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iForest -Biogeosciences and Forestry
IntroductionIn the past centuries elms (Ulmus spp.) became a very important part of the landscape in large areas of the Netherlands. Elms have a combination of highly valued characteristics such as no specific requirements concerning soil type, the ability to resist (sea) winds, the very good recovery from mechanical damage, its adaptability to city conditions including its ability to endure de-icing salts and closed pavements and its much appreciated open crown. In fact, the elm is the ideal street and landscape tree in large parts of the Netherlands. This made the elm, and especially U. × hollandica cultivars, the dominant tree in the coastal areas and cities in the western part of the country at the beginning of the 20 th century. The spread of Dutch elm disease (DED), caused by Ophiostoma ulmi (Buisman) and later Ophiostoma novoulmi (Brasier) during the 20 th century however resulted in millions of trees being lost (Hiemstra et al. 2006). Since 1928, elm breeding for resistance to DED has resulted in the release of several more or less resistant cultivars to the European market. Especially in the Netherlands -where the first cultivar ("C. Buisman") was released in 1936 -and the USA, large and long-term selection programs have been performed resulting in a number of hybrid clones with moderate to very good resistance. Also, more recently, other European programs on elm breeding were set up, e.g., in Italy, Spain and France (Mittempergher & Santini 2004). Today a wide range of clones of different DED-resistance and different parentage is available on the market. However, the use of these new elm cultivars in the Netherlands is still limited. "Vegeta" and "Commelin" elms that were planted in very large numbers in the second part of the twentieth century because of their level of resistance to DED caused by O. ulmi were attacked by O. novo-ulmi. Apparently, the lasting problems with DED in these earlier released cultivars, such as the sanitation costs for removing and replacing diseased and dead trees, has led to a lack of confidence in the resistance of newly released cultivars among urban foresters and landscape managers. As a result and despite its glorious past, around the year 2000 the elm was not even listed in the top 20 of street trees produced by Dutch tree nurseries (Geurts & Hiemstra 2002). However, the outcome of a survey among Dutch trees nurseries was that there is no other species that can replace the elm and it was concluded that confidence in the elm should be restored. None of the substitute tree species has the same set of characteristics that makes the elm so well-suited as a street tree; therefore the best replacement for the classical Dutch elm is still an elm, provided it is resistant to DED (Hiemstra et al. 2006). This paper reports on a study aimed at restoring the posi-