Temporal and Spatial Dynamics Characteristics of Microclimate in Subalpine Birch Forest in Western Sichuan

Solar radiation,air temperature,air relative humidity and soil temperature of different soil layers within the subalpine birch forest(elevation 2 540 m) located at the Wanglang National Nature Reserve((λ(E) 103°55′~104°10′,φ(N) 32°49′~33°02′)in western Sichuan,were continuously observed in situ from June 2002 to May 2003.The main results showed: 1) Daily course of solar radiation under canopy and over canopy was markedly different,and the intensity of solar radiation under canopy and over canopy was mainly controlled by the solar altitude.Total radiation of a day under canopy was influenced by solar altitude and air transparence.It was also influenced by forest structure because the forest structure,and leaf area and characteristics were different in different seasons.In spring,summer and fall,the mean total radiation of a day under canopy was reduced by 53.1%,39.8% and 55.8%,respectively.2) In summer,the diurnal air temperature was higher than soil temperature,but the nightly air temperature was lower than soil temperature.With the increase of solar radiation,the air and soil temperature also increased.The arisen time of extreme air and soil temperatures were significantly different.The sensitivity of air temperature to solar radiation was stronger than that of soil temperature.3) In spring,degree-day4 ℃ of soil 5 cm layer was higher than that of air and soil 15 cm layer.In summer,degree-day 4 ℃ of air was higher than that of soil 5 cm and 15 cm layers.But in fall,degree-day4 ℃ of soil 15 cm layer was higher than that of air and soil 5 cm layer.The line correlation between daily mean air temperature and soil temperature were markedly significant(P0.001).4) The air relative humidity under canopy was higher than that over canopy.Furthermore,the daily and seasonal change range of air relative humidity under canopy was lower than that of air relative humidity over canopy.Fig 7,Tab 1,Ref 19