Factors affecting the circular distribution of the leafless mistletoe Tristerix aphyllus (Loranthaceae) on the cactus Echinopsis chilensis

We describe the pattern of emergence of the holoparasitic mistletoe Trister ix aphyllus from its cactus host Echinopsis chilensis in a semiarid Chilean ecosystem. The observed circular distribution of the parasite inflorescenc e differed significantly from a uniform distribution based on a random proc ess. We quantified the circular distribution of the seeds defecated on the cactus surface by Mimus thenca, the only bird responsible of seed dispersal . Our data did not support the idea of a directional seed deposition by the bird. To test the hypothesis that the observed circular distribution can b e attributable to a differential seed survival due to microsite temperature variation, we infected cacti with seeds of T. aphyllus every 30 degrees an d quantified the temperature associated to each angle. Our results revealed that even though seeds located in angles with higher sun exposure had the lowest haustorial disk formation, this variation in mortality is not suffic ient to explain the angular polarity observed in this species. Inspection o f inflorescences of T. aphyllus that emerged 17 months after the experiment al infection, revealed mean angular values indistinguishable from the natur al circular distribution. Assessment of the anatomical structure at two opp osing angles of the cactus revealed striking differences in epidermal const itution, such as a four-fold thicker epidermis in north than in south facin g samples due to formation of highly lignified bark. We suggest that bark f ormation is likely the most important factor determining the biased circula r distribution of T. aphyllus.