Las comunidades de Salix pyrenaica en el valle de Aran.

Se aportan datos referentes a dos comunidades de la alianza Primulion intricatae en que participa el subarbusto Salix pyrenaica: Dryado-Salicetum pyrenaicae Chouard ex Vanden Berghen 1970 y Carici-Geetum pyrenaici Chouard ex Vanden Berghen 1970. Ambas se conocían de áreas más atlánticas de la cordillera pirenaica, y principalmente dentro de territorio francés, si bien se disponía de muy pocos inventarios fitosociológicos concretos. En el valle de Aran, aparecen localmente en los afloramientos calizos, y contienen diversos táxones poco comunes o raros en los Pirineos, como por ejemplo Arctostaphylos alpina. Además, ambas comunidades muestran un carácter relativamente particular en comparación con las afines descritas previamente. Por ello, se propone considerarlas como sendas nuevas subasociaciones: Dryado- Salicetum pyrenaicae salicetosum reticulatae, subass. nov., y Carici-Geetum pyrenaici anthyllidetosum boscii, subass. nov.

How are riparian plants distributed along the riverbank topographic gradient in Mediterranean rivers? Application to minimally altered river stretches in Southern Spain.

Species structure and composition in Mediterranean riparian forests are determined by hydrological features, longitudinal zonation, and riverbank topography. This study assesses the distribution of four native riparian plants along the riverbank topographic gradient in three river stretches in southern Spain, with special emphasis on the occupation of adult and young feet of each species. The studied stretches suffered minimal human disturbances, displayed semi-arid conditions, and had wide riparian areas to allow the development of the target species: black alder (Alnus glutinosa), salvia leaf willow (Salix salviifolia), narrow-leafed ash (Fraxinus angustifolia), and oleander (Nerium oleander). Thalweg height was used to define the riverbank topographic gradient. The results showed a preferential zone for black alder and salvia leaf willow in the range of 0-150 cm from the channel thalweg, with adult alders and willows being more common between 51 and 150 cm and young alders being more common under 50 cm. Conversely, narrow-leafed ash and oleander were much more frequent, and showed greater development, in the ranges of 151-200 cm and 201-250 cm, respectively, whereas the young feet of both species covered the entire topographic range. Adult feet of the four species were spatially segregated along the riverbank topographic gradient, indicating their differential ability to cope with water stress from the non-tolerant alders and willows to more tolerant narrow-leafed ash trees and oleanders. Young feet, however, showed a strategy more closely linked to the initial availability of colonisation sites within riparian areas to the dispersion strategy of each species and to the distribution of adult feet. In Mediterranean areas, where riparian management has traditionally faced great challenges, the incorporation of species preferences along riverbank gradients could improve the performance of restoration projects.