Temporal Organization of Flowering Among the Hummingbird Foodplants of a Tropical Wet Forest

Abstract

This paper seeks to elucidate causal factors in the temporal organization of blooming patterns, both within and between years, of a coadapted group of plants sharing a group of pollinators: the hummingbird flowers of Finca La Selva, Costa Rica. Quanititative data on flowering of 42 species over a four-year period are presented, with supplementary data on an additional 17 species. Peaks of flower availability for hummingbirds occur in the dry and early wet seasons; a severe and predictable flower shortage occurs in November and December, the wettest months of the year. This pattern is repeated in several other tropical areas, among hummingbird foodplants and the plant communities in general. A severe drought in March-April 1973 severely affected flowering of many species for up to four months thereafter; some species were noticeably affected as much as a year later. Hummingbird foodplants of large light gaps and second growth have longer peak blooming periods than do those of forest understory; vines bloom for shorter periods than do other growth forms. Mean year-to-year variation in various temporal parameters of flowering was about two months among La Selva hummingbird flowers, and about one month among those of a California locality. If, as in other groups of organisms, the action of photoperiod is to define the time span within which plants may be stimulated to flower by other factors like rainfall, then La Selva plants respond to a daylength change about half as great as do California plants. Two mechanisms may help maintain a regular sequence of blooming peaks from year to year in the face of annual variations in rainfall: displacement of the entire sequence, and opposite and compensating responses of individual plant species. In California and probably in tropical dry forest, the first mechanism predominates; at La Selva, both mechanisms may be important.