Nathan P. Lemoine, John D. Dietrich, Melinda D. Smith
The first European settlers of the midwestern United States were awestruck by the productivity of tallgrass prairies. The fertility of this region was apparent from the number and density of flowering stalks, which can reach over 1.5 m in height for many tallgrass species. Yet species vary in both the yearly timing and amount of flowering, and many species flower erratically. For example, indiangrass (Sorghastrum nutans) and little bluestem (Schizachyrium scoparium) flower nearly every year, whereas big bluestem (Andropogon gerardii) produces flowering stalks sporadically. All species, however, vary considerably in both the number and density of flowering stalks produced each year. Unfortunately, we still know little about the relationship between flower production and the timing of rainfall for many of these ecologically and economically important grass species.
Using three decades of long-term data, we partitioned flowering of big bluestem, little bluestem, and indiangrass into three individual components: the probability of producing any flowers, the number of flowering stalks produced, and the individual size of each stalk. We used advanced statistical techniques to determine how the timing of rainfall influenced flowering for each of these three species. Our results suggest that each species has unique precipitation requirements that stimulate flower production.
Big bluestem, for example, was extremely sensitive to rainfall during the flowering stalk elongation period from late June – early August. Precipitation deficits during this period reduced both the probability of flowering and the number of flowers produced. A similar trend was observed for indiangrass, which flowered every year but did so more intensely in years of high mid-summer rainfall. Little bluestem, on the other hand, flowered consistently regardless of interannual variation in precipitation.
Many of these differences can be attributed to different growth strategies among these species. Both big bluestem and indiangrass are rhizomatous grasses and respond quickly to changes in environmental conditions. Little bluestem is a bunchgrass with large carbohydrate reserves, and is able to flower consistently regardless of rainfall. These differences in life history can buffer tallgrass prairie flower production despite considerable interannual rainfall variation.
Image provided by authors.