| The Loess Plateau faces severe ecological and environmental issues, such as soiland waterloss and vegetation degradation. Ziwuling forest region, in the center of the Loess Plateau, is an essential component considered to regulate regional climate, maintain ecological balance, and promote sustainable development of economy and ecology. Ziwuling forest region encompasses abundant types of forest communities, including natural secondary forests that recover naturally and artificial forests. Therefore, this region is an important site for restoration ecology research and forestry management work related to strategies that can accelerate the succession of forest communities, promote vegetation recovery, and improve the biological diversity and ecological service functions of forest communities. In this study, we focused on nine typical forest communities in Mt. Ziwuling. Structural characteristics, species diversity, and spatial pattern in different types of forest communities were investigated using classical plant ecology research methods of field investigation and sampling in a large area. The effects of microhabitat (shrubs and herbs), plant litter, and seed burial on the regeneration of the main constructive species, namely, Liaodong oak (Quercus wutaishanica) and Chinese pine (Pinus tabuliformis), in the forest region in different stages of succession were also investigated by field controlled experiments and monitored for two years. This study mainly aimed to compare the characteristics of different forest types in this region, reveal the secondary succession rule of forest communities, verify C-S theory of community succession, and provide specific recommendations relevant to vegetation restoration and protection in the Loess Plateau. The following results were obtained:(1) Structural characteristics of forest communities1) Considering plant species composition, we found that forest communities in Mt. Ziwuling were composed of abundant plant species. A total of57families,164genera, and279species of higher plants were recorded; herbs and shrubs dominated this region. Considering genus composition belonging to families, we observed that families with more than five genera were few; many families contained less than five genera. Furthermore, the majority of families comprised one genus. The species composition of families and the species composition of genera presented the same characteristics.2) In a tree layer, the important value composition structures of the main tree species were very high in pure forests, particularly in the artificial pure forests. The important values of constructive species in mixed forests were relatively low; the dominant species of shrub layer in different forest communities were different. Carex dahurica was dominant in the herb layer. Lianas were recorded frequently. The composition of dominant species of shrub and herb layer in artificial Robinia pseudoacaciaforest, Betula platyphylla forest, and Populus davidiana forest was greatly different from that of other forest community types. Q. wutaishanica was important in tree and shrub layer in all types of forest communities, especially in communities dominated by P. tabuliformis.3) Considering size structure of main tree species, we found that Q. wutaishanica presented a mature and stable inverse-J type structure. The DBH class frequency distribution of the main tree species in natural P. tabulaeformis forest, artificial P. tabulaeformis forest, and B. platyphylla-P. tabuliformis mixed forest showed an increasing L type structure. In Q. wutaishanica-P. tabuliformis mixed forest the populations of Q. wutaishanica and P. tabuliformis showed an increasing and stable type, respectively. P. davidianaforest and B. platyphyllaforest showed an unstable size structure. The main tree species of artificial R. pseudoacacia forest and deciduous broad-leaved mixed forest showed a unimodal type structure.4) Based on the height structure of the main trees, the individual distributions of the constructive species of Liaodong oak forest and natural Chinese pine forest were homogeneous in all height classes and the development of communities was stable. By contrast, P. davidiana forest declined in height and B. platyphylla showed volatility and instability. The height structures of two artificial forest types showed medium height classes in which individuals showed a concentrated distribution pattern, but artificial Chinese pine forest showed a relatively stable development trend. Among the three mixed forest types, Q. wutaishanica-P. tabuliformis mixed forest showed good stability in height structure; the height classes of B. platyphylla and P. tabuliformis presented niche overlap; P. tabuliformis was dominant. In deciduous broad-leaved mixed forests, B. platyphylla and P. davidiana declined in height structures.(2) Species diversity characteristics of forest communities1) The diversity index of forest communities with high species diversity in this region showed the following trend: mixed forest>natural pure forest>artificial forest. The artificial Chinese pine forest exhibited low levels of all species diversity indexes, particularly the artificial R. pseudoacacia forest showing the lowest species diversity indexes. As the climatic climax community, Liaodong oak forest showed low species diversity, although this forest was highly stable. B. platyphylla-P. tabuliformis mixed forest, Q. wutaishanica-P. tabuliformis mixed forest, and deciduous broad-leaved mixed forest showed higher species diversity levels in middle and late stages of succession.2) Community vertical layer diversity of all of the forest community types showed the following trend:shrub layer>herb layer>tree layer. However, the species diversity of P. davidiana forest and artificial R. pseudoacacia forest showed that shrub layer was lower than herb layer.3) Considering β diversity, we found that the dissimilarity of forest communities with similar constructive species composition was generally low, and the lowest dissimilarity was observed between the natural Chinese pine forest and the Q. wutaishanica-P. tabuliformis mixed forest. However, the artificial R. pseudoacacia forest was greatly different from other forest communities in terms of species composition.