Semester

Summer

Date of Graduation

2023

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

Stephen DiFazio

Committee Member

Jonathan Cumming

Committee Member

Donna Ford-Werntz

Abstract

Salix nigra is a common tree-form willow found throughout the eastern United States and Canada in riparian and wetland habitats. Like other members of the Salicaceae, it is dioecious with separate male and female individuals. While Salix has been thought to be strictly insect pollinated and Populus wind pollinated, several species of Salix have had wind pollination present in varying amounts by species. S. nigra’s pollination syndrome has not been studied extensively, though it is assumed to be insect pollinated. Unlike other willows with wind pollination, S. nigra is basal on the Salix phylogenetic tree. This study was conducted in a fragmented riparian forest in West Virginia University’s Core Arboretum. Other aspects of S. nigra reproductive biology were also studied, such as pollen: ovule ratio and pollen limitation. Pollen: ovule ratio can be indicative of pollination syndrome, and pollen limitation can vary based on pollination syndrome as well but can also be used to explain if dioecy can be maintained. Our research determined that S. nigra is likely insect pollinated by using pollinator exclusion bags on flowering catkins. A low pollen:ovule ratio of 260 also gives evidence for insect pollination. Pollen limitation was not found during this study, indicating the efficiency of pollinators in this population and a lack of pressure to change from a dioecious state. In addition, gene flow and factors influencing male reproductive success were measured to help explain how and why S. nigra is pollinated and which individuals produce offspring through a paternity analysis. Pollination within the population was influenced by distance between mother and father trees with other measured factors not having a significant differences when comparing all offspring and potential fathers within the defined population. Assigned paternities ranged up to 510 meters away from a mother tree and 12% of resulting offspring were due to gene flow from unidentified fathers outside of the sampled population. S. nigra pollinators are therefore capable of traveling great distances and linking together fragmented populations of S. nigra. Additional studies are needed to understand S. nigra pollinator foraging behaviors and its potential impacts on pollination and gene flow.

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