Effects of riparian buffers of North Mort of Beijing on air temperature and relative humidity

The study explored the effects of urban riparian buffers (RBs) with different structures on air temperature (T) and relative humidity (RH) in the North Mort of Beijing. From May to October in 2011, four RB types were selected on both the north and south RBs based on structural differences (such as width, slope, plant species, vegetation coverage, and distance and height to road). By both sides of the roads vertical to these RBs, the corresponding control areas (CAs) were set, adjacent to surrounding different urban land cover types. From 8:00 to 18:00, 10-hour measurements for T and RH were taken in different RB types and their CAs on sunny or cloudy days of late each month. Statistical analysis was used to test significance of differences in T, RH, and Thermohygrometric Index (THI) between RBs and CAs, between north and south RB types, and among different RB types and cross sectional zones. The results demonstrated that RBs had significant adjusting effects on T and RH compared with CAs, and could effectively improve human comfort. Firstly, from May to September, T and RH for RBs were extremely significantly lower and higher than those for CAs, respectively. Secondly, most daily variations in T for CAs showed a bimodal trend, but they had a single peak for RBs; the highest T for RBs occurred between 14:00 and 15:00, which was one hour later than that for CAs; RBs could significantly reduce the highest daily T, although it changed the lowest daily T little; meanwhile it could also effectively decrease high-temperature days. Thirdly, daily variations in T and RH for RBs were less than those for CAs. These differences depended on season, that is, the adjusting effects of RBs on microclimate range were especially strong in spring and autumn, but not in summer. Finally, THI for different RB types were significantly different from those for their CAs: all the CAs made human feel uncomfortable in the summer; the south RB types could effectively enhance human comfort level in June and August, and the north RB types could also do that in August. The results also showed that differences in adjusting effects on T and RH also existed in different RB types. Firstly, south RBs had stronger adjusting effects than north RBs, that is, T and RH for the north RB types were significantly higher and lower than those for the south RB types, respectively. It was mainly due to tree density, canopy structure, and riparian width. The wider south RB with higher tree density and den