Shifts of irrigation in Aleppo pine under semi-arid conditions reveal uncoupled growth and carbon storage and legacy effects on wood anatomy

Abstract

Climate models forecast progressive aridification in the Mediterranean region. These warmer and drier conditions will probably lead to a decrease in carbon fixation and growth rates of drought-prone forests. To improve our knowledge on how water availability drives radial growth, wood anatomy and carbon storage we compared changes during a 40-year period in tree-ring width, wood-anatomical traits (tracheid lumen area, cell-wall thickness and parenchyma area), and stem sapwood non-structural carbohydrates (NSC) concentrations. We studied the drought-tolerant Aleppo pine (Pinus halepensis) comparing trees subjected to three irrigation regimes (control trees, no irrigation; IR trees, irrigated trees; TI trees, temporarily irrigated trees). Tree-ring width of TI trees widely overtook growth rates shown by IR trees during the irrigation treatment (1980s), whereas transversal lumen area and cell-wall thickness presented similar values in both groups of trees. Non-irrigated trees showed the lowest growth rates and also formed tracheids with the smallest lumen area and the narrowest cell walls. After the irrigation period, TI and trees without irrigation showed similar year-to-year growth variability. Lumen area of TI trees took 4–5 years to show values similar to those of the non-irrigated group. The TI trees had lower ray parenchyma area percentages during the irrigation period; afterwards both TI and non-irrigated trees formed similar amounts of ray parenchyma which were always higher than in IR trees. NSC concentrations were lower for IR trees suggesting either a dilution effect due to their significantly larger biomass or a higher demand for NSC by other carbon sinks such as primary growth. Aleppo pine may coordinate carbon supply and demand (growth) under changing conditions of water availability but long-term modification in soil moisture lead to legacy effects on wood anatomy.