Renewal Structures and Shoot Growth of Three Species of Woody Mediterranean Dwarf Shrubs Growing Along an Altitude Gradient

Sara Palacio-Blasco^1*, Rubén Milla-Gutiérrez¹ and Gabriel Montserrat-Martí¹

¹ Department of Functional Ecology and Biodiversity. Pyrenean Institute of Ecology (CSIC), Av Montañana, 1005. Apdo. 202. 50080 Zaragoza, Spain.

^* Author for correspondence: sarap@ipe.csic.es

The morphology of renewal structures and shoot growth were studied in three species of woody Mediterranean dwarf shrubs growing along an altitude gradient in the Spanish Western Pyrenees. The selected species were the evergreen Satureja montana, which inhabits at the lowest altitudes; the winter-deciduous Ononis fruticosa, which grows at mid altitudes; and the thorny cushion-plant Echinospartum horridum, which occupies the highest altitudes within the studied gradient. The climate of this region is characterized by the presence of short summer-drought periods and the occurrence of low temperatures in winter, with a high freeze risk that increases with altitude. Most woody Mediterranean dwarf shrubs bear two kinds of shoots: long shoots or dolychoblasts and short shoots or brachyblasts. S.montana and O.fruticosa showed axillary brachyblasts as renewal structures, which bear both adult and embryonic organs. The species growing at the highest altitude (E.horridum) displayed axillary naked buds composed exclusively of embryonic organs as renewal structures. O.fruticosa showed its brachybasts protected by the stipules of the leaves of the dolychoblast, thus performing like naked buds. In the three species shoot growth stopped in autumn and was not resumed until spring, indicating the occurrence of rhythmic shoot growth. This results contrast with the observations of brachyblasts with continuous growth made by other authors in species of woody Mediterranean dwarf shrubs of warmer regions.