Propagule and seedling responses of three species naturalised in subtropical South Africa to elevated temperatures

- Seed traits of all species promote persistent seed banks but not seedling vigour.
- Elevated temperatures enhanced germination capacity and velocity in all species.
- Seedling growth responses to elevated temperature differed across species.
- Growth patterns that enhance productivity promoted survival at high temperatures.
- Not all species naturalised in warm areas will11 thrive under elevated temperatures.

The predicted 2-5 °C increase in global temperatures by the end of this century could enhance the invasive potential of some alien plant species. It is unclear whether aliens that originate from and/or are naturalised in warm areas are predisposed to respond positively to this increase. This study assessed the propagule and seedling responses of three alien plant species (viz. Oenothera biennis, Syncarpia glomulifera and Petiveria alliacea) naturalised in a subtropical region in South Africa to elevated temperatures. The species were characterised in terms of propagule (seed/achene) traits related to invasiveness before these were sown at ambient (daily maximum: 26.2 ± 0.3 °C; daily minimum: 17.9 ± 0.2 °C) and elevated (daily maximum: 30.6 ± 0.5 °C; daily minimum: 21.2 ± 0.3 °C) temperatures to assess germination capacity and rate. Seedlings subsequently produced were grown at these temperatures to assess growth and performance. Propagules of all species possess traits that facilitate persistent seed banks but their small size may not favour seedling survival under harsh conditions. Only O. biennis and P. alliacea exhibited high seed viability but elevated temperatures enhanced germination capacity and rate in all species. Seedlings of both O. biennis and S. glomulifera exhibited signs of stress at elevated temperatures (e.g. reduced biomass) though. However, while seedling production and survival were reduced at elevated temperatures in O. biennis, these parameters were enhanced at elevated temperatures in S. glomulifera (possibly due to increased shoot emergence rate). Elevated temperatures did not affect seedling production in P. alliacea but did enhance seedling survival, possibly by inducing biomass allocation patterns that enhance productivity. The ability of naturalised species to become invasive in a changing climate may be more dependent on phenotypic plasticity than the possession of a syndrome of invasive traits. Species that originate from, and are naturalised in, warmer climates may not be predisposed to respond positively to elevated temperatures.