EFFECTS OF BIRD INGESTION ON SEED-GERMINATION OF SORBUS-COMMIXTA

To determine the effects of ingestion by birds on seed germination, we performed germination experiments in the field and laboratory with So rbus commixta. The germination of four groups of seeds was compared: i ngested seeds, seeds defecated in feces after feeding of fruits to bir ds; extracted seeds, seeds deliberately extracted from the fruit pulp; juiced seeds, seeds plus the juice of the pulp after seeds had been d eliberately extracted from the pulp; intact seeds, seeds in untreated intact fruits. In the laboratory, intact and juiced seeds hardly germi nated, but ingested and extracted seeds germinated. Thus, the pulp and its juice appeared to inhibit germination, but seeds could germinate without ingestion by birds once the seeds had been manually extracted from the pulp. In the field, intact fruits did not germinate in the fi rst spring, because the seed was still covered with pulp. The pulp of intact seeds decomposed during the first summer, and thus, the seeds h ad the potential to germinate during the second spring. In fact, most intact seeds do not germinate during the second spring either, since t hey lose their viability during the first summer. Thus, under natural conditions, most seeds of Sorbus commixta cannot germinate without bir d ingestion.