Strong Coupling of Shoot Assimilation and Soil Respiration during Drought and Recovery Periods in Beech As Indicated by Natural Abundance δ13C Measurements

Drought down-regulates above- and belowground carbon fluxes, however, the resilience of trees to drought will also depend on the speed and magnitude of recovery of these above- and belowground fluxes after re-wetting. Carbon isotope composition of above- and belowground carbon fluxes at natural abundance provides a methodological approach to study the coupling between photosynthesis and soil respiration (SR) under conditions (such as drought) that influence photosynthetic carbon isotope discrimination. In turn, the direct supply of root respiration with recent photoassimilates will impact on the carbon isotope composition of soil-respired CO2. We independently measured shoot and soil CO2 fluxes of beech saplings (Fagus sylvatica L.) and their respective delta C-13 continuously with laser spectroscopy at natural abundance. We quantified the speed of recovery of drought stressed trees after re-watering and traced photosynthetic carbon isotope signal in the carbon isotope composition of soil-respired CO2. Stomatal conductance responded strongly to the moderate drought (-65%), induced by reduced soil moisture content as well as increased vapor pressure deficit. Simultaneously, carbon isotope discrimination decreased by 8 parts per thousand, which in turn caused a significant increase in delta C-13 of recent metabolites (1.52.5 parts per thousand) and in delta C-13 of SR (11.5 parts per thousand). Generally, shoot and soil CO2 fluxes and their delta C-13 were in alignment during drought and subsequent stress release, clearly demonstrating a permanent dependence of root respiration on recently fixed photoassimilates, rather than on older reserves. After re-watering, the drought signal persisted longer in delta C-13 of the water soluble fraction that integrates multiple metabolites (soluble sugars, amino acids, organic acids) than in the neutral fraction which represents most recently assimilated sugars or in the delta C-13 of SR. Nevertheless, full recovery of all aboveground physiological variables was reached within 4 days and within 7 days for SR indicating high resilience of (young) beech against moderate drought.