Interactive effects of waterlogging and atmospheric CO₂ concentration on gas exchange, growth and functional traits of Australian riparian tree seedlings

The ability to survive and thrive in repeatedly waterlogged soils is characteristic of plants adapted to riparian habitats. Rising atmospheric CO₂ has the potential to fundamentally alter plant responses to waterlogging by altering gas exchange rates and stoichiometry, modifying growth, and shifting resource-economic trade-offs to favor different ecological strategies. While plant responses to waterlogging and elevated CO₂ individually are relatively well characterized, few studies have asked how the effects of waterlogging might be mediated by atmospheric CO₂ concentration. We investigated interactive effects of elevated (550 ppm) atmospheric CO₂ and waterlogging on gas exchange, biomass accumulation and allocation, and functional traits for juveniles of three woody riparian tree species. In particular, we were interested in whether elevated CO₂ mitigated growth reduction under waterlogging, and whether this response was sustained following a refractory “recovery” period during which soils were re-aerated. We found species-specific effects of atmospheric CO₂ concentration and waterlogging status on growth, gas exchange, and functional traits between species, and no evidence for a general effect of elevated CO₂ in mediating plant responses to flooding. For one specie, Casuarina cunninghamiana, elevated CO₂ substantially increased growth, but this effect was entirely removed by waterlogging, and there was no recovery following a refractory period. Differential responses to combined waterlogging and elevated CO₂ among species may result in compositional changes to riparian plant communities and associated changes in ecosystem functioning.