β Diversity And Community Assembly Mechanism Of Temperate Forests At Different Spatial Scales In Northeastern China

The assembly mechanism of ecological communities has always been the central topic of ecology.In recent years,by studying on difference of β diversity among different communities,and explaining why some of the communities have great changes in species composition while some others have only a little,ecologists have obtained a comprehensive understanding of the ecological mechanism affecting community assembly,that the community structure of local communities is a result of both niche and neutral process,which is also limited by the size of species pool.However,despite the importance of scale effect on β diversity and the underlying mechanisms is increasing,there are many important issues are still unsolved: On one hand,at the local scale,the relative importance of niche process and neutral process on community assembly remains controversial;how the effects of these two processes vary with the size of sampling units is still unclear;the factors that affected β diversity in relation to local and species contributions to β diversity have not been analyzed.On the other hand,in the studies on β diversity along environmental gradients in a large study area,quantitative analysis of ecological processes at multiple scales has not been reported;and the relative importance of regional and local ecological process on β diversity pattern has not been verified.In this thesis,on the basis of the big plots and a network of circular plots in temperate forests,the ecological mechanism affecting the local community structure is analyzed comprehensively at the local and regional scale,and the relative importance of different ecological mechanisms is compared,the scale dependency of β diversity and the underlying mechanisms in temperate forests were discussed.The study will provide factual and theoretical supports for supplementing and improving the production and maintenance mechanism of biodiversity in the framework of ecological theory.The main results are summarized as follows:1.By comparing the differences of community structure between the random community simulated by null model and the actual community at the local scale,it is found that the similarity of species composition among subplots in the actual community is significantly lower than that in the random community,indicating that the distribution of species in space is aggregated.This aggregation pattern is a result of both niche process(dominated by environmental filtering)and neutral process(dominated by dispersal limitation).In the two mechanisms,the effect of neutral process is greater than that of niche process.2.By sampling at five different spatial grains,the β diversity was calculated,and the effects of environmental filtering and dispersal limitation were quantified by variation partition at the local scale.The results showed that both β diversity and the underlying mechanism are scale dependent: with the increase of sampling units,β diversity is decreasing,the effect of environmental filtering is increasing,and the effect of dispersal limitation has no obvious change.In addition,the relative importance of niche process and neutral process also changes with sampling scale,that when the sampling scale is broad,the niche process is more important,while when the sampling unit is fine,the effect of neutral process is more important.3.By decomposing β diversity into local and species contributions to β diversity(i.e.LCBD and SCBD)at the local scale,uniqueness of species composition of subplots and the relative importance of species contribution to β diversity were measured.The corelation between LCBD and species richness,abundance and environments were analyzed,as well as the corelation between SCBS and abundance,species biological and ecological characteristics.The results showed that LCBD was negatively correlated with species richness,positively correlated with the secondary term of species richness,positively correlated with abundance,positively correlated with altitude and slope.These relationships showed that the subplots with unique species composition usually had less species and more individuals,mainly distributed in places with higher altitude and steeper slope.SCBD had a significant positive correlation with species abundance,a significant negative correlation with niche position,and a significant correlation with growth type,indicating that the species significantly contributed to βdiversity usually has a large abundance,the number of its individuals varies a lot among subplots,and its biological and ecological characters have significantly effects on SCBD.4.β diversity decreases with increasing latitude at the regional scale.By comparing the differences of composition structure between the random community simulated by null models and the actual community at multiple scale,the ecological processes of community assembly were quantitatively divided into the environmental filtering at the region-zone,zone-area,area-district,district-plot scale and the spatial aggregation at the within-plot scale.The relative importance of ecological process at each spatial scale was compared,and their patterns along latitude were analyzed.The results showed that the pattern of β diversity along latitudinal gradient is a result of multiple-scale environmental filtering and spatial aggregation.Firstly,by environmental factors from broad to fine scales,environmental filtering processes progressively select the species which are best adapted to local conditions from the regional pool,then spatial aggregation processes ultimately determine the spatial organization of the selected species.Among them,the processes at region-zone,plot-district and within-plot scale are the dominant.The environmental filtering at the region-zone scale is mainly related to the factors of temperature,precipitation,wind speed,altitude and soil depth,of which the effects increase with latitudes;the environmental filtering at the district-plot scale is mainly related to the factors of temperature,precipitation,slope,slope direction,litter thickness,of which the effects decrease with increasing latitudes;besides,the effects of spatial aggregation at the within-plot scale decrease with the increase of latitudes.5.β diversity of trees,shrubs and herbs all decreased with the increase of latitudes at the regional scale.By comparing the differences of species composition structure between the random communities simulated by null model and the actual community,the ecological processes affecting the local community assembly were quantitatively divided into regional and local ecological processes,the relative effects of these two processes were compared and how their effects varied along latitudinal gradient were analyzed.The results showed that the relative effects of regional and local processes on the pattern of β diversity along latitudinal gradient were different among trees,shrubs and herbs.For trees,the regional and local processes both have a significant impact on the change of β diversity along the latitudinal gradient.The effect of regional process increases with the increase of latitude,the effect of local process decreases with the increase of latitude,and the effect of regional process is far stronger than that of local process.For shrubs,the effect of regional process increases with latitude,the effect of local process does not change with latitude,and the effect of regional process is greater than that of local process.For herbs,the effect of regional process increases with latitude,but the effect of local process does not change with latitude gradient,and the effect of regional process is similar to that of local process.