Abstract: This work described the microclimate characteristics of Sygera alpine treeline ecotones, and provided basic data for further study of alpine treeline maintenance mechanisms. Based on the data of automatic weather stations in two treeline ecotones (Abies georgei  var. smithii treeline and Juniperus saltuaria treeline) and the observations of temperature and humidity of air and soil in different habitats of both ecotones in the Sygera Mountain during 2018-2020, we analyzed the microclimatic characteristics of treeline ecotones and the variation characteristics of hydrothermal conditions of the three habitats (under forest, shrub, and forest gap). In both ecotones, mean temperature of the warmest month was 8.71-9.32 ℃. The mean temperature for the coldest month ranged from -7.23 to -9.08 ℃. The annual mean temperature was 0.55-0.88 ℃. The annual temperature difference was 15.95-18.40 ℃. The annual biological temperature was 2.79-3.33 ℃. The warm index was 9.15-12.94 ℃·month^-1 and the cold index ranged from -60.22 to -64.98 ℃·month^-1. The mean temperature and length of the growing season defined by air temperature were 7.45-8.31 ℃ and 106-136 days, while those defined by 10 cm soil temperature were 6.06-7.01 ℃ and 120-141 days, respectively. Air temperature and humidity in the different habitats of the two ecotones were similar, whereas soil temperature and moisture were highest in the forest gap, with large daily amplitude of soil temperature. Soil temperature in the forest was lower than that of the other habitats, with a “cool foot” effect. The growing season defined by soil temperature lagged behind that defined by air temperature by about one month. The average air temperature, average soil temperature, and soil water content of the forest gaps were higher. The freezing injury events in the forest gaps and shrubs were frequent, strong, and lasting. The differences in microclimate characteristics among different habitats in the treeline ecotones of Sygera Mountain were mainly reflected in soil temperature, growing season temperature, and occurrence of freezing injury events in the growing season.

Key words: Sygera Mountain, treeline ecotone, climate indicator, temperature, growth season