Xiaoan Wang

Distinguishing the importance between habitat specialization and dispersal limitation on species turnover

Understanding what governs community assembly and the maintenance of biodiversity is a central issue in ecology, but has been a continuing debate. A key question is the relative importance of habitat specialization (niche assembly) and dispersal limitation (dispersal assembly). In the middle of the Loess Plateau, northwestern China, we examined how species turnover in Liaodong oak (Quercus wutaishanica) forests differed between observed and randomized assemblies, and how this difference was affected by habitat specialization and dispersal limitation using variation partitioning. Results showed that expected species turnover based on individual randomization was significantly lower than the observed value (P < 0.01). The turnover deviation significantly depended on the environmental and geographical distances (P < 0.05). Environmental and spatial variables significantly explained approximately 40% of the species composition variation at all the three layers (P < 0.05). However, their contributions varied among forest layers; the herb and shrub layers were dominated by environmental factors, whereas the canopy layer was dominated by spatial factors. Our results underscore the importance of synthetic models that integrate effects of both dispersal and niche assembly for understanding the community assembly. However, habitat specialization (niche assembly) may not always be the dominant process in community assembly, even under harsh environments. Community assembly may be in a trait-dependent manner (e.g., forest layers in this study). Thus, taking more species traits into account would strengthen our confidence in the inferred assembly mechanisms.

Differences in plant species diversity between conifer (Pinus tabulaeformis) plantations and natural forests in middle of the Loess plateau

We compared differences in plant species diversity between conifer (Pinus tabulaeformis) plantations and natural secondary forests in the middle of the Loess plateau. The goal of the study was to examine the differences in the effect of stand development on species diversity and in species responses to changes between forest types and between forest layers. To clarify the effects of differences in forest management, we emphasized the functional types of plant species occurring in each forest type. The result as follow: (1) The H′ and S of tree layer were significantly lower in natural conifer forest than old conifer and secondary forest, but were not different compared with mid aged conifer forest. The H′ and S of shrub layer were significantly lower in mid aged conifer forest compared with other forest types. The H′ of herb layer showed no significant differences in the four forest types. The evenness index (J′) of tree layer of mid aged conifer forest was lower than other forest communities and its J′ of shrub layer was highest although its richness of shrub layer was lower than in the other forest types. (2) The analysis of β diversity index also indicated large differences between conifer plantations and natural forests. Although the tree layer species were similar in old plantation and natural conifer forests, they differed greatly between the natural conifer and secondary forests. The natural conifer and secondary forest species composition in shrub layer differed significantly from those in plantation and secondary plots. Tree species were significantly less common in plantations than in abandoned coppice forests. Species composition in the herb layer of different forest types was similar. (3) The management of P. tabulaeformis plantations alters plant species composition considerably; the number of sub tall-tree species is increased in old aged conifer forest, especially species dispersed by animals. Plantation management appears to affect ecological processes through seed dispersal. From the perspective of management, the change in the structure and composition of the canopy in plantations could affect the behavior of dispersers and regeneration.

Determining the recruitment limitation of three native woody species in the Chinese pine (Pinus tabuliformis Carr.) plantations on the Loess Plateau, China

This research aimed to quantify the relative importance of the multiple processes that limit the recruitment of three native woody species (Quercus wutaishanica Mayr, Betula platyphylla Suk., and Acer ginnala Maxim.) in the Chinese pine (Pinus tabuliformis Carr.) plantations in the Ziwuling Mountain on the Loess Plateau, China. In total, 216 seed traps and 1080 seedling plots with seed rain and seedling establishment monitoring were set up in three Chinese pine plantations and three hardwood forests for a three-year period. In addition, a seed-sowing and an acorn-tracing experiments were performed to evaluate the roles of litter and rodents in seedling recruitment and acorn dispersal. Despite the short distance between the conifer plantation and seed sources, our study revealed low efficiency of seed dispersal from hardwood forests to conifer plantations, whether they were dispersed by wind or by animals. The proportion of rodent-dispersed acorns with respect to acorn secondary dispersal was much lower in conifer plantations than in hardwood forests (i.e. 8% vs. 26%). In the conifer plantation, the thick litter constituted a barrier for seedling emergence, but favored the survival of Q. wutaishanica and A. ginnala seedlings. The indexes of recruitment showed that seed and establishment limitations were powerful restrictive forces in conifer plantations for the recruitment of the three species.

The effects of microhabitat, plant litter, and seed burial on the regeneration of Quercus wutaishanica and Pinus tabulaeformis

The effects of microhabitat (shrubs and herbs), plant litter, and seed burial on the regeneration of Liaodong oak (Quercus wutaishanica Mayr) and Chinese pine (Pinus tabulaeformis Carr.) were studied in three typical stands (Liaodong oak forest, Chinese pine plantation, and grassland) in the Loess Plateau, China. We monitored the establishment and growth of seedlings of these two woody species in sown experimental plots, in which shrubs and herbs, plant litter, and seed burial were manipulated. In the grassland, shrubs and herbs facilitated Liaodong oak establishment, with no effect on the establishment of Chinese pine. In the two forest stands, shrubs and herbs primarily had an inhibition effect on the establishment of these trees. The effects of plant litter were facilitation or inhibition, depending on the target species and the habitat. Seed burial had a positive effect on seedling establishment. In all three habitats, shrubs and herbs had inhibition effects on seedling growth of both tree species. Plant litter and seed burial did not influence seedling growth in either species. Liaodong oak and Chinese pine use different regeneration strategies during early stages of succession and similar strategies during late stages of succession.

The relative contributions of rare and common species to the patterns of species richness in plant communities

Understanding how overall patterns of spatial variation in species richness are affected by species distributional is one of the key questions in species diversity research. In the present study, we investigated the relative contributions of common and rare species to overall plant species richness in the Liaodong oak (Quercus wutaishanica) forest, which is located in the Ziwu Mountains of Loess Plateau, northwestern China. Based on species frequency distribution, we developed rank sequences of the most common to the most rare and the most rare to the most common species. We then correlated the rank sequences with cumulative species distributions. Our results showed that common species had a higher correlation with the cumulative species distribution in comparison with rare species. Moreover, common species had stronger effects on species α diversity and species β diversity as compared with rare species. Although the number of rare species was greater than that of common species, the overall species richness pattern was better predicted by common species than rare species. Therefore, common species were confirmed to be good indicators of species richness pattern and need to be protected priority.

The distribution pattern of different patch types and heterogeneity of the light and temperature: Larix chinensis Beissn in Qinling Mountains (China)

Based on a field investigation in the sample of an old growth subalpine coniferous forests in Qinling Mountains (China), we recognized the gap-forming processes in Larix chinensis forest for four representative patches (gap phases (G), building phases (B), mature phases (M) and degenerate phases (D)). The distribution pattern of four different patches and heterogeneity of the light and temperature were analyzed. The percentages of four different patch types (G, B, M and D) were 32.3%, 24.5%, 23.5% and 19.7%, respectively. The light and temperature had significant change among four patch types. From 6 to 20 o’clock, gap patch types changed more significantly than other three phases. The light and temperature regimes in mature patch type had a relatively mild change. Such micro-environmental heterogeneity among four different patches can lead to species niche partitioning, and some deterministic factors are benefit to the coexistence of various species with different characteristics within forest community.