Why are some invasive plant species so successful in nutrient-impoverished habitats in south-western Australia: a perspective from their phosphorus-acquisition strategies

Invasive plants are a major cause of the global biodiversity crisis; it is therefore crucial to understand mechanisms that contribute to their success. South-western Australia is a global biodiversity hotspot with extremely low soil phosphorus (P) concentrations. In this region, a large proportion of native plant species release carboxylates that mobilise soil P. Many widespread invasive plant species in southwestern Australia are arbuscular mycorrhizal (AM). We hypothesised that some of these invasive AM plant species exhibit similar P-acquisition strategies as native carboxylate-releasing P-mobilising species which allows them to thrive in P-impoverished soils. To test this hypothesis, we collected 23 common invasive species in the field and assessed their leaf manganese concentration [Mn], relative to that of native reference species at different locations, as a proxy for carboxylate release. In addition, we cultivated seven of the invasive species in hydroponics at different P supplies to measure their root carboxylate exudation. Furthermore, we measured leaf P concentration and photosynthetic P-use efficiency (PPUE) of five invasive species in the glasshouse. In the field investigation, almost all invasive species exhibited significantly higher leaf [Mn] than the negative references, which do not release carboxylates, indicating carboxylate release of the invasive plants. Leaf [Mn] of a few invasives even exceeded that of positive references, which exhibit significant carboxylate release, indicating substantial carboxylate release of these invasives. All glasshouse-grown invasive species with high field leaf [Mn] released root carboxylates under low P supply. Most of the tested invasive plant species also exhibited greater PPUE than native plants under low P supply. Invasive AM plant species exhibited root exudation of carboxylates as a P-acquisition strategy, which very likely allows their successful invasion of severely P-impoverished habitats.

Methods
We have submitted the raw data related to the publication in Functional Ecology. Dataset including Field leaf data_Tang et al_2024_Functional Ecology.csv and Glasshouse_root carboxylate data_Tang et al_2024_Fucntional Ecology.csv.

Field_leaf data

We measured leaf manganese concentration ([Mn]) of representative invasive and native species (as references) from multiple sites in the field, using it as a proxy for carboxylate release. The fieldwork was conducted at eight locations (11 sites) in southwestern Australia. There were seven sites that were in good condition with minimal disturbance and four in poor condition with remarkable disturbance. The whole study area experiences a Mediterranean climate characterised by dry and hot summers from December to February, and wet and cool winters from June to August.

We also measured leaf phosphorus (P) and nitrogen (N) concentration. "n/a" indicates not measured in this study.

A total of 34 species were selected across 11 sites comprising 23 invasive species (Arctotheca calendula, Asparagus asparagoides, Aira elegantissima, Briza maxima, Briza minor, Bromus diandrus, Bromus rubens, Bromus madritensis, Ehrharta calycina, Gladiolus caryophyllaceus, Gladiolus undulatus, Lupinus angustifolius, Lupinus albus, Lolium perenne, Monoculus monstrosus, Oxalis pes-caprae, Pelargonium capitatum, Senecio condylus, Trifolium angustifolium, Ursinia anthemoides, Vicia sativa, Vulpia bromoides and Zantedeschia aethiopica) and 11 native reference species (Banksia telmatiaea, Banksia attenuata, Banksia nivea, Banksia sessilis, Conostylis aculeata, Clematis linearifolia, Eucalyptus astringens, Eucalyptus wandoo, Hardenbergia comptoniana, Macrozamia fraseri and Xanthorrhoea preissii).

Glasshouse_root carboxylate: in fresh weight (FW)

Parallel low-P (0.5 µM P) and high-P (50 µM P) treatments were applied for seedlings of A. asparagoides, E. calycina (Poaceae), G. undulatus, O. pes-caprae, Z. aethiopica and C. aculeata; whereas B. maxima (Poaceae) and G. caryophyllaceus received only a single low-P (0.5 µM P) treatment. The response under low-P treatment was the primary study aim, but we also aimed to explore the effect of a high-P treatment on some of the seedlings. Hence, high-P treatments were used for some species from the same genus (G. undulatus) or family (E. calycina). When the seedlings had formed ample new roots and mature leaves for analyses (two weeks for B. maxima and G. caryophyllaceus, four weeks for A. asparagoides, G. undulatus, O. pes-caprae and C. aculeata, six weeks for E. calycina, nine weeks for Z. aethiopica), the plants were harvested.