On May 21, 2020, White Tank Mountain Regional Park experienced a 128-acre wildfire within the park’s boundaries. Thankfully, the wildfire didn’t cause harm to people or structures. However, the wildfire did leave the park with an abundance of scorched plants in its wake. Many questions remain as to the long-term effects the fire will have on the flora, fauna, and overall Sonoran Desert landscape.
Most people don’t think wildfires can devastate natural desert areas, and until recently they would be right. Desert climates are harsh, and native plants are usually far from each other. Even if there was a spark from a human or lightning strike, a fire would have a hard time finding continuous fuel.
But, things have changed over the past 30 years as invasive plants have come along. These plants are not native to the Sonoran Desert and often end up causing harm to the areas they invade. Things like non-native thistles, grasses, and other weeds fill in the exposed soil and rock areas between native plants.
How did these invasive species reach the Sonoran Desert in the first place? Invasive species are primarily spread by human activities such as shipping, where seeds hitch a ride intentionally or unintentionally and then spread quickly. As more people move to the Sonoran Desert, invasive plant species numbers are rising with it.
To remedy the invasive species problem, volunteers with the White Tank Mountains Conservancy (WTMC) Desert Defenders Initiative have spent time locating, mapping, and removing invasive plants to prevent wildfires.
Unfortunately, with a good amount of precipitation this past winter (and for the last three consecutive years), these invasive species plants grew in abundance, only to dry out when temperatures soared, basically becoming dense fuel masses.
Assessing the Damage
A few weeks after the fire, a group of volunteers led by the Maricopa County Parks and Recreation Department natural resource specialist, Juanita Armstrong-Ullberg, returned to the burned area to perform post-fire assessment and document the damage. With the help of WTMC, Master Naturalists, and park volunteers, they monitored sampled plots counting the percent of scorch on the native trees, shrub, and cacti.
How do you measure the percentage of scorch on a specimen? This is typically done by visually inspecting the trees and shrubs. For the saguaro cacti, the amount of scorch around the base and trunk of the cacti typically is a good indicator of survival. Cacti with less than 30% scorch often survive, while those with greater than 30% scorch typically succumb in the following years.
In addition, the assessment team also identified and tagged a handful of saguaros for future research to study the long-term effects wildfires have on the species.
Why are these types of assessments important? One critical piece of data not readily available yet is how or if plants like cacti can recover after it is slightly or significantly scorched. And since the mighty saguaro grows slowly—it can take takes 50+ years to grow its first arm—identifying this information is critical to the overall health of the park and any desert landscape.
As Armstrong-Ullberg explains, “The saguaro is a keystone species of the desert habitat here, a sort of micro-climate if you will. The tall cacti provide shade for desert animals and it also provides shelter for insects and critters. Birds that flock to the White Tanks circle the cacti in search of food and spread seeds in their droppings. Take away the cacti, and the ecosystem may shift.”
From the assessment results, Armstrong-Ullberg determined the White Tank fire was patchy and of low intensity, with some small areas of moderate intensity. The plan is to return to various tagged saguaro locations to see how specific plants are faring. Will the cacti recover and how quickly, if at all? A study by the Tonto National Forest1 shows that saguaros who incur more than thirty percent of scorch damage will never recover.
Rebuilding a Desert Habitat
To help the burnt area recover faster, the Maricopa County Parks and Recreation Department staff and volunteers, WTMC volunteers, and others spent several days this spring collecting and planting native plant species in other areas of the park. These seeds include species like Brittlebush, Triangle-leaf Bursage, Desert Senna, Palmers Indian mallow, and others. The seeds were planted in portions of the burned area to help recolonize the much-needed plant life.
Beyond that, the key takeaway from the May 2020 fire is outreach and education. Many people who settle in the Phoenix area are new to the Sonoran Desert and are not aware that their actions could cause damage. Even though the landscape is not as full of plants as other areas around the country, a small spark could lead to more charred trees and saguaros. A fire could alter the desert landscape, which could take upward of 80 years to recover, even with optimal climate conditions.
Before going out and enjoying any park, check for park alerts such as burn bans. Be aware of agency park rules such as no smoking and always drive and park in designated areas to avoid contact with dry brush and vegetation.
“Post fire Saguaro Injury in Arizona’s Sonoran Desert” by Ruth C. Wilson, Marcia G. Narog, Bonnie M. Corcoran, and Andrea L. Koonce. Published 1993. https://www.fs.fed.us/psw/publications/4403/PostfireSaguaro.pdf