Conversion of the aging black pine plantations into ecologically more stable broadleaf forests is an important goal in many Mediterranean countries as well as in the sub-Mediterranean part of Slovenia. Six experimental plots, split between the more and the less productive site, were established near Divača in southwestern Slovenia in an area of mostly pure black pine stands. In November 2012 they were planted with six native broadleaf species (Celtis australis L., Quercus petraea (Mattuschka) Liebl., Fagus sylvatica L., Prunus avium L., Juglans regia L. and Acer pseudoplatanus L.). Experience gained from the test plantations will be useful for the potential large-scale forest conversion as well as for forming small initial nuclei from which the introduced broadleaf species could later naturally spread into the aging, partly decomposed pine plantations. Of all planted species P. avium had the highest survival rate (95 %) and was least susceptible to poor field conditions after the first growing season. Without exception, all species showed lower survival rate on the less productive site. Unlike other species, P. avium and J. regia seedlings had considerable height increment, while F. sylvatica had the lowest one. Although the results after only one growing season are insufficient to draw firm conclusions, all tested species except F. sylvatica showed promising potential for future conversion of mature pine stands in the Slovenian Karst. We can conclude that the introduction of broadleaves into pine plantations should not begin until conifers have already considerably improved soil conditions.