Elevated environmental humidity modulates aquaporin expression and impacts leaf hydraulic efficiency in silver birch ( <i>Betula pendula</i> )

Climate change is projected to increase environmental humidity in northern latitudes, yet its effects on tree hydraulics remain underexplored. We investigated how elevated air relative humidity and increased soil moisture influence water relations, gas exchange, and aquaporin (AQP) expression in silver birch (Betula pendula) at the free air humidity manipulation experiment. We applied air humidification and soil irrigation treatments and measured leaf hydraulic conductance, gas exchange parameters, and AQP transcript levels in leaves to assess physiological and molecular responses. Both treatments significantly decreased leaf hydraulic efficiency, that is the capacity of leaves to transfer water in liquid phase. However, AQP expression responded divergently: air humidification downregulated most AQP transcripts, whereas soil irrigation upregulated them. Despite these changes, gas exchange remained stable, but stomatal sensitivity to vapour pressure deficit declined under both treatments. Our findings suggest distinct regulatory mechanisms of AQPs in response to atmospheric vs edaphic moisture, while resulting in convergent physiological outcomes. The reduced stomatal sensitivity and decoupling between hydraulic performance and photosynthetic activity under nonstress conditions indicates a shift in water-use strategies and highlights the pronounced hydraulic plasticity of B. pendula. These results have implications for predicting tree and forest resilience under future climate scenarios, where increasing humidity may compromise hydraulic safety and water-use regulation during extreme weather events.