Trees (2004) 18:615–621 DOI 10.1007/s00468-004-0325-z ORIGINAL ARTICLE Pedro M. Díaz-Fernµndez · JosØ Climent · Luis Gil Biennial acorn maturation and its relationship with flowering phenology in Iberian populations of Quercus suber Received: 25 September 2003 / Accepted: 28 January 2004 / Published online: 5 May 2004  Springer-Verlag 2004 Abstract Since the XIX century, there is a controversy about the existence of biennial maturation of the acorns in Quercus suber L. While some authors recognised biennial cycles as an adaptation to habitats with short vegetative periods, other authors discarded the biennial pattern. Successive flowering events from spring to autumn and annual acorn ripening are proposed as an explanation of the multiple acorn crops typical of Iberian forests. To clarify this discussion, the presence of annual and biennial acorns was assessed in seven cork oak stands, covering a wide range of environmental conditions. In each stand, 100 individuals were sampled once in spring and once in autumn. Biennial acorns were observed with variable frequencies in all populations. There was a significant and positive relationship between latitude and the percentage of trees with biennial acorns within northern and central populations. On the contrary, this trend was not signifi- cant among southern populations. The hypothesis that the presence of biennial acorns in Quercus suber is related to individual female flowering phenology was confirmed in four populations located in the southwestern Iberian Peninsula. Unregarding local differences in the distribu- tion of phenological stages anticipated trees bore signif- icantly less biennial acorns than delayed individuals of the same stand. This result is coherent with the idea that the length of the vegetative period plays a crucial role in the frequency of annual and biennial acorn ripening pat- terns. The relationship between annual and biennial ripening cycles and the multiple acorn crops is discussed. Keywords Cork oak · Ripening cycle · Phenology · Quiescent flowers · Iberian Peninsula Introduction Cork oak (Quercus suber L.) is an outstanding sclero- phyllous species in mediterranean forest ecosystems. Its main area of distribution is located in the Atlantic regions of Morocco and the Iberian Peninsula with small scattered areas also appearing near the Tyrrhenian Sea, eastern France and Spain, Italy, Algeria and Tunisia (Díaz- Fernµndez et al. 1995). Cork oak forests play a crucial role to maintain wildlife and cattle in vast sparse forests (Costa Tenorio et al. 1997). In addition, sustainable cork extraction for wine bottle stoppers provides significant economic value to stands of this species, thanks to its rare ability to reconstruct bark after peeling. However, in spite of its economic and ecological relevance, some aspects related to acorn maturation in Quercus suber are still unclear. Within the genus Quercus, two patterns of acorn de- velopment have been described, annual and biennial (Cecich 1997a). In the annual pattern, the elongation of the pollen tube occurs immediately after spring pollina- tion; fecundation and embryo development continue after a short interruption of 1 or 2 months. Acorn maturation and dispersal takes place later on during the first autumn after flowering. In the biennial pattern, fecundation does not occur immediately after pollination, but in the fol- lowing spring, after a quiescent period. Thus, acorns mature during the second autumn after flowering. Within genus Quercus, a biennial pattern is characteristic of the groups Cyclobalanopsis, Lobatae (red oaks) and Proto- balanus, while an annual pattern is characteristic of white oaks (Kaul 1985; Kleinschmit 1993; Nixon 1993). The Cork oak is the only taxon of the Cerris group in the west of the Mediterranean basin while most other species in the Cerris group spread through Asia and a few others reach the eastern Mediterranean. In spite of this group being traditionally classified in the Quercus (white oaks) section (Nixon 1993), biennial acorn maturation has been described in more than 75% of the species (Camus 1938). Phylogeny of the Cerris group has been recently revised based on molecular markers and its separation P. M. Díaz-Fernµndez · J. Climent · L. Gil ( ) ) Unit of Plant Anatomy, Physiology and Genetics, ETSI Montes (UPM), Ciudad Universitaria, 28040 Madrid, Spain e-mail: lgil@montes.upm.es Tel.: +34-91-336-7113 Fax: +34-91-336-6386