How Native Plants Team Up to Repel Invaders

A new study by Benjamin Schlau and colleagues in Plant Ecology shows how native plants can work together to fend off invasive species. Researchers found that two common plants in California’s coastal sage scrub habitat actively reduce the growth of black mustard, an aggressive invasive weed, when they interact together. However, when each native plant interacts with the invasive weed individually, they actually help the weed grow bigger.

“Our previous work indicates the CSS [California coastal sage scrub] dominant native facilitatory perennial California buckwheat (Eriogonum fasciculatum) benefits from the competitive effects of its nearly exclusive beneficiary, the codominant native perennial California sagebrush (Artemisia californica), write Schlau and colleagues in their article. “Greenhouse and field experiments suggest E. fasciculatum acidifies CSS’s calcareous soils and alleviates alkalinity stress for A. californica.”

Between them, buckwheat and sagebrush dominate the California coastal sage scrub. California Buckwheat can outcompete most plants for water, spreading roots over three metres across and up to two and a half metres deep. However, California Sagebrush can live with it because the buckwheat acidifies alkaline soils. The sagebrush has fibrous, shallow roots that can grab the morning dew fast and any unexpected rains. These roots also exude terpenes that prevent the germination and growth of many species.

Schlau and colleagues say the two plants behave unusually when they’re together. Usually, it’s not a great idea to help a perennial to grow as, once they get established, they tend to outcompete their helper. Yet the authors note that the buckwheat and sagebrush tend to be found together, even when they get over a metre across. The authors argue that this indicates the two plants have a positive or at least neutral interaction into their late stages. One possibility is that the sagebrush protects the buckwheat from accidentally helping other plants to benefit from the buckwheat’s assistance with alkalinity. As a result, the pair working together can dominate their habitats.

Black mustard is a significant threat to this ecosystem, as it outcompetes native plants for resources. Schlau’s team conducted greenhouse experiments to investigate how the three-way interaction between the two natives and the invasive weed influences their growth. They also examined how the plants facilitate or inhibit recruitment in some field experiments at the UC Irvine Coastal Sage Scrub Ecological Preserve.

As expected from the greenhouse experiments, buckwheat helps the mustard to grow. What surprised the authors was that the sagebrush also helped the mustard to grow. Yet, put the two losers together, and it was the black mustard that suffered. Buckwheat and sagebrush together managed to reduce the black mustard’s specific leaf area, height and inflorescence count.

Schlau and colleagues also found that while specific leaf area was smaller, total mass was not. How can a smaller leaf weigh the same as a larger leaf? The answer is that the smaller leaf is thicker. The authors note that this is a sign of nitrogen stress or water stress. The team also found that mustard had to put more resources into its stem, despite having less height, suggesting that the buckwheat was outcompeting the mustard for water.

The field results showed how the sagebrush also helps fight mustard’s invasion. When the buckwheat dominance of the water supply doesn’t matter, like when there’s enough rain for everyone, there’s no real competition for water. That’s when the sagebrush’s traits count. The sagebrush’s terpenes leaching into the soil prevent the mustard seeds from germinating.

Schlau and colleagues write in their article that their results show the importance of taking pairwise interactions and studying them in their broader ecological context. “In the Anthropocene, this means considering species interactions within intensely invaded habitats. Most surprisingly, the tertiary species here—a fast-growing, allelopathic invasive (B. nigra)—appears to reduce competitive effects of a heterospecific beneficiary (A. californica) upon its facilitator (E. fasciculatum) during seedling-to-juvenile growth stages. During late growth stages, the competitive beneficiary protects the facilitative plant from recruiting the harmful invasive.”

Ironically, in the past, buckwheat and sagebrush reduced the biodiversity of the California scrub, but now might be equipped to save it. Schlau and colleagues conclude: “If this is true of E. fasciculatum and A. californica, as results presented here suggest, then the niche of the two native species may shift from suppressor to protector of native biodiversity. In more applicable terms, restoration of disturbed CSS habitat may benefit from cultivated co-disperal of these native perennials, especially into re-occurring populations of B. nigra—and likely any invasive annual. Indeed, as fire intervals decrease and habitat becomes continually isolated, species interactions between seedlings of dominant perennials will likely become even more determinant to post-fire vegetative succession.”

READ THE ARTICLE

Schlau, B.M., Huxman, T.E., Mooney, K.A. and Pratt, J.D. (2023) “Three-way species interactions reverse the positive pairwise effects of two natives on an exotic invader,” Plant Ecology. Available at: https://doi.org/10.1007/s11258-023-01304-6.