discuss the implication for coexistence.Prior to pilferage,CWR harvested and ate more seeds than KFM.CWR tended to larder hoard seeds,whereas KFM preferred to scatter hoard seeds.Following pilferage,CWR increased consumption,decreased intensity of hoarding,and pilfered more caches from KFM than they lost,while KFM increased consumption more than they hoarded,and they preferred to hoard seeds in low and medium competition areas.Accordingly,both of the 2 rodent species increased their total energy consumption and hoarding following pilferage.Both rodent species tended to harvest seeds from the source,rather than pilfer caches from each other to compensate for cache loss via pilferage.Compared to CWR,KFM consumed fewer seeds when considering seed number,but hoarded more seeds when considering the seeds’relative energy(energy of hoarded seeds/rodent body mass2/3)at the end of the trials.These results suggest that asymmetric competition for food exists between CWR and KFM,but differentiation in hoarding behavior could help the subordinate species(i.e.KFM)hoard more energy than the dominant species(i.e.CWR),and may contribute to their coexistence in the field.

Density dependence and habitat preference shape seedling survival in a subtropical forest in central China

Aims seedlings are vulnerable to many kinds of fatal abiotic and biotic agents,and examining the causes of seedling dynamics can help understand mechanisms of species coexistence.To disentangle the relative importance of neighborhood densities,habitat factors and phylogenetic relatedness on focal seedling survival,we monitored the survival of 5306 seedlings of 104 species>15 months.We address the following questions:(i)How do neighborhood densities,habitat variables and phylogenetic relatedness affect seedling survival?What is the relative importance of conspecific densities,habitat variables and phylogenetic relatedness to seedling survival?(ii)Does the importance of the neighborhood densities,habitat variables and phylogenetic relatedness vary among growth forms,leaf habits or dispersal modes?specially,does the conspecific negative density dependence inhibit tree and decidu-ous seedlings more compared with shrub and evergreen species?Does density dependence affect the wind and animal-dispersed species equally?Methods We established 135 census stations to monitor seedling dynamics in a 25-ha subtropical forest plot in central China.Conspecific and heterospecific seedling density in the 1-m2 seedling plot and adult basal area within a 20-m radius provided neighborhood density var-iables.mean elevation,convexity and aspect of every 5-×5-m grid with seedling plots were used to quantify habitat characteristics.We calculated the relative average phylodiversity between focal seed-ling and heterospecific neighbors to quantify the species related-ness in the neighborhood.Eight candidate generalized linear mixed models with binominal error distribution were used to compare the relative importance of these variables to seedling survival.akaike’s information criteria were used to identify the most parsimonious models.Important Findingsat the community level,both the neighborhood densities and phylogenetic relatedness were important to seedling survival.We found negative effects of increasing conspecific seedlings,which suggested the existence of species-specific density-dependent mortality.Phylodiversity of heterospecific neighbors was negatively related to survival of focal seedlings,indicating similar habitat preference shared among phylogenetically closely related species may drive seedling survival.The relative importance of neighborhood densities,habitat variables and phylogenetic relatedness varied among ecological guilds.Conspecific densities had significant negative effect for deciduous and wind-dispersed species,and marginally significant for tree seedlings>10 cm tall and animal-dispersed species.Habitat variables had limited effects on seedling survival,and only elevation was related to the sur-vival of evergreen species in the best-fit model.We conclude that both negative density-dependent mortality and habitat preference reflected by the phylogenetic relatedness shape the species coex-istence at seedling stage in this forest.

Differential impacts of adult trees on offspring and non-offspring recruits in a subtropical forest

An important mechanism promoting species coexistence is conspecific negative density dependence(CNDD),which inhibits conspecific neighbors by accumulating host-specific enemies near adult trees.Natural enemies may be genotype-specific and regulate offspring dynamics more strongly than non-offspring,which is often neglected due to the difficulty in ascertaining genetic relatedness.Here,we investigated whether offspring and non-offspring of a dominant species,Castanopsis eyrei,suffered from different strength of CNDD based on parentage assignment in a subtropical forest.We found decreased recruitment efficiency(proxy of survival probability)of offspring compared with non-offspring near adult trees during the seedling-sapling transition,suggesting genotype-dependent interactions drive tree demographic dynamics.Furthermore,the genetic similarity between individuals of same cohort decreased in late life history stages,indicating genetic-relatedness-dependent tree mortality throughout ontogeny.Our results demonstrate that within-species genetic relatedness significantly affects the strength of CNDD,implying genotype-specific natural enemies may contribute to population dynamics in natural forests.

