| Multi-species coexistence within a local community is a key issue in exploring the maintenance mechanism of biodiversity distribution and a hot topic in ecology.However,the universal mechanism of species coexistence is still unclear.Due to the significant differences in species richness among different natural ecosystems,different forest types may have different mechanisms for maintaining species coexistence.Therefore,specific studies are needed in a given forest.Current studies on the mechanisms of species co-existence are mainly focused on tropical or subtropical forests,but less on boreal forests(taiga forests).It is generally believed that resource competition is an important factor to promote species coexistence.However,in boreal forests with relatively harsh natural conditions,is the interaction between species also an important factor to promote the stable.coexistence of species? According to the resource ratio hypothesis and nutrient-load hypothesis based on niche partitioning and resource competition,relative ratios and absolute contents of limiting resources are the main factors determining the coexistence of species in natural communities.These two hypotheses are of important reference value for the study of the mechanisms of species diversity maintenance,and have been verified in controlled experiments and temperate grassland ecosystems.Thus in boreal forests with relatively simple species composition,do variations in nutrient ratios and loads also important factors for maintaining species diversity?Kanas Mountain in Xinjiang is the only typical coniferous forest distribution area of West Siberia in China.Due to its relatively southerly latitude and relatively abundant precipitation,this coniferous forest in this region has a higher diversity than most of typical Taiga forests,thus providing an excellent platform for studying the spatial distribution patterns,habitat associations and co-existence mechanisms of tree species at a local scale.In this study,the size and spatial distribution of all woody plants with DBH ≥1cm in a 12-ha(400m×300m)boreal forest dynamic plot in Kanas Nature Reserve,Xinjiang,completed in 2019 were investigated and measured.300 0.04 ha(20m×20m)quadrats were taken as the basic unit,the environmental factors in terms of topography,soil properties and canopy openness were obtained.We assessed the population distribution and diameter at breast height(DBH)structure,as well as spatial point patterns and interactions,of the four most abundant tree species(Larix sibirica,Picea obovata,Pinus sibirica,and Betula pendula)using g(r)function of point pattern analysis.Torus translation tests were used to analyze associations between dominant tree species and specific microhabitats.Linear mixed models were used to assess how soil nutrient availability and nutrient ratios affected the abundance and basal area of specific tree species,as well as how these effects varied across life stages.1.Spatial distribution patterns of these species were aggregated at small spatial scales,and that the degree of aggregation decreased with increasing spatial scale.Aggregation was the highest for the less common species(i.e.,B.pendula pendula).Saplings(1cm≤DBH<5cm)and juveniles(5cm≤DBH<15cm)were more aggregated than larger individuals(DBH≥15cm).Only P.obovata adults(DBH≥15cm)showed a regular(i.e.,hyperdispersed)distribution because of the intense intraspecific competition at small scales(i.e.,<5m).Intraspecific interactions between individuals were either positive or not significant.In terms of interspecific competition,there were significant negative associations among large(DBH≥15cm)and small adults(5cm≤DBH<15cm)for the two shade tolerant species(P.obovata and P.sibirica).In contrast,we observed positive interspecific interactions between individuals of different DBH classes and different shade tolerance species.Thus,most of the intra-and interspecific interactions showed positive or no significant correlations.Negative associations were generally less common than positive associations both within and between species.2.Two shade-tolerant species,P.obovate and P.sibirica,exhibited opposite relationships with soil nitrogen(N)and phosphorus(P)content.Picea obovate was associated with microhabitats with high N:P ratios,while P.sibirica preferred microhabitats with low N:P ratios.The light-demanding B.pendula was most abundant at low N and P sites.Another lightdemanding species,L.sibirica,did not show any significant habitat preferences.Furthermore,the N:P ratio mainly affected species abundance,while N and P content largely impacted basal area.Soil nutrient effects on species distributions weakened with tree age(from saplings to adults).Our results show that competition and facilitation simultaneously influence the spatial patterns of plants in this community.Moreover,positive interactions between tree individuals in this temperate region are likely to be more common than competitions for survival and growth of tree stems.Different microhabitats in the boreal forest had distinct soil resources.Both nutrient ratios and loads were correlated with the distribution of dominant boreal tree species,though these effects weakened as trees grew,especially for shade-tolerant species.Overall,niche partitioning due to resource variation may alleviate interspecific competition and contribute to species coexistence,and this study provides convincing proof of the importance of both nutrient ratios and loads on maintaining boreal forest diversity. |
