An Experimental Study On The Effect Of Dense Ferns,Diplopterygium Glaucum,on Tree Regeneration In Tiantong Evergreen Broad-Leaved Forest

Forest regeneration affects the stability and sustainable development of forests,and is the basis for the coexistence of community species.As an "ecological filter",dense fern understory could change the microenvironment of the community,thereby affect the regeneration of tree species,which is an important factor that potentially affects forest community structure and function.However,there are few studies on the effects of dense fern understory on the process of community regeneration,especially the temporal differentiation effect of different regeneration stages is still unclear.As an important part of dense fern understory,the effects of fern litter layer on tree species regeneration also needs to be explored.In this study,seeds and seedlings of 4tree species,Hovenia acerba,Schima superba,Quercus glauca and Lithocarpus glaber,were selected from Tiantong evergreen broad-leaved forest in Zhejiang Province.Under the dense Diplopterygium glaucum layer,3 treatments were set,including D.glaucum retention（Fern）,litter of D.glaucum retention（Litter）and D.glaucum removal（Clear）.A 7-month period of seed retention（3600 seeds）and seed germination experiments（3600 seeds）and a 6-month period of seedling survival and seedling growth（720 individuals）are carried out respectively,and the regeneration of two stages is tracked and monitored.In addition,environmental factors such as light,physicochemical factors and nutrient factors are collected regularly.Analysis of variance,generalized linear mixed effects model,linear mixed effects model and structural equation modeling are used to compare the 3 experimental treatments to explore the effects of dense D.glaucum layer and D.glaucum litter layer on the understory environment,the regeneration of seeds and seedlings of different tree species,and to reveal the ecological filtering effect and temporal differentiation effect of dense D.glaucum layer.The main results and conclusions are summarized as follows:（1）The dense D.glaucum layer significantly changes the physicochemical environment and nutrient conditions under the forest,it reduces light,soil moisture and soil microbial biomass,and increases soil available nitrogen content.One year after the D.glaucum was removed,the soil organic carbon,total nitrogen and total phosphorus contents decrease significantly in the Litter plots,and the total nitrogen and total phosphorus contents decrease significantly in the Clear plots,while there is no significant difference in the Fern plots.（2）At the seed stage,the dense D.glaucum layer inhibits the seed retention of Quercus glauca and Lithocarpus glaber,and the factor of animal activities plays a signififcant role;The seed germination of Quercus glauca is inhibited,and the structural equation modeling showes that the potential factors may be the changes of light and soil temperature;The seed retention and seed germination of Hovenia acerba and Schima superba are not significantly affected by the dense D.glaucum layer.In addition,D.glaucum litter layer has no significant effect on the regeneration of seed stage of 4 tree species.（3）At the seedling stage,the dense D.glaucum layer could inhibits the seeding survival of Hovenia acerba and seedling growth of Lithocarpus glaber,and has no significant effect on the seedling survival and growth of Hovenia acerba and Quercus glauca.Structural equation modeling showes that the seedling survival of Schima superba is affected by the dense D.glaucum layer directly,and by light and soil moisture.The soil nutrient environment has a significant effect on the seedling growth of Lithocarpus glaber.In addition,the D.glaucum litter layer could directly promote the seeding survival of Hovenia acerba,and has no significant effect on the seedling survival and growth of the other three.（4）At different regeneration stages,the dense D.glaucum layer has different effects on different species,and has a temporal differentiation effect.It mainly affects the seeding survival process of Schima superba;It affects the two processes of seed retention and seed germination of Quercus glauca;It affects the two processes of seed retention and seeding growth of Lithocarpus glaber.The dense D.glaucum layer has no significant effect on the seed regeneration and seeding regeneration of Hovenia acerba.These results indicate that different tree species have different responses to the dense D.glaucum layer,which confirms the ecological filtering effect.The responses of the same tree species to the dense D.glaucum layer in different regeneration stages and different processes are also inconsistent,and the ecological filtering effect of the dense D.glaucum layer also has a temporal differentiation effect.In addition,The D.glaucum litter layer has no significant inhibitory effect on tree species regeneration and even promote some tree species regeneration.In general,the dense D.glaucum layer has a significant impact on the understory environment and tree species regeneration,and would change the composition of understory species differently through the effects on different tree species and different regeneration stages.