COS 98-6 - Drought memory of soil microbes and their effects on tree physiological function

Abstract: The degree to which forests provide ecosystem services in the wake of global change depends, in part, on how they cope with stressors like drought. Most research on drought tolerance of trees has focused on the physiological and morphological adaptations trees possess to survive water stress. However, emerging research suggests that edaphic conditions and plant-associated microbes can influence tree drought tolerance. Using a greenhouse experiment, we investigated whether soil microbes exposed to reduced moisture in the past affect the sensitivity of hardwood trees to experimentally-imposed drought. We hypothesized that trees growing in soils with a history of dry conditions experience less water stress in subsequent drought events due to the upregulation of drought resilience mechanisms in the microbial communities. We planted saplings of tulip poplar, chokecherry, and red oak in field soil exposed to ambient moisture or field soils exposed to a 55% reduction of moisture (relative to ambient) for 5 years. Then, we exposed trees to either weekly or biweekly watering regimes for 10 weeks and quantified weekly changes in photosynthetic assimilation rate, stomatal conductance, stem elongation, and stem diameter. In a subset of time periods, we measured soil and leaf water potential to determine whether soil history affected overall plant water-use strategies (i.e., degree of isohydry vs. anisohydry).

Overall, soil history affected tree species sensitivity to drought for chokecherry but had no effects on tulip poplar and red oak. When planted in soil with a history of reduced moisture, chokecherries were buffered from the effects of a 14d drought, demonstrating 73% greater photosynthetic assimilation rates (p=0.0096), 78% greater stomatal conductance (p=0.0173), and 34% greater leaf water potential (p=0.0001) than chokecherries planted in soil with an ambient moisture soil history. These effects were not apparent in the tulip polars and red oaks, which respectively showed 83% and 66% reductions in photosynthetic assimilation rates when water was withheld regardless of soil moisture history. Soil history had no effects on where tree species fell along the isohydric to anisohydric spectrum, as well as on stem diameter growth and stem elongation. Collectively, our study highlights how trees may differentially interact with associated soil microbes, leading to interspecific variation of trees’ responses of drought stress.