The effect of elevated CO2 on aboveground and belowground carbon allocation and eco-physiology of four species of angiosperm and gymnosperm forest trees

Abstract

Although atmospheric CO₂ concentration ([CO₂]) continues to rise, the question of how tree carbon (C) allocation is affected by this change remains. Studies show that C assimilation increases under elevated CO₂ (eCO₂). Yet, no detailed study has determined the fate of the surplus C, i.e., its compartment and physiological process allocation, nor in multiple species together. In this project, we grew 2-year-old saplings of four key Mediterranean tree species (the conifers Cupressus sempervirens L. and Pinus halepensis Mill., and the broadleaf Quercus calliprinos Webb. and Ceratonia siliqua L.) to [CO₂] levels of 400 or 700 p.p.m. for 6 months. We measured the allocation of C to below and aboveground growth, respiration, root exudation, storage and leaf litter. In addition, we monitored intrinsic water-use efficiency (WUE), soil moisture, soil chemistry and nutrient uptake. Net assimilation, WUE and soil nitrogen uptake significantly increased at eCO₂ across the four species. Broadleaf species showed soil water savings, which were absent in conifers. All other effects were species-specific: Cupressus had higher leaf respiration, Pinus had lower starch in branches and transiently higher exudation rate and Quercus had higher root respiration. Elevated CO₂ did not affect growth or litter production. Our results are pivotal to understanding the sensitivity of tree C allocation to the change in [CO₂] when water is abundant. Species-specific responses should be regarded cautiously when predicting future changes in forest function in a higher CO₂ world.