Extended seed rain period of Adenostoma fasciculatum impacts diverse seed predators

Aims The principal chaparral species in California, Adenostoma fasciculatum, an evergreen, sclerophyllous shrub, is broadly distributed and provides habitat and food resources for a large and diverse animal community. The effects of climate change, including elevated temperatures, fire frequency and severity, along with increased urban encroachment, have placed pressure on chaparral habitats in California. Our goal is to investigate aspects of reproductive ecology as a measure of the potential resiliency of A. fasciculatum. We focus on seed rain (all seed falling into the seed traps regardless of origin) and seed banks in the context of plant-animal interactions and regeneration. Methods Stand recovery following disturbance is achieved through both resprouting and germination from established persistent soil seed banks. In this study we focus on seed ecology using a series of experiments to document the length and quantity of seed rain, seed predation, parsing the importance of the community of granivores, and evaluating the connection between stand age and germination rate from soil seed banks. Important findings Our research documented an 8-month seed rain duration with over 1 million seeds per m2, multiple seed predators including passerines (songbirds) and rodents, and points to the possibility of native ants playing a role in the seed dispersal process. This is important given the recent advancement of the invasive Argentine ant (Linepthema humile) into Californian chaparral. This research demonstrates a clear relationship between A. fasciculatum and both resident and migratory granivores in the chaparral. We documented that a 39-year-old stand had higher germination rates than those which were 16, 20, 41 and 71 years old and how seed banks play a major role in assuring resiliency following fire. These findings are important for wildland managers to assure the continued resiliency of A. fasciculatum.

Post-fire regeneration strategies and flammability traits of California chaparral shrubs

Fire behaviour is strongly influenced by fuel load and structure; however, efforts to describe fuel patterns have largely ignored differences among species or post‐fire regeneration strategies. In California chaparral, evergreen shrubs can be grouped into three post‐fire regeneration strategies that correlate with a wide variety of physiological and demographic characteristics including seasonal water status and the timing of reproduction in response to fire. To test if regeneration strategy is also associated with flammability, we compared the fuel loads and structure of two post‐fire seeders, Adenostoma fasciculatum and Ceanothus cuneatus, and two obligate resprouters, Heteromeles arbutifolia and Prunus ilicifolia. Species and post‐fire regeneration strategies did not differ in total fuel per area, or bulk density. The proportion of fuels smaller than 6 mm in diameter differed among species, but not consistently with regeneration strategy. However, species with a post‐fire seeding regeneration strategy had higher proportions of dead branches. We discuss how this difference could have arisen from evolutionary, demographic, or physiological processes.

Photosynthate Allocation Patterns Along a Fire-Induced Age Sequence in Two Shrub Species From the California Chaparral

In the prolonged absence of fire, shrubs of the California chaparral have sometimes been reported to demonstrate an increase in the proportion of standing dead biomass and a decrease in productivity (''senescence''). To investigate the possible physiological basis of this phenomenon, seasonal patterns of the allocation of 14C-labeled photosynthate were studied in leaves of two chaparral species, Adenostoma fasciculatum H. & A. and Ceanothus greggii var. perplexans (Trel.) Jeps., along a fire-induced age sequence. Harvested leaves labeled with 14C were analyzed for percent of 14C in storage, structural, metabolic, and defense compounds. No age-specific trends in photosynthate allocation were found except in the spring. During spring, when demands on photosynthate for growth were high, shrubs in older stands allocated a much lower proportion ot their photosynthate to storage compounds compared to shrubs in younger stands. Older shrubs are apparently less able to meet the concurrent demands of storage and growth during spring than younger shrubs. This observation is consistent with the notion of physiological senescence in older shrubs, and has implications for questions about the optimum length of the fire-free interval for stands managed with prescribed burning.