Shifting Importance Of Abiotic Versus Biotic Filtering Across Intact Mature Forests To Post-clearcut Secondary Forests In Tiantong Area

Context: Understanding community assembly or biodiversity maintenance processes across natural to human-modified systems is a central topic in ecology and is germane to anthropogenic land management.Although ecologists often emphasize the roles of external-environment versus internal-biotic filter in structuring forest communities,these processes often act simultaneously instead of acting individually or sequentially.Most importantly,the relative importance of processes may vary among undisturbed versus disturbed habitats,especially in the context of natural versus post-clearcut secondary forests,and the details remain poorly understood.Moreover,clearcutting practice can be of diversified forms per se,some removed all standing vegetation,followed by plantation raising,others might only remove commercially valuable stems and keeping residual natural vegetation.Therefore,such a contrasting approach to clearcutting leads to different secondary forests,including plantation forests and shrublands.The subtropical forest communities of eastern China include both intact mature forests and post-clearcut secondary forests(e.g.,shrubland and plantation forests),which gave us a suitable site to bridge the knowledge gap of how the relative importance of externalenvironment versus internal-biotic filter that varied among undisturbed versus disturbed forests.Objective: The relative importance of environmental versus biotic filtering structuring among post-disturbance and undisturbed forest communities should differ markedly,in part,due to varying environmental properties as well as interspecific and intraspecific competitions.One of the ways to verify such assertions is to employ a trait-based approach and look at forest communities through the lens of external-environment versus internal-biotic filter.We gathered data on important leaf traits and divided them into within-community(alpha)and among-community(beta)components,analyzing community-level and species-level trait attributes along ecological gradients.Utilizing intact mature forests(control),shrublands with moderate disturbance history and plantations with severe disturbance history that share a regional species pool from subtropical eastern China,employ a trait-based approach,we would like to address the following four questions:(1)Do community-level trait mean,trait range and trait-trait covariation reflective of external-environment filtering vary among intact mature forest versus post-clearcut secondary forests?(2)Do community-level inter and intraspecific trait variation reflective of internalbiotic filtering vary among intact mature forest versus post-clearcut secondary forests?(3)Do species-level alpha-and beta-trait values,as well as niche breaths,vary among intact mature forests versus post-clearcut secondary forests?(4)How do underlying environmental conditions modulate the community-level trait mean,range,trait-trait covariation,and trait variations in forest communities?Methods: This study was conducted in the subtropical forests at Tiantong National Forest Park in Ningbo,Zhejiang.We selected four sites from each of three forest types—post-clearcut plantation and shrubland and intact mature forest—for a total of 12 sites and established six contiguous plots of 20 m × 20 m per site for a total of 72 plots.We identified all woody species at each plot and recorded their abundance,focused on eight leaf traits:(i)mean leaf area(MLA),(ii)specific leaf area(SLA),(iii)leaf dry matter content(LDMC),(iv)leaf thickness(LT),(v)leaf nitrogen concentration(LNC),(vi)leaf phosphorus concentration(LPC),(vii)leaf carbon to nitrogen ratio(C:N)and(viii)leaf nitrogen to phosphorus ratio(N:P).To assess the site’s environmental conditions,we measured(i)soil moisture content(%),(ii)the depth of organic matter(cm),(iii)canopy opening(%)and(iv)soil compaction(kg/cm2)at each plot.We examined whether the community-level mean,range,interspecific variation,intraspecific variation,and covariation of leaf functional traits reflecting environmental and biotic filtering vary among control versus post-disturbance forests.We repeated our analysis at the species level using Trait Gradient Analysis(TGA)to examine whether the frequency distribution of species trait mean value,alpha trait value,beta trait value and niche breadths vary among forest types.Lastly,we conducted a Principal Component Analysis(PCA)to summarize the plot-level variations in site environmental conditions and assessed how the underlying environmental conditions modulate the communitylevel trait mean,range,covariation,and variations in forest communities.Results: The PCA analysis indicated that plantations and shrublands with a disturbance history had lower site productivity than the intact mature forest.Plantations had higher MLA,SLA,LNC and LPC but lower LT,LDMC,C:N ratio and N:P ratio than the shrubland or intact mature forest,suggesting a clear dominance of resource acquisition strategy in plantations versus resource conservation strategy in the mature forests.Community-level trait ranges for most traits was substantially narrower for plantation than shrubland or mature forest.Plot-level trait-trait covariation was significantly higher in plantation than shrubland or mature forest.Multivariate inter and intraspecific trait variations across forest communities accounted for 67.8% and 32.2 % of the variance,respectively.However,mature forests and shrublands had slightly higher interspecific trait variations in LT than in the plantation.In contrast,the plantation had slightly higher intraspecific trait variations in LDMC and leaf C:N ratio than in shrublands.Compared to plantation or shrubland,the intraspecific trait variation in LT was also significantly higher at mature forests.The overall pattern of resource acquisitive versus conservative traits was similar through species’ beta-trait value which is reflective of environmental filtering,but blurred when through species’ alpha-trait value which is reflective of competition.The frequency of species having higher niche breadth was substantially higher at mature forests than at the plantation,suggesting the improved site conditions at mature forests.Conversely,the higher frequency of species with lower niche breadth may point towards stringent environmental filtering at the plantation.The association between plotlevel trait mean or trait range and site conditions appeared to be trait-as well as environmental property-dependent.Conclusion: Overall,our study demonstrates the shifting importance of site conditionmediated environmental filtering and intraspecific competition-mediated biotic filtering in the plantation to interspecific competition-mediated biotic filtering in the intact mature forests and shrublands.Our study also validates that ecological processes leave their signatures on plant functional traits.Apart from a further understanding of forest biodiversity maintenance processes,our results can help inform forest management.For instance,plantation forestry can benefit from thinning operations to reduce intraspecific competition and minimize soil disturbance to improve site productivity and diversity.