DOI: 10.5061/dryad.v9s4mw75z
title: Data from: Antagonistic biotic interactions mitigate the positive effects of warming on wood decomposition

description: Global change drivers such as habitat fragmentation, species invasion and
 climate warming can act synergistically upon native systems; however,
 global change drivers can be neutralized if they induce antagonistic
 interactions in ecological communities. Deadwood comprises a considerable
 portion of forest carbon, and it functions as refuge, nesting habitat and
 nutrient source for plant, animal and microbial communities. We predicted
 that thermophilic termites would increase wood decomposition with
 experimental warming and in forest edge habitat. Alternately, given that
 predatory ants also are thermophilic, they might limit termite-mediated
 decomposition regardless of warming. In addition, we predicted that a
 non-native, putative termite-specialist ant species would decrease termite
 activity, and consequently decomposition, when replacing native ants. We
 tested these hypotheses using experimental warming plots (~2.5°C above
 ambient) where termites, and their ant predators, have full access and
 vary in abundance at microscales. We found that termite activity was the
 strongest control on decomposition of field wood assays, with mass loss
 increasing 20% with each doubling of termite activity. However, both
 native and non-native ant abundance increased with experimental warming
 and, in turn, appeared to equally limit termite activity and,
 consequently, reduced wood decomposition rates. As a result, experimental
 warming had little net effect on the decomposition rates – likely because,
 although termite activity increased somewhat in warmed plots, ant
 abundances increased more than five times as much. Our results suggest
 that, in temperate southern U.S. forests, the negative top-down effects of
 predatory ants on termites outweighed the potential positive influences of
 warming on termite-driven wood decomposition rates.
description: Twenty-four study plots were established, using a random blocked
 design, in a 2,500-m<sup>2</sup> area at Whitehall Forest in
 2009, with 12 plots located within forested habitat (hereafter “forest
 plots”) and 12 located in an adjacent manually cleared area at the edge of
 a powerline cut (hereafter, “edge plots”). The minimum and maximum
 distances between plots was 2 and 60 m. We used both edge and interior
 plots because temperate forests have, for a long time, been impacted
 strongly by human development, resulting in forest landscapes composed of
 a great deal of edge as well as interior forest which vary in their
 temperature regimes. Further, edge environments may facilitate non-native
 ant invasions (Ness 2004; King and Tschinkel 2016; Warren II et al.
 2020).  In 2009, resistance heating cables were buried
 10-cm deep and 20-cm apart in 18 of the 24 plots, which were built into
 18-m<sup>2</sup> rectangular (4 x 4.5 m) open-top chambers
 (1.5 m height) sided with greenhouse PVC sheeting attached to wooden
 frames (Supplement 1). As a control to account for potential “chamber
 effects” of plastic siding and the disturbance of burying heating cables,
 chicken wire was used for the sides of the final six plots and no
 resistance cables were buried within. For warming treatments, proximate
 chambers were randomly assigned within clusters of four-chamber ‘plots’ so
 that each block contained a non-chambered control plot (chicken wire
 sides), a control chamber with no warming (but with buried cables), a
 chamber with targeted warming 3°C above ambient and a chamber with
 targeted warming 5°C above ambient (<em>n</em> = 6 total
 replicates for each chamber/plot treatment; 24 chamber/plots in total).
