The saguaro – the classic symbol of the American southwest – is the largest cactus species in the United States and only grows in the Sonoran Desert. Straddling the city of Tucson, AZ, Saguaro National Park protects over 90,000 acres of its namesake cactus. While visiting there recently, I learned how this unique environment is being impacted by climate change.
The Southwest is the hottest and driest part of this country and climate change is making it even more so. According to the US National Climate Assessment, 2001 through 2010 was the region’s warmest decade on record with temperatures almost 2°F above historic averages. Looking ahead, the report projects continued increases in average annual temperature and a decrease in precipitation, especially in the southernmost part of the region. In a National Park Service (NPS) report on arid lands, the agency says it has already observed a widespread winter and spring warming trend and a lengthening of the frost-free season in the Sonoran Desert.
These warmer conditions are causing a variety of environmental changes in Saguaro National Park. While fewer cacti are dying from severe freeze events, evaporation rates are increasing, making an already parched area even drier. “Arid ecosystems,” the NPS says, “are particularly sensitive to climate change and climate variability because organisms in these regions live near their physiological limits for water and temperature stress.” Even slight changes in the environment can dramatically alter the distribution and abundance of many desert species. On average, the region currently receives less than 12 inches of rain per year.
The phenology, or the timing of natural events, is also being thrown out of sync as the climate changes. The vital rains of the region’s monsoon season traditionally begin in early July, but are projected to arrive later and later in the coming years. This can be a problem for the saguaros, because they have evolved to produce fruit in the beginning of July. While adapted for the dry desert environment, the cacti still depend on adequate rain during the summer growing season for both their own survival and the establishment of seedlings.
Another serious climate change challenge for the Sonoran Desert is the spread of non-native plant species. Buffelgrass, a native of Africa, was introduced to Arizona in the 1940’s for cattle forage. Thriving in the heat, this invasive species is crowding out the native plants (including the cacti), changing the look of the landscape, and increasing the risk of wildfires.
The Sonoran Desert did not evolve with fire as an ecological factor. Lush with vegetation by desert standards, the region usually had large gaps between groups of plants. This helped keep any wildfires small. Now, as the buffelgrass fills in those open spaces, fires can burn hotter and spread much more easily. Adding insult to injury, the buffelgrass grows back after a fire has killed the native plants.
The changing environment is also problematic for the animals that depend on the saguaros for food and shelter. These include a wide array of species, ranging from woodpeckers to coyotes.
Scientists are continuing to monitor and research how additional changes in temperature and precipitation will affect this beautiful and unique place.