(3) Spatial pattern characteristics of forest communities1) In monodominant forest communities, the regular spatial distribution pattern of constructive species manifested aggregated distribution on small scales and random distribution on large scales. The spatial patterns of the constructive species of Liaodong oak forest and natural Chinese pine forest were completely in conformity with this characteristic. In P. davidiana forest, the distribution pattern of P. davidiana was highly unstable, showing that aggregated and random patterns were observed on large and small scales. B. platyphylla population in B. platyphylla forest showed aggregated distribution pattern on small scales and random distribution pattern on large scales; after a particular period, regular distribution was observed. In the two artificial forest types, P. tabuliformis population showed weak aggregation on small scales, regular distribution on middle scales, and random distribution on large scales. By contrast, R. pseudoacacia population mainly showed aggregated distribution pattern, showing intense aggregation on small scales and random distribution on large scales.2) In the three forest communities with co-dominant species, different forest types presented different spatial pattern characteristics. The co-dominant species of Q. wutaishanica-P. tabuliformis mixed forest were not associated completely, and each species showed stable niche. This community type may be stable for a long period, In deciduous broad-leaved mixed forest, B. platyphylla population was negatively correlated with P. davidiana population on large scales, and the co-dominant species exhibited repellency. The co-dominant species of B. platyphylla-P. tabuliformis mixed forest showed a complicated interspecific relationship and intense competition. This community type may eventually become a monodominant community.(4) Regeneration mechanism of main constructive species in forest communitiesThis study confirmed the presence of facilitation, inhibition, and tolerance effect during the regeneration of forest communities in Ziwuling forest region. This study also verified the C-S theory of community succession.1) Shrubs and herbs facilitated Liaodong oak seedling establishment without affecting the seedling establishment of Chinese pine in the grassland. In the two forest stands, because of the competition of light and water, shrubs and herbs primarily had an inhibition effect on the establishment of these trees. The effect of plant litter was either facilitation or inhibition depending on target species and habitat. Furthermore, seed burial positively affected seedling establishment.2) Shrubs and herbs elicited inhibitory effects on the seedling growth of Liaodong oak and Chinese pine species in all of the three habitats; seed burial elicited different effects on Liaodong oak in different habitats. Plant litter did not influence seedling growth in either of the species. Shrubs and herbs, litter, and seed burial also exhibited different effects (facilitation, inhibition, or tolerance) on root:shoot biomass allocation, which depended on microsites and species.3) Liaodong oak and Chinese pine used different regeneration strategies during early stages of succession and similar strategies during late stages of succession. This observation was related to the positive and negative influences of shrubs and herbs on seedling regeneration. These effects were attributed to competition for environmental resources, such as light and water.In the series of succession of forest communities in Mt. Ziwuling, community structural characteristics and spatial pattern characteristics of P. davidiana forest and B. platyphylla forest reflected instability and volatility during early stages of succession. In middle and late stages of succession, Q. wutaishanica-P. tabuliformis mixed forest was the most stable mixed forest community type; B. platyphylla-P. tabuliformis mixed forest exhibited an evident interspecific competition and likely formed P. tabuliformis-monodominant community. Deciduous broad-leaved mixed forest declined gradually. The three types of mixed forest showed the highest species diversity level. In the late stages of succession, Liaodong oak forest and natural Chinese pine forest showed stable community characteristics. Two artificial forests demonstrated similarities and differences. The artificial Chinese pine forest was relatively stable; by contrast, artificial R. pseudoacacia forest greatly differed from other forest types in community characteristics and showed a declining development tendency. The two artificial forest types showed lower species diversity level than other forest types. In vegetation restoration, specific tending measures that promote regeneration should be considered on the basis of different responses of Liaodong oak and Chinese pine in dissimilar habitats; these measures included sowing by seed burial, reducing the densities of shrub and herb vegetation and plant litter in forests, increasing the coverage of shrubs, herbs, and plant litter in seedling emergency stage, and reducing coverage during the early growth stage of seedlings in grasslands. These pertinent measures may effectively promote the regeneration of forest communities and accelerate community succession and vegetation restoration in this region. |
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