The structure of natural microbial enemy-victim networks

Introduction:The persistence of generalists and specialists is a topical question in community ecology and results from both ecological and evolutionary processes.At fine taxonomical scales,ecological specialisation,i.e.organisms preferentially exploiting a subset of available habitats,is thought to be a driver promoting niche diversity.It is not clear,however,how different mechanisms interact to shape specialist-generalist coexistence.Methods:We reconstruct the structure of five bacteria-phage networks from soil isolates,and perform an analysis of the relationships between host phylogenetic diversity,parasite specialism,and parasite performance.Results:We show that the co-occurrence of species on a continuum of specialism/generalism is influenced by niche overlap,phage impact on bacterial hosts,and host phylogenetic structure.In addition,using a null-model analysis we show that infection strategies of the phages have more explanatory power than bacterial defenses on key structural features of these antagonistic communities.Conclusions:We report that generalists have more impact on their hosts than specialists,even when the phylogenetic heterogeneity of hosts is controlled for.We discuss our results in the light of their implications for the evolution of biotic interactions.

Indirect facilitation by a liana might explain the dominance of a small tree in a temperate forest

Aims Lianas are expected to influence composition,structure and func-tioning of forest systems due to unequal distribution across the potential set of host plants.However,our understanding of mecha-nisms associated with preferences for specific hosts is still limited,and so is our ability to discern between endogenous and exogenous forces driving forest dynamics in the long run.In this paper,we eval-uated whether the dominant liana Hedera helix can indirectly con-tribute to the eventual dominance of the small multi-stemmed tree Corylus avellana in a remnant temperate forest in central Iberian Peninsula from comparatively reduced liana infestation on C.avel-lana relative to co-occurring woody species.Methods through principal component analysis and co-occurrence ana-lysis,we studied the distribution and spatial association between woody species and the liana H.helix.We analyzed the relationship between the number of species in a plot and the number of species infested by the liana to test the hypothesis that H.helix is a gen-eralist liana.through generalized linear mixed models,we tested the dynamic-multi-stemmed growth form of C.avellana as a plaus-ible life strategy to withstand,in the long run,the liana infestation.In particular,we tested(i)the relationship between stem size and the probability of H.helix infestation including all the tree species within plots and(ii)the relationship between stem size and mortal-ity as evidence of the stem turn over in the tree C.avellana.Important Findings Our results indicate that H.helix and C.avellana significantly co-occur in mature stands of this remnant temperate forest where pion-eer woody species are absent.Hedera helix severely infests all the woody species whenever stem size exceed≈25 cm perimeter and there is physical contact at the base of the stem.this implies that all the trees in the community are potential hosts for H.helix.Mixed models indicate that both,infestation by H.helix and stem mortality,are positively related to C.avellana stem perimeter.Reduced long-term infestation of the liana by means of a multi-stemmed growth form with high stem turnover in C.avellana might be an advantage with respect to unipodial tree species.thus,the liana-tree coexistence pattern may be interpreted as an indirect positive interaction that,contrary to previous findings,results here in species dominance instead of species coexistence.

Invasive weed disrupts facilitation of nutrient uptake in grass-clover assemblage

Nutrient constraints in low-fertility soil were modified by different species combinations.Grass-clover assemblages benefited both species in terms of nutrient procurement.Interplay of competition and facilitation is demonstrated.An invasive weed removed essential nutrients from the grazing cycle.To investigate the interplay of competition and facilitation between plants in low-fertility pasture grasslands of New Zealand,we compared nutrient uptake and acquisition of key nutrients of three species from different func-tional groups.Combinations of Pilosella officinarum(mouse-eared hawk-weed,an invasive weed),Trifolium repens(white clover,a nitrogen fixer)and Dactylis glomerata(cocksfoot,a pasture grass)were planted into a soil with low-to-deficient concentrations of key nutrients.Highest yields were achieved by the grass growing alone but,when the clover and grass had grown together,there were complementary benefits in terms of procurement of a wide range of nutrients from soil despite lower root biomass.The inva-sive weed negated these benefits,and soil nutrients were exploited less efficiently when Pilosella had grown alone or in a mixture with the other species.Competition from the weed removed the benefits of grass-legume coexistence.These findings are interpreted to suggest that requirements for legumes to be the main source of nitrogen in pasture grasslands may be compromised unless competitive weeds are controlled to avoid disrupted procurement of key nutrients.It is likely these constraints to nutrient procurement would similarly impact conservation grasslands.