Arboriculture & Urban Forestry 45(5): September 2019 Thornthwaite scheme was therefore used in this study when matching site conditions in the five forest reserves in the Republic of Georgia with those in the selected five large cities in Western Europe. This modification involves estimating daily reference evapotranspiration taking as input the normal climate temperature for determining the I and a thermal indi- ces, and has a performance almost identical to that of the more robust and highly recommended Penman- Monteith FAO-56 model (PM-56) despite including fewer variables (Pereira and Pruitt 2004). When screening regions that can host potential urban trees for Western Europe, such simple and easy models that enable many large areas to be screened are needed. Through our water net calculations, it was possible to detect suitable tree species not only for a specific city, but also for a specific urban site situation. A greater catalogue of suitable tree species is needed for street environments than park environments due to the much more constricted growing conditions in urban streets. Our calculation approach makes it pos- sible to pinpoint a specific use category when analyz- ing forest systems. This is likely to be helpful and save much time in the subsequent selection process where the focus is on highly promising species/geno- types, rather than randomly selecting species without this additional information. The present compilation of water net differences in five forest reserves is far from providing a complete overview of the Republic of Georgia’s potential as a source of trees for urban environments. However, from this study it was possi- ble to identify specific region and climate types that are a close match to those in paved environments in Copenhagen, Denmark, which makes it easy to rec- ognize further regions and sites of interest. Carpinus betulus was found in both Oni and Akhmeta forest reserves, but the differing conditions in these two reserves indicate different use capacity of the species. Earlier research by Bauerle et al. (2003) and Sjöman et al. (2015) has shown that there are great differences in drought tolerance between different genotypes and ecotypes of different climate and site origins. Carpinus betulus is exposed to much warmer and drier conditions in Akhmeta than in Oni, so the potential of the Akhmeta ecotypes as an urban street tree can be assumed to be much higher than that of the ecotype from Oni. This study should be regarded as a first step in a long chain of efforts before the proposed trees are 231 released to the market. The test period, combined with production and evaluation, can take a long time, as it has to cover aspects such as evaluation of the species/genotypes capacity for different growing sites, propagation issues, establishment conditions, early maintenance demands, etc. However, the major- ity of the species found in Georgian forest reserves in the present study (Table 3) are not new to horticulture in Western Europe and North America, as they can all be found in nursery production and/or in tree collections. From these nurseries and plant collections, there is already a rather good knowledge of their propagation, establishment and maintenance needs, mature size, etc. Using this existing knowledge can make the selection process much shorter once proper ecotypes from the forest reserves are available and tested (Sjöman et al. 2012a). Moreover, even if Carpinus betulus is a com- mon species in cultivation in the CNE region where the genetic material originates from, a much more humid and cooler forest habitat in the region com- pared to the plant material in the study in Georgia means that even for a common species, an “upgrade” of its tolerance for challenging sites will be necessary in order to increase its use potential towards paved environments in a future climate. In contrast, there is a lack of experience in Western Europe and North America of cultivation of species from, e.g., China, which will lead to a much longer selection process due to the many aspects that need to be investigated before any trustworthy recommendations can be made. A shorter selection process would help meet the urgent need for a larger plant stock for urban environments. Furthermore, while this study focused on finding tree species that are tolerant to warm and periodically dry habitats, i.e., typical growing conditions in paved urban areas, further technical solutions and design will be necessary in order to develop sustainable and long-lasting tree plantings, especially for a future cli- mate. Measures that decrease water runoff and increase water infiltration will be needed, which means that combining stormwater management and tree planting will be absolutely essential in paved environments in a future climate. All trees investi- gated in forest reserves in this study have a generous rooting space (more than 50 cm [19.5 in] in depth), which is not the case in street environments. Thus in paved environments, trees experience faster and more severe drought than in natural habitats, which means that choice of tolerant tree species or ecotypes must ©2019 International Society of Arboriculture
September 2019
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
