Biennial acorn maturation and its relationship with flowering phenology in Iberian populations of Quercus suber

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 significant 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 distribution of phenological stages anticipated trees bore significantly 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 patterns. The relationship between annual and biennial ripening cycles and the multiple acorn crops is discussed.

Acorn production in Spanish holm oak woodlands

“…Gómez (2004) fits acorn weight to a log-normal distribution, with mean varying in two years from 2.25 ± 0.02 g to 2.64 ± 0.03 g, also under a continental Mediterranean climate, however 200 mm more humid in average than Afzal-Raffii et al's continental site. The reduction of acorn size as a response of the shorter, cooler growing season has been reported elsewhere for other oak species (Aizen and Woodcock, 1992;Díaz-Fernández et al, 2004).…”

“…Some authors have suggested the hypothesis that woody species with longer intermast interval are likely to produce larger seeds, likely as an adaptation to increase seedling survival (Aizen and Woodcock, 1992;García et al, 2000;Sork, 1993). It has been shown in some oak species how acorn size correlates positively with the length of its development period and with rainfall (Sork, 1993;Díaz-Fernández et al, 2004).…”

“…Acorn production ranged from 0.74 dry matter g/crown-m 2 to 332.85 g/crown-m 2 (average 155.6 g/ crown-m 2 in the un-pruned trees vs. 81.2 g/crown-m 2 in the pruned) and was significantly higher in unpruned trees just the years production was above the average. Q. suber flowering and fruiting phenology is more complex than in holm oak (Díaz-Fernández et al, 2004), and this could influence any comparisons between the species.…”

“…This behavior would be particularly important in regions with Mediterranean climates. Intense drought can lead to losing of female flowers (Díaz-Fernández et al, 2004), which would negatively influence both acorn production, and most probably also acorn size for the surviving flowers.…”

Opportunity for hybridization between two oak species in mixed stands as monitored by the timing and intensity of pollen production

“…Since cork oak bears both annual and biennial fruiting (Corti, 1955;Díaz-Ferná ndez et al, 2004), the latter was excluded simply by starting the observations simultaneously with holm oak fruiting.…”