Ziwuling Temporal Pattern And Process Plant Community Species Diversity

Aims Plantations are established for a variety of reasons including wood production, soil and water conservation, and carbon sequestration. However, their implications for species diversity are considerably debated. To assess restoration effect of species diversity in plantations, we characterized and compared species diversity between plantations and natural forests at multiple scales, including sucessional time scales; quantifying the relative contributions of space and the environments on community composition to demonstrate different structuring processes. At last, the program intends to answer the following research questions:what governs community assembly and the maintenance of biodiversity? Plantations should be considered as ’green deserts’or valuable habitats for indigenous species? How to strengthen the restorational functions of plantations? Importantly, we address the long-term recovery of the plantations while taking into account the multiple spatial scales, compared with old growth forests as a baseline, providing a comprehensive understanding to recover in plantation forests. Meanwhile, it will be endued with practical significance for vegetation restoration on the Loess Plateau.Location Ziwu Mountains, in the middle of the Loess Plateau, northwestern ChinaMethods Using the method of’substituting temporal changes with spatial difference’, alpha and beta diversity patterns along a successional gradient were investigated in the Ziwu Mountain of Loess Plateau, northwestern China. The patterns of two beta diversity components (i.e. nestedness component and species replacement component) were also studied using additive partitioning. Species diversity patterns of Liaodong oak (Quercus wutaishanica) forests and Chinese pine (Pinus tabulaeformis) plantations were analyzed by additive partitioning across three spatial scales (subplot, plot and site scales). We examined how community assembly was affected by environmental filtering and dispersal limitation using canonical variation partitioning and Mantel tests, i.e. partition the variation of community composition/dissimilarity distance between environmental and spatial factors/distance. We repeated the analysis for three species groups (herb, shrub and tree layers) that differ in traits of likely importance for environmental filtering and dispersal limitation. Results (1) Herb layer species richness, evenness and species diversity decreased gradually during community succession (P<0.05); Shrub layer species diversity and richness showed a J-shaped trend, while evenness did not change significantly (P>0.05); Tree layer species richness, evenness and species diversity did not change significantly (P>0.05); There was a significant hump-shaped pattern in species richness along the temporal gradient, which agrees with the intermediate species-richness hypothesis.(2) There was a significant hump-shaped pattern in beta diversity of herb layer (P<0.05) during the process of community succession. Beta diversity of shrub layer decreased gradually over the successional stages, while that in tree layer increased gradually. Both species replacement and nestedness component had significant effects on beta diversity, and varied throughout the successional stages and among different layers. Overall, replacement component had significant dominant effects on beta diversity at all stages and layers. For conservation purposes, we suggest devoting efforts to a large number of different sites, but not necessarily the ones with richest diversity.(3) All forest types were simultaneously governed by environmental control and spatial processes; together, these processes explain29%to65%of the species composition variation (P<0.01). However, the effects of these two processes were varied among species groups; shrub layer and tree layer species were dominated by spatial process while there was similar importance between two processes for herb layer. Mantel and partial Mantel tests showed significant correlations between community dissimilarity and environmental dissimilarity/geographical distance (P<0.05). Herb layer showed high correlation with environmental dissimilarity while shrub and tree layer displayed high correlations with geographical distance.(4) Species diversity in Liaodong oak (Quercus wutaishanica) forests displayed significant non-random patterns at all scales. Species richness increased with increasing spatial scales, and β richness at site scale contributed the most. While Shannon diversity varied across scales and the greatest contribution to y diversity was from a at the finest scale (i.e., subplot scale). Species richness pattern in forest floor was beta-dominated across scales while for canopy layer and understory layer showed a similar importance between α and β components. By contrast, Shannon diversity patterns in all three layers were alpha-dominated. In conclusion, larger scales (plot and site scales) drive overall species richness patterns. While finer scale (subplot scale) determine Shannon diversity patterns. Habitat specialization explains a larger proportion of the variation in species composition between plots than does dispersal limitation; together, these processes explain28%to60%of the variation. The significant environmental variables differed among different layers and mainly functioned as spatially structured habitat. The effects of between habitat specialization and dispersal limitation were also varied among species groups, and both processes were stronger for the herb layer than for the shrub layer and canopy layer species.