Unusual suspects in the usual places: a phylo-climatic framework to identify potential future invasive species

Differences in vegetative growth of two invasive hawkweeds at temperatures simulating invaded habitats at two altitudes

Hieracium pilosella and H. aurantiacum are invading alpine regions in New South Wales, Australia. In a glasshouse experiment we investigated germination and growth rates of these two species at temperatures simulating the altitudes where invasions are occurring from autumn to spring. We measured germination rates, growth rates and the development of stolons and ramets using seedlings and plantlets from established plants. Germination was low in H. aurantiacum and unaffected by altitude or seed age. H. pilosella showed site to site variability in germination but had greater germination. No species produced flower spikes. Both species grew rapidly and put at least twice as much biomass into roots compared to shoots. H. aurantiacum could begin to produce stolons after 27 days and seedlings grew a little larger than for H. pilosella. Hieracium aurantiacum put significantly more resources into ramets, allocating between 4–15% of biomass. H. pilosella produced 2.6 stolons month⁻¹, in contrast to 9.8 stolons month⁻¹ for H. aurantiacum. Furthermore, plantlets from established plants had vastly different growth rates. Plantlets of H. aurantiacum produced 2.1 leaves day⁻¹ from late summer to winter where H. pilosella was 3 times slower for the same period but faster following winter. Both species were able to maintain strong growth over cooler months suggesting hawkweeds have the capacity for fast growth in the invaded range under high nutrients and lower competition. H. aurantiacum is likely to be a more effective invader than H. pilosella spreading through stolons and the development of weed mats.

Evolutionary changes in growth, regrowth and carbohydrate storage in an invasive plant

We hypothesized that due to the absence of specialist herbivores in introduced ranges, invasive plants have evolved decreased allocation to carbohydrate storage for regrowth ability and as a consequence allocate more to growth. In this study, we compared plant growth, carbohydrate storage and regrowth ability of invasive and native Jacobaea vulgaris in response to complete shoot defoliation. We used invasive J. vulgaris genotypes from three geographically and climactically distinct regions and compared these with native genotypes from Europe. We found that invasive genotypes initially grew larger while native genotypes regrew larger after defoliation. Before defoliation, the carbohydrate storage in roots of invasive genotypes was 38% lower than native genotypes. Biomass after regrowth increased with root carbohydrate storage while it decreased with structural root mass, showing that it is crucial to study root storage and structural components separately in order to investigate plant regrowth. All studied traits of invasive populations from the three geographically and climatologically distinct regions changed in the same expected direction suggesting that the shifts in herbivore guild were causal to the observed change in growth and regrowth ability rather than environmental factors.