The “isohydric trap”: A proposed feedback between water shortage, stomatal regulation, and nutrient acquisition drives differential growth and survival of European pines under climatic dryness

Abstract

Climatic dryness imposes limitations on vascular plant growth by reducing stomatalconductance, thereby decreasing CO2 uptake and transpiration. Given that transpiration-driven water flow is required for nutrient uptake, climatic stress-induced nutrientdeficit could be a key mechanism for decreased plant performance underprolonged drought. We propose the existence of an "isohydric trap," a drynessinduceddetrimental feedback leading to nutrient deficit and stoichiometry imbalancein strict isohydric species. We tested this framework in a common garden experimentwith 840 individuals of four ecologically contrasting European pines (Pinushalepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature andlow soil water availability. We measured growth, survival, photochemical efficiency,stem water potentials, leaf isotopic composition (d13C, d18O), and nutrient concentrations(C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower d18O and higher d13C values than the other species, indicatinghigher time-integrated transpiration and water-use efficiency (WUE), along withlower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greaterdry-biomass (up to 63-fold) and survival (100%).