Isotopic and Water Relation Responses to Ozone and Water Stress in Seedlings of Three Oak Species with Different Adaptation Strategies

The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O₃) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity—WS and WW treatments, respectively) and O₃ exposure (1.0, 1.2, and 1.4 times the ambient concentration—AA, 1.2AA, and 1.4AA, respectively) were carried out on Quercus robur L., Quercus ilex L., and Quercus pubescens Willd. seedlings, to study physiological traits (1. isotope signature [δ¹³C, δ¹⁸O and δ¹⁵N], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, A_sat, and stomatal conductance, g_s]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased A_sat in Q. robur and Q. pubescens while water stress decreased it in all three oak species. Ozone did not affect δ¹³C, whereas δ¹⁸O was influenced by O₃ especially in Q. robur. This may reflect a reduction of g_s with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O₃ on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by δ¹³C and by δ¹⁸O in all three oak species, while δ¹⁵N did not define plant response to stress conditions in any species. The δ¹³C signal was correlated to leaf water content (LWC) in Q. robur and Q. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O₃ exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy. View Full-Text