DOI: 10.5061/dryad.qjq2bvqkj
title: How detritivores, plant traits and time modulate coupling of leaf versus woody litter decomposition rates across species

description: 1. Plant functional traits are increasingly used to understand
 ecological relationships and (changing) ecosystem
 functions. For understanding ecosystem-level biogeochemistry, we need to
 understand how (much) traits co-vary between different
 plant organs across species, and its implications for litter
 decomposition. However, we do not know how the degree of
 synchronous variation in decomposition rates between
 organs across species could be influenced by different keystone
 invertebrates decomposing different senesced plant organs,
 especially in warm-climate forests. Here we asked
 whether interspecific patterns in wood and leaf decomposition
 rates and in the spectra of resource
 economics traits underpinning them,
 co-vary across woody species; and how (much) the
 keystone invertebrate decomposers of the litter of these organs
 enhance or lower such co-variation of decomposition
 rates through time.  2. We
 addressed these questions through an 18-month “common-garden”
 decomposition experiment using leaf, twig and branch litter of 41 woody
 species in two distant subtropical forest sites in east China. We
 quantified the effects of leaf,
 twig, and branch functional traits and their
 respective key invertebrates (moth larvae, termites) on the decomposition
 rates of those organs.  3. Interspecific
 variation in wood traits was partly decoupled from that in leaf
 traits across species, while strong coupling was found between twigs and
 branches. The co-variation between leaf and woody organ
 decomposition rates was altered dynamically through the
 shifting activities of the key decomposers, which created
 non-linear relationships of invertebrate litter consumption as a function
 of species rankings along the resource economic trait spectra of leaves
 and branches. 4. The deviations from coupling of decomposition
 rates between organs were likely caused by combinations of three
 mechanisms: (1) (de-)coupling between organs of other traits, not commonly
 considered in resource economics spectra (e.g., resins) (2) leaf
 and wood decomposers having specific diet requirements, and (3) temporal
 patterns of the decomposers’ activity. 5. Synthesis. Our study
 highlights the importance of considering the different ways by
 which invertebrate detritivores drive decomposition
 processes through time. Under the ongoing biodiversity
 decline, future research would benefit from a better understanding
 of the role of the dynamic interactions between
 detritivore activities and plant functional traits on the carbon turnover
 in ecosystems.
description: The twig component of this experimental work, including the huge
 twig-related dataset for traits, termite consumption and decomposition
 across 41 species in two sites, is entirely new and critical to testing
 our conceptual model and hypotheses. The work and datasets for leaf and
 branch traits and decomposition rates overlap strongly (but not
 completely; see our new data subsets on leaf and branch decomposition
 below) with those used in two previous studies on single tree organs to
 test different hypotheses unrelated to organ co-variation of traits and
 decomposition (Guo et al., 2020; 2021); for these organs we provide brief
 methods here with reference to these two studies for further
 details. Study sites We conducted
 the in situ decomposition experiment Funlog in two subtropical evergreen
 broad-leaf forest sites in Zhejiang Province, E-China (details in Suppl.
 Table S1; Guo et al., 2020; 2021), both with subtropical monsoon climate:
 i) Tiantong National Forest Park (TT) (29°52′N, 121°39′E) on the mainland,
 with Schima superba as the predominant tree species; ii) Putuo island (PT)
 (29°97′N, 121°38′E), in the Zhoushan archipelago, with Quercus
 glauca being predominant. Based on previous observations (Guo et al.,
 2020), the moth Arippara indicator Walker is a key leaf litter detritivore
 in the PT forest litter layer. While 3-4 generations inhabit the litter
 layer from May to November (Leraut, 2013), larval abundance and
 consumption activity peak from August through November (details in
 Appendix S1: Fig. S1). In both sites, termites are the main detritivores
 of deadwood including branches in the litter layer (details in
 Appendix S1: Fig. S2; Guo et al., 2021). Tree species
 and sampling In October-November 2017, we collected
 leaf, twig, and branch samples from 41 common woody species in TT and PT,
 8 evergreen trees, 12 evergreen shrubs (including short understory trees),
 11 deciduous trees, 5 deciduous shrubs and 5 conifer trees (see Guo et
 al., 2021, details in Appendix S1: Table S2). In total, 195 individual
 healthy adult trees/shrubs were selected, i.e., 27 species × 3 tree
 individuals and 19 species × 6 shrub individuals. The rationale for
 selecting living trees was (1) to standardize the initial, undegraded
 phase for all samples (Cornelissen et al., 2012); (2) to mimic
 typhoon-induced wind-throw and logging as predominant agents of deadwood
 formation on the forest floor. We chain-sawed 20 cm long branch sections
 of approx. 5cm diameter; cut twig samples into 10 cm long sections of
 approx. 2 cm diameter (to standardize diameter as a possible covariate of
 decomposition rate). Adjacent to each end of a branch we cut off a 2 cm
 thick disk for initial branch trait analyses, while for twigs and leaves
 we randomly selected subsamples from each individual per species for
 initial trait measurement. All leaf litter and twig samples were stored
 air-dried until further use. Due to space/time constraints, branch samples
 were weighed at field moisture before sealing them into litter-bags (see
 below). Litter decomposition experiment
 Similar sub-experiments were set up simultaneously at both sites.
