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Article

Arthropod Recolonization of Soil Surface Habitat in Post-Fire Mulch Treatments

by
Christine Mott
*,
Anita Antoninka
and
Richard Hofstetter
School of Forestry, Northern Arizona University, 200 East Pine Knoll Drive, Flagstaff, AZ 86011, USA
*
Author to whom correspondence should be addressed.
Forests 2023, 14(7), 1421; https://doi.org/10.3390/f14071421
Submission received: 19 May 2023 / Revised: 3 July 2023 / Accepted: 4 July 2023 / Published: 11 July 2023
(This article belongs to the Special Issue Forest Management Impacts on Soil Biological, Chemical and Physical Properties)
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Abstract

:
Increasing size, severity, and human proximity to fires in the western US are driving a need for more effective ecosystem restoration in the immediate post-fire period. Surface treatments, such as mastication of logging slash, reduce erosion and improve soil nutrient and water retention on steep slopes. However, few studies have investigated the impact of these treatments on arthropod communities over time. Our objective was to determine which insect communities return to these treated areas and if the mulch changes the community structure over time. We surveyed arthropod abundance using pitfall traps in mulch treatments in a landscape-scale fire near Flagstaff, Arizona, and a controlled split-plot experiment outside of the larger fire footprint. Predatory beetles were more abundant in mulch in the large landscape treatment, with no differences in abundance in the split plots. Fungivores had no significant mulch preference, and several native bark beetles were more abundant in the untreated sites. We found that the size of the fire footprint and distance to the intact forest matrix likely impact arthropod community composition over time. We were unable to fully evaluate vegetation recovery, but further work will allow us to understand how surface treatments impact the interaction of arthropods and vegetation.

1. Introduction

The recent increase in wildfires across the western United States has resulted in widespread plant mortality, increased carbon emissions, loss of wildlife, and substantial fire suppression expenditures [1,2]. The severity and frequency of these fires can lead to significant long-term consequences, such as reduced water retention, soil degradation, and loss of litter and nutrients [3,4,5,6]. Reforestation following large-scale tree mortality events such as wildfires has long been a high priority for the US Forest Service. However, current practices lack enhancements for wildlife such as arthropods in the recovery process.
Fire directly affects arthropod communities, with positive or negative ecological outcomes depending on the size and severity of the fire, ecosystem type, fire regime, and the pre-fire arthropod community [7,8]. Less mobile insects, such as those in their pupal stages, ground-dwelling or immobile larvae, non-flying insects, and soil-dwelling insects, are vulnerable to extreme high fire temperatures, while highly mobile adults or insects in deeply burrowing life stages may survive [9]. Post-fire arthropod community recovery is an area of research interest that is expanding under current and future climatic conditions. While many studies address single arthropod groups [10,11,12,13,14,15,16,17] in the recolonization of burned areas after fire, some demonstrate both positive and negative community-level responses among arthropods within the first year after a fire [18,19,20,21,22,23].
As human encroachment into forested areas brings fires closer to infrastructure and recreation, managers are being forced to perform emergency large-scale treatments of severely burned areas to protect human interests in lower watersheds with significant expenses for taxpayers and state or county infrastructure budgets [24]. Burned Area Emergency Response (BAER) units throughout the western United States use wood shreds or commercially available wood straw to stabilize soils, especially on high-angle slopes where lighter materials may wash or blow away more quickly [25,26,27,28]. Locally sourced logging slash mulch has several advantages over commercial straw mulch in post-fire landscapes. It lasts longer on site, protecting soil from erosion and trapping moisture in damaged soil over multiple growing seasons [27,29]. In addition, the complex, irregular surface area of logging slash mulch can provide a microhabitat for surface insects and promote fungal growth and wood decay. These two components are essential for many surface insects, either for their own food or for the breakdown of wood to return nutrients to the soil surface. Several studies have evaluated the effects of wood mulch on post-fire soil and vegetation recovery in small- and large-scale fires [28,30,31,32]. However, little is known about its interactions with surface-dwelling animals. To address this gap, we investigated the impact of different types of mulch on post-fire arthropod communities.
Some post-fire studies have suggested that surface vegetation and other understory variables have little influence on ground-dwelling arthropods, but others suggest that understory characteristics, such as forbs, duff, and litter, can affect arthropod community structure [18,19,20,21,33]. Similarly, Nadel et al. [34] found that some arthropods can survive in residual slashes after wildfire events, demonstrating the importance of coarse woody debris in arthropod biodiversity. However, a study by Lavoura Puga et al. [35] showed that bark mulch applied immediately after a fire had little effect on total arthropod abundance, richness, diversity, and evenness. They also suggested that mulching could, in fact, reduce overall abundance, although this may have been driven by the abundance of one group (ants) in unmulched plots in their study. Each study also suggested that the time since the occurrence of a fire likely affects community structure, with immediate effects either dissipating over time or community structure taking longer to recover in more complex ecosystems [21,35,36].
Post-fire ecosystems face significant challenges in the recovery of arthropod communities and their associated functions, and no consensus has been reached regarding community structure and function. This study aimed to answer the question of whether arthropods return to these burned landscapes within two years after the fire and whether different post-fire erosion treatments alter the arthropod community composition. The objective of this study was to examine the overall diversity of arthropod communities and the effect of mulch treatment on insect community diversity in post-fire landscapes, including large-scale wildfire and small-scale slash-pile scars. We hypothesized that adding a secondary mulch component, such as needle and leaf materials from shredded slash piles or other small-diameter slash with foliage, would enhance moisture retention and support a richer insect community than xylem-only mulch treatments or untreated areas. In turn, this may promote soil stability and increase plant establishment, potentially reducing the need for repeated treatments or soil amendments. Improving nutrient cycling could be a key factor in promoting ecosystem recovery after wildfires, as many erosion control techniques rely on vegetation to stabilize the soil over time.

