Plant Functional Traits’ Spatial Variability And Its Scales Dependent Correlations With Environment And Biotic Competitions In Evergreen Broad-leaved Forests

Environmental filtering and biotic interactions are the main ecological processes influencing community assembly and species coexistence. Trait-based approach is a new method to explain the ecological processes and strategies. Previous researches demonstrate that environmental filtering play a key role in community assembly at global and regional scales, while biotic interactions mainly take effect at micro-scales. The recent simulation studies indicate that environmental filtering and biotic interactions contribute to community assembly simultaneously, but their relative contributions have not been verified from field researches. Analyzing the spatial variation of plant functional traits across spatial scales and testing the associations between plant functional traits and environmental variables will offer insight into the mechanisms of species coexistence and community assembly.We selected a 5 ha forest plot in subtropical evergreen broadleaved forests located in Tiantong National Forest Park, Zhejiang Province. Four leaf traits including, diameter at breast height (DBH), individual leaf area (ILA), specific leaf area (SLA), and leaf dry matter content (LDMC) were measured for 19603 individual trees with DBH≥1cm. Soil properties were measured included:contents of soil total nitrogen, soil total phosphorus, soil total carbon, soil pH value, soil volumetric water content, bulk density, and humus depth. Topographic variables were measured included elevation, slope and convexity. Firstly, we used semi-variogram function to characterize the spatial structure and spatial variability of plant functional traits under the effects of environmental filtering and biotic interactions. Secondly, RLQ and Fourth-corner analyses were employed to test the significant associations between plant functional traits and environmental variables. Finally, we used Pearson correlation coefficient to evaluate the correlations among plant functional traits, environmental variables, and biotic competitors. Boosted regression trees (BRT) models were applied to visualize and quantify the correlations and relative contributions of environmental filtering and biotic interactions to plant functional traits. The main results and conclusions of this study are listed as follow:(1) Plant functional traits have larger variation at smaller spatial scales, and decreased with increasing spatial scales. For environmental variables, a reverse result was presented, indicated that environmental variables prefer have larger variations at large spatial scales than smaller spatial scales declared environmental filtering more likely occur at larger spatial scales.(2) RLQ analysis indicated that dominant plants have lager specific leaf size mostly distributed in a low slope and valley habitat with higher pH and BD; species grow in high valley always have a smaller leaf size. Plants in a nutrient depletion high slope habitat always with bigger LDMC, and palnts in fertile low valley habitat with bigger DBH.Fourth-corner analysis showed that DBH were significantly positive correlated with High slope (p<0.05); and ILA and Low slope have a significant positive relationships (p<0.05); SLA were significantly positive correlated with pH value, BD (p<0.05), and significant negative correlations were showed between SWCV, HD, C, Low slope, High ridge (p<0.05); LDMC were positively correlated with SWCV, HD, C, High ridge (p<0.05), and significantly negative correlated with pH value, BD, and Low slope (p<0.05). The correlations between other variables were not significant (p>0.05).(3) Combining analysis of RLQ and Fourth-corner, we found that considering the range of plant functional traits, SLA was significantly negative correlated with AxcR1 (p=0.002), SWCV, HB, C significant positively correlated with AxcQl (p< 0.05), pH value, BD were significantly negative correlated with AxcQ1 (p<0.05). When we used the variance of plant functional traits, there were significant positive correlations between LDMC and AxcQl (p=0.002). AxcQl were significant positively correlated with SWCV, HB, C, Low ridge (p<0.05), and negatively correlated with pH value and BD (p< 0.01). Other associations among plant functional traits and environmental variables were not significant (p>0.05). When used the mean of traits, AxcRl were significant positive related with LDMC (p< 0.01). AxcQl were positively correlated with TN, TC, and negative correlated with C (p<0.05). When CV calculated for traits, there were significant positive correlations among AxcQ1 and SWCV, HD, Low slope, and negative correlated with BD (p< 0.05). AxcQ2 were significantly negative with Low ridge (p<0.05).(4) Associations between plant functional traits and environmental variables showed that DBH correlated with SWCV and HD; ILA was sensitive to E and S; SLA and pH value, BD have stronger correlations; LDMC have bigger correlations with TC, TN, TP, High ridge, and Low valley.(5) Plant functional traits and environmental variables were significant correlated (p<0.001) in a local region whose correlations increased with the scales. However, the relationship between plant functional traits and biotic competition decreased with the scales.(6) BRT analysis demonstrated that environmental filtering and biotic competitions influenced community assembly simultaneously, whereas their relative contributions varied with spatial scales. Contributions of environmental variables increased across spatial scales contrary to that of biotic competitions.The contributions of biotic predictors had a peak at of 5 m x 5 m scales. As a whole, except for ILA at 5 m×5 m scales, contributions of environmental filtering were consistently larger than biotic competitions at the same spatial scales.In conclusion, we analyszing the spatial scales of plant functional traits and their spatial scales correlations to environment and biotic competitions in a subtropical evergreen board-leaved forest, in Tiantong Zhejiang province. It is found that environment, biotic competition, and spatial scale are the important sources of plant functional traits’variability. It demonstrated the processes and mechanisms of spceis coexistence and community assembly of plant in a local region. Comprehensive conclusion is that environmental filtering and biotic competition play a role to community assembly and ecological strategies simultaneously in a local region, and have the complementary effect of themselves.