Abstract
The effects of pollen limitation on reproductive success in plants have been well-documented using pollen supplementation experiments. However, the role of local demographics in determining pollen limitation, particularly in terms of the additive and interactive effects of pollen availability and competition are not well known. We measured fruit set in the dioecious shrub Canada buffaloberry (Shepherdia canadensis) in Central Alberta, Canada to evaluate whether local demographics measured at three spatial scales (25, 50, and 100 m2) affect fruit set in buffaloberry. We test whether density-dependence (population density), pollen donor (measured as male density, distance to nearest male plant and size of nearest male plant), female competitor (measured as female density and distance to nearest female plant), or the combined pollen donor and competitor hypotheses best explain natural variations in fruit set for a population of Canada buffaloberry. Support was highest for the combined pollen donor and competitor hypothesis at an intermediate spatial scale of 50 m2. Proportion fruit set increased with male shrub density (pollen donors) and decreased with female shrub density (pollen competitors), but was more affected by the presence of males than females. This illustrates that access to male shrubs within a 3.99 m radius affects pollen availability, while nearby females compete intra-specifically for pollen.
Highlights
Fruit set is commonly limited by pollen availability, in dioecious species due to the isolation of male and female reproductive organs (Burd, 1994; Knight et al, 2005)
Pollen quantity is known to be related to distance to nearest conspecific male plant (Kay et al, 1984; De Jong, Batenburg & Klinkhamer, How to cite this article Johnson and Nielsen (2014), Demographic effects on fruit set in the dioecious shrub Canada buffaloberry (Shepherdia canadensis)
The most supported candidate models explaining fruit set in buffaloberry were the pollen donor and competitor hypothesis (+♂D −♀D) at the 50 m2 scale and the pollen donor hypothesis again at the 50 m2 scale (+♂D50) (Akaike weights, wi = 0.244 and 0.089 respectively; Table 3) supporting both the pollen donor and female competitor hypotheses
Summary
Fruit set is commonly limited by pollen availability, in dioecious species due to the isolation of male and female reproductive organs (Burd, 1994; Knight et al, 2005). Demographic factors have been shown to play a role in determining fruit set through regulation of the quantity and quality of pollen available to females (House, 1992; Kunin, 1993; Knight et al, 2005). Pollen quantity is known to be related to distance to nearest conspecific male plant (Kay et al, 1984; De Jong, Batenburg & Klinkhamer, How to cite this article Johnson and Nielsen (2014), Demographic effects on fruit set in the dioecious shrub Canada buffaloberry (Shepherdia canadensis). The role of female competitors in regulating pollen availability, as well as the additive effects of pollen donation from males and competition from females, has received much less attention
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