Bryophytes are a group of green land plants and include liverworts, hornworts, and mosses. Scientists know that bryophytes were one of the first plants to take root on land and are still important: They are also the second largest group of land plants after flowering plants. Overall, they constitute a major part of the biodiversity in moist forests, wetlands, mountains, and even tundra ecosystems. Thus, all three types also play a significant part in the global carbon budget and carbon dioxide exchange, nutrient cycling, and water retention, to name just a few. They are also important environmental indicators and are even used as predictors and prognosticators of past—and now future—global climate changes. (For more about bryophytes, see the chapter “Plant Diversity.”)