Double- and Single-Seeded Acorns of Bur Oak (Quercus macrocarpa): Frequency and Some Ecological Consequences

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Abstract
This study of bur oak (Q. macrocarpa) in east-central Illinois [USA] examines some ecological consequences of the variation in seed number per acorn. The frequencies of double-seeded acorns ranges from 0-20% per tree (n = 17 trees). Insect damage of acorns per tree ranges from 43-100%. Such infestation reduces the probability of germination per seed. At least one seed escapes insect damage more often in double- than single-seeded acorns. In double-seeded acorns, a seed''s weight is .apprx. 1/2 that of single-seeded acorns of the same tree, but the proportion of acorn biomass allocated to seeds is similar in the 2 types of acorns. Germination of the 2 seeds is fairly synchronous in double-seeded acorns. The resulting 2 seedlings are smaller in size and biomass at 10 mo. than single seedlings but survive equally well under greenhouse conditions. Conflicting selection pressures have the potential to maintain some variability in seed number per acorn a single tree produces.

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