Root-Associated Fungi of Fremont Cottonwood Mitigate the Negative Effects of Tamarisk Legacy by Promoting Investment in Root Systems
Julia Hull1*, Catherine Gehring1,2, Gery Allan1,2, and Thomas Whitham1,2
1 Northern Arizona University, Department of Biology, Flagstaff, Arizona, United States; jbh232@nau.edu
2Merriam Powell Research Station, Flagstaff, Arizona, United States
Climate change is having a significant negative impact on the southwestern United States, including its riparian ecosystems which are already under threat from invasive species such as tamarisk (Tamarix spp). Even after it is removed, tamarisk can negatively affect vegetation because it leaves a legacy by altering chemical and biological soil properties and by disrupting obligate, mutualistic relationships of native plants and their beneficial root-associated fungi. These effects may remain for years following the removal of tamarisk. Additionally, stands of cottonwoods that have experienced tamarisk invasion are less genetically diverse than stands of cottonwoods that have not, resulting in a legacy of tamarisk invasion. Although often overlooked, beneficial root-associated fungi may play an important role in the effective mitigation of climate change and tamarisk legacy in the riparian systems of the southwest. We tested the hypothesis that beneficial root-associated fungi would help mitigate the negative effects of tamarisk legacy and climate change on the performance of the native foundation species, Fremont cottonwood (Populus fremontii). Using a fully factorial greenhouse experiment, we manipulated air temperature (ambient and +3.5o C), cottonwood history (experienced with tamarisk or naïve), beneficial fungi (sterile or live inoculum) and soil legacy (tamarisk or not). Three major patterns emerged: 1) Mortality was significantly higher in the heated treatment, especially when the cottonwoods were grown in tamarisk legacy soil. 2) The addition of root-associated fungi improved the performance of cottonwoods up to 4 times. 3) Cottonwoods originating from sites with co-occurring tamarisk invested more heavily in root systems than cottonwoods from sites without tamarisk, especially when grown in tamarisk legacy soil. Based on this and other studies, we suggest that the addition of beneficial root-associated fungi become incorporated into restoration practices following tamarisk removal.