Blue fan palm distribution and seed removal patterns in three desert oases of northern Baja California, Mexico

Information on the current processes controlling the distribution patterns of relict palm populations at the limit of their northwestern distribution in America are poorly known. We explored the importance of post-dispersal seed removal by vertebrates, recruitment, and distribution patterns of the blue fan palm, Brahea armata, in the Northern Baja California peninsula, by evaluating (i) the levels of blue fan palm seed removal by vertebrates at two spatial scales and the initial fate of dispersed seeds, (ii) the spatial distribution and association of seedlings and adults at two spatial scales, (iii) seed removal levels and seedling densities based on density and distance to adult palm trees, and (iv) the population age structure. Overall, seed removal levels were low at all sites, varied at regional but not at local scales, with small rodents apparently being responsible of most removals. Adult density and distribution differed among oases, but seedling densities did differ locally. Whereas seed removal did not vary with distance from seed sources or parent trees, a weak positive association between seedlings and adults at the whole patch level indicated that establishment tended to occur in or near those grid cells where adults had established successfully. However, the analysis showed a negative association between seedling and adult densities within the patches, indicating that within the grid cells where growth was most successful, seedlings established preferentially in relatively open spaces. All life-history categories were adequately represented within each site and populations seemed to be in a fairly good conservation state. We did not find evidence to establish that post-dispersal seed removal by rodents is a main factor defining the recruitment patterns in these oases. In contrast, the effect of flood pulses seems to be significant and may have strong and conclusive effects on palm seedling distributions. We suggest that local biotic (nurse plants) and/or abiotic (canyon physiography, nurse objects) factors at each particular canyon have the potential to affect post-dispersal seed removal activity patterns by rodents, as well as to provide vital protection for palm seedling establishment from the extreme floods. This baseline information will help in further investigations on the possible mechanisms that sustain palm populations at their distribution limits and in stressful environments.