Persistent soil seed banks currently are thought to develop slowly in Arctostaphylos resulting in a period of vulnerability for obligate seeders termed “immaturity risk”. After seedlings establish following fire, survivors generally take several years to 10 years before they begin producing seeds. Seeds are slowly incorporated into the soil and must reach a sufficient depth in the soil to survive the heat of wildfire. The extreme temperatures of fire kill many seeds in the top 2-3 cm but may not reduce viability below that. The time required to both produce seed and get them deep enough to survive fire is the period of immaturity risk. Fires from Summer 2008 have revealed a second mechanism of seed bank formation among Arctostaphylos (Ericaceae) species from California chaparral; that rodents cache Arctostaphylos seeds enabling higher rates of seed survival during fires. I documented seedling establishment in post-fire sites of five different Arctostaphylos species, tabulating seedlings emerging from rodent caches separately. Plot fire intensity was estimated using the smallest branch diameter method. To assess rodent caching, fruit were collected from two species of Arctostaphylos and placed beneath shrubs in unburned chaparral dusted with UV fluorescent powder.
Results/Conclusions
Results indicated that rodent caching is critical for seed survival in wildfires. While overall seedling density declined with fire intensity, the proportion of seedlings emerging from caches increased from about 40% to above 90%. In the caching experiment, all marked fruit were eaten or cached, 70% of the caches were buried deeper than 2 cm, and 54% deeper than 3 cm, with the average depth of 4.1 cm (2.4 sd). These results have important implications in the context of climate change scenarios developed for California; which suggests an increase in frequency and intensity of wildfire. Caching of Arctostaphylos seeds may shrink the period of time currently considered ‘immaturity risk’ due to more rapid burial of seeds. Secondly, rodent caching may bury them to sufficient depth so that they can survive higher intensity wildfires.