Species-specific reliance of trees on ectomycorrhizal fungi for nitrogen supply at an alpine treeline

Low temperatures near alpine treelines limit microbial release of soil nitrogen and tree growth. Ectomycorrhizal fungi can increase nitrogen supply for trees, but the importance of this exchange of carbon for nitrogen at the treeline remains unclear. Our bomb radiocarbon measurements indicated that trees transferred carbon fixed <2 years previously to fungi. The allocated carbon likely included sugars involved in starch synthesis, as δ¹³C in fungal caps closely resembled that of fine-root starch. Mass balance of nitrogen isotopes along the plant-fungi-soil continuum revealed that Larix decidua trees relied less on fungal nitrogen (0–35% of N uptake), compared to Pinus mugo trees (up to 41%). We estimated that treeline trees allocate up to 18% of photoassimilates to fungi. Our findings suggest that at alpine treelines, trees allocate to ectomycorrhizal symbionts relatively high amount of carbon compared to the reverse nitrogen flux, but the exact exchange is tree species-specific.