THE INFLUENCE OF CACTUS SIZE AND PREVIOUS INFECTION ON BIRD DEPOSITION OF MISTLETOE SEEDS

Tristerix aphyllus (Loranthaceae) is a bird-dispersed mistletoe that i nfects several cactus species in central and north-central Chile. At L as Chinchillas National Reserve it parasitizes two species of columnar cacti (Echinopsis chilensis and Eulychnia acida) and its seeds are di spersed exclusively by the Chilean mockingbird Mimus thenca. The preva lence of T. aphyllus on cacti depended on host species and size. Echin opsis chilensis was parasitized at a higher, and E. acida at a lower, frequency than expected from their relative abundances. In both specie s, the incidence of T. aphyllus parasitism increased with cactus heigh t. Birds perched and deposited seeds primarily on parasitized cacti, w hich resulted in an aggregated distribution of seeds. By preventing bi rd perching and thus reducing seed deposition, the long spines of E. c hilensis and E. acida may help protect: cacti against T. aphyllus infe station. However, the seeds of T. aphyllus produced a slow-growing rad icle that reached up to 91 mm long in 43 days. Radicles of this length are unique among mistletoe species and may have evolved to overcome t he barrier posed by cactus spines.