By Julie Kray and Dana Blumenthal

Globally, rising atmospheric CO₂, warming temperatures, increasing nitrogen deposition and shifting precipitation patterns are changing the game for plants. These and other global changes influence the availability of key plant resources, which may affect the relative success of different plant species, and transform the composition of many plant communities. What we don’t yet know is which plant communities will change the most, and what types of plants will dominate those communities in the future.

There is concern that global changes, especially those that increase plant resource availability (e.g. elevated CO₂ and higher nitrogen deposition), may favor invasive plant species more often than not^1,2. Invasive plants often thrive in new and changing environments because of their rapid resource acquisition and fast growth rates, ability to disperse to new habitats, and broad environmental tolerances. However, even the most prolific invasive plants can fare poorly when faced with resource-poor or highly competitive environments. The challenge is to predict which ecosystems will become more susceptible to invasion as climates and landscapes change.

In semiarid environments, global change effects are particularly complicated because many of them not only influence plants directly but also alter water availability. Higher atmospheric CO₂ allows plants to partially close stomatal pores on their leaves, which reduces transpirational water loss and increases plant water use efficiency, leaving more water in the soil. This could favor fast-growing invasive species with high resource requirements.  Warming, on the other hand, increases evapotranspiration and reduces soil water, which might inhibit invasive species.

At the Prairie Heating and CO₂ Enrichment (PHACE) experiment in Wyoming, we are working to understand how the combination of higher CO₂ and warmer temperatures will influence the relative success of native and invasive plants in semiarid mixed-grass prairie. In the first of several studies, we focused on Dalmatian toadflax (Linaria dalmatica), one of the most common invasive plants in northern mixed-grass prairie. Toadflax seedlings were transplanted into native prairie and exposed to 4 years of higher CO₂ and temperature. Under elevated CO₂, toadflax plants grew 13 times larger, produced 32 times as many seeds, and 7 times as many vegetative sprouts from belowground roots³. Toadflax far out-grew the native species it was growing with, and showed the strongest response to elevated CO₂ observed yet for an invasive plant in the field. And even though warming dried the soil, its effects were much smaller than those of CO₂ , such that elevated CO₂ and warming together strongly increased toadflax survival, growth, and reproduction.³

Why did toadflax do so well? It seems to have profited from the direct effect of higher CO₂ (carbon fertilization), and the indirect effect of increased soil water availability—toadflax was tapping the water saved by native grass species, including western wheatgrass (Pascopyrum smithii). There were clear physiological differences between the two species in response to high CO₂: native western wheatgrass showed only a moderate increase in photosynthesis (23%) compared with a large increase in toadflax (87%). Further, western wheatgrass partially closed leaf stomatal pores at high CO₂, saving more water in the soil, while toadflax did not. In other words, the native grass grew a little more and conserved a lot more soil water under higher CO₂, whereas toadflax did not conserve water, and simply used the extra soil water and CO₂ to grow faster and scale up reproductive effort.

The bottom line is that toadflax is likely to benefit from the higher CO₂ and warmer temperatures forecast for northern mixed-grass prairie later this century, by virtue of its rapid growth and less conservative water use relative to native grasses that currently dominate the prairie. More broadly, these results suggest that increasing CO₂ may have particularly large effects on invasion in water-limited regions, posing challenges for food production and conservation of biological diversity.

For more perspective on this story, we recommend this Harvest Public Media radio broadcast by Luke Runyon with KUNC (or listen via New Phytologist’s news page). Learn more about invasive toadflaxes from Colorado State’s Ag Extension website.

You can catch up on earlier posts about the PHACE study here, here, and here.

EcoPress will highlight the latest discoveries from the PHACE experiment in future posts–check back soon!

¹Dukes, J.S. and Mooney, H.A. 1999. Does global change increase the success of biological invaders? Trends in Ecology & Evolution 14, 135-139.

²Bradley, B.A., Blumenthal, D.M., Wilcove, D.S. and Ziska, L.H. 2010. Predicting plant invasions in an era of global change. Trends in Ecology & Evolution 25, 310-318.

³Blumenthal, D. M., V. Resco, J. A. Morgan, D. G. Williams, D. R. LeCain, E. M. Hardy, E. Pendall, and E. Bladyka. 2013. Invasive forb benefits from water savings by native plants and carbon fertilization under elevated CO2 and warming. New Phytologist 200: 1156–1165.