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

Abstract

Premise Understanding recruitment processes of invasive species is central to conservation and management strategies. Iris pseudacorus, an emergent macrophyte, has established invasive populations across a broad global range, and reduces biodiversity in wetland ecosystems. Climate change is altering germination cues, yet studies on the invasion of wetland macrophytes often ignore germination ecology despite its importance to their establishment and spread. Methods We explored germination of seeds from invasive I. pseudacorus populations in California in response to seed coat presence or absence, and several environmental factors. Using experimental results in a thermal time model, we derived germination temperature thresholds. Results Germination of I. pseudacorus seeds did not require cold or warm stratification, and was not affected by seed coat presence or absence. Germination occurred in the dark, although germinability was two‐ to threefold times greater under light. At constant temperature, thermal time model estimates included 18.3 ± 1.8°C base germination temperature ( T b $({T}_{b}$); 28.2 ± 0.5°C optimal temperature ( T o $({T}_{o}$); and 41.0 ± 1.7°C ceiling temperature ( T c $({T}_{c}$). Seeds exposed to 36.0°C achieved over 10% germination, and embryos of ungerminated seeds presented 76% viability. Overall, germinability remained relatively low at constant temperatures (≤25%) but was close to 90% under alternating daily temperatures. Conclusions Exposure to diurnally fluctuating temperatures is essential for this species to achieve high germination rates. Our study reveals that I. pseudacorus has a broad germination niche supporting its establishment in a relatively wide range of environments, including at high temperatures more frequent with climate change.

Wrack Burial Limits Germination and Establishment of Yellow Flag Iris (Iris pseudacorus L.)

“…Iris pseudacorus shows increases in its germination percentage by ca. 70% from daily constant to alternating temperatures [ 39 ]. The drastic limitation in seedling emergence we observed occurred even though I. pseudacorus seedlings emerged along a single axis, relying on the elongation of the mesocotyl, an anatomical feature of monocots.…”

“…The ability of a few I. pseudacorus seedlings to emergence from wrack burial depths as high as 8 cm contrasts with other emergent monocot species, such as Spartina densiflora Brongn., a wetland grass that has much smaller seeds and is not able to emerge from wrack depths higher than 1 cm [ 50 ]. Moreover, wrack burial increased the time to first seedling emergence of I. pseudacorus , which could be related to a longer time lapse to grow through the wrack from deeper depths and to an increase in the germination period of this species at lower temperatures [ 39 ]. Additionally, the seedling growth rate increased with wrack burial depth as reported previously for sediment burial due to stimulation of root growth that could help seedlings absorb more nutrients [ 51 ].…”

“…In total, 20 containers (5 treatments × 4 replicates) were set up in a randomized complete block design. Because I. pseudacorus shows its highest germination percentages under high humidity conditions [ 39 ], containers were carefully irrigated twice a week to ensure the substrate remained waterlogged. Iris pseudacorus is sensitive to salinity [ 38 ].…”

“…The experiment continued until no additional emergence was observed in the control treatments for at least 30 days. This duration of the experiment was the usual time needed for the germination of I. pseudacorus in freshwater conditions [ 38 , 39 ].…”

“…The mass of I. pseudacorus seeds is relatively high, ca. 60–70 mg dry weight (DW), and their maximum germination percentages are achieved in freshwater and light conditions under daily alternating temperatures [ 38 , 39 , 40 ]. Large-seeded species may be less affected by wrack burial because large-buried seeds that germinate have more reserves needed by seedlings to reach the soil surface [ 4 , 41 , 42 ].…”