Effect Of Bursaphelenchus Xylophilus Invasion On Masson Pine Forest Ecosystem In The Three Gorges Reservoir Region,China

The pine wood nematode Bursaphelenchus xylophilus(Steiner and Buhrer)Nickle(Nematoda:Aphelenchoididae),which originates in North America and causes the destructive pine wilt disease(PWD),is identified as the most destructive forest invasive alien species in China.Therefore,PWD poses an extremely grave threat to the Masson pine forest resource in the Three Gorges reservoir region.The primary objective of this study was to investigate the effects of the invasion of B.xylophilus on Masson pine forest ecosystem in Yiling district.We addressed the following questions for damaged Masson pine forest sites:(1)To estimate the damage scale after the invasion of B.xylophilus on the Masson pine forest ecosystem.(2)To analyze the relationship between weather variables and PWD variables.(3)What are the effects of the invasion of B.xylophilus on the photosynthetic characteristic and resource-use efficiency of P.massoniana trees?(4)What are the effects of Masson pine mortality on forest composition and community structure?(5)To analyze the relative importance environmental variables in structuring plant populations after infection by the pine wood nematode.(6)To investigate the immediate response and subsequent trajectories of biomass,carbon(C),and nitrogen(N)storage in the stand-level major ecosystem compartments of Masson pine forest ecosystems.The main results in this research are as follows:1.The damage scale of the invasion of B.xylophilus on the Masson pine forest ecosystem had been determined in Yiling district,which belonged to "system bifurcation or crash" type.Namely,once infected by pine wood nematode,the Masson pine forest ecosystem in this area can destroyed or crashed.2.The relationship between weather variables and PWD variables had been determined in Yiling district.The principal component analysis and redundancy analysis had taken in order to research the effect of weather variables on PWD epidemic situation in this area.The study area shows high temperature,low relative humidity and low precipitation during the years of serious PWD damaged degree(2006,2011-2013).However,the study area shows high relative humidity and precipitation during the years of light PWD damaged degree(2007-2010).In addition,the number of PWD damaged sites,numbers of PWD damaged Masson pine stems,and the ratio of damaged stems to damaged areas were three major factors indicating the intensity of PWD epidemic degrees in Yiling district.The higher relative humidity may promote the PWD spread among Masson pine forest ecosystem.With the increase of high temperature,mean temperature and the hours of sunshine,the number of PWD damaged Masson pine stems and damaged areas may increase.3.This study highlights the impact of photosynthetic characteristics,foliar carbon isotope ratios,and resource-use efficiency of PWD-induced Masson pine trees,which can help identify PWD infestations at the photosynthetic and physiological levels.Results showed that net photosynthetic rate,transpiration rate,stomatal conductance,and internal CO2 concentrations decreased in the infested trees compared with controls.The results of the variation of maximum net photosynthetic rate and dark respiration rate in this study indicate that P.massoniana need to consume more photosynthetic products during the middle infection phase in order to defend against pine sawyer beetle feeding and PWD infection.The significant decrease of resource-use efficiency in PWD-induced P.massoniana can attributed to the closure of needle stomatal pores and the inactivation or loss of both Rubisco and other key Calvin cycle enzymes.Both of the net photosynthetic rates and foliar 813C linearly correlated with the foliar N content.4.The composition and distribution of plant community structure and plant diversity had investigated in various Masson pine forest stands damaged by PWD.The results indicate that the rapid spread of PWD kills all year sizes(5-40 age classes)of P.massoniana,but the majority of the infected Masson pine trees were located in 5-20 age classes.The selective cutting of Masson pine trees infected by B.xylophilus opened canopy gaps in the forest,which has facilitated significant changes in forest composition and community structure.Masson pine does not become re-established following PWD-induced mortality but instead replaced throughout the research area by broad-leaved tree species,which has implications for shifting forest composition and community structure.In addition,we also found that the invasion of pine wood nematode had influence on the biological diversity of arbor,shrub and herb.Among the examined environmental variables,the plant community structure principally related to MPS and the content of soil K+.5.The biomass,C,and N storage of different forest ecosystem compartments have experienced certain variations following the PWD epidemic,which is vital to understand the shifts in stand-level C and N allocation in PWD-damaged forest stands,as well as for predicting the responses of regional and global C and N cycling.The C and N storage of aboveground trees,tree roots,and the aboveground ecosystem decreased with the extent of PWD infection.However,the C and N contents of the understory,forest floor,and total mineral soil initially declined after PWD infection before recovering over the following several years.