Reproductive biology of Larrea tridentata : A comparison between core shrubland and isolated grassland plants at the Sevilleta National Wildlife Refuge, New Mexico

Expansion of diploid creosote shrubs (Larrea tridentata (Sesse & Moc. ex DC.) Coville)) into grassland sites occurs exclusively through seed production. We compared the reproductive biology of Larrea shrubs located in a Chihuahuan desert shrubland with isolated shrubs well-dispersed into the semiarid grasslands at the Sevilleta National Wildlife Refuge. Specifically, we examined (1) reproductive success on open-pollinated branches, (2) the potential of individual shrubs to self-pollinate, and (3) bee pollinator guild composition at shrubland and grassland sites. Sampling of the bee guild suggests that there are adequate numbers of pollinators at both locations; however, the community composition differs between shrub and grassland sites. More Larrea specialist bee species were found at the shrubland site as compared with the isolated shrubs. Large numbers of generalist bees were found on isolated grassland bushes, but their efficiency in pollinating Larrea is currently unknown. Higher percent seed fill of unbagged, open-pollinated shrubs at the shrubland site, compared with isolated grassland shrubs (76 versus 57 percent) suggests that bee specialists may increase plant pollination success. Isolated grassland shrubs varied greatly in the number of seeds produced in pollinator-exclusion bags, whereas the number of self-pollinated seeds produced by shrubland plants was more uniform. Overall, the difference in seed produced by bagged and unbagged branches of isolated shrubs was much less than the difference produced by plants located at the shrubland site. These trends will be explored in greater detail in future years.