Ecogeographic patterns of seed size in two fire-adapted genera accross the California Floristic Province

Background/Question/Methods

Arctostaphylos and Ceanothus are the two most taxonomically diverse woody taxa in the California Floristic Province and share very similar ecologies that are independent of evolutionary history.  Coincident with taxonomic diversity, there is a relatively large amount of diversity in Arctostaphylos seed morphology, a key life-history trait, but considerably less so in Ceanothus.  Previous work has exposed putative seed dispersal syndromes in Arctostaphylos based on seed morphology, which assumed that dispersal agents are the single selective force on seed morphology.  No work has thus far studied seed dispersal syndromes in Ceanothus.  One hypothesis yet to be falsified, however, is that fire frequency and severity have affected the morphology of seeds in fire-prone ecosystems.  Here, we take an ecogeographic approach to determine the relationship between seed mass and environmental conditions (including fire characteristics) across California to infer if or how these conditions have affected interspecific seed morphology between two flagship genera of the Californian chaparral, a vegetation type with frequent, recurrent fires. 

Results/Conclusions

Assuming no dispersal modality differences between the two genera, we found no correlation between the seed mass of Arctostaphylos species and latitude or fire frequency.  We did, however, find a negative correlation between the seed mass of Ceanothus species and latitude and fire frequency.  Curiously, Arctostaphylos and not Ceanothus showed a strong positive correlation between seed mass and elevation.  We interpret these results as two lines of evidence (first within Arctostaphylos and second compared to a functional ecological analog) supporting the idea that fire frequency and severity have played a marginal role, if any, in the evolutionary shaping of seed mass in Arctostaphylos.  Laboratory studies are in progress using artificial heat and charate sources to further test our finding.  Lastly, we will speculate on the role that we suspect dispersal has played in the impressive taxonomic diversity within Arctostaphylos and Ceanothus.