DOI: 10.5061/dryad.z8w9ghxdk
title: Litter decomposition rates across tropical montane and lowland forests are controlled foremost by climate

description: The “hierarchy of factors” hypothesis states that decomposition rates are
 controlled primarily by climatic, followed by biological and soil
 variables. Tropical montane forests (TMF) are globally important
 ecosystems, yet there have been limited efforts to provide a biome-scale
 characterization of litter decomposition. We designed a common litter
 decomposition experiment replicated in 23 tropical montane sites across
 the Americas, Asia, and Africa and combined these results with a previous
 study of 23 sites in tropical lowland forests (TLF).
  Specifically, we investigated (1) spatial heterogeneity in
 decomposition, (2) the relative importance of biological factors that
 affect leaf and wood decomposition in TMF and, (3) the role of climate in
 determining leaf litter decomposition rates within and across the TMF and
 TLF biomes. Litterbags of two mesh sizes containing Laurus nobilis leaves
 or birchwood popsicle sticks were spatially dispersed and incubated in TMF
 sites, for 3 and 7 months on the soil surface and at 10-15 cm depth. The
 within-site replication demonstrated spatial variability in mass loss.
 Within TMF, litter type was the predominant biological factor influencing
 decomposition (leaves &gt; wood), with mesh and burial effects playing
 a minor role. When comparing across TMF and TLF, climate was the
 predominant control over decomposition, but the Yasso07 global model
 (based on mean annual temperature and precipitation) only modestly
 predicted decomposition rate. Differences in controlling factors between
 biomes suggest that TMF, with their high rates of carbon storage, must be
 explicitly considered when developing theory and models to elucidate
 carbon cycling rates in the tropics.
description: <strong>Study
 sites</strong> We conducted a large-scale field
 experiment across 23 TMF sites (column B in data file, rows 2-24) located
 in 14 countries in the Americas, Asia, Africa, and several islands in the
 Caribbean and the Pacific . These sites spanned a wide range of elevations
 (600-3202 m), mean annual temperatures (MAT, 3.0-23.0 ⁰C), mean annual
 precipitations (MAP, 335-5010 mm), and dry season length (0-6 months). We
 augmented our study by including a previously published TLF dataset
 (Powers et al. 2009) that comprised of 23 tropical sites in 14 countries
 in Asia, Latin America, and Oceania, spanning a wide range of elevations
 (5-1200 m), mean annual temperatures (MAT, 18.5-26.5 ⁰C), mean annual
 precipitations (MAP, 760-5797 mm), and dry season length (0-10 months)
 (column B in data file, rows 25-45).  To examine
 climatic control over decomposition, we used three homogenized global
 datasets (1-km resolution) to characterize each site based on 21 variables
 (Table S2).  WorldClim (Hijmans<em> </em>et al. 2005;
 https://www.worldclim.org/data/v1.4/worldclim14.html) provided 19
 bioclimatic variables describing annual trends, seasonality, and extreme
 conditions and CGIAR CSI provided potential evapotranspiration
 (Zomer<em> </em>et al. 2007, Zomer<em> </em>et al.
 2008; https://cgiarcsi.community/data/global-aridity-and-pet-database/). 
 Due to the importance of clouds in TMF, we used mean annual cloud
 frequency (MODCF mean annual; % cloudy days) derived from MODIS (Wilson
 & Jetz 2016; http://www.earthenv.org/cloud). <strong>Experimental
 design</strong> The <em>CloudNet</em>
 study followed the general design of the Powers et al. (2009) study to
 allow a combined analysis, with two exceptions: litter type and sampling
 intervals. Following the TLF study we used bay leaves, but instead of
 using raffia leaves we used wood as a second substrate (Powers<em>
 </em>et al. 2009). In contrast to Powers et al. (2009) that
 incubated the litterbags for 1, 3, 5, 7, and 9 months, we only incubated
 the bags for 3 and 7 months, times selected based on the observed
 decomposition rate for bay leaves in the Powers et al. (2009) dataset. Bay
 leaf (<em>Laurus</em> <em>nobilis</em>) (Frontier
 Co-op, www.frontiercoop.com) was the common leaf material in the TMF and
 TLF studies, and white birch (<em>Betula</em>
 <em>papyrifera</em>) wooden popsicle sticks (1 cm x 11.5 cm;
 Ben Franklin, Hilo, HI, USA) the common wood material for the TMF sites.
