Early development of Acacia longifolia is more severely impacted by water and nutrient stress in invasive than native seedlings

Acacia longifolia (Andrews) Willd. is a legume native to southeast mainland Australia and Tasmania and has two described subspecies: A. l. subsp. longifolia and A. l. subsp. sophorae. The species has been introduced around the world and is considered invasive in several Mediterranean-type climate regions, including in South America, South Africa, and southern Europe. Previous studies comparing native and invasive populations of A. longifolia have focused on its reproductive ecology and population genetics, and little information exists on the species' early life development and how abiotic factors influence it. Here, we performed a glasshouse experiment to compare the phenotypic responses of native and invasive (in Portugal) A. longifolia seedlings to different levels of water and nutrient availability. We found that seedlings of both subspecies responded similarly to different water and nutrient availability conditions in terms of biomass accumulation, root length, the number of phyllodes produced, phyllode water content, and root-to-shoot ratio. However, compared to native seedlings, invasive seedlings had limited capacity for stress responses. We found that invasive seedlings had lower drought tolerance than native seedlings, and thus the speed of invasion by A. longifolia into drier parts of Portugal may be hindered. Our results also hint of a possible role of seed "imprinting" in this species' early growth responses, resulting in different resource allocation strategies such as favouring early growth and development over drought resistance in the invaded range. Further studies are required to better understand the species' abiotic stress responses at the intraspecific level and their relation to its invasiveness.