Seed masting describes when extremely high seed production across individuals in a population is followed by one or more years of extremely low seed production. It occurs across large geographic regions and can occur within and amongst species. Unfortunately, the drivers of masting and the implications of masting are often poorly known. To explore causes and consequences of masting, we use 7 years of seed production data for six focal species (Abies amabilis, Callitropsis nootkatensis, Pseudotsuga menziesii, Thuja plicata, Tsuga heterophylla, Tsuga mertensiana) collected from 108 seed traps located in 18 stands at Mt. Rainier National Park. We address three objectives. First, we ask whether focal species exhibit mast seeding behavior (i.e. synchronized seed production across all stands, annual variation in seed production exceeds annual climate variation). Next, we assess whether seeds produced in mast years are more likely to be viable (as indicated by being ‘filled’), implying that pollination efficiency may drive masting. We also assess how masting patterns and the relationship between seed quality and masting years varies among our focal species. Finally, we tested seed viability of Tsuga heterophylla (our focal species with the highest seed production) for 7 locations and four years (including 2 mast years).
We found strong indications of seed masting among all six focal species, with mixed effects models supporting similar annual trends in seed production in geographically disparate sites for all species. Specifically, we found that seed production was 5 to 12 times greater in mast years, for all focal species. Additionally, years of high seed production were synchronized over large spatial scales, with stands as far apart as 35 kilometers exhibiting similar annual trends in seed production. Within our 7 year data set, 2010, 2012, and 2014 emerged as mast years for 4 of our 6 focal species (with the exception of weak seed production events for Pseudotsuga menziesii and Callitropsis nootkatensis in 2010 and 2012). The year 2014 emerged as an extremely high seed production year for all species. We also found a significant increase in the proportion of viable seeds produced in masting vs. non masting years (verified with germination tests on one species), suggesting pollination efficiency might be a driver of masting. In all, our results suggest that highly variable annual interspecific seed production is occurring. Production variation of this magnitude has direct implications on seedling recruitment, tree population demography, and influences ecosystem dynamics.