| Forests are one of the most important ecosystems on Earth,providing a wide range of important ecosystem services to human society.The impact of human disturbances on forest ecosystems has been received much more attention in recent years,with studies showing that human disturbances are one of the most important drivers leading to changes in the structure of forest ecosystems and loss of biodiversity worldwide.Therefore,clarifying the community structure of forest ecosystems and elucidating how they respond to human disturbances has become one of the hot issues in current ecological research.The subalpine forest areas on the Qinghai-Tibetan Plateau is fragile and unique,and is of great significance in the realization of China’s major strategic goals,such as the protection of biodiversity,the construction of an ecological security barrier and the building of an ecological civilization.However,compared with the global tropical and subtropical regions,the research on the effects of human disturbances on the community diversity of subalpine forest ecosystems on the Qinghai-Tibetan Plateau is still relatively weak,and the role played by forest soil seed bank in response to human disturbances has rarely reported.Focusing on the subalpine region in the northeastern part of the Qinghai-Tibetan Plateau,43(30×20 m)Picea purpurea forest samples with different levels of anthropogenic disturbances(mild,moderate,and severe)were classified and investigated by using measured canopy closure and five indicators(distance to nearest village,number of livestock and population within 2 km,the number of years of protection and of cessation logging of sampling site)of principal component analysis(PCA)to corroborate each other.We studied the growth status,species composition and community structure of the plants in the P.purpurea forest,analyzed in depth the influence of human disturbance on the growth status of the plants in the P.purpurea forest,the diversity of tree,shrub and herbaceous plant communities in different dimensions(species diversity,phylogenetic diversity,and functional diversity),and the soil seed bank,and explored the relative contribution of the changes in the environmental factors such as climate,habitat,and soil to the diversity pattern of the P.purpurea forest and the soil seed bank and their regulatory mechanisms under different backgrounds of human disturbances.The main results are summarized below:(1)The P.purpurea forest community in the northeastern part of the Qinghai-Tibetan Plateau mainly consists of young trees with diameter at breast height ranging from 15 to 20 cm(more than 90%of the tree basal area of less than 20 cm~2),and more than 90%of the shrubs have a ground diameter of less than 4 cm.A total of52 species of woody plants were identified,belonging to 18 families and 28 genera,including 25 endemic species in China.Among them,there are 9 species of trees,and the dominant species,in addition to the P.purpurea,are Picea asperata,Abies fabri and Sabina przewalskii;Shrubs 11 families,22 genera,43 species;The total number of herbaceous plants is 48 families,105 genera and 151 species,including 50 endemic species in China,among which three species,Fritillaria przewalskii,Goodyera bomiensis and Asarum sieboldii,are classified by International Union for Conservation of Nature(IUCN)as endangered species.(2)Human disturbance altered the plant growth,species composition,richness and phylogenetic diversity of woody communities.Severe disturbance reduced the number and proportion of tall trees and reduced the individual size of shrubs.Disturbance did not change the species composition of the plant community in the tree layer,but significantly changed the species composition of the plant community in the shrub layer,with moderate and severe disturbance increasing the abundance of Pertya sinensis and Daphne odora in the shrub layer.Woody plants at moderate disturbance levels had the highest species richness and phylogenetic diversity compared to mild and severe disturbance.(3)The herbaceous layer is the main component of the forest plant community,accounting for 74%of the species composition of the total forest community.The response of species richness and phylogenetic diversity of herbaceous plants to human disturbance was similar to that of woody plants,both reaching a maximum at moderate disturbance levels,influenced by light availability and soil organic matter,respectively.The effect of human disturbance on the functional diversity of herbaceous communities was latitude-dependent,i.e.the interaction of latitude and disturbance level could significantly alter the functional diversity of herbaceous communities:The functional diversity of herbaceous communities increased with increasing latitude under moderate disturbance and decreased with increasing latitude under mild disturbance,and this change was mainly caused by a combination of temperature and resource limitation(e.g.,available nitrogen).(4)The non-significant effect of human disturbance on soil seed bank richness and density,which is different from the response of above-ground plant communities,suggests that the seed bank have a lag in response to human disturbance and climate change,and contribute to the maintenance of species diversity through a"storage effect".In contrast,soil seed bank density increased with mean annual precipitation and was driven by soil and litter properties,implying that climate change(e.g.,drought)would reduce the potential of soil seed bank to maintain community stability.This study covers several perspectives of plant community ecology:for vegetation stratification,three layers of vegetation are considered:trees,shrubs and herbs;for biodiversity,different dimensions based on species diversity,functional diversity and phylogenetic diversity are included,and the soil seed bank,which is known as the“hidden”plant diversity,is also considered.Despite their small size,herbaceous plants are the important contributors to coniferous forest community diversity and are strongly influenced by human disturbances,therefore,herbaceous plants,an easily neglected plant group,should be fully considered when conserving the diversity of forest ecosystems. |
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