[en] The resistance of indigenous elm species to Dutch Elm Disease (DED) has been studied since 1991 with emphasis on natural forests. The situation varies with altitude, local conditions, species and with stand origin. The variability in morphological characteristics is important and some varieties are described. The taxonomy of elms in Romania, as in the whole of Europe, seems to be confusing (Ulmus ambigua was described in 1952 by Beldie in Flora of Romania). Natural hybrids between field elm and mountain elm were observed both in natural and artificial stands, especially in border zones for the altitudinal distribution of the species. Seedlings from the natural provenances and open pollinated seedlings from hybrids produced in the Dutch breeding programme were artificially inoculated with Ophiostoma ulmi- Buism.. Most of the seedlings died, but results show an important number of surviving individuals from natural provenances. In some natural and artificial stands, it was observed that beside transmission of the disease by insects (Scolytus spp.), it was spread through root sprouts especially in field elms, from the dead "mother tree" within a few years, as well as by root grafts to proximal healthy trees ("biogroups"). Because of the genetic incompatibility of individuals in some cases, the grafting cannot succeed and neither can infection. In the natural forest where the ecosystem is in equilibrium and functions normally, DED is present but its damages are not so important. The study of elms in natural forests provides valuable information for the problems caused by DED.

[es] La resistencia a la grafiosis de las especies nativas de olmo ha sido estudiada desde 1991, haciendo especial &eacute;nfasis en los bosques naturales. La situaci&oacute;n var&iacute;a con la altitud, las condiciones locales, las especies y el origen de la poblaci&oacute;n. La variabilidad morfol&oacute;gica es grande, habi&eacute;ndose descrito algunas variedades. La taxonom&iacute;a de los olmos de Ruman&iacute;a, como en el conjunto de Europa, es confusa (Ulmus ambigua fue descrito en 1952 por Beldie en Flora of Romania). Se ha observado la existencia de h&iacute;bridos naturales entre el olmo campestre y el olmo de monta&ntilde;a, tanto en rodales naturales como en artificiales, sobre todo en la franja altitudinal de contacto entre ambas especies. Se ha inoculado artificialmente con Ophiostoma ulmi Buism. Una serie de brinzales de origen natural y de polinizaci&oacute;n abierta de h&iacute;bridos procedentes del programa holand&eacute;s de mejora gen&eacute;tica. Aunque la mayor parte de los brinzales muri&oacute;, los resultados muestran que hubo un n&uacute;mero importante de ejemplares supervivientes de origen natural. En algunos rodales, tanto naturales como artificiales, se observ&oacute; que, adem&aacute;s de la trasmisi&oacute;n de la enfermedad por insectos (Scolytus spp.), hubo tambi&eacute;n, especialmente en olmo campestre, transmisi&oacute;n a trav&eacute;s de rebrote de ra&iacute;z, a los pocos a&ntilde;os de la muerte del &laquo;&aacute;rbol madre&raquo;, as&iacute; como por injerto de ra&iacute;z con &aacute;rboles pr&oacute;ximos sanos (&laquo;biogrupos&raquo;). Debido a la presencia en algunos casos de incompatibilidad gen&eacute;tica de los individuos, el injerto no tiene &eacute;xito ni causa infecci&oacute;n. En bosques naturales con el ecosiostema en equilibrio y que funcionan de una forma normal, la grafiosis est&aacute; presente pero sus da&ntilde;os son poco importantes. El estudio de los olmos en bosques naturales proporciona informaci&oacute;n valiosa sobre los problemas causados por la grafiosis.