Black Locust Love CO2

Generally speaking, there is enough water and sunlight to support more photosynthesis than what is observed today. Carbon dioxide (CO2) is the limiting variable in the real world, and when its concentration is increased, photosynthesis correspondingly increases. Some plants go a bit further and promote fungi growth around their roots and then use byproducts of the fungi for additional nutrients in their biomass production. This mycorrhizal colonization at the root level is critical in the health these plants.

One such plant is the black locust, a tree whose range originally spanned the continent from the Appalachian Mountains to eastern Oklahoma. Today it can be found throughout the United States.

Black locust wood is very strong, hard, and heavy. It is used for fence posts, poles, railroad ties, paper pulp, and mine timbers. Technically, black locust belongs to the pea or legume family. That’s right. Think of it as 100-foot tall pea plant! Black locust possesses virtually all of the characteristics that define a typical weed. However, the black locust has nodules or knoblike growths on its roots that enable it to add nitrogen to the soil in the same way such other common legume crops as clover and soybeans do. Black locust reproduces prolifically from root sprouts and dominates early forest regeneration in native forest stands and areas disturbed by man.

Two biologists from West Virginia University have examined the interactions between mycorrhizal colonization and elevated CO2 in black locust. Olesniewicz and Thomas grew black locust seedlings in controlled-environment chambers with atmospheric CO2 levels set at 350 ppm and 710 ppm. Half of the seedlings grew in sterilized soil with no mycorrhizal fungi while the other half had the benefit of the fungi.

After 56 days of growth, the seedlings revealed amazing benefits from the elevated CO2. The black locust with the mycorrhizal fungi showed a 53% increase for whole seedling mass, 95% greater leaf mass, 9% greater stem mass, a 136% increase in nodule mass, and 48% more nodules. The percentage were even greater for the stressed black locust growing in the absence of the fungi under a doubling of CO2: 189% greater whole seedling mass, 220% more leaf mass, 192% greater stem mass, a 114% increase in root mass, 95% more nodule mass, and a 78% increase in the number of nodules.

Olesniewicz and Thomas also note "elevated CO2 enhanced mycorrhizal colonization, increasing the proportion of fine roots colonized by 32% over that of seedlings grown in ambient CO2."

This experiment shows us that mycorrhizal fungi can be important in determining how elevated CO2 levels ultimately impact tree growth. However, the experiment also shows us that black locust may be safely added to the long list of plants positively affected by elevated atmospheric CO2 concentrations.

Olesniewicz, K.S. and Thomas, R.B. 1999. Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide. New Phytologist, 142, 133-140.