Characteristics Of Microclimate In Treeline-Shrubline-Alpine Meadow And Its Effects On Seedling Growth Of Abies Georgei Orr Var. Smithii

The driving mechanism of alpine treeline dynamics in the context of global climate change has attracted considerable interest.Numerous ecologists have produced extensive reports on the coupling relationship between environmental variables and alpine treelines,but investigations on the mechanism of functional traits in this relationship are scarce.However,the unique geographical position and hard conditions of the alpine treeline have made it extremely difficult for domestic and international scientists to install meteorological instruments and conduct research.This study is based on the automatic weather stations in the treeline ecotone of Sygera Mountain from 2018 to 2020,as well as the continuous monitoring of air-soil temperature and humidity in various habitats.Analyzed were the characteristics of the general microclimate in the ecotone of two treelines,the change of temperature and water conditions in three habitats(understory,shrub,and forest gap),as well as their variances during the growth season.On the basis of previous meteorological observations,630 seedlings of different age groups(4-6,7-9,and 10+)were transplanted to three habitats(4300–4500 m,canopy-in,canopy-out,and meadow)in the alpine treeline ecotone on the southeast slope of Sygera Mountain.Environmental variables,seedling leaf functional traits,and seedling survival status were investigated and measured in order to evaluate the strategy selection of seedlings with environmental change and show extrinsic factors(climate)drive the variation of seedling intrinsic factors(leaf functional traits)to mediate the ecology of tree species distribution in alpine treelines process.The results indicate:(1)The mean temperature of the warmest month is 8.71～9.32 ℃ in the two treeline ecotones in Sygera Mountain,the mean temperature of the coldest month is-7.23～-9.08 ℃,the annual average temperature is 0.55～0.88 ℃,and the annual temperature range is 15.95～18.40 ℃,the annual biological temperature is2.79～3.33 ℃,the warm index and cold index are 9.15～12.94 ℃·month,-60.22～-64.98 ℃·month,respectively,and the average temperature and length of the growing season defined by air temperature are 7.45～ 8.31 ℃,106-136 days,the average temperature and length of the growing season defined by 10 cm soil temperature are 6.06-7.01 ℃,120-141 days;The above-ground temperature and water conditions were comparable between different habitats in the two treeline ecotones,whereas the optimal soil temperature and water conditions in the subsurface layer were forest gaps.However,the daily amplitude of soil temperature was large,and the soil temperature in the forest was lower than in other habitats,demonstrating a “cold foot” effect.The growing season defined by soil temperature and its length fell behind the definition defined by air temperature by approximately one month.In forest gaps,the mean air temperature,mean soil temperature,and soil water content were all higher.Meanwhile,frost damage occurrences in forest openings and shrubs became more frequent,severe,and prolonged.The differences in microclimate features of distinct habitats in the treeline ecotones of Sygera Mountain are mostly represented in soil temperature,growth season temperature,and the occurrence of freezing damage events in the growing season.(2)After transplantation,the levels of non-structural carbohydrates,starch,and C:P in leaves increased dramatically,whereas leaf size fell significantly.In addition,PCA,Pearson,and other statistical techniques revealed that the resource use strategy of seedlings gradually turned towards conservatism,with the underlying process being mostly mediated by physiological features(nonstructural carbohydrates and plant nutrients).(3)At the same time,this study found that environmental factors did not directly affect the survival and growth of seedlings through structural equation modeling but indirectly affected the growth and survival of seedlings by mediating the variation of morphological traits.At the same time,the high solar radiation in the non-growing season in the high altitude area indirectly affected the morphological traits of the seedlings,producing a “lethal effect” on the seedlings that further offset the “positive effect” brought by warming,so that the upper limit of the distribution of alpine tree lines in the Sygera Mountain did not significantly change.