DOI: 10.5061/dryad.mcvdnck5b
title: Short-term effects of wildfire on high elevation stream-riparian food webs

description: Understanding how wildfires affect food web structure and function remains
 an important challenge, especially at high elevations that historically
 have burned infrequently. In particular, fires may alter the magnitude of
 reciprocal cross-ecosystem subsidies, leading to indirect effects on
 aquatic and terrestrial consumers. We quantified characteristics of
 high-elevation (2500 to 3000 m) stream-riparian food webs at 10 locations
 in the southern Rocky Mountains less than one year following
 high-intensity, stand-replacing wildfires. Using a paired
 'burned-unburned' stream survey design, we assessed benthic
 periphyton, aquatic macroinvertebrate community structure, trout
 population characteristics, trout stomach contents, inputs and emergence
 of insects to and from streams, and abundance of predatory riparian
 spiders that consume aquatic insects. Benthic macroinvertebrate density,
 flux of emerging aquatic insects, and riparian spider abundances were
 lower at burned sites. Fluxes of insect inputs entering the stream did not
 differ with burn status, despite the loss of riparian vegetation due to
 fire. Trout were somewhat less abundant, but larger on average at burned
 sites and did not differ in body condition. These results suggest
 mortality of smaller trout from fire disturbance and/or recolonization of
 burned sites by larger individuals. Trout showed subtle changes in diet
 composition with burn status, but no change in biomass or number of prey
 consumed. In general, burned sites showed greater variation in community
 characteristics than unburned sites, which may reflect differences in the
 timing and magnitude of post-fire flooding, erosion, and scouring of the
 stream bed. Taken together, our results suggest that short-term effects of
 fire disturbance strongly altered some food web responses, but others
 appeared relatively resilient, which is notable given the high severity of
 the wildfires in the study area.
description: <em><strong>Stream
 surveys</strong></em> – We surveyed five pairs of streams in
 late July and August of 2021 in the northern Colorado Rockies within the
 Arapaho and Roosevelt National Forests.  Three of the burned sites were
 within the perimeter of the Cameron Peak Fire, one was within the East
 Troublesome Fire, and one was within the Williams Fork Fire. Each burned
 site was paired with an adjacent unburned site outside of the fire
 perimeter. At each site, we calculated the percentage of upstream
 watershed area that had been burned. We also visually estimated local tree
 mortality along the riparian corridor (% of trees killed by fire). To
 characterize the stream environment, we measured stream widths, depths,
 discharge, chlorophyll a from periphyton, canopy cover, adjacent burn
 severity, water chemistry (dissolved oxygen, pH, water temperature,
 conductivity), and turbidity at each site at the time of the survey. We
 attempted to select sites that were similar in elevation, aspect, stream
 size, and valley classification (Carlson 2009) between the burned and
 unburned pairs. To test this assumption, we assessed whether the burned
 and unburned sites differed in elevation, wetted width, maximum depth, or
 discharge using paired t-tests. There were no significant differences in
 these environmental variables (all p &gt; 0.05). Trout species
 composition at all sites included nonnative brook trout
 (<em>Salvelinus fontinalis</em>) and/or brown trout
 (<em>Salmo trutta</em>), except for one site that supported
 cutthroat trout (<em>Oncorhynchus clarkii</em>).
 Macroinvertebrates were sampled using benthic Surber samplers,
 emergence traps, and pan traps. Five Surber samples (0.09
 m<sup>2</sup> in area, 248 mm mesh each) were evenly spaced
 along each reach in approximately the center of the stream, focusing on
 riffle habitat. We deployed three emergence traps and three pan traps
 within each reach on the same day as surveys were conducted. Traps of both
 types were collected after approximately 70 hrs (range = 66 to 72 hrs).
