Studying Resilience in Southwestern Forests
by Paulina Jenney
University of Arizona researcher Donald Falk and his students are studying hundreds of plots in southwestern mountain ranges to understand why some forests bounce back from devastating wildfires while others give way to grassland and shrubs.
The goal of his project, he said, is to provide guidance to park and forest managers faced with restoring post-fire landscapes, or allowing other species to move in.
“Typically in the past, after a disturbance event, land managers would try to direct the landscape back to its original state,” said Falk, associate professor in the School of Natural Resources and the Environment. By asking big questions about resilience—the ability of an ecosystem to recover to its pre-disturbance state—Falk and his team are out to determine whether this approach is still ecologically appropriate, or even possible.
“We want to understand the characteristics of a species that enable resilience and understand what determines the point of no return,” he said. “From the perspective of land managers, it’s important for them to understand this. If you are in the middle of a drought and then have a fire, is it your job to put a system back to how it was, or let it become something different? Managers are looking to science for guidance.”
Drought has gripped the southwestern United States for more than a decade, and average temperatures are rising: four of Tucson’s six warmest years on record have happened in the past six years, with 2014 topping the list. In combination with warmer temperatures and less precipitation, disturbances such as wildfire, forest die-off caused by insect infestations, and diseases can unhinge an ecosystem in a matter of days or weeks.
“Overlay climate change and a large disturbance and you have a recipe for very, very rapid change,“ Falk said. “A few decades ago, 50,000 acres burned was huge for fires in Arizona. Now they are 400,000 acres and larger. When big events like that occur amid a multi-year drought, you have a potential tipping point moment for the entire ecosystem.”
Post-fire soils can become inhospitable to the very seedlings trying to take root and restore the forest, and different vegetation moves in. Falk points to a section of the Catalina Highway, the road to Mount Lemmon, where shrubs and grasses have replaced trees.
“The consequences of this are huge,” Falk said. “We’re looking at the significant loss of forested areas, the wholesale conversion of many southwestern ecosystems.” Shrublands and grasslands may sequester less carbon than forests, accelerating the effects of changing climate.
To better understand what governs the transformation of an ecosystem, Falk and his team are using computer simulations and empirical observations—what vegetation grew there before a fire and what is there now—in the Catalina, Chiricahua, and Jemez Mountains.
“We are looking at why we are getting a tipping point in one area but not elsewhere, and the different pathways ecosystems follow after a disturbance,” he said. “We are trying to understand how this is playing out so we can advise land managers to try to restore the pre-fire condition, or accept change and manage for a new community,” Falk said. His team has started to interpret the first round of results with more to come later this year.
Falk got the project off the ground in fall 2014 with a Udall Center Faculty Fellowship, awarded jointly by the UA’s Institute of the Environment and Udall Center. He discussed his work during a seminar in September at the Udall Center. “I was really impressed with the comments I received,” Falk said. “The quality feedback helped shape the trajectory of the project. The release from teaching responsibilities was a great asset and allowed me to concentrate on moving this work forward.”