Species Diversity of Oak Stands and Its Significance for Drought Resistance

Interspecific Drought Cuing in Plants

Plants readily communicate with their pollinators, herbivores, symbionts, and the predators and pathogens of their herbivores. We previously demonstrated that plants could exchange, relay, and adaptively utilize drought cues from their conspecific neighbors. Here, we studied the hypothesis that plants can exchange drought cues with their interspecific neighbors. Triplets of various combinations of split-root Stenotaphrum secundatum and Cynodon dactylon plants were planted in rows of four pots. One root of the first plant was subjected to drought while its other root shared its pot with one of the roots of an unstressed target neighbor, which, in turn, shared its other pot with an additional unstressed target neighbor. Drought cuing and relayed cuing were observed in all intra- and interspecific neighbor combinations, but its strength depended on plant identity and position. Although both species initiated similar stomatal closure in both immediate and relayed intraspecific neighbors, interspecific cuing between stressed plants and their immediate unstressed neighbors depended on neighbor identity. Combined with previous findings, the results suggest that stress cuing and relay cuing could affect the magnitude and fate of interspecific interactions, and the ability of whole communities to endure abiotic stresses. The findings call for further investigation into the mechanisms and ecological implications of interplant stress cuing at the population and community levels.

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, Asat, and stomatal conductance, gs]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased Asat 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 gs 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.