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


French, Kris; Watts, Evi (2020), Differences in vegetative growth of two invasive hawkweeds at temperatures simulating invaded habitats at two altitudes, Dryad, Dataset,


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.


Data was from laboratory trials using seeds collected from a range of sites and plants from pots grown in glasshouse conditions. Two species were investigated: Hieracium aurantiacum (orange hawkweed, OHW) and Hieracium pilosella (mouse ear hawkweed, MEHW)

Three kinds of data are presented:

1.  Germination data based on 25 seeds placed into petridishes and followed through time.  Petri dishes were either placed in cabinets at ambient temperature, temperatures simulating 1000m or 1700m asl.  Germination was followed through time.  Old seeds collected from sites in previous years are contrasted with new seeds collected in the season prior to experimentation.

2. We followed growth through time for 20 germinants grown at either 1000m or 1700m.  Dry biomass of above ground and below ground parts are presented.

3. Using ramets from plants grown in pots and kept in the glasshouse for some years, growth rates (no. leaves through time) are measured at temperatures representative of the altitude where populations have invaded in the field (wither 1700m or 1000m). Only a subset of plants were grown through the latter part of the experiment after repotting.