As annual atmospheric CO2 concentration continues its monotonic increase, the resultant changes in climate are on track to increase global temperature by > 2 °C by 2100. Temperature increases are expected to be substantially greater in the boreal and arctic biomes, which could have significant impacts on net ecosystem carbon uptake and atmospheric feedbacks. At the southern edge of the boreal forest large, open-topped chambers are exposing a natural peatland ecosystem to whole-ecosystem warming × CO2 enrichment (https://mnspruce.ornl.gov/). After three years of warming (+0, +2.25, +4.5, +6.75, +9 °C) and two years of CO2 (+0, +500ppm) treatments, we assessed physiological and hydraulic responses of the deciduous Larix laricina and evergreen Picea mariana trees, as well as the semi-evergreen Rhododendron groenlandicum and Chamaedaphne calyculata shrubs. We hypothesized there would be differential hydraulic and carbon stress due to differential plant functional strategies and degree of physiological plasticity. Thus, we tracked seasonal sap flow, water potential, non-structural carbohydrates (NSC), water-use efficiency, photosynthetic pigments and fluorescence trait responses between July 2018 and July 2019. Observational damage indices were also noted and photographed, and particular attention was given to assessing visibly damaged versus undamaged trees.
Results/Conclusions
There was some branch tip and top dieback observed on several L. laricina and P. mariana trees in the +9 °C, ambient CO2 plot, but not in the +9 °C, elevated CO2 plot, despite a lower tree density. Damaged trees displayed lower stored stem NSC and increased water potential stress. Compared to pretreatment measurements, the warming treatments induced substantially lower water potentials for L. laricina and C. calyculata but had little effect on P. mariana or R. groenlandicum. L. laricina in the warmest plots (+6.75 and +9 °C) displayed excessive hydraulic stress where midday water potentials dropped below -2.5 MPa. In contrast, P. mariana maintained hydraulic safety with water potentials below the turgor loss point, but the fluorescence parameter Fv/Fm indicated reduced quantum efficiency of PSII versus L. laricina, and reduced C uptake could lead to excessive carbon stress. C. calyculata also had lower Fv/Fm than R. groenlandicum suggesting greater carbon stress. Excessive hydraulic and carbon stress in C. calyculata as compared with R. groenlandicum, and alternate hydraulic (L. laricina) or carbon (P. mariana) stress in the trees may lead to a shift in community composition and net C uptake in this ecosystem under future warming.