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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
Authors:Diego Salazar‐Tortosa  Jorge Castro  Pedro Villar‐Salvador  Benjamín Viñegla  Luis Matías  Anders Michelsen  Rafael Rubio de Casas  José I Querejeta
Institution:1. Departamento de Ecología, Facultad de Ciencias, Universidad de Granada, Granada, Spain;2. Forest Ecology and Restoration Group, Departamento de Ciencias de la Vida, Universidad de Alcalá, Madrid, Spain;3. Departamento de Biología Animal, Biología Vegetal y Ecología, Facultad de Ciencias Experimentales, Universidad de Jaén, Jaén, Spain;4. Terrestrial Ecology Section, Department of Biology, University of Copenhagen, Copenhagen ?, Denmark;5. Centro de Edafología y Biología Aplicada del Segura (CEBAS‐CSIC), Murcia, Spain
Abstract:Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO2 uptake and transpiration. Given that transpiration‐driven water flow is required for nutrient uptake, climatic stress‐induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an “isohydric trap,” a dryness‐induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ13C, δ18O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ18O and higher δ13C values than the other species, indicating higher time‐integrated transpiration and water‐use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry‐biomass (up to 63‐fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ18O and lower δ13C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.
Keywords:climatic change  hotter drought  nutrients  stable isotopes  stoichiometry  stomatal behaviour  water use efficiency
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