Trees first inhibit then promote litter decomposition in the subarctic

Trees affect organic matter decomposition through allocation of recently fixed carbon belowground, but the magnitude and direction of this effect may depend on substrate type and decomposition stage. Here, we followed mass loss, chemical composition, and fungal colonization of leaf and root litters incubated in mountain birch forests over four years, in plots where belowground carbon allocation was severed by tree girdling or in control plots. Initially, girdling stimulated leaf and root litter mass loss by 12 and 22%, respectively, suggesting competitive release of saprotrophic decomposition when tree-mediated competition by ectomycorrhizal fungi was eliminated (Gadgil effect). After four years, girdling instead hampered mass loss of root litter by 30%, suggesting late-stage priming of decomposition in the presence of trees, in parallel with increased growth of shrubs and associated fungi following tree elimination. Hence, different mechanisms driving early- and late-stage litter decom..., The study was carried out in Abisko, Swedish Lapland (68°20´60´´ N, 18°49´48´´ E), on a mountain slope 500-600 m above sea level 5 km southeast of Abisko (68°19´08´´to 68°18´31´´N and 18°49´00´´to 18°50´24´´E), in the treeline ecotone, with patchy stands of mountain birch (*Betula pubescens* Ehrh. ssp. *czerepanovii* (Orlova) Hämet Ahti). Mean annual temperature and annual precipitation at the Abisko Scientific Research Station during the study period (2018-2022) were 0.69 °C and 269 mm, respectively. On September 9, 2018, leaf and root litter was collected from the study area. Leaves of mountain birch were collected from the ground beneath several birch individuals and were brought back to the laboratory. To obtain root litter, organic layer material was sieved in the field, and root material was brought back to the laboratory, where fine roots (≤ 2 mm in diameter) were recovered and cleaned in distilled water to remove external organic and inorganic particles. Roots and leaves were dr..., , # Data from: Trees first inhibit then promote litter decomposition in the subarctic

[https://doi.org/10.5061/dryad.k6djh9wgn](https://doi.org/10.5061/dryad.k6djh9wgn)

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