DOI: 10.5061/dryad.k6djh9wgn
title: Data from: Trees first inhibit then promote litter decomposition in the subarctic

description: 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 decomposition should be considered in climate-feedback evaluations
 of forest expansion.
description: 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 dried at 40 °C for 48 hrs.  
 To measure litter mass loss in the field, we used litter bags
 (10.0 x 6.4 cm, internal dimensions, and 1.0 x 0.1 mm in mesh size). In
 each litter bag, we placed either 1.0 g of leaf litter or 0.35 g of root
 litter (± 0.001 g, for both litter types), and then closed the top with a
 wire. The leaf litter was 100% mountain birch leaves, whereas the root
 litter consisted of a mixture of the most common vascular plant species in
 the mountain birch forest at the study area (Parker *et al*. 2020), i.e.,
 75% of *Empetrum nigrum* L. ssp. *hermaphroditum* (Hagerup) Böcher, 15%
 mountain birch, 7% *Vaccinium vitis-idaea* L., and 3% *V. myrtillus* L.,
 with proportions estimated from DNA analyses (see below) of pre-incubation
 litter. Subsamples of both litter types were dried at 60 °C for 48 hrs,
 and weighed, to determine the dry mass. Based on this, the dry mass of
 litter in each of the litter bags was 0.929 g and 0.330 g for leaf and
 root litter, respectively.  
 In early June 2017, six paired plots of 20 m diameter were set
 up in the study area. Each pair consisted of one control (untreated) plot
 and one plot in which the mountain birches were girdled in mid-June 2017,
 to reduce the transfer of photosynthate from the canopy to belowground.
 For more detailed information on the study site and the plots. In
 September 2018, two root litter bags were deployed in the upper organic
 soil layer (~5 cm below the soil surface), together with two leaf litter
 bags on top of the organic soil, in each of these paired plots. Each of
 the four litter bags were attached with a metal wire (~5 cm long) to a
 single, plastic stick stuck into the ground, to ease detection, and a
 metal clamp was placed over each leaf litter bag to keep it in place and
 to ensure contact with the organic soil surface. Such a set of four litter
 bags was deployed at four locations (approximately 5 m apart) in each
 plot, making it a total of 16 litter bags in each plot. Retrievals of
 litter bags were made over two years, once in early summer and once in
 early autumn, i.e. June 6 and September 10, 2019, and June 17 and August
 20, 2020. At each date, one randomly chosen pair of leaf and root litter
 bags were retrieved from each plot. At retrieval, the litter bags were
 brought back to the laboratory, freeze dried, and then stored in a freezer
 (-20 °C) before being sorted from exogeneous material and weighed. In
 addition, on August 10, 2022, 47 months after deployment, the remaining
 litter bags were retrieved from all plots.
   <em>Analyses of litter chemistry
 and fungal abundance and community composition</em>
   For organic matter quality evaluation,
 we performed solid state Nuclear Magnetic Resonance (NMR) and C and N
 analyses of litters before deployment as well as after incubations up to
 23 months. Fungal abundance was estimated in all litters based on
 quantitative real-time PCR assays of the fungal ITS2 region, and fungal
 community composition was based on PacBio sequencing of the same marker
 gene. Our PCR primers also targeted all dominant plant species in our
 system, and the relative abundance of different plant species in the root
 mixture before deployment as well as in incubated litters was estimated
 based on the ITS2 sequencing output.
description: # 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) ## Description of the data and file structure The description of the data can be found in Metafile_for_data_files.xlsx. ## Code/software To view the data, Excel or similar is needed.