2. Materials and Methods

2.1. Research Sites

The large-scale study was conducted within the footprint of the Museum Fire near Flagstaff, Arizona. The Museum Fire began on 21 July 2019 at approximately 11:15 a.m., approximately one mile north of the city of Flagstaff, Arizona, in an area known as the Dry Lake Hills. InciWeb showed the final fire boundary as 793.6 hectares (1961 acres) and listed various fuels involved, including ponderosa pine (Pinus ponderosa Engelm.), for which there is heavy dead and down debris in the lower elevations due to ongoing forest treatment/thinning, and mixed conifer trees—primarily ponderosa pine, Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco), and scattered individuals of the Southwestern white pine (Pinus strobiformis Engelm.) and limber pine (Pinus flexilis James) hybrid zone, which are generally referred to as Pinus strobiformis in this area [37,38]—at higher elevations. The fire was 100% contained on 12 August 2019 [39], and a Burned Area Emergency Response (BAER) team reported to the area to survey requirements for infrastructure and watershed protection for the city of Flagstaff and the Coconino National Forest. The final soil burn severity (SBS) maps provided by the US Forest Service showed that, of the 1961 acres involved in the fire, 95.5 hectares (236 acres or 12%) were considered to have very low SBS, 380 hectares (940 acres or 48%) had low SBS, 220 hectares (546 acres or 28%) had moderate SBS, and 93.9 hectares (232 acres or 12%) had high SBS. As indicated in Figure 1, much of the moderate and high soil burn severity was on steeper slopes and at higher elevations.
The BAER team used on-site materials to treat the fire footprint for soil stabilization [38]. The area near site A (Figure 1) and several other locations near research sites were used as staging areas for log decks from thinning operations prior to the fire. These logs were partially burned in the fire so that only dry xylem remained, and they provided significant material to masticate into wood mulch to stabilize steep slopes in the critical Spruce Avenue Wash watershed leading directly into east Flagstaff [39]. Porter et al. [39] found that there was ample sediment buildup in all watersheds, and there was concern regarding flood runoff into city infrastructure and housing [39]. The crews treated steep, high-elevation slopes with moderate and high SBS (sites C and D) using helicopter drops of wood mulch where the BAER team indicated a depth of no more than 3 inches (7.62 cm), while lower and shallower slopes (site B) were treated with masticated slash piles also left from thinning operations within the unburned sections of the treatment unit. As we were unable to match mulched and unmulched transects within the Museum Fire footprint, we also set up a small mulch experiment in the Centennial Forest on small-scale slash-pile burn scars, as described below. This experiment was intended to help us understand the difference between treated and untreated burned areas, as well as the different mulch types, in a controlled setting.

