PS 14-139 - Oaks on a warming coast: Exploring how mutualistic soil fungi may help oak seedlings survive drought

Background/Question/Methods

Ectomycorrhizal fungi form symbiotic relationships with tree roots to trade nutrients and water for sugars produced via photosynthesis. Trees depend heavily on their fungal partners, especially in drought conditions when fungal water redistribution is critical. Fungal communities associated with arid sites are thought to confer more drought tolerance to plant hosts than fungal communities associated with mesic sites due to differing ecological traits in the fungi. In this project, I measure the extent to which soil communities change seedling physiology in drought versus non-drought conditions. I hypothesize that fungal communities from arid sites buffer the growth of Quercus lobata (valley oak) seedlings experiencing drought stress, while fungal communities from mesic sites do not. To test this hypothesis, I planted 340 Quercus lobata seedlings in soil from three arid sites and three mesic sites across a climatic gradient in the Tehachapi Mountains in Southern California. After allowing symbioses to establish, I exposed half of seedlings to drought stress, and continuously measured growth and photosynthesis during the experimental drought. Following destructive harvest of the seedlings, I quantified seedling biomass, fungal colonization, and fungal community composition, to link fungal taxonomic identity to seedling performance.

Results/Conclusions

In my study, controlled watering introduced a 10-15% reduction in soil moisture. Within 8 weeks, seedlings experiencing drought stress exhibited a 41.2% decline in photosynthetic rate compared to well-watered seedlings, indicating that drought stress altered seedling ability to acquire carbon. Differential responses in seedlings inoculated with fungi from arid vs mesic sites suggest that fungal community composition can buffer these responses, perhaps through shifts in the relative abundance of ectomycorrhizal partners with stress-tolerant traits. Drought-tolerant traits include the ability to transport water, and to forage for and transport resources over long distances. Our results speak to the ability of host trees–particularly a key California species, Q. lobata–to tolerate a changing climate. Climate change is affecting rainfall patterns around the world and many areas are becoming warmer and drier. Purposefully inoculating Q. lobata and other trees with fungi known to provide protection from drought is a way to minimize the effects of climate change, especially in ecologically sensitive areas and restoration sites.