Elevated CO2 Exacerbates Flooding-Induced Mortality in Black Spruce While Enhancing Adventitious Root Formation in Tamarack

Anthropogenic increases in atmospheric CO concentrations, coupled with climate change-induced alterations in hydrological regimes, are increasing flood frequency across many boreal regions.Prolonged flooding creates oxygen-deprived soils that force roots to shift from aerobic respiration to anaerobic fermentation, an inefficient metabolic pathway that compromises carbohydrate utilization and leads to root dieback.We examined the interactive effects of flooding and CO levels on two ecologically important boreal species: black spruce (Picea mariana [Mill.]Britton, Sterns & Poggenb.) and tamarack (Larix laricina [Du Roi] K. Koch).Seedlings were exposed to a 28-day flooding event under ambient (AC, 400mol mol) and elevated (EC, 800mol mol) CO concentrations.Flooding significantly reduced root respiration (-16% during treatment; -36% 35 days post-recovery), root biomass (-66%), root volume (-58%), and overall growth (-70%).While Photosystem II quantum efficiency declined under flooding, photosynthetic rates remained stable.Species responses diverged markedly: flooded black spruce exhibited 2.1 higher mortality under EC (59%) versus AC (28%), whereas tamarack developed adventitious roots 2.9 more frequently under EC (53%) than AC (18%) with no mortality.Contrary to our hypothesis, EC failed to mitigate flood stress or enhance recovery through carbohydrate supplementation.These findings demonstrate that elevated CO exacerbates flood vulnerability in black spruce while promoting tamarack's adaptive capacity through morphological plasticity.Such differential responses suggest future climate conditions may drive compositional shifts in boreal forests, favoring tamarack in increasingly flood-prone landscapes.