Variation of red oak acorn production

Abstract

Acorn production is characterized by extreme variation among years and among individual trees. The size of acorn crops affects many components of the ecosystem, and both annual and individual variation in acorn production influence the regeneration and management of oak forests. In order to assess the variation within a stand, we sampled the acorn production of 120 red oaks (Subgenus Erythrobalanus) for 11 years. Half of the trees were treated by thinning 2 years before the study began, and half were unthinned controls. Trees in the thinned part of the stand produced more acorns per tree than those in the unthinned area, but the treatment effect was small in comparison with individual and annual variation. Differences in acorn production between the most productive and least productive individual trees were about 11-fold for thinned and 28-fold for unthinned trees. The largest yearly acorn collection exceeded the smallest by 135 times for the thinned treatment and by 109 times for the unthinned treatment. The two best years (1991, 1993) accounted for 55% of the total 11-year acorn collection, while the five poorest years provided only 10% of the total collection, suggesting that efforts to enhance regeneration should coincide with mast years. Thinned and unthinned sample populations exhibited synchronous masting patterns, and good and poor producers within each population also exhibited year-to-year synchrony in acorn production. There was no evidence of cyclic acorn production by this population. Half of the total acorn collection was produced by about one-third of the trees. On average, we were able to identify 87% of the better-than-average acorn producers by monitoring individual tree production for any three consecutive years. This method of identifying superior acorn producers is effective and conceptually simple but it requires at least 3 years to implement.