2.2. Arthropod and Vegetation Data Collection

2.2.1. Museum Fire Transects

To study the impact of BAER soil stabilization practices on the return of ground arthropods and vegetation in the aftermath of a severe wildfire, we planned a survey of four sites throughout the Museum Fire footprint in moderate and high soil burn severity areas. We identified four transects in late summer 2020 (Figure 1), approximately one year after the fire and treatment efforts, using hydrology monitoring transects as a guide [38] and those referenced by Porter et al. [39]. For two transects, sites B and C, we were able to find the hydrological markers and set our transects approximately 25 m to one side. For sites A and D, we were unable to locate the markers but set a transect in an area with a similar canopy structure, aspect, and slope to those described by hydrology personnel [40]. For each transect, we installed six pitfall traps designed to minimize both soil disturbance and bycatch [41] in a line perpendicular to the elevation contour, with a 3–5 m (10–15 foot) elevation gain between traps. Each trap was loaded with a 50/50 glycol mix, as in Cobb et al.’s [41] protocol, to ensure preservation of delicate specimens over the 7-day period. For each trap, we noted the GPS location and elevation (Table 1), and we placed a metal tent stake approximately 1 m below each trap as a backup location marker for follow-up visits.
The initial status of each transect was documented by observing the treatment type around the transect. Sites C and D at higher elevations were treated with xylem-only wood mulch from the burned log decks as these areas were inaccessible and treated by helicopters. Site B showed significant mixing of green and needle mulch with the xylem mulch. Many trees in both sites A and B were not killed by the fire directly but died and lost their needles in the following weeks. Site A was not treated with xylem but had the most significant needle coverage on the soil surface over the course of the two-year study.
We successfully collected data for the Museum Fire for late summer (post-monsoon) 2020 and both early (pre-monsoon) and late summer (post-monsoon) 2021. We collected specimens from the glycol in the traps at the end of the 1-week trap period and transferred them to 75% ethanol in the field for transport to the laboratory. We sorted and identified all specimens to the order level, with taxa of further interest curated for further identification and study. The remaining specimens were stored in ethanol in an entomology laboratory. We also collected vegetation data during insect specimen collection using a 0.5 m × 0.5 m quadrat placed 1 m to the left and 1 m to the right of the trap on the same elevation contour to avoid the potential seed bank disturbance area created by the installation of the trap. We counted and recorded vegetation that was visible at the surface without significantly disturbing the mulch and photographed each plot for further review if species could not be identified in the field. Due to limited regrowth at this early stage of fire recovery, we counted single stems and identified the species where possible or the functional group (grass, forb, shrub, tree), as there was extremely limited cover in all areas.

