Effects Of Dwarf Mistletoe (Arceuthobium Sichuanense) Infection On Natural Spruce Forest And Key Factors Of Its Outbreak In Qinghai Province

Arceuthobium sichuanense (H. S. Kiu) Hawksworth & Wiens is a hemiparasite angiosperm that infects Picea crassifolia Kom., Picea purpurea Mast., Picea likiangensis var balfouriana (Rehd.et Wils.) Hilliex ex Slawin and Picea spinulosa (Griff.) henry in Sichuan, Qinghai Province and Xizang Autonomous Region, China, and causes severe damage to spruce forests in Qinghai-Tibet Plateau. For carrying out scientific and efficient control to the spread of dwarf mistletoe(A. sichuanense), it is essential to study the characteristics of A. sichuanense infection on its host trees, and analyze the key inducible factors of its outbreak in Qinghai Province. For revealing the growth limited factors of infected host trees, we evaluated the effects of A. sichuanense infection on needles and current-year shoots of spruce trees (P. crassifolia & P. purpurea); based on the death and regeneration influence of spruce forest by the infection of A. sichuanense, we interpreted the various infection characteristics by A. sichuanense to different ages host trees; depended on the key inducible factors about large area spread of A. sichuanense, we analyzed A. sichuanense outbreak factors in local natural spruce forest; and chemical control of A. sichuanense infection were also studied. The main results were as follows:The most apparent effect of A. sichuanense infection on two species hosts (P. crassifolia and P. purpurea trees) and different ages of hosts (mature and young trees of P. crassifolia) was a significant reduction in needle size and current-year shoot in the infected branches (P< 0.001); and the dwarf mistletoe infection also resulted in a significant decrease in needle N concentration andδ13C value of host trees (P< 0.001), which leaded the host trees growth under water and nitrogen limited in some extent. Furthermore, we inferred that the death of infected host trees may increase in drought years because of the limited water resources.Based on the investigation of 3 years (2008-2010), we researched the dwarf mistletoe infection influence to the death and regeneration of natural spruce forest. Compared to the uninfected spruce stands, there was a significantly higher death rate of spruce trees (mature and young trees of P. crassifolia) in heavily infected spruce stands (P< 0.001). The death rate of young infected trees was higher than that of mature trees (35.5% vs.13.6%), the death of young trees were all resulted from A. sichuanense infection, and the death of mature trees were mostly resulted from dwarf mistletoe infection and bark beetle attack. The local dominant species of bark beetle were Ips nitidus, Pseudips orientalis andⅠ. shangria, and the attack from dominant bark beetle species accelerated the death of heavily infected mature trees by A. sichuanense. The preliminary results showed that spruce trees in the stands severely infected by A. sichuanense were most likely killed associated with the attack from bark beetles.Principle Component Analysis and Regression was applied to evaluate the key environmental factors inducing the large area infection by A. sichuanense on spruce forest. Canopy density was significantly negative correlated with infection index of spruce forest (P< 0.001). A. sichuanense infection was likely emergence in the spruce forest with low canopy density, higher coverage of vegetation and thinner moss layer. DBH of spruce trees was significantly positive correlated with the infection index (P< 0.001). The mixed forest was less susceptible to the infection by A. sichuanense than pure spruce forest, especially the mixed forest by spruces and broad leaf species. There was a positive correlation between the infection index and elevation of study site (P< 0.05). The occurrence of A. sichuanense infection was more frequent in down slope than in middle and upper slope. Soil organic carbon and total nitrogen were both positive correlated with the infection index (P< 0.05).For carrying out scientific and efficient chemical control to the spread of A. sichuanense, four kinds of plant hormones including 40% ethephon aqueous solution,50% daminozide water solution powder,20% 1-naphthylacetic acid dust and 90.8% abscisic acid were applied to the flowering buds of A. sichuanense in May. The 50% daminozide and 20% NAA (diluted 200 fold) killed the flowering buds successfully and the mortality of flowering buds reached more than 40% and 60%, respectively. The 90.8% abscisic acid fell off the flowering buds successfully and the falling rate reached more than 80%, but it can not kill the flowering buds. The results showed that all diluted 40% ethephon (diluted 100,200 and 400 fold) killed the flowering buds successfully and the mortality of flowering buds reached nearly 100%. The 40% ethephon aqueous solution (dilution of 1:400) was the best choice because it was harmless to spruce needles and branches.