Have you removed weeds and found despite all your efforts, they come back as strong as ever? A study by Agostina Torres and colleagues investigated removing Sweetbriar Rose (Rosa rubiginosa) and Scotch Broom (Cytisus scoparius) from Isla Victoria in Nahuel Huapi National Park, Argentina. Their results, published in the Journal of Ecology suggest that removing invasive species successfully may also depend on timing.
Sweetbriar Rose and Scotch Broom are European plants that have become pests in Patagonia. The plants grow rapidly and can reproduce sexually or asexually. They also can affect other plants, not just by taking resources, but allelopathy, changing the chemical balance of the soil to harm native plants.
While the plants need removing, it’s not clear what the best way is. Torres and colleagues decided to test when to remove the plants by testing removals during the summer (February) and autumn (May). They set up a combination of plots, so they could test the effects of early removal of the rose or the broom, and left some undisturbed as control plots.
The early plant removals happened when the warm weather and long days gave plants the best conditions for growth. Surprisingly, removing the Scotch Broom from the plots in this period was bad for the native plants. It turns out that the open space was colonised by new invasive plants. In contrast, removing the Scotch Broom in the autumn meant that the native plants had less competition for the new space. Torres and colleagues write that the reason for the invasive success might be due to conditions in the soil.
Although untested in this study, it is likely that a resource pulse generated by its early removals favoured the establishment and growth of nonnative species with advanced phenology rather than natives species (Marushia et al., 2010; Wolkovich & Cleland, 2011). In such a case, our results suggest that nonnative species capitalized released resources more quickly than co-occurring species, hindering the growth of natives and even that of the rose (Torres et al., 2022).Torres et al. 2023.
For the Sweetbriar Rose, early removal aided the Scotch Broom. The obvious reason would be that there was more space and light, but Torres and colleagues rule this out, as the rose tends to be shorter and has an open architecture, so it shouldn’t be shading the broom out. Instead, they suggest that the battle is belowground, with the rose outcompeting the broom for water. They also note that early removal of the rose harmed the native plant species, indicating that while Sweetbriar Rose is a problem, it does help suppress the Scotch Broom, and so indirectly aid the native plants.
Torres and colleagues argue that their results show the need for community-level approaches to invasive plants. Both the rose and the broom are serious problems but, in this case, the rose is also a problem for the Scotch Broom. They also show that plant relationships can vary with the season. Torres and colleagues write:
Importantly, our work shows that plant-to-plant interactions are not static but dynamic over time. We argue that such overlooked temporal dynamism of nonnative interactions can explain part of the current unpredictability of community assembly after removal of invasive species. In doing so, management actions aimed at restoring native communities could benefit from removing invasive species late in the growing season, thus maximizing the benefits provided by nonnative biotic resistance while giving native species a better chance of recovering.Torres et al. 2023.
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Torres, A., Morán-López, T., Rodriguez-Cabal, M.A. and Núñez, M.A. (2023) “Timing of invasive species removal influences nonnative biotic resistance and trajectories of community reassembly,” The Journal of Ecology. Available at: https://doi.org/10.1111/1365-2745.14168.
Cover: Nahuel Huapi National Park. Image: Canva.