The productivity of economically valuable focal trees in mixtures is often improved by inclusion of lower- value nursing species, but the mechanisms underpinning such effects are poorly resolved. This gap in understanding limits the capacity to develop efficient planting strategies for forests and woodlands to contribute to net-zero and other critical ecosystem functions.

Here, we undertook a plant-soil feedback experiment to test the hypothesis that feedback effects improve the biomass of Sitka spruce (Picea sitchensis (Bong.) Carr) in soil conditioned by monocultures of heterospecific nurse species, and a mixture comprising Sitka spruce and heterospecifics, compared to soil conditioned by only Sitka spruce.

Sitka spruce saplings had greater biomass in soil conditioned by Scots pine monocultures (Pinus sylvestris L.) and the mixture compared to their own soil or soil conditioned by silver birch (Betula pendula Roth). Statistical models showed that colonisation of ectomycorrhizal fungi on tree roots in the feedback phase was positively related to seedling biomass, and significantly influenced plant growth strategy.

Soil inorganic nitrogen concentrations were strongly affected by monocultures and mixtures in the conditioning phase, but these effects were not related to biomass of seedlings in the feedback phase. However, the positive associations among microbial biomass nitrogen, extractable ammonium in soil, and the activity of the nitrogen-degrading enzyme N-acetylglucosaminidase may influence seedling biomass in the longer-term by stimulating nitrogen cycling.

Synthesis and Applications:

Our findings show that in the crucial early phases of tree growth, plant-soil feedback plays an important role in shaping productivity of Sitka spruce and the nitrogen cycle in forest soils, the latter which may have important consequences for tree biomass in the longer-term. Our findings demonstrate the importance of the nursing effect in the early stages of plant growth and provide a mechanistic explanation both for long-held observations of nursing effects in forestry systems and for biodiversity effects in natural woodlands. The nursing effect was largely driven by the activity of symbiotic ectomycorrhizal fungi, and therefore it is important for practitioners to ensure nursing species monocultures or mixtures support ectomycorrhizal fungi that are compatible with high-value target species.