Resistance to, or tolerance of the main threats to urban trees, were mostly assessed through field trials, observation plots and surveys. These methods are less costly, but still require adequate resources and a long timescale to ensure useful results. However, methods for early assessment have been developed and some of these are reported below.
Resistance to diseases is tested using artificial infection techniques. Austria, Belgium, Italy and France are testing the resistance of elms against Dutch elm disease (Ceratocystis ulmi). Austria reported artificial infection methods for screening the susceptibility of Sophora japonica L. towards canker. Spain and France are using them for the assessment of resistance to anthracnose on Platanus L. Italy and France reported artificial infection to test susceptibility of Cupressus sempervirens L. for cortical canker. An artificial infection technique to test resistance of willows to Watermark Disease is under development in Belgium.
Nearly no early assessment methods were reported to test resistance to, or tolerance of pests. However, pot experiments are used in the UK, Sweden and Austria to test the susceptibility of horse chestnut toward leaf miner (Cameraria ohridella).
In general, field trials are used, involving many species. In Italy, isozyme studies in combination with field trials are used for Quercus L., Juglans L. and Prunus avium L. Frost tolerance of Acerplatanoides L. and Pinus halepensis Mill. is tested by studying electrolyte leakage after freeze testing plant material. Hydroponics was used to test drought resistance of this species. Iceland reported freeze testing of Picea sitchensis (Bong.) Carr. and Abies lasiocarpa (Hook.) Nutt.
Surveys, monitoring and in situ trials are common methods of testing the general performance of trees in the urban environment (UK, Germany, The Netherlands, Greece), salt resistance (UK, Iceland), resistance to photochemicals (Greece), stress resistance (Finland, Greece) rooting ability (Spain) and shape of the crown (Sweden). The UK and Belgium are using hydroponics to test the tolerance of salt and heavy metals in Salix clones. In Austria, pot experiments are used to screen the tolerance of de-icing salts of Acer L., Tilia L., Picea spp. and Pinus nigra Arnold. The growth reaction of Tilia cordata Mill., Acer platanoides L. and Pinus nigra Arnold is also being studied in this way.
The ornamental value, including habit, fruit color, flowering, autumn color, covering capacity and competing capacity of a whole range of species and varieties is mainly assessed visually. Breeding programs were also reported.
Most of the participating countries are running programs on the conservation of genetic resources in trees. Inventories, within and outside conservation projects exist, as well as studies on genetic diversity. In Sweden, selection is done in Tilia L. in historical plantations in combination with the establishment of trial plots and DNA-studies.
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