(5) Species diversity of both herb and shrub layers in the Chinese pine plantations (Pinus tabulaeformis) displayed significant non-random patterns at all scales. Species richness increased with the spatial scales, and beta richness at the site scale contributed the most. While Shannon diversity displayed a distinct opposite trend as compared with species richness patterns, and the greatest contribution to y diversity was from a at the finest scale (i.e., subplot scale). The spatial distribution of diversity in Chinese pine plantations was simultaneously driven by environmental filter and dispersal limitation. However, the relative contributions of these two processes varied with different layers. The herb layer was dominated by spatial process (spatial structured environmental factors and purely spatial variables), while the shrub layer dominated by environmental filter, including spatial structured environmental factors and purely environmental factors. In the herb layer, altitude, slope position, soil organic matter, and soil available potassium were significant environmental factors; in the shrub layer, altitude, slope aspect, soil available nitrogen, available potassium and available phosphorus were the significantly contributing environmental variables. In conclusion, larger scales drive overall species richness patterns while finer scales determine Shannon diversity patterns, which may be explained by distribution patterns of the common and rare species. These results suggest that the pine plantation alters plant species composition considerably after long-term succession and should not be considered as’green deserts’. High beta diversity at the site scale demonstrates the importance of this scale in preserving biodiversity. However, different protection measures should be applied to species in different layers with different species traits.(6) Overall, these two forest types showed significant different species compositions at the subplot and plot scales, but displayed similar accumulation of species diversity across spatial scales. The contribution of species diversity components increased with the spatial scales, and both types displayed lower alpha diversity at the subplot scale but higher beta diversity at the plot and site scales. Alpha diversity was not significantly different between these two forests at all three scales. By contrast, the pine plantations displayed significant higher beta diversity than old-growth forests at finer scales (i.e. subplot and plot scales). The diversity of both two forest types were simultaneously governed by environmental control and spatial processes, but dominated by spatial processes, with the exception of a greater contribution of each component in the pine plantations. The litter depth and altitude were two significant environmental factors that differed species composition between the two forests.Main conclusions (1) The importance of beta diversity. As mentioned above, beta diversity is the most important component to support total species diversity, no matter old-growth forests or plantations, especially at larger scales. In addition, beta diversity links ecological patterns with ecological processes. Assigning the different beta diversity patterns to their respective biological phenomena is essential for analyzing the causality of the processes underlying biodiversity. Thus, the differentiation of the spatial turnover and nestedness components of beta diversity is crucial for our understanding of central biogeographic, ecological and conservation issues.(2) Overall, replacement component had significant dominant effects on beta diversity at all stages and layers, which may be caused by the harsh environments in this area. Thus, reducing the intensity of interspecific competition is the most important task at present to effectively restore species diversity, especially at local scales (such as the subplot and plot scales in present study). Therefore, improving the quality and quantity of availability resource, and even habitats heterogeneity may be an important approach. In addition, as mentioned above, keep the spatial discontinuity of habitats may be also one of effective ways.(3) The assembly of plant communities in the Loess Plateau appears to be simultaneously driven by environmental filtering and dispersal limitation. This case study shows the importance of the dispersal limitation in understanding the forests assembly in the Loess Plateau. Consequently, it is important that management planning for restorations of natural vegetations take into account both habitat heterogeneity and geographical differences. (4) Our study did reveal a significant role of spatial scales in assessing biodiversity restoration function of plantations. Although beta diversity is a key component supporting total biodiversity in the both forest types, higher contribution beta diversity in pine plantations at finer scales (at the subplot and plot scales) suggested that beta diversity was more important than that in the old-growth forests to support total species diversity. These results support the argument that pine plantations alter plant species composition considerably and should not be considered as’green deserts’. However, to effectively restore species diversity, pine plantations should also be managed for enhancing high levels of species co-existence at the local scale (i.e. alpha diversity at the subplot and plot scales in the present case); a pattern that characterizes by not only the higher beta diversity but also higher alpha diversity may be our final restoration goals.(5) Our results also indicate that the spatial discontinuity of habitats acts in concert with stark environmental gradients to dictate species coexistence and diversity for both forest types. Although habitat fragmentation is often associated with a loss in biodiversity, studies indicate that fragmentation per se, as opposed to habitat loss, may actually have positive effects on biodiversity. Similarly, in our study system, natural fragmentation of the landscape promoted beta diversity at the regional scale (between sites), which probably enhances regional coexistence and diversity beyond what would be expected in contiguous habitats.