Calcicolous rock-outcrop lime forests in the Cantabrian Mountains and the Western Pyrenees

We present the first study of the forest vegetation dominated by Tilia platyphyllos growing on calcareous rock outcrops in the northern part of the Iberian Peninsula. This vegetation is confined to steep rocky slopes of mainly northern aspect in limestone areas. Based on a dataset of 53 releves recorded during the summer of 2019 and compared with so far described associations of the order Aceretalia pseudoplatani (118 releves), we describe the new association Helictotricho cantabrici-Tilietum platyphylli, distributed in the area stretching from the Picos de Europa in the west to the French Pyrenees in the east. Beside common species of mesophilous and thermophilous forests, numerous rockoutcrop specialists occur in this association. This vegetation is species-rich and of a high conservation value, as it also contains a high number of endemic species of the northern Iberian Peninsula, such as Antirrhinum braun-blanquetii, Helictotrichon cantabricum, Oreochloa confusa, Petrocoptis pyrenaica, Saxifraga canaliculata and Seseli cantabricum. Two subassociations of the Helictotricho-Tilietum are distinguished: H. c.-T. p. typicum represents more mesophytic stands occurring in the BasqueCantabrian Mountains and the Pyrenees, while H. c.-T. p. lauretosum nobilis, with drought-tolerant submediterranean species, was recorded in the Cantabrian Range. We classify the HelictotrichoTilietum in the Melico-Tilion platyphylli (order Aceretalia pseudoplatani, class Carpino-Fagetea), an alliance including xero-mesophilous scree and rock-outcrop forests of the European temperate zone.