 Each chamber and plot were outfitted for temperature and soil moisture
 monitoring and the real-time soil temperature data used to target the
 warming of treatment soils. During the months of the current study the
 real-time monitoring revealed that actual warming was ~2.5°C above ambient
 for all warming chambers (Supplement 2a), so we pooled the faunal data
 from these targets together. Given that there was also no difference in
 soil temperature between the non-chambered (chicken wire) control plots
 and the unheated control chambers (Supplement 2a), we also pooled faunal
 results from the non-chambered control plots and unheated control
 chambers. Overall, then, our design consisted of 12 warming chambers and
 12 non-warmed ambient-temperature plots equally distributed between forest
 and edge habitats.          <em>Deadwood
 assays</em> Two milled wood assays with no bark
 (12 x 14 x 2 cm), hereafter “assays,” were placed 1 m apart in each plot
 (<em>n</em> = 48 total) in April 2018 with half collected in
 June 2019 (<em>n</em> = 24) and half collected in June 2020
 (<em>n</em> = 24) [Supplement 3]. The assays were made of
 <em>Pinus taeda</em> L. (loblolly pine), which is native and
 abundant in the study area. Each assay had a “G”-shaped chamber (width:
 2.5 cm; depth: 2.5 cm) routed out and open to the outside to create
 potential nesting chambers for ant colonies, and each assay was placed in
 contact with the forest floor soil to promote termite foraging. Prior to
 placement in the field, the air-dry mass of each assay was recorded and
 ranged from 90 to 151 g, with a mean of 111 g. We note that the assays
 ranged in mass because of natural variation in wood density but all assays
 had the same dimensions, and the wood was not treated (e.g., chemically or
 by pressure) in any way. After weighing, each assay was topped with clear
 acrylic plastic and an opaque ceramic tile following established protocols
 from previous research in the area (Warren II et al. 2012, 2015b; Bradford
 et al. 2014). The clear top allowed periodic monitoring of the nests
 before collection, but the tile blocked light otherwise (because the study
 termites and ants are negatively phototropic). We
 assessed ant abundance when the deadwood assays were collected by
 freeze-killing and counting ant abundance in each of the two years. Ants
 make their nests in the deadwood assays whereas termites channelize and
 feed on the wood. Termites in the study region are the only wood
 decomposers that import mineral soil into the wood [bonded with saliva and
 fecal material – often replacing the structure of consumed wood] (Ulyshen
 et al. 2014; Myer and Forschler 2019). We assessed termite activity by
 drying the emptied assays at 65°C for 7 d, weighing each assay, and then
 we calculated the ash-free dry mass by burning off the remaining wood at
 500°C in a muffle furnace. Hence, termite activity was represented by the
 remaining mineral soil (g ∙ assay<sup>-1</sup>). This estimate
 likely better represents an estimate of integrated termite activity given
 that the termites forage in the assays, but nest underground. Termite
 abundance at the time of assay collection may only capture a snapshot of
 termite activity and a subset of colony workers. For example, from this
 study, 54% of the assays showed signs of termite activity (tunnels, soil
 filling) and live termites were found in 46% of the assays when collected
 in the first year; in the second year, however, 100% of the assays showed
 signs of termite activity, but termites were only found in 63% of the
 assays when collected. Ants and termites were identified by species with
 reference to voucher specimens and taxonomic guides. We note that ants and
 termites occurred naturally in the plots. Ants are the main predator of
 <em>R. flavipes</em> (Beard 1973; Warren II et al. 2015).
 Whereas many ant species occur at the study site (Warren II et al. 2022;
 Merchlinksky et al. 2023), few use wood nests to the degree of
 <em>A. rudis</em> and <em>B. chinensis</em> (we
 only found single colonies of <em>Camponotus</em> and
 <em>Crematogaster </em>spp. during the study).
description: # Antagonistic biotic interactions mitigate the positive effects of
 warming on wood decomposition **Data file description:** * The data file
 included in the submission is Warren.etal.data.csv **Variables:** Ident =
 line number Treatment = experimental warming treatment (Warming = warming
 treatment; ambient = control) Habitat = habitat type (Gap, Forest) Block =
 experimental block Year = year of data collection Nest.g = Starting nest
 weight in grams Nest.dry.wgt = final nest weight Nest.perc.soil = percent
 mineral soil in final nest weight Ant.sp = ant species (BRCH =
 Brachyponera chinensis; APRU = Aphaenogaster rudis) Ant.pres = ant
 presence/absence Ant.abund = ant abundance Term.pres = termite
 presence.absence Term.abund = termite abundance Missing data: NA