Density-Dependent Pollination and Germination in the Patchy Vernal Pool Species Lasthenia californica

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International Journal of Plant Sciences

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Premise of research. The response of animal pollinators to visual characteristics of plants is crucial to understanding how plant-pollinator interactions contribute to plant reproductive processes. For plant species that aggregate in patches, the spatial distribution and density of flowering plants in and around patches may affect pollinator visitation to a greater degree than the characteristics of individual flowers do. We examine how patch characteristics of the self-incompatible, vernal pool species Lastheniacalifornica (California goldfields) impact pollinator visitation, using pollen load size and pollen germination rates to infer visitation patterns.Methodology. Flower density and the size and distribution of patches were estimated from aerial images captured during drone surveys of an upland prairie ecosystem in southern Oregon. Pollen load sizes and the number of germinated grains were quantified using Alexander stain. We assessed the effects of flower density on pollen deposition and the number of germinated grains for increasing neighborhood areas using multiple regression models.Pivotal results. We found that pollen deposition on stigmas decreased with flower density within a 3- to 20-m-radius neighborhood area. Pollen germination frequency increased with the density of flowers within an individual patch, but the frequency decreased with total flower density within a 0.5- to 1-m radius around the patch.Conclusions. Our results imply that increasing flower density can have negative consequences for pollination services, which could result in reduced seed set when pollinators are scarce. Although pollen deposition appears to increase in dense patches, as neighborhood density around the focal patch increases, pollen deposition rates decline, which indicates a lower pollinator visitation rate. When neighboring flower patch density is higher, pollinators probably fly shorter distances and consequently deposit higher proportions of incompatible pollen on stigmas. Pollen deposition and germination patterns demonstrate patch and neighborhood size and density-dependent effects on plant reproductive success.

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