Inviable Seed Set Affects Arthropod Damage to Seeds of Western Juniper (Juniperus occidentalis)

Abstract

Abstract Retention and development of fruits containing inviable seeds has been considered anomalous, because such seeds cannot contribute to plant fitness. Although trees in the genus Juniperus are conifers and thus not true fruiting plants, junipers package seeds within fruit-like female cones commonly referred to as “berries,” and juniper seeds often exhibit high levels of inviability. We tested the prediction that inviable seeds in western juniper (J. occidentalis) reduce levels of pre-dispersal seed predation by arthropods. From 2009 to 2014 we assessed production of berries and seeds in two northeast California populations of western juniper and documented presence of four arthropods (three seed predators, one fruit predator) as well as condition of seeds. The number of inviable seeds per juniper berry was directly associated with numbers of the pulp-feeding frugivore in berries collected both in fall and spring, suggesting that trees may abort seeds within berries experiencing heavy frugivore damage. Consistent with the hypothesis that inviable seeds can reduce seed predation, numbers of a granivorous mite were inversely related to numbers of inviable seeds in fall berry collections. However, the pattern switched to a direct relationship between mites and inviable seeds in spring-collected berries, perhaps due to the timing of mite infestation during berry development. Inviable seeds occurred most frequently during a year of mast production of juniper berries, and the lowest levels of seed damage by a granivorous chalcid wasp occurred during the mast year. The following year saw the greatest levels of seed damage by granivorous moths and mites, possible hold-over effects from large populations built during the mast year. The production of inviable seeds, together with satiating granivores through masting, may represent complementary mechanisms for reducing pre-dispersal seed predation.