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Flowering Phenologies in a Shrub Community: Competition and Constraints

Beverly Rathcke
Journal of Ecology
Vol. 76, No. 4 (Dec., 1988), pp. 975-994
DOI: 10.2307/2260627
Stable URL: http://www.jstor.org/stable/2260627
Page Count: 20
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Flowering Phenologies in a Shrub Community: Competition and Constraints
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Abstract

(1) Flowering phenologies of fourteen shrub species were recorded for five years in The Great Swamp, Rhode Island, U.S.A. Flowering phenologies were significantly aggregated and average overlap per species was $> 1.0$. Ten species flowered during June and early July and only one species flowered in late July-August. When this last species was excluded and a shorter flowering season was used in the analyses, flowering was significantly aggregated in three of the five years and significantly less than random in two years. Observed overlaps were always much greater than those calculated from an even dispersion of flowering times. Flowering sequences of species were highly correlated among years so consistent interactions were possible. (2) The lack of July flowering is perplexing, especially as this could decrease flowering overlap. Climatic conditions seem favourable, pollinators are abundant if plants are available, and time is available for fruiting, especially for wind-dispersed species. This pattern is found for shrubs throughout New England and the reason for this common constraint remains to be determined. (3) The shrubs seemed likely to compete for pollination because most species overlapped in flowering, depended upon insect pollinators for seed set and shared bumblebees or small, solitary bees. If the species were divided into the two major pollination guilds (bumblebee-pollinated or small bee-pollinated), flowering remained either aggregated or random in different years and average overlap per species was high (0.5-0.9). Of the six congeneric species pairs, only two pairs had little overlap ($< 0.1$), three had high overlap (0.5-0.9), and one was intermediate (0.1-0.2) during the five years. (4) Competition for pollination seemed likely and pollination limitation of fruit set was detected in four of the nine species tested; however, pollination limitation could be attributed to interspecific competition in only one species. The high overlap in flowering and pollinator use can be maintained because current competition for pollination is rare or mild. (5) Although competition is generally assumed to select for divergence and specialization in resource use, avoidance of competition by these shrubs was achieved by three major characteristics: self-pollination, long-lived flowers and diverse pollinator assemblages. Possible constraints on divergence are discussed. Divergence in time and specialization for pollinators increased, rather than decreased, the risk of pollination limitation.

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