Background/Question/Methods
Although divergence in floral traits within a species may be constrained by pollinator-mediated stabilizing selection, floral characters also can vary intraspecifically. Such variation may be due to directional selection or to phenotypic plasticity; local resource availability may influence floral size and abundance, for example. Dirca palustris, a deciduous shrub in understories of forests in eastern North America, forms petite yellow flowers early in spring, before emergence of leaves of most deciduous species in the overstory. Because D. palustris occurs at sites low in irradiance, grows slowly, and flowers by using energy stored from previous seasons, we asked whether vegetative growth and floral display compete for limited solar resources. Vegetative growth and the size and number of flowers were assessed in 2008 for 150 plants, 30 indigenous in each of five populations that spanned the distribution of the species along a northwest to southeast transect (northern North Dakota, southwestern Wisconsin, southern Illinois, northeastern Alabama, and northwestern Florida). Anthesis varied temporally from early February in Florida to early May in North Dakota. 
Results/Conclusions
Annual stem elongation and inflorescence count were correlated positively with percentage of open-sky radiation received by plants in Alabama and Florida, the two populations most exposed to radiation (mean radiation 10.4% vs. 3.9% among the other populations). Although D. palustris is shade-tolerant, number of inflorescences on deeply shaded plants in Alabama and Florida was restricted. Data from all five populations showed calyx length was correlated negatively with inflorescence count, with regression predicting a 35% decrease in calyx length from the maximum to the minimum inflorescence counts we observed. An increase in calyx length with decreasing inflorescence count might represent a mechanism for maintaining sufficient floral display for the attraction of pollinators despite insufficient resources for production of additional inflorescences. Inflorescence count was correlated positively with stem elongation the previous year, evidence that vegetative growth and the production of inflorescences are not directly competitive. Differences observed in inflorescence count, calyx length, calyx width, and protrusion of pistils and stamens among populations suggest divergence in floral characters. Such divergence could be due to nonadaptive genetic drift or to selection by pollinators that vary in size or morphology; it also may represent a plastic response to contrasting environments.