| Pinus yunnanensis is a coniferous tree species endemic to southwest China.P.yunnanensis forests have important ecological and economic value.The central Yunnan plateau,which is the main distribution area of P.yunnanensis,is a habitat with the characteristics of fire-dependent ecosystem.Fire disturbance is one of the important ecological factors in this region.To explore how fire disturbance affects P.yunnanensis is very important to understand the distribution and development of P.yunnanensis.In this study,we took the natural forest of P.yunnanensis in Yunlong Tianchi National Nature Reserve,Yunlong County,Dali Bai Autonomous Prefecture of Yunnan Province as the research object.We selected two different slopes and set three sample plots with an area of 30m*30m respectively.A total of 814 trees in all plots were investigated,including 753 P.yunnanensis plants,and tree ring samples of 310 P.yunnanensis plants were collected.Through the "tree ring-fire scar" analysis,we traced the history of fire disturbance in two natural populations of P.yunnanensis,and analyzed the relationship between the age structure of P.yunnanensis population and fire disturbance and the effect of fire disturbance on the radial growth of P.yunnanensis.Based on the above studies,we discussed the effect of fire disturbance on the population dynamics of P.yunnanensis and the maintenance mechanism of P.yunnanensis population.The main conclusions were as follows:（1）Age structure characteristics of P.yunnanensis population: the investigated sample plots on two slopes are almost pure forests of P.yunnanensis.The age structure of P.yunnanensis natural population on two slopes showed a discontinuous multi-peak structure.The population of P.yunnanensis on the south slope of Tianchi Lake showed a declining trend to some extent,while the population of P.yunnanensis on the north slope showed an increasing trend.（2）Fire disturbance history of P.yunnanensis natural forest: the analysis of tree ring fire scar samples showed that there was periodic fire disturbance in P.yunnanensis natural forest in Yunlong Tianchi National Nature Reserve.The forest of P.yunnanensis investigated in this study experienced fire disturbance at least three times.The occurrence time of forest fire disturbance in the south of Tianchi Lake is different from that in the north of Tianchi Lake.The years of fire disturbance in P.yunnanensis forest on the south slope of Tianchi Lake were about 1919,about 1942,and about 1975,respectively,and the interval between fire disturbance was 23 to 33 years.The three fire disturbances were all ground fires and some of the plants survived after being disturbed by fire.The years of forest fire disturbance of P.yunnanensis on the north slope of Tianchi Lake were about 1929,about 1958,and about 1975,respectively,and the interval between fire disturbance was 17 to 29 years.The three fire disturbances were all ground fires and some of the plants survived after being disturbed by fire.（3）Relationship between age structure and fire disturbance in a natural population of P.yunnanensis: The discontinuous multi-peak structure of the age structure of P.yunnanensis natural population is related to fire disturbance to a certain extent.The obvious peak of individual number appeared after the occurrence of fire disturbance,and after the peak of individual number,the number of individuals of P.yunnanensis population decreased sharply.The emergence of the peak of P.yunnanensis population is dependent on fire disturbance to a certain extent.（4）The fire disturbance with a short time interval may have led to less regeneration of P.yunnanensis population between the two fires,or only a small number of individuals regenerated in this period have survived.However,even under the condition that there was no fire disturbance for a long time,the self-thinning within the P.yunnanensis populations may lead to the decrease of survival individuals.（5）Radial growth of P.yunnanensis population on each slope of a different side of Tianchi Lake: the chronology of tree ring width of P.yunnanensis on each side of Tianchi showed there is a certain similarity in the radial growth trend of P.yunnanensis between the two populations.However,the years in which the growth peak and trough appeared were slightly different,and the ring width index（RWI）of P.yunnanensis was quite different between the two populations.（6）Resistance index and resilience index of radial growth of P.yunnanensis to fire disturbance events in two populations in Tianchi area: There were differences in radial growth resistance and resilience index of plants of different ages in two different populations.The radial growth of individuals in both populations has a certain degree of resistance to fire disturbance.Fire disturbance showed an overall inhibition trend on the radial growth of P.yunnanensis.The occurrence of ground fire decreased the radial growth of P.yunnanensis but did not lead to the stagnation of its radial growth.After five years of fire disturbance,the radial growth of P.yunnanensis could not be restored to the growth level of five years before the fire disturbance.There was periodic fire disturbance in the natural forests of P.yunnanensis.P.yunnanensis has certain resistance and recovery ability to fire disturbance.A certain number of individuals can survive after being disturbed by fire.Five years after fire disturbance,the radial growth of P.yunnanensis could not be restored to the level of five years before fire disturbance.The natural population of P.yunnanensis had a large number of regenerations after the fire disturbance,and the fire disturbance may have cleared some seedlings.Even under the condition that there was no fire disturbance for a long period,the self-thinning within the P.yunnanensis populations may lead to the decrease of survival individuals,and due to the lack of new individuals supplement,the population of P.yunnanensis may gradually show a declining trend.Fire disturbance maintains the long-term persistence of P.yunnanensis natural forest. |
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