Urban climates are generally drier, hotter than those in adjacent rural areas. Trees growing in such environments may suffer from severe stress. The impact of urban climate on tree water use in the central area in Beijing, China, was investigated by studying four common species at the whole-tree level by heat dissipation method from April to September in 2008. On bright days, sap flow densities exhibited high variations from one species to another. Higher rates (0.006-0.010 cm.s-1) were observed on G.biloba, intermediate values (0.004-0.006 cm.s-1) on P.tabulaeformis and C. deodara and lower values (0.002-0.004 cm.s-1) on R.pseudoacacia. Moreover, the seasonal pattern of sap flux (cm3.cm-2.h-1) differed a lot among species. The sap flux for G.biloba in the summer was higher than that in the spring which was higher than that in the autumn; the sap flux for R.pseudoacacia in the autumn was higher than that in the summer; the sap flux for P.tabulaeformis and C.deodara in spring was higher than that in the summer which was higher than that in the autumn. Responses of daily JS to the environmental patterns, particularly variations in responses to Rs and D, are evident. While relative day to day variability is small in evergreen species P.tabulaeformis and C.deodara, large changes in deciduous species G.biloba and R.pseudoacacia are evident. Responses of daily sap flow to D were similar at all soil moisture levels for all species except G.biloba.
Results/Conclusions
Gc increased sharply between 0700h and 1100h; afterwards, a tendency to decrease was observed, indicating stomatal closure. However, several differences were also found as follows: firstly, the time when gc of G.biloba, R.pseudoacacia reached the maximum was 0700-0900, while reached the maximum was 0900-1100 in P.tabulaeformis and C.deodara; Secondly, gc of C.deodara stayed steadily during most of the afternoon; thirdly, gc in P.tabulaeformis showed obvious decrease in the noon; lastly, the stomata kept open even after sunset, especially in P.tabulaeformis. Gc for the four species was dependent on global radiation (positive effect), and on D (negative effect). However, the analysis of dependence of gc to the climate factors showed only a poor relationship with global radiation. When 0< Rs <200 W m-2, the negative relationships weren’t obvious for G.biloba, R.pseudoacacia, while they were obvious for P.tabulaeformis and C.deodara. When Rs > 200 W m-2, the negative relationships were very obvious. For R.pseudoacacia P.tabulaeformis and C.deodara, the negative relationships varied much, while varied little for G.biloba. The hyberbola relationships of Gc to Rs varied much among species and D classs.
