Functional Biogeographic Pattern And Community Assembly Mechanism Of Woody Plants In Zhoushan Archipelago

Marine islands are considered the ideal ecosystem for studying the distribution patterns and causes of island biodiversity due to their disproportionate species amount and unique functional characteristics.Over the past century,human land use and biological invasion have led to a great loss of biodiversity on islands globally,which have profoundly changed species composition,structure,and function of island's biomes.However,few studies focused on the combined effects of land use and biological invasion on island plant diversity.Studying this question will contribute to clarifying whether traditional island biogeography laws can still effectively predict the patterns of plant diversity in the context of human disturbance,and thus shed light on the underlying mechanisms of island plant community assembly in the Anthropocene.In the past 30 years,the drastic land use changes and pinewood nematode invasion led to a large number of pine forest losses in Zhoushan Archipelago,which have profoundly influenced the plant species diversity,functions,and community assembly process.In this study,we examined all woody plant individuals within 78 vegetation plots across 43 islands in Zhoushan Archipelago.Based on the effects of island area,isolation,land use intensity,and pine forest loss on plant diversity(species diversity,functional diversity,and community functional composition),the best explanatory factors and their relative importance were obtained by model selection and variance partitioning.Meanwhile,the structural equation model was used to test the direct and indirect effects of island area,isolation,land use intensity,and pine forest loss on plant diversity.Finally,RLQ and Fourth-corner methods were used to examine the global relationships among environments-plant traits-species abundance,as well as the mechanism of plant community assembly based on functional traits.The main results and conclusions are as follows:(1)Land use intensity increased significantly with the increase of island area,and pine forest loss increased significantly with the increase of isolation(the distance from the mainland).Species diversity of plant community increased significantly with the increase of island area and land use intensity and reduced significantly with the increase of isolation and pine forest loss.Isolation indirectly reduces species diversity in pine forest loss and aridity,while land use intensity and pine forest loss indirectly decrease species diversity via forest succession(evaluated by wood plant basal area)and stand structural complexity.These results indicated that the human land use effect strengthens the island area effect,jointly increases the plant species diversity,while biological invasion strengthens island isolation effect,thus reducing plant species diversity.(2)Functional diversity of plant community increased significantly with the increase of island area but decreases with the increase of isolation.Land use intensity significantly increased and pine forest loss significantly declined species diversity.Similarly,plant functional diversity increased significantly with the increase of plant species diversity.Island area increases and isolation decreases plant species diversity,respectively,which indirectly increased and decreased plant functional diversity.Land use intensity indirectly reduces plant functional diversity by increasing soil nitrogen content,and pine forest loss indirectly reduces plant functional diversity by reducing plant species diversity.These results suggest that island area,isolation,pine forest loss have an indirect effect on plant functional diversity via plant species diversity,while land use intensity has an indirect effect on plant functional diversity by increasing soil nitrogen content.(3)Island area and isolation not only directly but also indirectly regulate the functional composition of plant community by mediating pine forest loss,soil electrical conductivity,and aridity.In general,on larger islands,plant communities are dominated by species with higher tree height,wood density,and seed mass.The more isolated islands,plant community is dominated by species with lower wood density,specific leaf area,and seed mass.For islands with more pine forest loss,their plant communities are dominated by species with lower tree height and wood density,while land use intensity did not significantly affect functional composition of plant community.These results indicate that plant community functional composition has been significantly changed under the influence of land use intensity and pinewood nematode invasion,while island area,and isolation are still the important driving factors for determining the functional composition of plant community.(4)Island area and land use intensity significantly promote the diversity of resource acquisitive species but did not affect the diversity of resource conservative species.Isolation and pine forest loss significantly reduced the diversity of resource acquisitive species and resource conservative species.The economic spectrum of resource acquisitive species was positively correlated with their occurrence,but did not correlate with the economic spectrum of resource conservative species.These results demonstrate that the island area-human land use effect can significantly enhance the distribution region of resource acquisitive species,thereby strengthening the island area effect.(5)Global relationships of environments-plant traits-species abundance showed that species with greater tree height,wood density,leaf carbon content,and seeds adapt to unassisted dispersal mode had greater abundance distribution in larger islands with lower soil electrical conductivity.However,in more isolated islands,soil electrical conductivity and pine forest loss are higher,and species with higher leaf phosphorus content were more distributed.Islands with higher land use intensity have low soil fertility,with larger leaf area,specific leaf area,leaf nitrogen content,Huber value,specific twig length,and more wind dispersal species.Islands with higher soil fertility,the forests succession degree is higher(i.e.,more in middle and late succession stages),the plant community structure is more complex,and the distribution ratio of evergreen plants with larger seeds and seeds dispersed by animals has a great abundance.These results indicate that island area,isolation,human land use,pine forest loss,and environmental factors jointly determine the distribution pattern of plant species abundance on island.In conclusion,this study systematically illustrates the variation and driving mechanisms of species and functional diversity,community functional composition,and distribution of species abundance in Zhoushan Archipelago.These results showed that island area,land use intensity mediated by island area,isolation,and pine forest loss influenced by isolation together drive the species composition and functional structure of island plant communities.The results of this study provide important evidence for further understanding of Anthropocene island biogeography and functional biogeography and provide a theory and reference for the conservation of plant diversity in Zhoushan Archipelago.