 Leaf, twig, and branch subsamples from each individual per species were
 weighed air-dried and oven-dried (75 °C) for the calculation of initial
 dry mass of the litter-bag samples via water content. For the leaf litter
 incubation, details are in Guo et al. (2020), but here we added a harvest
 after 18 months, which yielded a large new dataset compared to that study.
 Briefly, 10g of pre-weighed air-dried leaves were sealed into 1-mm mesh
 nylon litter-bags, with broadly similar packing density among species. For
 the twig incubation, we sealed two pre-weighed twig sections per sample
 into 10 × 20 cm nylon litter-bags with 4 mm mesh. For branch incubation
 (details in Guo et al., 2021, but here we added an additional 12 woody
 species to the decomposition dataset), we used 4 mm mesh nylon
 litter-bags. These different mesh sizes were based on achieving the best
 compromise in terms of allowing free access to the main detritivores
 (allowing ranking of the “natural” contribution of invertebrates to
 decomposition for each organ) while preventing litter particles from
 falling out throughout the decomposition trajectory for each of the tree
 organs – the latter was especially important for leaf litter of
 small-leaved conifers and fragile-leaved deciduous species. Based on field
 surveys before the experiment, the main leaf litter detritivores were moth
 larvae (Guo et al., 2020), which could enter the 1 mm mesh easily.
 Termites, which are estimated to be responsible for &gt; 90 % of the
 invertebrate contribution to decomposition, were found to have body
 diameters between 1.21 mm and 2.67 mm (authors’ measaurements), leading to
 the choice of 4 mm mesh. The only other possible significant contributor
 to leaf and woody litter decomposition in our study sites, large
 millipeds, have body diameters around 7 mm, so these had to be excluded
 from the experiment altogether to prevent litter particle losses from the
 litter-bags. In total, 2469 litter-bag samples were used, based on 46 tree
 species (including 5 repeated species) × 3 organs × 3 replications (plots)
 × 2 incubation sites × 3 harvest times. The litter-bag
 samples were distributed over three replicate forest plots in each sites,
 in January 2018 (details in Guo et al., 2020; 2021). The litter-bags of
 each species were pinned onto the forest floor in their respective
 subplots within each of the three replicate plots randomly, and each
 species’ replicate had three litter-bags per organ one for each harvest.
 Harvests were after 6 (Jul. 2018), 12 (Jan. 2019), and 18 months
 (Jul. 2019).  We carefully cleaned the collected leaf
 samples with a brush. Each twig and branch sample was put in a large tray
 with steep, tall edges to retain the termites. We cut the sample into
 small pieces and removed soil (brought in by termites) with a brush
 carefully. All samples were then oven-dried at 75 °C to constant mass and
 weighed (dry mass). Plant organ trait
 measurements Details of leaf trait measurements
 relevant to the LES, including leaf thickness, leaf chemistry, specific
 leaf area (SLA) and leaf dry matter content (LDMC), were reported by Guo
 et al. (2020). For initial wood traits considered
 relevant to the WES and to decomposition, each 2 cm subsample of branch or
 10 cm long twig was stored cool in a sealed plastic bag between collection
 in the field and processing. Within 12h, after bark removal, a subsample
 was cut from each disk of branch and twig to obtain fresh mass and
 determine initial volume (Williamson & Wiemann, 2010). All wood
 subsamples were dried at 75 °C for 72h. Initial wood density (WD) was
 calculated as dry mass per volume. For chemical trait
 measurements of leaf and woody litter, initial leaf, twig and branch
 subsamples were ground to powder. Thereafter, 0.2g sub-samples were
 digested using concentrated H2SO4 to determine N and P concentrations on
 an infrared spectrophotometer (Smartchem 200, Alliance, France). For
 branches, lignin content was determined by acidolysis-titration, and
 cellulose content was determined by anthrone-sulfuric acid
 colorimetry. Moth larvae and termite activity
 measurements Upon harvesting of the leaf litter bags,
 macrofauna (Arippara indicator larvae) were collected and counted, and
 moth larvae faeces extracted, oven-dried at 75 °C and weighed for dry mass
 (details in Guo et al., 2020). The feeding intensity of larvae was defined
 by larval abundance and divided into six classes: (0), (1), (2), (3), (4)
 indicated 0, 1, 2 3, 4 larvae, respectively; (5) ≥ 5 larvae. 
 We visually scored the termites’ feeding intensity of branches
 and twigs after cleaning the litter-bag samples. We (see Guo et al., 2021
 for details) adjusted the method that classifies termite feeding intensity
 based on the bite marks (Liu et al., 2015). Briefly, we measured the
 percentage surface area loss due to termite activity using a visual grid
 method. After removing soil brought in by termites, we estimated the depth
 of termite damage in five random spots along the sample surface. The area
 loss and foraging depth were used to estimate the percentage of sample
 volume of twig or branch consumed by termites, distinguishing 5 classes;
 (1) 1-10% (2) 11-20%, (3) 21-30%, (4) 31-40%, (5) &gt; 41% volume
 loss.