The Regularity Of Fire Occurrence In Natural Yunnan Pine Forest And The Effect Of Fire On Population Dynamics

In order to explore the role of fire in the population persistence of Yunnan pine.We sampled five plots in the old natural forest of Pinus yunnanensis in Yunlong Tianchi National Nature Reserve in Yunnan Province,and three post-fire regeneration plots.The height,DBH and location of all trees were recorded.The DBH of P.yunnanensis was classified byl cm scale to study the population size class structure.The core samples of P.yunnanensis with obvious burnins scars on the base trunk were analyzed and reconstruct a fire history and to analyze the relationship between the fire history and the popultion size class structure.The density of post-fire regenerated saplings was counted,the year of regeneration was confirmed,the height and basal diameter were recorded.The main objective of this study was to investigate how to persistence of populations of P.yunnanensis in the fire-prone ecosystem,the regeneration resularity and growth trend of.The main results are as follows:(1)The density of Yunnan pine saplings in the burned area was significantly higher than that of the non-burned area.The order was:155/100mlin medium fire intensity plot>136/100m2in high fire intensity plot>55/100m2in low fire intensity plot>4+2.17/100m2in non-burned plot.The seedlings in the burned area were regenerated within a few years after fireAfter fire,the peak of seedling recruitment appeared in P.yunnanensis population.The new seedlings declined sharply and maintained at low level,even with intermittent years.(2)The growth of P.yunnanensis seedlings in burned area was significantly higher than that in the unburned forest.In the burned area with high intensity,the average height of annual,biennial and triennial seedlings were 29.88±8.06cm.55.67±16.76cm and 83.24121.77cm.It was about ten times more than that in the unburned forest.The seedlings of P.yunnanensis continued to grow at a relatively high growth rate after fire.(3)The population size class structure of P.yunnanensis in the old natural forest showed a multi-peak structure with poor continuity,it looked like a pyramid-shaped around the first large peak at the DBH class of 6-10 cm.and showed a better continuity at the middle and low diameter level.At the high diameter level,it presented regular peaks and discontinuities,and the period of peaks was about 2-4 diameter class.The size class structure of P.yunn.anensis population on different slope aspects was significantly different.The number of size class on sunny slope was less than that on shady slope,and its continuity of size class was better in middle and low size class.The distribution of P.yunnanensis in different size groups was closely related to its quantity.The individual distribution area of DBH-class trees with peak number is wide,while the individual of DBH-class trees with small number is clustered in several fixed areas.(4)The analysis of fire scar tree rings on two slope aspects showed that both plots had more than once of fire in the pass.The last fire on sunny slope was in 1997,and the interval between forest fires was about 20-25 years;on shady slope,the last fire was in 1990,the fire return interval(years)was about 25-30 years.The regenerating time of the first peak in the population size class structure was agree with the first site in fire chronology.Fire played a decisive role in the persistence of P.yunnanensis population.The regeneration of P.yunnanensis population was strongly dependent on forest fire.Repetitive fire disturbance could maintain the population of P.yunnanensis in the central Yunnan plateau for a long time.