FACTORS CONTRIBUTING TO THE SOIL SEED BANK SIZE OF TWO OBLIGATE SEEDING CEANOTHUS SPECIES IN NORTHERN CALIFORNIA

Abstract

In fire-prone vegetation, non-sprouting, obligate-seeding species depend on a persistent seed bank to reestablish post-fire populations. Thus, for such seeds, seed development and survival in the soil are critical life history stages. The objectives of our study were to examine the structure and composition of Ceanothus seed banks, analyze the viability of seeds in the soil, and determine rates of predation before and after seed dispersal. This study focused on populations of Ceanothus jepsonii var. albiflorus (J. Howell), which is restricted to serpentine soils. We also included data on Ceanothus cuneatus (Hook.) Nutt., a widespread, non-restricted species, for comparison. We found that the seed bank of C. jepsonii loses seeds due to destruction or removal at several stages of development. Pre-dispersal seed predators attacked 37% of inflorescences included in experimentation and up to 50% of shrubs from which fruits were collected, post-dispersal predators removed over 70% of seeds in the litter. There was a 30% reduction in viability of older intact C. jepsonii seeds in the soil compared to seeds collected from plants during the study. A beetle in the genus Zabrotes, not previously known to oviposit on Ceanothus, was determined to be a causal agent of pre-dispersal seed predation, with other pre-dispersal predators and parasites also possible. These findings suggest that Ceanothus seed banks are quite dynamic and subject to a variety of seed losses during several stages prior to germination, which may ultimately be responsible for soil seed banks remaining relatively constant rather than building up over time.