 To determine initial mass of the litter types for the litterbags, the bay
 leaves (with absent or very short petioles) were oven dried at 70° C for
 at least 48 hours and subsequently weighed and placed into mesh litter
 bags (Figure 1b); the wood was initially dried at 120 °C to release
 volatiles following the standardizing wood pre-treatment protocol
 (Cheesman<em> </em>et al. 2018), and then dried in the oven at
 70°C for 48 hours. Mesh bags (10 x 15 cm) contained
 either 1.00 ± 0.01 g (mean ± 1 SD) dry mass of whole bay leaves or one
 popsicle stick (1.34 ± 0.12 g). Two nylon mesh sizes were used (52 μm-mesh
 and 1920 μm-mesh; Component Supply Company, Fort Meade, FL) to selectively
 exclude mesofauna (defined as 100 μm to 2 mm) and fine roots. Litterbags
 were assembled in Hawai‘i and mailed to all participants.
 The experiment started at each of the 23 TMF sites between May
 and Dec 2017 and concluded between Dec 2017 and July 2018. At each site,
 we incorporated within-site replication by having litterbags deployed
 along a 50 m transect, subdivided into six stations 10 m apart. At each
 station, we deployed 16 bags: 2 litter types, 2 burial depths, 2 mesh size
 treatments, and 2 collection times for a total of 2208 bags (Figure 1b).
 The burial depths were on top of the litter layer at the soil surface and
 10–15 cm depth. We used a knife or machete to cut into the soil at a
 45<sup>o</sup> angle, inserted the bag and filled the hole. At
 each station, bags were anchored to a central stake using fishing line to
 facilitate retrieval.  The initial bag placement occurred right before or
 at the beginning of the wet season (May-Dec 2017) for all sites with the
 exception of the Ngel Nyaki site, where the start of the experiment
 occurred in Nov 2017 at the beginning of the November-March dry season.
 After retrieval at each harvesting time, the bags were cut open, the
 contents removed, and gently washed with tap water. Any soil or root
 material attached to the litter type was removed using tweezers. Samples
 were dried at 70<sup> o</sup>C and weighed to ± 0.01
 g.  To compare decomposition rates
 between tropical montane and lowland forest, we subsampled the TLF dataset
 to include only bay leaves and match the 3- and 7- months sampling
 times.  In the TMF dataset, the 3- and 7-month harvests
 occurred after 95 ± 8 days (mean ± 1 SD) and 226 ± 23 days, respectively;
 in TLF, those harvest times were 106 ± 22 days and 218 ± 15 days. This
 resulted in a subset of 483 bags (3 bags x 2 burial depths x 2 mesh
 treatments x 2 times) in the TLF dataset.
 ​​​​​​​<strong>Data analysis</strong> We conducted extensive
 exploratory data analyses to clean the dataset, identify outliers and
 investigate the source of their distinctiveness, and examine collinearity
 among variables to be used in the main analyses (Zuur & Ieno
 2016). We received information for 2200 out of the 2208 TMF bags that were
 deployed out in the field. After careful examination of the data, we
 retained 2162 for the analysis. For quality control, we subtracted the
 final weights of the litterbags from the initial weights with the
 expectation that differences should be positive; if the reported final
 weight of the litterbag was 10% greater than the initial weight, the point
 was discarded.  In some instances, we contacted the relevant co-author to
 clarify problematic data points that could not be resolved by the lead
 authors (e.g., mislabeled samples). Thus, 11 bags were discarded due to
 larger final versus original weights, and another 27 were discarded due to
 other reasons (e.g., lost, ruptured, unearthed bags). We used R version
 4.0.3
 (2020-10-10) to perform all subsequent analyses (R Core Team
 2018).
description: There is a ReadMe file to explain the variables in the
 Datafile.