By Kara Barron, 2016-17 McDowell Sonoran Conservancy biodiversity fellow
The immediate aftermath of a fire produces obvious evidence. Vegetation is scorched or dead, and animals have fled to safer places. It takes time for the land to return to its pre-fire state, and the key to the area’s ability to do this lies hidden in the blackened soil. It is the seeds.
Seeds are the future potential vegetation of an ecosystem, but can be wiped out by fire. They reenter the burned area in various ways though— on the wind, or on the backs and through the stomachs of animals. The process can be facilitated by humans too, through replanting or reseeding after a burn. Some ecosystems, such as ponderosa pine forests, are known to be fire-adapted. This means that fires occur often enough that the plants have evolved ways to thrive despite this disturbance. The Sonoran Desert, however, historically incurred fires only once every 100 to 1,000 years, so it is not considered fire-adapted. The migration of humans and invasive species into this ecosystem is contributing to more frequent fires.
Scottsdale’s McDowell Sonoran Preserve has had four fires over the last 30 years, burning approximately 11,000 acres, or one-fourth, of the land. With this change in fire frequency, are the plant communities in the Sonoran Desert going to return to their pre-fire
state?
The areas where the fires occurred on the Preserve were left to recover naturally, so they presented an opportunity to begin to answer the above question. Because no data was available about what those plant communities looked like before the fires, they are being compared to adjacent unburned plant communities in order to determine the actual effects of the fires. To do the comparison, three sites were established within the burn perimeters, and three sites were established as controls on nearby unburned land. Each site contained 10 plots along a 100-meter transect. In each plot, botanists and stewards measured percent cover (the portion of space the species occupy within the plot) for each perennial species, and recorded frequency (presence or absence) for all annual species.
Measurements were taken in the fall and spring of 2016–17. Results show that, when it comes to cover, plant communities in the burned areas appear to be approaching their pre-fire state. However, there are changes in cover and frequency for individual species. Specifically, there seems to be a reordering of perennial species. Species that normally dominate the Sonoran Desert, such as little-leaf palo verde (Parkinsonia microphylla), creosote (Larrea tridentata), and cacti, showed a significant decrease in cover in the burned plots. They were replaced by increases in species such as globemallow (Sphaeralcea ambigua) and brittlebush (Encelia farinosa). Frequencies of annuals, such as scorpion weed (Phacelia crenulata), broadfruit combseed (Pectocarya platycarpa), and fiesta flower (Pholistoma auritum), showed a significant decline in the burn plots too, while the western tansy mustard (Descurainia pinnata) increased in frequency.
So, are the plant communities in the Sonoran Desert able to return to their pre-fire state? The answer isn’t as straightforward as one might like it to be. Some experts think that deserts need 75 years or more to fully recover after a fire, but each desert is unique. So, testing this hypothesis in the Sonoran Desert is valuable. It’s not hard to imagine that if an area were to sustain multiple burns, some species might drop out of the plant community completely. Conversely, if these areas are given another decade undisturbed and monitored again, the pre-fire dominant species might be closer to their pre-fire cover. This is something that will need continued investigation. But this study informs what steps can be taken now to continue to preserve the beauty and diversity of places like the McDowell Sonoran Preserve for generations.