Description
The Mexican flannel bush (Fremontodendron mexicanum) is a rare and federally endangered plant species known to currently exist on one mountain within the United States. The goal of this study was to identify components to the reproductive biology of F. mexicanum that would better assist in managing the remaining plants and address the apparent lack of dispersal out of the current recorded range. To identify potential limiting traits in the reproductive life cycle (phenology) of the species, various stands of F. mexicanum were analyzed for pollination, seed dispersal, and germination characters in 2010 and 2011. The European honey bee (Apis mellifera) was the predominant floral visitor to F. mexicanum flowers comprising 88.5% of all floral interactions in both 2010 and 2011 combined. Ultraviolet floral pigmentation in a pattern similar to that reported for the sister taxa, Fremontodendron decumbens was identified on all F. mexicanum flowers examined (n = 20). Seed set appeared to be robust with each fruit producing approximately nine seeds, indicating that seed production is most likely not the cause of limited dispersal. One of the most important finds of the study was the identification of an elaiosome structure on 97.4% of all seeds examined. An elaiosome is indicative of ant dispersal (myrmecochory), and is found in both California flannel bush (Fremontodendron californicum) and Pine Hill flannel bush (F. decumbens) with 93.6% and 100% occurrence, respectively. While examining seeds in a scanning electron microscope (SEM), stomata were found on the elaiosome. The purpose of the seed stomata is unknown; however, they could act as a secondary pathway for water imbibition signaling mesic germination conditions are present. A graniverous ant species, Messor andrei, was found between two of the canyons where F. mexicanum is currently found. This same species of ant was found to be the primary seed disperser for F. decumbens. To assess the possibility of native ant dispersers facing competition from invasive species, the three canyons were F. mexicanum is found was surveyed for the occurrence of the invasive Argentine ant (Linepithema humile); none were found. A side by side comparison of germination rates under different conditions was conducted on both F. mexicanum and F. californicum. The treatments were based off of a study assessing the reproductive attrition of F. decumbens and included: control, scarification, stratification (5° C for eight weeks), heat (100° C for five min.), heat + soil amended with charate, and heat-stratification-charate. The heat treatment had the highest germination totals for F. mexicanum (47.5%) and the heat + charate treatment had the highest germination totals for F. californicum (53%). It could be possible that an alteration in the fire frequency or severity could explain the current limited distribution of F. mexicanum. The results of this study found the reproductive cycle of F. mexicanum appears to be functioning. A large amount of seed production was recorded, native graniverous ants known to disperse sister taxa seeds were present in the local area, and seeds successfully germinated under a variety of conditions. It could be that the historic herbarium specimens from the northern portions of the F. mexicanum range were misidentified. Species determinations for the historic, northern range specimens were made exclusively from morphological traits of leaves and flowers. A genetic analysis should be performed on both the extant stands and historical samples of F. mexicanum, as there are many morphological similarities between F. mexicanum and F. californicum. The genetic analysis would allow agencies to manage the remaining stands of F. mexicanum appropriately and help better understand the species ability to disperse.
