Comparison of Chinese Pine Transpiration in a Suburban Versus an Urban Environment

Table 4.3 summarizes transpiration in Chinese pine trees in a suburban area and in the center of Beijing (Liu 2008; Ma et al. 2006; Ma and Wang 2002; Ma et al. 2003; Nie et al. 2005; Wang et al. 2008). Both daily Et and the maximum Js observed in this study were much higher than those reported for Chinese pines with similar DBH in the suburban area of Beijing. The results suggest that the urban environ­ment may promote plant water loss. The significant differences in transpiration between Chinese pine in the suburban and urban area may be attributable to the following factors: (1) increased Beijing urban air temperatures (Xiao et al. 2007) may enhance plant water loss (Wang et al. 2005); (2) low plant density could increase transpiration in both potted plants (Hagishima et al. 2007) and trees (Jimenez et al. 2008) because of the “clothesline effect” (van Bavel et al. 1962); (3) SWC is a very important factor for Chinese pine transpiration, particularly when the soil water is in deficit (Liu 2008), and the increased transpiration in urban areas could be attributable to a large increase in Et in response to higher SWC; and (4) the

trees sampled in this study were shorter than the trees reported in the suburban area. A previous study reported that taller trees had lower mean canopy stomatal con­ductance (Gs, ~ 60 mmol/m2 leaf area s-1) than shorter trees (Gs, ~320 mmol/m2 leaf area s-1) (Schafer et al. 2000).