Hybridization of Non-Native Dune-Building Beachgrasses on the U.S. Pacific Northwest Coast: Characterization of Functional Morphology, Hybrid Swarm Composition, and Ecological Consequences of Ammophila arenaria × A. breviligulata

The rates of biological introductions and invasions are increasing, driving up the associated harms to ecosystems and economies. The spread and effects of invasive organisms depend on the specifics of the introduction, the character of the invaded ecosystem, and multiple traits of the invasive organism itself. One mechanism by which invasive organisms, especially plants, can develop the traits necessary to be successful invaders is hybridization. In this dissertation I describe the functional morphology and population genetics of a newly discovered hybrid beachgrass (Ammophila arenaria × breviligulata) and assess its ecological interactions with its parent species. Two species of non-native beachgrasses, Ammophila arenaria and A. breviligulata, were introduced to the US Pacific Northwest in the early 1900s to stabilize what was a dynamic shifting sandy environment. These beachgrasses rapidly colonized the sandy beaches in Oregon and Washington, which make up 45% of the coastline, stabilizing sand and building tall, linear dunes that provide ecosystem services of coastal protection and carbon sequestration. Additionally, the beachgrasses displaced some native endemic plants and contributed to the loss of habitat for shorebirds. The two beachgrasses have different ranges and biophysical effects on coastal dunes: A. arenaria is the sole beachgrass species from central Oregon to the south, where it builds tall narrow dunes, while A. breviligulata is the dominant beachgrass species from central Oregon through central Washington, where it builds shorter wider dunes. The distributions of the two species overlap on the sandy beaches of central Oregon through Washington. Although the congeners are evolutionarily diverged, it was recently discovered that they are hybridizing in their range overlap in the Pacific Northwest coast. Using a combination of observations and experiments, I examine how the distribution and abundance, population genetics, functional morphology traits, and competitive ability of the novel hybrid may affect its invasion trajectory and ultimately the ecosystem structure, function, and services of Pacific Northwest dunes. In Chapter 2, I describe for the first time the genotypes, genome size, and functional morphology A. arenaria × A. breviligulata in comparison to its parent species. I find that all tested hybrid individuals were first generation crosses between the two parent species and that none of the hybrids had undergone polyploidy. Additionally, I find that the hybrids expressed some exceptional morphological traits that could increase sand capture and thus impact dune shape and associated ecosystem functions and services. In Chapter 3, I explore the population genetic diversity, parentage and direction of hybridization, and recombinant types present in the hybrid population. I determine that the hybrid population is made up of 56% first generation individuals and 42% second generation or later generation individuals, indicating that the hybrid is fertile and reproductively compatible with other hybrid individuals. I document the presence of one A. arenaria backcross out of 43 individuals, suggesting that the hybrid is able to be reproductively compatible with at least one parent species. Finally, I find that either beachgrass species can act as the maternal parent such that there are no barriers to hybridization, or to hybrid fertility, based on sex of the parent species. Thus, we show that a hybrid swarm (i.e., a population of fertile hybrids producing later generation hybrids and backcrosses) has developed and that the consequences for Pacific Northwest dunes could be far-reaching. Finally, in Chapter 4, I assess the intrinsic morphological differences between, and ecological interactions of, the hybrid and its parent species using a common garden experiment. This large-scale species interaction experiment revealed that the hybrid grows taller, more densely, and at a higher biomass than either parent species. The hybrid also successfully competes with both parents, causing declines in stem density and morphology. The hybrid is forecast to competitively exclude A. breviligulata and to exist in an unstable equilibrium with A. arenaria. We predict that the hybrid will increase in abundance on the Pacific Northwest coastline and will build dunes that are taller and more stable than either parent species with consequences for coastal protection, carbon sequestration, and biodiversity conservation. Overall, the findings of this dissertation describe a novel beachgrass hybrid A. arenaria × A. breviligulata that could shape the evolution and invasion trajectory of the Ammophila species in the Pacific Northwest. Future research should continue to monitor the hybrid to assess changes in the distribution and abundance, population genetics and genome size, and ecological impacts of this emerging invader. Exploring the consequences of hybridization on invasion success can not only serve to better explain the evolution of novel invaders but also to help guide future management actions

