Flowering phenology and reproductive output in two sister species of Ferocactus (Cactaceae)

Flowering phenology is often strongly constrained by phylogenetic history: many closely-related plants have very similar phenologies. On the other hand, divergent flowering phenologies can function as isolating mechanisms, which may be reinforced if related plants occur sympatrically. I investigated flowering phenology and reproductive output of sister species of barrel cacti, Ferocactus cylindraceus and F. wislizeni, where they occur sympatrically in the Sonoran Desert surrounding Tucson, Arizona. Ferocactus cylindraceus began blooming in May, and continued until early or mid-October, with a bimodal pattern of flowering amplitude. Individuals in the study population were moderately well-synchronized phenologically. Ferocactus wislizeni began blooming in July, and also continued until early or mid-October, with a single peak of intensity; individuals in the study population were well- synchronized phenologically. In both species, the vast majority of individuals bloom every year. Plant size was positively correlated with flowering amplitude in both species, and with flowering onset in F. wislizeni. The study population of F. cylindraceus was strongly affected by a flower-eating caterpillar in all years, with the earliest flowers most likely to be destroyed. For F. wislizeni, seed number per fruit was highest for flowers open in the middle of the blooming season in 1998. Other components of individual plant phenology, including among-plant synchrony, had little influence on reproductive output.