Asymmetric phenological changes projected for U.S. rangelands
Cows grazing on Pawnee National Grassland.
The impact of climate change on rangeland phenology is relatively understudied at the national level. The sustainability of goods and services derived from rangeland ecosystems is sensitive to differing growing seasons and phenological patterns. The diversity of growing seasons encompassing the extent of United States rangelands ensures that key resources, such as forage for domestic and wild ungulates, will be available at different times in different places. Climate change is threatening the phenological patterns that many of our management systems have been built upon for the last 100 years.
As a result, this research investigating the potential impacts of climate on future phenological patterns is critical to build more effective management protocols that promote greater adaptive capacity. The timing and duration of plant growth in rangeland ecosystems is an important factor influencing the global carbon cycle, many aspects of landscape ecology, and land uses such as grazing.
Changes in growing season timing are already detectable globally, but projections of future shifts and research in rangelands specifically have been limited. Researchers used remotely-sensed data from eMODIS to associate growing season timing in western U.S. rangelands with annual climate from 2000 to 2014. We then applied this model to future climate projections to predict how growing season timing and length will change in the future. Results suggest plant growth will begin 10 to 17 days earlier by the end of the 21st century across the study area, depending on the emissions scenario. Similarly, plant growth could end 12 to 24 days earlier. This corresponds to a shortening of the overall growing season by 2 to 7 days by the end of the century.
These changes may impact plant fitness and landscape ecology broadly, and would require adaptation in grazing and other land uses. Growing seasons are expected to be shorter in the future by 3 to 6 days in the early and mid-century periods respectively under the RCP 4.5 ensemble scenario, and 4 to 10 days shorter respectively under the RCP 8.5 ensemble scenario. Earlier projected shifts to the start of the rangeland growing season are most pronounced in eastern Washington and Oregon and throughout the Great Basin, relative to other areas. Earlier projected shifts to the end of the growing season are more pronounced than changes to the start of the growing season, especially on the southern plains of Texas and Oklahoma where the end of the season is projected to occur up to 31 days earlier by 2070 under RCP 8.5.