Researchers at the Institute of Ecology and Botany of the ELKH Centre for Ecological Research and Eötvös Loránd University studied changes in the growth, seed yield and mass of an invasive horseweed (Conyza canadensis) in a field experiment on precipitation manipulation in Kiskunság. The results of the research were published in the journal Oecologia.

Plant invasions, defined as a constant increase in the distribution area and population size of non-native plant species in habitats favorable to them, can cause serious problems both for nature conservation and for forestry, agriculture and health. Today’s rapid environmental changes such as climate change are expected to contribute to the spread of many invasive plant species. Although more and more studies are emerging on the effects of climate change on the success of invasive species and the resilience of habitats to invasive species, responses to change in precipitation are often difficult to predict. As a climate becomes wetter, it may favor the invasion of some non-native species, while other invasive species are spreading in the wake of droughts, when water shortages reduce or destroy the original, native vegetation. Understanding the effects of changes in precipitation on plant invasion is particularly important in dry and semi-arid habitats, where the amount and distribution of precipitation is a major determinant of organic matter production and the stability of plant communities. In Hungary, for example, open sand grasslands are considered such habitats, where Andrea Mojzes and her colleagues carried out their research work.

Horseweed (Conyza canadensis), a member of the family of herbs, is an annual species introduced from North America that has now become a cosmopolitan weed. It is widespread in Hungary, predominantly in agricultural crops and vineyards, though it can often be found on railway embankments and on the side of roads. In sandy areas, it can collect on abandoned fallows, for example, but it can also penetrate natural open sandy grasslands and multiply as a result of disturbance.

In a field experiment in Kiskunság, the researchers investigated the effect of repeated precipitation manipulations in the experimental plots (moderate drought, severe drought, watering) on the height and seed yield of horsefeed growing in the plots, and examined whether these differences could be related to changes in plant mass as a result of treatment. Mild drought was defined as a month-long and severe drought a two-month lack of rainfall in summer. Irrigation was carried out once a month, from May to August.

In the second year of application of the treatments (2016), the researchers measured the height of the marked horseweed individuals in the experimental plots and estimated their seed production. After stopping the treatments, the proportion of dead individual plants was determined, as well as the above-ground biomass of the horseweed in each experimental plot.

Habitat image, rosettes and flowering and fruiting stage of horseweed (Conyza canadensis) in the sand grassland near Fülöpháza. The tiny, pappiferous fruits of the plant are quickly spread by the wind. Photos by Andrea Mojzes.

Shoot height (a), number of seeds per plant (b) and above-ground biomass (c) in 2016, in a precipitation manipulation field experiment

Surprisingly, the researchers found that the horseweed showed enhanced growth in drought-treated plots compared to control and watered plots. This difference was particularly marked in plots exposed to moderate (one month) drought, where individual plants grew 2.5 times taller than in control and watered plots (Fig. 1a). Similar differences were found in seed production: in moderately drought-treated plots, individuals produced 2–2.5 times more seeds than in the control and watered plots (Fig. 1b). The researchers believe the most likely explanation for the results is that the experimental drought had an indirect beneficial effect on the performance of the individual horseweed plants. The drought treatments of the previous year (2015) drastically reduced the mass of the originally dominant perennial grasses – the Hungarian fescue (Festuca vaginata) and the sand feathergrass (Stipa borysthenica) – and these grasses could not regenerate until the beginning of the summer of 2016. As a result, the annual horseweed, freed from the oppressive effects of perennial grasses, presumably gained more soil moisture.

The mass of the horseweed observed in the experimental plots is in line with the individual responses presented above. Although the mortality of outbreaks was highest in plots exposed to severe drought (40%, compared to 7–13% in control plots and other treatment plots), its above-ground biomass was 4–14 times higher in drought-treated plots, and the shoot weight measured in the control and watered plots (Fig. 1c). The results obtained in 2016 are confirmed by the fact that the above-ground biomass of the horseweed in the highly drought-treated plots also exceeded the values measured in the control and watered plots in the following two years (2017 and 2018).

The results thus show that the horseweed was found to be sensitive to the two-month summer drought, but its growth and abundance were mainly determined by how the abundance of the species dominant in the grassland changed as a result of precipitation changes. Based on the results obtained so far, the researchers predict that the expected increase in the length and frequency of summer droughts with climate change will favor the survival of the horseweed and even help increase its mass increase in open sand grasslands.

Reference to the paper:

Mojzes, A., Ónodi, G., Lhotsky, B., Kalapos, T., Kröel-Dulay, Gy. (2020) Experimental drought indirectly enhances the individual performance and the abundance of an invasive annual weed. Oecologia 193: 571-581.