A recent analyses of ancient mammoth and bison hair, teeth and bones revealed new insight on the expansion of grassland plants during the latter part of the Neogene, a geologic era that includes the present.
Oregon State University researchers found that the shift from what are known as C3 grasses to C4 grasses was likely driven by increased rainfall during the growing season. This represents one of the most significant changes in the Earth's vegetation.
"The point of the work was to understand what drove one of the most dramatic biological transitions in the past 65 million years, and also to better understand the past so that we can make predictions about the future," explained Jennifer Cotton, assistant professor at California State University, Northridge. "We know that the balance between C3 and C4 grasses is controlled by both atmospheric carbon dioxide and climate, but the relative influence of each of these factors has not been clear."
Cotton led the study as a post-doctoral researcher at the University of Utah and in the College of Forestry at Oregon State University.
Scientists have long known that some C4 grasses became more abundant during this period, including the ancestors of corn, sugar cane and sorghum. Compared to C3 grasses, these species use a different method of metabolism via photosynthesis. They tend to thrive under warm, moist conditions, in addition to low levels of carbon dioxide in the atmosphere.
When analyzing the hair, teeth and bones of mammoths and bison, researchers found that a changing climate - particularly increasing rainfall, rather than just atmospheric carbon dioxide - explains the widespread expansion of grassland plants during the latter part of the Neogene.
Researchers analyzed the carbon isotopes of 632 samples of bison and mammoth tissues from across North America over the past 18,000 years, corresponding to the time between the peak of the last ice age to the present. This showed that, over time, the animals' diets shifted toward more C4 plants and those plants gradually spread north.
Therefore, researchers concluded that increasing precipitation during the growing season was the single most important factor in the spread of C4 grasses. Similar transitions can be seen today, with increased rainfall and rising temperatures enabling farmers to grow corn in the upper Midwest, where wheat crops generally dominate.
"Both atmospheric carbon dioxide and climate have been changing and will continue to change in the future," Cotton added, "and many have suggested that additional carbon dioxide in the atmosphere will benefit C3 grasses, causing them to outcompete C4 grasses. Our results suggest that climate, rather than carbon dioxide fertilization, will drive future changes to C3 and C4 grass distributions, which will likely benefit C4 grasses in much of the Great Plains."
Their findings were recently published in the journal Science Advances.