2.2.2. Centennial Forest Mulch Plots

To further inform our efforts in the larger fire footprint and to address some potential outcomes of our previous pile burn scar research, we selected 11 plots in the Centennial Forest for experimental mulch treatments on existing pile burn scars 4–5 m (13–16 ft) in diameter in a known research area. Each plot was split in half using a strip of landscape edging. Edging provided only a visual cue for the plots and did not impede movement through or around the burn scar. Half of each plot was treated with one of three mulch treatments: green, xylem, or mixed. For the xylem, we shredded the boles of several ponderosa pine trees felled from the site to create a wood chip mulch, whereas the green mulch was the remainder of the material, including a mix of branches, small twigs, and needles common to hand-built slash piles created by thinning in the local area. The mixed treatment comprised an equal volume, as measured in 5-gallon (18.9 L) buckets, of the green mulch and xylem mixed with a thatch rake on the soil surface.
Due to the small size of the individual pile scars as compared to the larger hydrology transects at the Museum Fire site, we installed four of the same pitfall traps as in the Museum Fire site at each plot: two on the treated side and two on the untreated (control) side. We sampled on the same timeline as for the larger Museum Fire site in the fall of 2020, early summer 2021, and fall 2021, although significant logistical and storage issues prevented accurate use of 2020 data for the Centennial Forest plots. Arthropod specimens were collected, identified, and stored in the same manner as for the Museum Fire transects. We collected vegetation data at the same time as insect specimens, with a 0.5 m exclusion zone placed around each trap to exclude the disturbance area created by trap installation from the sample. Due to the small size of each plot and the slow regrowth on the pile scars, we counted vegetation over the entire plot minus the exclusion area in four quadrants labeled A, B (treated side), C, and D (control side). As in the Museum Fire transects, we identified vegetation to the species level where possible or as functional groups if necessary and photographed all quadrants for further review.

2.2.3. Specimen Identification

Here, we only included adults of the orders and families indicated in each analysis, and the few recognizable larvae (caterpillars, nymphs) were assigned to the “other” category due to low numbers and the difficulty of subadult identification for most local Hemiptera and Lepidoptera. We categorized insects into functional groups based on our knowledge of their feeding habits or primary ecosystem services, as in the study by Beiber et al. [8]. We used four major ecosystem functions—herbivore (HERB), omnivore (OMNI), predator (PRED), and fungivore (FUNG)—with one additional group, herbivore B (HERBB, bark beetles), due to their importance in western forest ecosystems. A large number of the fungivores found in this system also eat sap and rotting fruit, but owing to the lack of fruit or live trees, we chose to classify them as FUNG for this ecosystem. The HERBB category was considered separately from other herbivores due to the unique habitat requirements and feeding habits in these dry forest ecosystems, as well as the special interest for forest managers in a post-fire system.
We focused on order-level diversity and diversity and function among the beetles, Coleoptera, as they were the most abundant order in our study. There are several studies on the baseline beetle diversity within northern Arizona ponderosa pine forests and some ongoing research on small-scale fire scars that allow for better conclusions about returning diversity [11]. Beetles also provide one of the widest arrays of ecosystem functions within the ponderosa pine forest and can be used as an indicator of overall forest functioning [11].

2.2.4. Analysis

For our initial analysis of the insect groups, we used PCORD 6 to perform an indicator species analysis at the order level, as well as by beetle family and functional group. We also used the NMS feature to ordinate beetle communities and functional groups and visualize any factors, such as time-period or mulch-type effects or significant overlap in community composition. We applied the Multi-Response Permutation Procedure (MRPP) in PCORD for the beetles to determine if there was a significant difference between the sampling units based on mulch type and time period. We also used the indicator species function to determine if any beetle functional groups or families stood out as indicators for any particular treatment or time period. Owing to the limitations of PCORD with zeroes in a full row of data, we added a single dummy-variable column to the order, family, and functional group data. In this case, zeroes are just as important as positive integer counts, as a lack of insects could also have an effect on overall ecosystem recovery. In addition, due to a non-normal distribution in the count data across all transects and traps at both sites, we also used the Kruskal–Wallis test in R to determine if there was a significant (p < 0.05) difference in the median abundance between the different treatments after examining the initial NMS and MRPP results in PCORD.

3. Results

We first analyzed arthropod communities to assess the overall diversity of the post-fire landscape at an elevation of approximately 2500 feet in two ecosystems. We found that, across all three trap periods at the Museum Fire site, the top three groups to return to the post-fire area were beetles, flies, and ants. Beetles were the most abundant group to appear in our ground surveys after a severe wildfire, as shown in the large proportion of total capture here (48% for the Museum Fire compared to 37% in the smaller pile burn scars in Centennial Forest). The second highest abundance for combined capture periods in the Museum Fire footprint was for flies (26% of the total capture), and for the Centennial Forest sites, it was for ants (36% of total capture) (Table 2).

