262 Sjöman et al.: Habitat Studies Identifying Potential Trees for Urban Paved Environments developed an extensive plasticity and tolerance of a range of environmental conditions, while others have specialized in certain habitat types (Rabinowitz 1981; Gurevitch et al. 2002). For instance, steep south-facing mountain slopes with thin soil layers represent a distinct habitat type where the environmental parameters that define the particular habitat and separate it from other habitats have shaped the evolu- tion of plants. Such environmental parameters also screen out many potential colonizing species not suited to the par- ticular habitat. Investigating the ecological background and performance of species growing in habitats where they experience drought during the growing season and winter temperatures similar to the CNE-region could be a great help in the identification of trees for future selection for use at urban sites (Flint 1985; Ware 1994; Ducatillion and Dubois 1997; Sæbø et al. 2005; Roloff et al. 2009). Extensive fieldwork was carried out in the Qinling Moun- tain range, China, in 2008, to obtain an overview of the spe- cies composition, structure, and dynamics of forest systems at altitudes where the climate is similar to that of the inner city across the CNE-region. The study specifically focused on: * Identification of habitats in the Qinling Mountains where tree species are exposed to seasonally dry and harsh condi- tions. * Characterization of the performance of the tree species in these habitats. * Presentation and discussion of promising tree species for further research and testing regarding use in urban paved sites in northern Europe. The study forms part of a four-year research program initiated by the Swedish University of Agricultural Sci- ences to examine selection of site-adapted species for urban paved sites in the CNE-region. Other case study areas are located in northeast Romania and the Caucasus (Geor- gia). The underlying hypothesis in this selection program is that ‘new’ tree species for urban use can be identified through studies of natural habitats where trees are exposed to stresses similar to those in the urban paved environment. MATERIALS AND METHODS Case Study Area China is considered the most species-rich region in the world (Körner and Spehn 2002; Tang et al. 2006). The Qinling Mountain range in the central, temperate part of the coun- try forms a botanic border between the southern and north- ern regions of China, and consequently hosts a species-rich flora (Ying and Boufford 1998). Shaanxi province, where the Qinling Mountains are situated, is reported to have 1224 indigenous woody species (Kang pers. comm. 2009), com- pared with only 166 in the Scandinavian countries (Moss- berg and Stenberg 2003). The relatively northern location of the mountain range combined with its high altitudes means that plants are exposed to cold winters and warm summer months with periods of intense drought (Takhtajan 1986; Breckle 2002), conditions comparable to the climate in ur- ©2010 International Society of Arboriculture ban paved sites of the CNE-region. This is especially the case in the many steep, south-facing rocky and craggy slopes. Site Description The field work was carried out within three different ar- eas in the north of the Qinling Mountain range – Taibai For- est Reserve (34°05’10”N, 107°44’46”E), the Red Valley Forest Reserve (34°05’08”N; 107°44’52”E), and Siboshan (33°42’08, 30”N; 106°47’16, 69”E). These three sites were compared against two different site situations (urban paved and urban park) in the inner city environment of Copenha- gen (Denmark), which was used as an example to illustrate growth conditions in a large city of the northern CNE-region. Urban paved sites in Copenhagen currently have a mean annual temperature of 8°C–12°C when the urban heat is- land effect is included (+1°C–3°C) (DMI 2009; U.S. EPA 2009) and mean annual precipitation of 525 mm (DMI 2009). Based on climate data for the Qinling Mountains (Liu and Zhang 2003; Tang and Fang 2006), the altitude zone 1000–1500 m above sea level (asl.) was identified as the area where mean annual temperature and precipitation matched the climate of urban paved sites in the CNE-region. Mean annual temperature in the area is 9°C–12°C, mean annual precipitation is 830 mm and 50% of the precipitation occurs predominantly during May–July, mainly as heavy rainstorms (Liu and Zhang 2003; Tang and Fang 2006). Botanical ex- perts from the Northwest Agriculture and Forestry University in Yangling assisted in the initial work in order to obtain an overall understanding of the local site conditions. Here, spe- cies composition, structure, and dynamics are strongly gov- erned by both altitudinal gradients and cardinal directions. Woodland Systems Between 1000 and 1500 m asl. The forest system between 1000 and 1500 m asl. comprises deciduous broadleaved oak forest (Liu and Zhang 2003). Quercus aliena var. acuteserrata is the main canopy species throughout the zone. In the lower part of the zone (<1200 m asl.), Quercus aliena var. acuteserrata co-dominates with Quercus variabilis, while in higher parts it co-dominates with Quercus wutaishanica. These oak species are particu- larly dominant on slopes, regardless of direction, whereas the moist river valleys are characterised by mixed broad- leaved forests with a large number of other canopy species. From the steep, south-facing slopes down to the moist and more shaded river valleys, there is a gradual change in the species composition and density of the vegetation. Fig- ure 1 illustrates this change in species composition and den- sity of the vegetation from steep south-facing slopes down to moist and more shaded river valleys. Quercus aliena var. acuteserrata dominates in the canopy layer throughout the slope. Closer to the slope bottom, a dense canopy layer consisting of numerous co-existing broadleaved tree spe- cies occurs, in this case Quercus aliena var. acuteserrata co-dominating with the moisture-demanding Toxicodendron veniciflua. In the understory layer, a distinct change oc- curs in the species composition and density of the vegeta- tion down the slope. The upper part the understory layer is sparse and consists of small trees of Acer davidii and Kalopanax
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