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Abstract Invasive plants formed via hybridization, especially those that modify the structure and function of their ecosystems, are of particular concern given the potential for hybrid vigor. In the U.S. Pacific Northwest, two invasive, dune‐building beachgrasses, Ammophila arenaria (European beachgrass) and A. breviligulata (American beachgrass), have hybridized and formed a new beachgrass taxa (Ammophila arenaria × A. breviligulata), but little is known about its distribution, spread, and ecological consequences. Here, we report on surveys of the hybrid beachgrass conducted across a 250‐km range from Moclips, Washington to Pacific City, Oregon, in 2021 and 2022. We detected nearly 300 hybrid individuals, or an average of 8–14 hybrid individuals per km of surveyed foredune. The hybrid was more common at sites within southern Washington and northern Oregon where A. breviligulata is abundant (75%–90% cover) and A. arenaria is sparse and patchy. The hybrid displayed morphological traits such as shoot density and height that typically exceeded its parent species suggesting hybrid vigor. We measured an average growth rate of 30% over one year, with individuals growing faster at the leading edge of the foredune, nearest to the beach. We also found a positive relationship between hybrid abundance and A. arenaria abundance, suggesting that A. arenaria density may be a controlling factor for hybridization rate. The hybrid showed similar sand deposition and associated plant species richness patterns compared with its parent species, although longer term studies are needed. Finally, we found hybrid individuals within and near conservation habitat of two Endangered Species Act‐listed, threatened bird species, the western snowy plover (Charadrius alexandrinus nivosus) and the streaked horned lark (Eremophila alpestris strigata), a concern for conservation management. Documenting this emerging hybrid beachgrass provides insights into how hybridization affects the spread of novel species and the consequences for communities in which they invade.

Abstract 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.