Researchers found that the overall protein concentration of goldenrod pollen fell about one-third from the onset of the Industrial Revolution to the beginning of the 21st century. Previous studies have shown that increases in carbon dioxide can lower the nutritional value of plants such as wheat and rice — staple crops for much of the … More Rising levels of atmospheric carbon dioxide have reduced protein in goldenrod pollen, a key late-season food source for North American bees, a Purdue University study shows.