Discovery of a dune-building hybrid beachgrass (Ammophila arenaria × A. breviligulata) in the U.S. Pacific Northwest

The production of novel hybrid zones is an ecologically important consequence of globally increasing rates of species introductions and invasions. Interspecific hybridization can facilitate gene flow between parent species or produce novel taxa that may alter invasion dynamics or ecosystem services. The coastal sand dunes of the U.S. Pacific Northwest coast are densely populated by two non-native, congeneric, dune-building beachgrasses (Ammophila arenaria and A. breviligulata). Here, we present morphological, cytological, and genetic evidence that the two beachgrass species have hybridized in this globally unique range overlap. The A. arenaria × A. breviligulata hybrid has been found at 12 coastal sites in Washington and Oregon. It is a first-generation hybrid between the beachgrass species as evidenced by genome size comparisons and single nucleotide polymorphism genotyping. It is intermediate between the parent grasses in many morphological characters but exceeds both parents in shoot height, a trait associated with dune-building potential. Understanding the ecological and population genetic consequences of this novel hybridization event is of the utmost importance in a system where any change in dominant beachgrass species can have large effects on both biodiversity management and coastal protection

Species-Specific Functional Morphology of Four US Atlantic Coast Dune Grasses: Biogeographic Implications for Dune Shape and Coastal Protection

Coastal dunes arise from feedbacks between vegetation and sediment supply. Species-specific differences in plant functional morphology affect sand capture and dune shape. In this study, we build on research showing a relationship between dune grass species and dune geomorphology on the US central Atlantic Coast. This study seeks to determine the ways in which four co-occurring dune grass species (Ammophila breviligulata, Panicum amarum, Spartina patens, Uniola paniculata) differ in their functional morphology and sand accretion. We surveyed the biogeography, functional morphology, and associated change in sand elevation of the four dune grass species along a 320-kilometer distance across the Outer Banks. We found that A. breviligulata had dense and clumped shoots, which correlated with the greatest sand accretion. Coupled with fast lateral spread, it tends to build tall and wide foredunes. Uniola paniculata had fewer but taller shoots and was associated with ~42% lower sand accretion. Coupled with slow lateral spread, it tends to build steeper and narrower dunes. Panicum amarum had similar shoot densities and associated sand accretion to U. paniculata despite its shorter shoots, suggesting that shoot density is more important than morphology. Finally, we hypothesize, given the distributions of the grass species, that foredunes may be taller and wider and have better coastal protection properties in the north where A. breviligulata is dominant. If under a warming climate A. breviligulata experiences a range shift to the north, as appears to be occurring with U. paniculata, changes in grass dominance and foredune morphology could make for more vulnerable coastlines

Literature-based latitudinal distribution and possible range shifts of two US east coast dune grass species (Uniola paniculata and Ammophila breviligulata)

Previous work on the US Atlantic coast has generally shown that coastal foredunes are dominated by two dune grass species, Ammophila breviligulata (American beachgrass) and Uniola paniculata (sea oats). From Virginia northward, A. breviligulata dominates, while U. paniculata is the dominant grass south of Virginia. Previous work suggests that these grasses influence the shape of coastal foredunes in species-specific ways, and that they respond differently to environmental stressors; thus, it is important to know which species dominates a given dune system. The range boundaries of these two species remains unclear given the lack of comprehensive surveys. In an attempt to determine these boundaries, we conducted a literature survey of 98 studies that either stated the range limits and/or included field-based studies/observations of the two grass species. We then produced an interactive map that summarizes the locations of the surveyed papers and books. The literature review suggests that the current southern range limit for A. breviligulata is Cape Fear, NC, and the northern range limit for U. paniculata is Assateague Island, on the Maryland and Virginia border. Our data suggest a northward expansion of U. paniculata, possibly associated with warming trends observed near the northern range limit in Painter, VA. In contrast, the data regarding a range shift for A. breviligulata remain inconclusive. We also compare our literature-based map with geolocated records from the Global Biodiversity Information Facility and iNaturalist research grade crowd-sourced observations. We intend for our literature-based map to aid coastal researchers who are interested in the dynamics of these two species and the potential for their ranges to shift as a result of climate change