Compounding Environmental Stressors Cause Governing Hydraulic Behaviours to Shift From Roots to Leaves in Avicennia germinans

Abstract

Mangrove forests are expanding poleward due to ongoing climate change. Near the range limits of mangrove expansion, stressors such as salinity and vapour pressure deficit play a critical role in shaping ecosystem carbon and water fluxes. These stressors, which often compound, are expected to become more severe with ongoing climate change. Here, we analysed the independent and coupled impacts of salinity and VPD stresses on plant hydraulics and photosynthesis in Avicennia germinans in a greenhouse experiment. We exposed A. germinans grown in low-salinity (10 parts per thousand [ppt]), midrange-salinity (20 ppt) and high-salinity (40 ppt) regimes to a 30-ppt NaCl treatment and to high- and low-VPD conditions. Plants experiencing high osmotic stress had a stronger relationship between ψ_s and VPD than plants experiencing lower osmotic stress, highlighting the impact of compounding stressors on plant hydraulics. Under osmotic stress and non-limiting VPD conditions, root traits regulated gas exchange and water movement. Under high VPD, the most dominant water-regulating traits shifted from roots to leaves, with increased stomatal closure acting to conserve water at the cost of reduced photosynthetic uptake. Isohydricity in A. germinans was revealed to be dynamic. Under increased atmospheric and osmotic stress, plants become more isohydric. While under low stress, they behaved more anisohydrically. Plants maximized carbon gain when chances of embolism were low and minimized water loss at the expense of carbon gain under high-stress scenarios. Dynamic shifts acted as a resilience mechanism against cavitation, allowing plants to survive under a wide range of conditions. Our results highlight the plasticity of A. germinans' hydraulic strategy and its ability to cope with combined salinity and VPD stresses.