The reproductive biology of the rare shrub Fremontodendron mexicanum

Background/Question/Methods

The Mexican flannel bush (Fremontodendron mexicanum) is a federally endangered and rare plant restricted to the northern slopes of Otay Mountain in southwestern San Diego County. The genus, a relic from California’s past tropical forests, is common in today’s Mediterranean climate. With a historic range from northern Baja California, Mexico to Santa Clara County, California, today F. mexicanum is only known to exist in three canyons in the United States. Anticipating an issue in one of the plant's reproductive life stages we conducted an in-depth analysis of pollination, seed dispersal, and germination traits. This range retraction might have been caused by a number of reasons: small seed set due to inadequate/unsuccessful pollination by floral visitors, improper dispersal of seeds away from the parent plant, and/or unfavorable seed recruitment conditions. Floral visitor species and behavior as well as counts of reproductive structures were analyzed to understand the interplay of various factors in the reproductive cycle. Since related species have ant-dispersed seeds, ants were trapped and identified to determine their potential effect on F. mexicanum dispersal. Finally, we used related Fremontodendron californicum to examine seed recruitment under a variety of ecologically relevant pre-treatments and with two different soil types.

Results/Conclusions

F. mexicanum did not appear to be pollinator limited. We observed a generalist pollinator syndrome dominated by European honey bee (Apis mellifera) (n=786, 88.5% total visits) that resulted in abundant seed set. Seeds apparently were dispersed gravitationally directly from the fruit. One ant species known to disperse the sister taxa Fremontodendron decumbens was found in close proximity to F. mexicanum populations and a small elaiosome (ant attraction structure) was identified on virtually all F. mexicanum seeds examined. It appears that fire may play a critical role in seed recruitment in Fremontodendron due to enhanced seed germination when exposed to 100^oC dry heat for at least five minutes. A previously unrecorded trait of stomata on the elaiosome was identified on all F. mexicanum seeds examined. These stomata could indicate an alternate pathway to allow imbibition of water signaling mesic germination conditions are present. While results of this study did not definitively explain the range retraction, the data do provide life history details that will refine and improve future F. mexicanum recovery plans.