Functional adjustment under lethal drought and physiological memory to water stress of two phylogenetically close and coexisting conifers

Abstract

Forest die-off can primarily affect the least drought-tolerant species and mortality be modulated by physiological legacies left by drought. To characterize the functional response and physiological memory to water stress of two phylogenetically and functionally close conifers with current contrasting field mortality and regeneration dynamics, we conducted a drought experiment in which seedlings were driven to death. Both species exhibited isohydric behavior with small hydroscapes. Yet, hydraulic performance, osmotic adjustment and gas exchange dynamics under rising water stress consistently showed that Pinus pinea was more resistant to drought than Pinus pinaster. Under water stress, P. pinaster exhibited greater water use efficiency, but higher photochemical damage and more constrained photosynthetic and stomatal conductance dynamics than P. pinea. Xylem conductivity loss under water stress and the wilting point were lower in P. pinea. Osmotic potential at full turgor decreased in P. pinea but it increased in P. pinaster with increasing water stress below a water potential threshold. Consequently, lethal water stress levels were higher for P. pinea than for P. pinaster. Synergistically, cumulative water stress induced physiological legacies curtailing carbon uptake and hydraulic performance in both species. Negative physiological legacies left by memory to water stress in plants, might explain delayed tree mortality years after drought. The functional responses to drought characterized agree with current field demographic dynamics under climate change in mixed evergreen forests where more drought-tolerant taxa like P. pinea are replacing formerly dominated P. pinaster stands.