Summary: Understanding how plant water uptake interacts with acquisition of soil nitrogen (N) and other nutrients is fundamental for predicting plant responses to a changing environment, but it is an area where models disagree. We present a novel isotopic labelling approach which reveals spatial patterns of water and N uptake, and their interaction, by trees. The stable isotopes¹⁵N and²H were applied to a small area of the forest floor in stands with high and low soil N availability. Uptake by surrounding trees was measured. The sensitivity of N acquisition to water uptake was quantified by statistical modelling. Trees in the high‐N stand acquired twice as much¹⁵N as in the low‐N stand and around half of their N uptake was dependent on water uptake (²H enrichment). By contrast, in the low‐N stand there was no positive effect of water uptake on N uptake. We conclude that tree N acquisition was only marginally dependent on water flux toward the root surface under low‐N conditions whereas under high‐N conditions, the water‐associated N uptake was substantial. The results suggest a fundamental shift in N acquisition strategy under high‐N conditions.

The new European Union Forest Strategy for 2030 aims to plant an additional 3 billion trees on non-forest land to mitigate climate change. However, the choice of tree species for afforestation to achieve the maximum climate benefit is unclear. We compared the climate benefit of six different species in terms of carbon (C) sequestration in biomass and the harvested wood substitution in products to avoid carbon dioxide (CO₂) emissions from fossil-based materials over the 100-year period by afforesting about ¼ of the available area in northern Europe. The highest climate benefit was observed for larch, both at a stand scale (1626 Mg CO₂ eqv. ha⁻¹) and at the landscape level for the studied scenario (579 million Mg CO₂ eqv.). Larch was followed by Norway spruce, poplar, hybrid aspen and birch, showing a climate benefit about 40–50% lower than that for larch. The climate benefit of willow was about 70% lower than larch. Willow showed 6–14-fold lower C stocks at the landscape level after 100 years than other tree species. The major climate benefit over the 100-year period comes from wood substitution and avoided emissions, but C stock buildup at the landscape level also removes significant amounts of CO₂ already present in the atmosphere. The choice of tree species is important to maximize climate change mitigation.