Transgressive overyielding in mixed compared with monospecific Scots pine (Pinus sylvestris L.) and oak (Quercus robur L., Quercus petraea (Matt.) Liebl.) stands – Productivity gains increase with annual water supply

•Mixed Scots pine-oak stands show 15% higher standing volume and 14% higher volume productivity than adjacent monocultures.•Productivity gains increase in years with higher water supply.•Both Scots pine and oak modify their morphology in mixture.•Admixture of Scots pine significantly increases size partitioning of oak.