Synergistic interactions of ecosystem stressors and mitigation by mycorrhizal fungi on the foundation species, Fremont cottonwood

The combined ecological stressors of climate change and invasive species are threatening native ecosystems, each independently and may interact in unpredictable ways. My greenhouse study examined the potentially synergistic interaction of climate change and invasive species legacy soil on the fitness and performance of a foundation tree species in the threatened riparian ecosystems in the southwestern United States. I also examine methods for improving restoration outcomes for the foundation tree species, Fremont cottonwood (Populus fremontii), when planted in invasive tamarisk (Tamarix spp.). The first study found that increased temperature and invasive tamarisk legacy soils had a synergetic negative interaction on cottonwood survivorship, but a positive synergistic interaction on cottonwood root investment. Root investment typically positively correlates with fitness in arid environments; therefore, the combination of higher temperatures and tamarisk legacy soil may allow surviving cottonwoods to perform well in these novel environmental conditions. The second study found that the addition of whole soil inoculum, including important root symbionts known as mycorrhizal fungi, from a nearby reference site increased cottonwood dry biomass by 45% compared to those given sterile inoculum and increased mycorrhizal colonization by 53%. I found that cottonwoods from sites with co-occurring tamarisk (experienced) showed better performance than cottonwoods from sites without tamarisk (naïve) when grown in tamarisk legacy soil. However, when given live whole soil inoculum from a reference site, there were no longer differences between experienced and naïve cottonwoods. Although I expected to see experienced cottonwoods respond less to inoculation than naïve cottonwoods, I found no difference. I was surprised to find that cottonwoods, independent of source, responded more positively to inoculation when grown in tamarisk legacy soil than in agricultural legacy soil. The results from this greenhouse study suggest that Fremont cottonwood restored in tamarisk legacy soil will show increased root investment, especially when given whole soil inoculum from a reference site, which may make them better suited to the increasing aridity in the American southwest due to climate change