 Emergence traps (Cadmus et al. 2016) were 0.37 m<sup>2</sup>
 in area and incorporated an opening at the top that directed insects into
 a Nalgene bottle of 80% ethanol. Emergence traps were situated in
 relatively calm water, typically below a riffle. For pan traps, we used
 shallow plastic bins 0.30 m<sup>2</sup> in area supported on
 frames of PVC plastic tubing. Pan traps were situated in shallow water at
 the edge of the streams and were filled with stream water and
 approximately 5 mL of unscented dish soap. Insects were removed from pan
 traps using a handheld dipnet (750 mm mesh) and preserved in 80%
 ethanol. To survey riparian spiders, we followed
 methods described in Benjamin et al. (2011). Spiders were surveyed along a
 50-m reach that was undisturbed by electrofishing, either immediately
 upstream or downstream of the electroshocking survey reach. Because the
 focal spiders are nocturnal, spider surveys were initiated at night after
 2100 h. Two observers wearing headlamps, one on each side of the stream,
 recorded all visible spiders within approximately 1 m of the wetted stream
 edge and up to 2.5 m in height. Based on their morphology, spiders were
 identified as Tetragnathidae, Araneidae, or "other" for
 individuals that could not be identified (&lt;2% of all spiders).
 Prior studies indicated that detection probabilities using these survey
 methods were &gt;90% (Benjamin et al. 2011). Because spider abundances
 may be related to habitat availability for web-building, we also
 quantified the density of branches along each stream reach (focusing on
 branches &lt;5 cm diameter and &gt;50 cm in length). Branch
 density was estimated in categories (0, 1-5, 6-25, 26-50, &gt;50
 branches) for every 2 m along both sides of the 50 m reach (Benjamin et
 al. 2011). We used the midpoint of each category as the estimated branch
 density. For the highest branch density category, we used 75 as the
 midpoint. Branch density data for a single site (South Fork Poudre) was
 lost, resulting in a comparison between four unburned and five burned
 sites. We quantified trout relative abundance,
 population size structure, body condition, and stomach contents. At each
 site, we conducted a single-pass survey using a backpack electroshocker
 within a designated 50 m stream reach. The selected reaches were composed
 primarily of riffle and run habitats, which helped standardize habitat
 conditions between sites. All trout caught were anesthetized with Aqui-S,
 nonlethally lavaged for stomach contents, measured for total length
 (nearest mm) and weight (nearest 0.1 g), and then released after recovery
 from anesthesia. Fish were lavaged using either a laboratory wash bottle
 with an affixed plastic straw (for larger fish) or a 60 ml syringe with a
 blunt 18 gauge needle (for smaller fish). Fish stomach contents were
 preserved in 80% ethanol until processing in the laboratory. At some
 sites, we collected fewer than three trout in the designated reach, so we
 continued electroshocking upstream and downstream of the reach until we
 obtained 20 to 30 trout for diet analyses (mean = 27). We calculated trout
 relative abundance within each reach as the number of fish caught per
 minute of the electroshocker running. The number of people electroshocking
 (n=4) and number of backpack electroshockers (n=1) were standardized
 within the reach at each site. At one site (Corral Creek), problems with
 the electroshocker affected shocking efficiency, so we omitted this site
 from estimates of trout relative abundance but included it for all other
 analyses. <em><strong>Sample processing
 </strong></em>– Macroinvertebrates from Surber samples were
 identified to family, while macroinvertebrates from trout stomach
 contents, emergence traps, and pan traps were identified to either family
 or order (Ward et al. 2002, Merritt et al. 2008). Macroinvertebrates from
 trout stomach contents, emergence traps, and pan traps were also assigned
 as either 'aquatic', indicating they have one or more aquatic
 life stages, or 'terrestrial', indicating they have no aquatic
 life stage. Invertebrates identified to order generally included adult
 life stages of certain taxa (e.g,, Diptera) and invertebrates from trout
 stomachs that were partially digested and had lost key features. A subset
 of 10 intact invertebrates of each taxon from each sample were measured
 for body length to the nearest 0.5 mm. We then used published
 length-to-mass regressions to estimate dry biomass for individual
 invertebrates (Benke et al. 1999, Sabo et al. 2002), which was then summed
 for each Surber to obtain biomass per unit area.