The advantages of canopy seed retention (serotiny) for plants inhabiting fire-prone ecosystems are well documented. However, very few species are completely serotinous and non-fire induced opening of serotinous fruits is commonly observed (weak serotiny). Two non-mutually exclusive causes are envisaged to contribute to this process: mechanical changes in serotinous fruits mediated by climatic conditions (e.g. drought) or the costs of maintenance for the plant of these long-lasting structures. However, their relative contribution to the spontaneous opening of serotinous fruits remains elusive as well as the consequences for the build-up of the canopy seed bank and inter-individual differences in serotiny.
In this study we monitored the dynamics of cone production and cone opening in the weakly serotinous Pinus halepensis for five years (2004–2008), including two severe drought episodes (2005, 2006). Drought decreased the production of conelets, increased the abortion of immature cones, reduced the seed quality in the cohorts of cones produced during these years, and increased the opening of serotinous cones. During the first drought episode, a higher proportion of serotinous cones opened in those pines bearing a larger crop of younger cones. This suggests that not only passive changes induced by drought but also competition among cones for resources (e.g. water) might be involved in this process.
The opening of serotinous cones in pines bearing more cones made inter-individual differences in the size of the canopy cone bank to narrow or even to reverse from 2004 to 2008. These results may help to understand the decrease in serotiny when pines grow and accumulate more cones and the large inter-individual variability in the degree of serotiny observed in P. halepensis forests. In addition, the negative effects of drought episodes for the size of the canopy cone bank and the seeds contained can be an unexplored cause of post-fire regeneration constraint.