Data from: Plant-soil feedback drives the nursing effect on Sitka spruce

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.