Germination niche breadth of invasive Iris pseudacorus (L.) suggests continued recruitment from seeds with global warming

Authors: GILLARD, MORGANE - University Of California, Davis; CASTILLO, JESUS - University Of Seville; MESGARAN, MOHSEN - University Of California, Davis; Futrell, Caryn; Grewell, Brenda

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2022
Publication Date: 6/18/2022
Citation: Gillard, M.B., Castillo, J.M., Mesgaran, M.B., Futrell, C.J., Grewell, B.J. 2022. Germination niche breadth of invasive Iris pseudacorus (L.) suggests continued recruitment from seeds with global warming. American Journal of Botany. 109(7):1108-1119. https://doi.org/10.1002/ajb2.16026.
DOI: https://doi.org/10.1002/ajb2.16026

Interpretive Summary: Wetland ecosystems provide critical ecosystem services such as flood water attenuation and biogeochemical cycling of nutrients. The world’s wetlands are negatively impacted by biological invasions and climate change. Iris pseudacorus, an emergent macrophyte, has established invasive populations across a broad global range that reduce biodiversity in wetland ecosystems. In northern California, the recent spread of I. pseudacorus from the Sacramento – San Joaquin Delta to multiple sites in the sensitive tidal wetlands of Suisun and Carquinez Straits of the San Francisco Estuary represents a concern considering the invasiveness of the species and the vulnerability of these ecosystems. Understanding recruitment processes of invasive species under a range of environmental conditions is central to conservation strategies and invasion management. Environmental temperature is the primary regulator of seed germination. Seed dormancy and germination responses of wetland macrophytes are expected to change in response to global climatic warming. The potential for increased germination capacity, seed dispersal and geographic range expansion of invasive plant species is a particular concern relevant to the conservation of native biodiversity. A functional view of seed ecology and germination is often ignored relative to the invasion ecology of wetland macrophytes, though long-distance seed dispersal often explains their spread. In this study, we investigated the germination characteristics of seeds sourced from five populations of invasive I. pseudacorus located in intertidal habitats along the species’ current estuarine distribution in the Sacramento-San Joaquin River Delta-San Francisco Estuary, and separated by 35-40 km. We explored the germination requirements of I. pseudacorus populations to various environmental factors, including responses to cold and warm temperature stratification, presence or absence of seed coat, presence or absence of light, and to eight constant temperatures and two alternating temperature regimes. Seeds from invasive I. pseudacorus populations in California did not require cold or warm stratification to germinate. Seed germination was not impacted by the presence or absence of the seed coat, and germination occurred in the dark, although germinability was two to three-fold times greater under light. At constant temperature, germination was optimal at +28 °C. Seeds exposed to +36 °C presented >10% germinability, and embryos of ungerminated seeds presented 76% viability. Overall, germinability remained relatively low at constant temperatures (= 25%), but was greatly enhanced under alternating daily temperatures. Exposure to diurnally fluctuating temperatures best supported germination of this species. The results improve knowledge of the seed ecology of I. pseudacorus relative to invasion risk, and reveal the invader’s ability to maintain germination under a wide variety of conditions, including high temperatures more frequent with global warming.

Technical Abstract: Iris pseudacorus, an emergent macrophyte, has established invasive populations across a broad global range that reduce biodiversity in wetland ecosystems. In northern California, the recent spread of I. pseudacorus from the Sacramento – San Joaquin Delta to multiple sites in the sensitive tidal wetlands of Suisun and Carquinez Straits of the San Francisco Estuary represents a concern considering the invasiveness of the species and the vulnerability of these ecosystems. Understanding recruitment processes of invasive species under a range of environmental conditions is central to conservation strategies and invasion management. A functional view of seed ecology and germination is often ignored relative to the invasion ecology of wetland macrophytes, though long-distance seed dispersal often explains their spread. In this study, we investigated the germination characteristics of seeds sourced from five populations of invasive I. pseudacorus located in intertidal habitats along the species’ current estuarine distribution in the Sacramento-San Joaquin River Delta-San Francisco Estuary, and separated by 35-40 km. We explored the germination requirements of I. pseudacorus populations to various environmental factors, including responses to cold and warm temperature stratification, presence or absence of seed coat, presence or absence of light, and to eight constant temperatures and two alternating temperature regimes. Seeds from invasive I. pseudacorus populations in California did not require cold or warm stratification to germinate. Seed germination was not impacted by the presence or absence of the seed coat, and germination occurred in the dark, although germinability was two to three-fold times greater under light. At constant temperature, germination was optimal at +28 °C. Seeds exposed to +36 °C presented >10% germinability, and embryos of ungerminated seeds presented 76% viability. Overall, germinability remained relatively low at constant temperatures (= 25%), but was greatly enhanced under alternating daily temperatures. Exposure to diurnally fluctuating temperatures best supported germination of this species. The results improve knowledge of the seed ecology of I. pseudacorus relative to invasion risk, and reveal the invader’s ability to maintain germination under a wide variety of conditions, including high temperatures more frequent with global warming.