Nutrient Regulation in Invasive and Native Grasses

Native vegetation in grasslands, which are critical habitats that provide ecosystem services and economic opportunities for South Dakotans, are widely being replaced by invasive grasses. The utility and stability of grassland services and opportunities are undergirded by their native vegetation, which is at risk. In order to preserve our grasslands and the value we derive from them, we must understand why invasive grasses can outcompete and replace native vegetation. However, the question of why invasive plants are capable of re-placing native plants is still an active area of research with many unanswered questions. Using a concept of stoichiometric homeostasis, we show that invasive plants’ success is in part due to how they manage their nutrition.

Just as your body regulates the quantity of various nutrients it takes up and retains, so too do plants regulate the amount of nutrients they take up into their tissues through their roots. A term for this nutritional regulation is stoichiometric homeostasis; that is, the regulation (homeostasis) of the balance (stoichiometry) of internal nutrients. Imagine a potted plant that gets fertilized weekly with an ever increasing concentration of nitrogen (N) fertilizer. If the concentration of N within the leaves of that plant were to remain the same over the course of several weeks, then that plant would have a high level of stoichiometric homeostasis (H for short).

Alternatively, you could say that the plant is highly regulatory with respect to N. Now imagine a plant whose leaf concentration of N steadily rises at a rate similar to the increasing rate of fertilizer concentration. Such a plant would have a low level of H and could be said to be poorly regulatory with respect to N. Recent research done in the Konza Prairie of Kansas showed that with fertilization at rates of 100 kilograms of N per hectare (about 90 lbs. per acre), highly regulatory (high H) plant populations decreased, while poorly regulatory (low H) plant populations increased. Can these observations help explain the success of invasive grasses?

We believe they can. In a greenhouse experiment we determined H in two native (western wheatgrass, Canada wildrye) and two invasive grasses (smooth brome, crest-ed wheatgrass) by cultivating them across a range of known soil nutrient concentrations and examining leaf nutrients. We found that the invasive grasses had a lower value of H than the native grasses. We suspect that this trend holds for many invasive and native grasses. What does this mean for our grasslands? Agricultural runoff, atmospheric deposition, and soil management practices can all lead to increased levels of N in soils, creating conditions that promote the success of invaders. The challenge to conserve our native vegetation, the value it provides, and the effort to prevent the spread of invasive grasses is then, in part, the challenge to maintain the chemical integrity of our prairie soils.