by Makari Krause
As global warming proceeds there will be numerous shifts in plant communities around the world. Blumenthal et al. (2013), realizing that the implications of these shifts are not well known, set about testing how reorganization of plant communities may affect invasive plants. They hypothesize that elevated CO2 levels and warming might strongly influence the success of invasive species in semi-arid grasslands. In order to test their hypothesis they placed Linaria dalmatica, an invasive forb, into grass prairie that was treated with free-air CO2 enrichment and infrared warming. They then measured survival, growth, and reproduction of the forb over a four-year period. The CO2 enrichment had huge effects on the growth and survival rates of L. dalmatica; biomass increased 13 fold, seed production increased 32 fold, and expansion increased seven fold. Warming had little effect. Blumenthal et al. then compared the leaf gas exchange and carbon isotopic composition between L. dalmatica and the native C3 grass, Pascopyrum smithii. The elevated CO2 decreased stomatal conductance in the grass but not in he forb and didn’t increase the photosynthesis rate in the grass nearly as much as it did in the forb. The invasive species benefited hugely from CO2 enrichment, which could have far reaching implications for future colonization.
Blumenthal et al. use free-air CO2 enrichment and infrared warming to test how elevated CO2 and warming, alone and in combination, influence the growth, survival, and reproduction of L. dalmatica. They also test whether the effects of elevated CO2 and warming were due to different physiological responses between the invasive species and the dominant, native C3 grass, P. Smithii. They did five replications of four different tests: control, free-air CO2 enrichment, infrared heating, and a combination of heating and CO2 enrichment. The authors let the transplanted invasive species grow for two growing seasons taking measurements every month.
Prior studies had been inconclusive about the effects of CO2 enrichment on invasive species. While invasive plants do respond positively to enrichment it is not known whether they are affected more or less than native plants. Invasive plants are often able to respond more positively to increases in resource availability in general and might be benefited more by CO2 enrichment than native plants. One of the mechanisms for this difference is that invasives usually have higher growth rates, which allow them to maintain high photosynthetic rates and low construction costs (carbon needed to produce each gram of plant tissue) under elevated CO2. The effect of warming on invasive plants is even less understood. While warming will most certainly increase the incidence of invasion, it is hard to say whether it will benefit the invasives in their new territories. Some invasive species are able to rapidly adjust their phenology and might be benefited by the warming while others may not.
Warming and elevated CO2 may also indirectly affect invasive species through their effects on water. Increased CO2 allows plants to maintain their photosynthetic rates while increasing water efficiency and conserving soil water. If water availability limits plant growth, increased water conservation by native plants may benefit invasive species, which usually have relatively small decreases in water use due to increased CO2.
Blumenthal et al. found that CO2 fertilization increased L. dalmatica biomass, seedpod production, and new shoot production. The main reasons for the increase in biomass and reproduction was an increase in the L. dalmatica percentage survival and average plant height. Warming, on the other hand, had almost no effect on biomass, seed production, or shoot production and decreased survival when combined with elevated CO2.
Elevated CO2 increased photosynthetic rates in both L. dalmatica and P. smithii. L. dalmatica photosynthetic rates were higher in warmed than in unwarmed plots. Elevated CO2 decreased stomatal conductance only in P. smithii and warming decreased stomatal conductance in L. dalmatica in the absence of elevated CO2 but had no effect in cases where CO2 was added. CO2 therefore limited the negative effects of warming on stomatal conductance.
In addition to the direct effects of CO2, water also affected plant success. Volumetric soil water content increased with CO2 and decreased with warming. Photosynthesis increased in L. dalmatica as soil water increased, photosynthesis in P. smithii increased to a certain point with increasing water and then started to decrease.
Over the course of the four-year study, L. dalmatica was significantly more successful than the native grasses. The reason for the success of L. dalmatica is its faster physiological response time, which allows it to take advantage of carbon fertilization and increases in soil water content. This success was not only attributable to the direct effects of carbon fertilization but also to the indirect effects of changing soil water content. The increase in soil water with CO2 enrichment occurs because native species conserve more water, thus benefiting the invasive species, which are generally less water conservative. The authors hypothesized that warming would inhibit invasive species but it had almost no effect, so with increasing CO2 levels, ecosystems where water is a limiting factor, such as semi-arid grasslands, may become much more susceptible to invasion by other species of plants.
Blumenthal, D. M., Resco, V., Morgan, J. A., Williams, D. G., LeCain, D. R., Hardy, E. M., … & Bladyka, E., 2013. Invasive forb benefits from water savings by native plants and carbon fertilization under elevated CO2 and warming. New Phytologist 200(4), 1156–1165