268 Sjöman et al.: Habitat Studies Identifying Potential Trees for Urban Paved Environments Table 4. Mean value for yearly height growth and diameter growth followed by mean tree height and DBH at age 15 and 50 years for tree species belonging to Group 2. Species A. altissima C. bungeana Yearly height Yearly diameter Size of growth (m) growth (cm) 0.26 C. turczaninowii 0.16 0.16 0.16 0.25 0.16 0.19 0.24 0.14 F. chinensis K. paniculata M. mongolica O. japonica Q. aliena Q. baronii Q. wutaishanica 0.25 0.22 0.21 S. folgneri S. pekinensis U. pumila U. glaucescens 0.17 0.12 0.36 0.26 0.21 0.23 0.40 0.30 0.32 0.36 0.23 0.35 0.23 0.30 0.22 0.20 3.9/5.4 2.4/3.9 2.4/3.2 2.4/3.5 3.9/6 2.4/4.5 2.9/4.8 3.6/5.4 2.1/3.5 3.8/5.3 3.3/3.5 3.2/4.5 2.6/3.3 1.8/3 Size of 15-year-old tree 50-year-old tree height/DBH height/DBH 13/18 8/13 8/10.5 8/11.5 12.5/20 8/15 9.5/16 12/18 7/11.5 12.5/17.5 11/11.5 10.5/15 8.5/11 6/10 species on wetter sites. However, species such as Ailanthus al- tissima, Koelreuteria paniculata, Quercus aliena, Q. wutaishan- ica, Sorbus folgneri, and Syringa pekinenis had a yearly height increment of over 20 cm in the study plots and develop into 10–13 m high trees in approximately 50 years, while the other species in this group can develop into almost 10 m high trees during same period (Table 4). The calculations presented in Table 4 are based on rather few individuals, but can still be used as an indicator of their growth in these climate and site conditions. DISCUSSION Site Conditions This study examined forest systems occurring between 1000– 1500 m asl. in the Qinling Mountains, in order to identify tree spe- cies growing and developing satisfactorily in natural habitats with similarities to those found in urban paved sites in the CNE-region. The mean annual temperature on south-facing slopes within the altitude band studied is comparable to that in inner city conditions in the CNE-region (8°C–12°C). The direct exposure to sunlight on the slopes also creates low air humidity and rapid drying of the soil, which is comparable to the situation on many urban paved sites (Sieghardt et al. 2005). In addition, the low organic matter content (mean 36.3 g/kg) and neutral pH (mean 6.7) of the study plot soils are similar to values reported for urban sites (Craul 1999). Among the multiple stress factors that characterize ur- ban paved sites, water stress is argued to be the main con- straint for tree growth and health (e.g., Whitlow and Bas- suk 1987; Craul 1999). The high silt content of the soils in the Qinling Mountains plots means they have good water-holding capacity, but as the surface of bare silty soil dries surface has a tendency to form a hard crust, which can cause extensive water runoff (Brady and Weil 2002). This characteristic is further exaggerated on steep slopes with sparse field layers (mean cover 27%) and frequent rock outcrops, like the plots studied (Table 2). In terms of estimated potential water stress, the forest sys- tems on steep south-facing slopes between 1000–1500 m asl. in the Qinling Mountains showed a clear discrepancy with paved sites of Copenhagen today. The net water deficit appeared in the study plots in April but only became severe in July, consider- ©2010 International Society of Arboriculture ably later and less severe than in the present situation in paved sites of Copenhagen (Figure 3). However since Copenhagen has a maritime climate while the study plots are in a continental climate, the match between the Qinling Mountains plots and paved sites of present day Copenhagen is probably closer than illustrated in Fig- ure 3. Moreover, as the water runoff data used in cal- culating the potential water stress levels were not taken directly from the Qinling Mountains, the estimated water stress levels should be interpreted with caution. Species Composition and Performance The plot data combined with general observations of tree species occurrence across woodland systems in river valleys and slopes with different orientations showed that some of the species present have developed an extensive plasticity and tolerance toward a range of environmental conditions (habitat generalists), while others have specialized in the dis- tinct habitat type on south-facing slopes (habitat specialists). Based on the observation together with botanical experts from the Northwest Agriculture and Forestry University, Yangling, the habitat generalists occurred on south-facing slopes, but have their main distribution in valleys and on slopes with other as- pects (Table 1). When these species grow on steep south-facing slopes they develop slowly and are primarily found in the un- derstory (Table 3), where the shading canopy reduces the tem- perature and is likely to increase the air humidity. In compari- son, the species that have specialized in the distinct habitat on south-facing slopes occur as tall canopy trees as well as younger individuals in the understory (Table 3). The capacity to develop into tall, old trees indicates a broad and long-standing adapta- tion of these species to the harsh conditions characterizing this type of habitat. As suggested by Flint (1985), Ware (1994), and Duhme and Pauleit (2000), this ecological background makes the group of habitat specialists identified interesting for future selection of trees for use in urban paved environments. Habitat specialists for warm and dry south-facing slopes at 1000–1500 m asl. in Qinling (Group 2) included Ailanthus altissi- ma, Carpinus turczaninowii, Celtis bungeana, Fraxinus chinen- sis, Koelreuteria paniculata, Morus mongolica, Ostrya japonica, Quercus aliena var. acuteserrata, Q. baronii, Q. wutaishanica, Sorbus folgneri, Syringa pekinensis, Ulmus glausescens, and Ul- mus pumila. These species have developed different ecological strategies to cope with warm and dry habitats where water stress occurs. Such strategies include development of leaves with a thick cuticle to prevent excessive transpiration, which is the case for Koelreuteria paniculata (Balok and Hilaire 2002). Large and deep-penetrating root systems enabling the plant to find water in lower soil layers is another successful strategy employed by many broadleaved oak species, such as Quercus aliena var. acu- teserrata and Q. wutaishanica (Spurr and Barnes 1980; Gale and Grigal 1987; Kozlowski et al. 1991). In addition, wintergreen oak species such as Quercus baronii can photosynthesize dur- ing cooler and moister periods of the year, which allows them to reduce activity in periods with severe water stress (Corcuera et al. 2002). In the CNE-region, the first two of these strategies would appear to be effective at urban paved sites but deep rooting would only be possible if they are given a deep planting bed. Further- more, the frozen ground during winter could cause dehydration problems for evergreen trees at dry urban paved sites. Moreover
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