One size fits some

Abstract

The size of seeds may determine not only how likely they are to be dispersed or consumed and to germinate but also how competitive their seedlings may be. In the case of oaks (Quercus spp), acorns are dispersed by jays and magpies (both in the Corvidae family) if they fit in the bird’s bill. Similarly, rodents select acorns of sizes they can carry to hoarding sites. Yet acorns are more likely to resist desiccation and tend to produce taller seedlings if they are large. Owing to divergent pressures on acorn size across life stages, acorn mass can vary by two orders of magnitude within a species. While each oak parent tree produces acorns of relatively similar sizes, there is remarkable variability among individual trees, as demonstrated in this image by acorns sorted in groups of three “siblings” from each of nine holm oak (Quercus ilex) parents. Offspring may be most competitive under the conditions in which the parent established, and high variability in acorn size across parents may result from, and persist under, heterogeneous conditions. Curiously, inter-parent variability also applies to acorn shapes. What are the dispersal implications of acorns being elongated, spherical, or thick at the bottom? What consequences could reforestation programs, which select for large acorns due to their greater probability of emergence, have on tree development and survival over generations? How is intraspecific diversity of oak populations affected if defaunation removes certain acorn disperser or consumer guilds? Does the ongoing oak colonization of conifer-dominated ecosystems under climate change select for certain acorn characteristics, and what does this imply for acorn-feeding guilds in these new communities?