Hua Wang, Zhiyun Ouyang, Weiping Chen, Xiaoke Wang, and Hua Zheng
Abstract Urban environments can significantly influence the transpiration of isolated plants. Therefore, optimal green space design, tree species selection, and tree maintenance require that the water use patterns of urban plants be quantified. In this study, the transpiration from individual Chinese pines (Pinus tabulaeformis) in the center of Beijing, China was measured continuously over a 2-year period. The response of whole-tree transpiration (Et) to environmental factors was investigated in multiple time scales. Maximum sap flux density (Js) ranged from 3.34E-05 to 8.2E-03 cm/s. Et was much higher in summer (32.93 kg/day) than in winter (6.22 kg/day). Et in the urban environment was much higher than that reported for Chinese pines with similar diameters at breast height (DBH) during 2000-2005 in suburban Beijing. Great differences were observed in the response of Et to environmental factors at different time scales. At the diurnal scale, hourly mean Js was linearly related to photosynthetically active radiation (PAR) and vapor pressure deficit (D), whereas at the daily scale, daily mean Et was linearly related to PAR, air temperature (Ta), and soil water content (SWC), and was curvilinearly related to D. At the annual scale, Et was similar in the growing seasons of 2008 (a wet year) and 2009 (a dry year), even though the annual precipitation (P) and irrigation times were significantly different (724.8 vs. 432.8 mm; 2 vs. 12).
State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
Institute of Forestry and Pomology, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100093, China e-mail: wanghuaphd@gmail. com
Z. Ouyang (*) • W. Chen • X. Wang • H. Zheng
State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China e-mail: zyouyang@rcees. ac. cn; wpchen@rcees. ac. cn; wangxk@rcees. ac. cn; zhenghua@rcees. ac. cn
N. Nakagoshi and J. A. Mabuhay (eds.), Designing Low Carbon Societies in Landscapes, Ecological Research Monographs, DOI 10.1007/978-4-431-54819-5_4, © Springer Japan 2014
From this result, it can be concluded that urban soil water conditions affected by both P and irrigation practice were a major cause of interannual Et variation.
Keywords Green space • Isolated tree transpiration • Sap flux density • Soil water content • Urban environment • Urban soil water
Urbanization significantly influences local and regional climate, water resources, the atmosphere, and land use (Wu 2008). Urban green spaces including parks, street trees, gardens, agricultural areas, rehabilitated areas, fragmented natural areas in a city, natural areas surrounding a city, and other open areas are providers of urban ecosystem service (Niemela et al. 2010). An urban green ecosystem can mitigate many of the environmental impacts of urban development by moderating climate, reducing atmospheric carbon dioxide, improving air quality, lowering rainfall runoff, and reducing noise levels (Nowak and Dwyer 2007). We see urban green areas as the most effective environmental protection tool and the foundation of the urban ecological framework (skeleton) or green infrastructure. Tree transpiration, in particular, cools the air and reduces storm water runoff (McPherson et al. 2005; Nowak and Dwyer 2007). At the same time, the transpiration pattern of trees in cities and surrounding areas may be significantly influenced by urban environmental changes (Gregg et al. 2003). Many metropolitan cities, including Beijing, expend great effort to improve tree cover. However, inappropriate green space design, tree species selection, and tree maintenance can increase water consumption. Furthermore, isolated urban plants are thought to be susceptible to “the clothesline effect,” which causes high rates of evapotranspiration (Hagishima et al. 2007; van Bavel et al. 1962). Therefore, it is imperative to study the transpiration patterns and factors affecting transpiration in the urban environment.
Many studies have examined the factors affecting urban plant transpiration, which factors include plant density, irrigation, energy exchange with building walls, and pollutant concentrations (Hagishima et al. 2007; Heilman et al. 1989; Martin and Stabler 2002; Montague and Kjelgren 2004; Neighbour et al. 1988). These studies have focused primarily on responses of potted plants to a single feature of the urban environment. However, the effects of changes in the urban environment on the water use of trees in situ have seldom been studied.
Time scale is an important component in evaluating the factors influencing transpiration. Photosynthetically active radiation and vapor pressure deficit and soil moisture content affect transpiration on a daily time scale, whereas leaf area varies within and among species on seasonal and interannual time scales (Ohta et al. 2008; Phillips and Oren 2001). Diurnal and seasonal variations in tree transpiration have been broadly studied. However, knowledge regarding interannual variation is insufficient, especially about trees in urban environments with irrigation. Soil moisture changes are also a major cause of interannual variation in the transpiration period (Yoshifuji et al. 2007). Garden management techniques such as irrigation are being adopted in urban environments, especially in arid and semiarid areas or under dry weather conditions. Thus, in irrigated systems the interannual variations in soil water content do not correspond to the amount of precipitation. The effects of irrigation on interannual variations in transpiration are poorly understood.
Basic data of urban green characteristics in Beijing were listed as follows: 658,914.07 ha forest area, 61,695 ha green area, 44.4 % urban green coverage, 22 parks, 315 public gardens, 100 boulevards, 100,000 m2 roof green area, and 210 ha community green area (Statistical Yearbook of Beijing 2010; www. bjyl. gov. cn). During the past decades, both urban green coverage (22.3-44.4 %) and green area (26,680-61,695 ha) have notably increased (Statistical Yearbook of Beijing 2010). Trees are essential components of all urban green spaces. More than 2,056 species of vascular plants (He et al. 1993) and more than 61 million trees (Beijing Municipal Bureau of Landscape and Forestry 2005) are planted across the city. Chinese pine (Pinus tabulaeformis), a species endemic to China, has been planted widely in northern Chinese cities because of its wide adaptability and aesthetic value. It is one of the top five evergreen tree species in Beijing in terms of number of individuals and ecological importance value (Beijing Gardening and Greening Bureau 2005; Meng 2004). In this study, we monitored the diurnal, daily, seasonal, and annual patterns of transpiration of Chinese pines in the center of Beijing with the help of the thermal dissipation probe method (TDP). The influences of urban environmental changes on the transpiration of Chinese pine were evaluated at different time scales.