Experiment

We set up an experiment in which artificial communities of native European plants were invaded by exotic knotweed. We attempted to make this experiment as ecologically realistic as possible—by using local, natural substrates and plant materials, and by working with communities rather than individual native plants—while maintaining experimental control over the total amount and temporal patterns of nutrient availability. For the native communities, we used five forbs (Geranium robertianum, Geum urbanum, Silene dioica, Symphytum officinale, Urtica dioica) that are common in the habitats invaded by knotweed. We used seed material from a regional supplier of wild-collected seeds (Rieger-Hofmann GmbH, Blaufelden-Raboldshausen, Germany). As invaders, we used the two most invasive knotweed taxa Fallopia japonica and the hybrid Fallopia × bohemica. We used rhizomes that had been collected in invasive populations in Germany and Switzerland, and propagated in a common garden for several years.

In May 2010, we filled 150 4 l pots with a 1:1 mixture of sand and field soil (RICOTER Erdaufbereitung AG, Aarberg, Switzerland). Into each pot we planted one seedling from each of the native species, plus one knotweed rhizome (8–10 cm, two nodes). In half of the pots, we planted rhizomes of F. japonica, in the other half Fallopia × bohemica. The rhizomes were buried 5 cm deep in the centre of the pot, and the natives planted in a circle around it (Fig. 1b). Rhizomes and seedlings were randomly assigned to pots, and the planting order of natives was randomized. The experiment was set up in a large greenhouse, where the sides had been removed, essentially under a glass roof, in Muri b. Bern, Switzerland.

One month after the setup, 15 pots per knotweed taxon were randomly assigned to each of five nutrient supply scenarios. We used liquid NPK (7:5:6) fertilizer, applied at 10 different time points, every 10 days, to create five different temporal patterns of nutrient supply (Fig. 2a): (i) constant low (20 kg ha−1), (ii) constant high (60 kg ha−1), (iii) gradual increase, (iv) single large pulse or (v) multiple pulses of different magnitudes. The same total amounts of nutrients were added in treatments (ii) to (v), only with different temporal patterns of supply. The large pulse started 2 months after experimental setup, at a time when all plants were growing vigorously. For the multiple pulses, one random temporal pattern of extra nutrient supply was used for all replicates of the treatment.

During the experiment, the pots were watered as needed, and all pots received the same amounts of water. In September 2010, we cut the above-ground biomass of all plants, separated it by species, dried it at 80 °C until constant weight and weighed it.

Data analysis

For each pot, we calculated total community biomass by summing up the biomasses of all species, and as a measure of invasion success we calculated the percentage of total biomass that was represented by knotweed. The data were analyzed with general linear models that initially included nutrient level (low versus high), variability (the four scenarios at high nutrient level) nested within nutrient level, Fallopia taxon, and their interactions, as fixed effects. However, as there never were any statistically significant differences between the Fallopia taxa, we dropped this factor, and its interactions, from the final analyses. The percentage data were arcsin square-root transformed before analyses. For the analyses we only used the 123 pots where Fallopia had resprouted (shoot appearance above ground) during the experiment (82% of the pots; no differences of resprouting between taxa or treatments).