3.1. Beetles of the Museum Fire

The NMS ordination showed no significant differences in the beetle community by function with mulch type as the variable but a significant difference in the beetle community during time period two (Figure 2 and Figure 3).

3.2. Indicator Species by Beetle Family and Function

We found several indicators for the different mulch types in the Museum Fire footprint (Table 2). At the Museum Fire site, Nitidulidae, Tenebrionidae, Trachypachidae, and one of the unknown groups were significant indicators for early summer 2021; Scarabaeidae were significant indicators for late summer 2020; and Coccinellidae were significant in late summer 2021. The significant indicator species by treatment were Curculionidae in the untreated transect, Tenebrionidae in the mixed mulch, and Carabidae in the xylem treatment. When we looked at indicator species with the interaction of period and treatment, we noted Nitidulidae in early summer 2021 with no treatment, Tenbrionidae in early summer 2021 in the mixed mulch, Trachypachidae and one of the unknown groups in early summer 2021 in the xylem treatment, and Carabidae and Coccinellidae in late summer 2021 in the mixed mulch. Indicator species in beetle functional groups showed that, by period, fungivores and predators were indicator groups in early summer 2021, and omnivores were indicators in late summer 2021.
The MRPP for the beetle functional groups (Figure 2 and Figure 3) in the Museum Fire showed significant differences in beetle communities (Table 3) for both time period (p < 0.0001) and mulch type (p = 0.0017), but the low A-value in each test (A = 0.11, A = 0.029) showed that it was not a strong effect in either case. We found similar results for beetle families, with significant effects of mulch type (p = 0.0112, A = 0.018) and time period (p < 0.0001, A = 0.11). While many of the treatment effects were hidden by the effect of sampling period and small sample sizes, we noticed several trends in the data after creating visual representations of the mean abundance in each treatment for both beetle families and functional groups (Figure 4a–g), especially in the Museum Fire transects. Among the families, Carabidae and Cryptophagidae were significantly more abundant in the xylem-only treatment, while Tenebrionidae were significantly more abundant in the mixed-mulch treatment and Curculionidae were significantly more abundant in the untreated area (Figure 4a–d). Among the functional groups, predator abundance was significantly different across all treatments, with the highest abundance in the xylem-only treatment (Figure 4e). The omnivores were significantly more abundant in the mixed mulch (Figure 4f), and bark beetles were significantly more abundant in untreated areas (Figure 4g). The Kruskal–Wallis test confirmed that there were several families with statistically and/or ecologically significant differences in abundance between treatment types (Table 4) for the following groups in the Museum Fire: Carabidae, Cryptophagidae, Curculionidae, Tenebrionidae, and Trachypachidae.

3.3. Beetles of the Centennial Forest Slash-Pile Scars

Indicator Species by Beetle Family and Function

We found no indicator species for the mulch types in the Centennial Forest but several indicator species by time period (Table 3). Similar to the Museum Fire site, Nitidulidae and Tenebrionidae were indicators in summer 2021 in the Centennial Forest site, with the addition of Staphylinidae in late summer 2021. Indicator species in the Centennial Forest site also showed that, by period, fungivores and predators were indicator groups in early summer 2021, and omnivores were indicators in late summer 2021.
The MRPP results for beetle functional groups in Centennial Forest showed no significant effect of mulch type (p = 0.6002, A = −0.0069) but a significant effect of time period (p < 0.0001, A = 0.31) for functional groups. The same trend appeared in the beetle family comparison (Table 2), with no significant effect of mulch (p = 0.72, A = −0.016) but a significant effect of time period (p < 0.0001, A = 0.26). There were no significant results in the MRPP for any beetle family with mulch type in the Centennial Forest plots.

4. Discussion

Forest fires can have significant effects on soil and vegetation [24,39,42], leading to water-repellent soil surfaces that can cause flash flooding and debris flows. In addition to these physical impacts, fires can also have ecological consequences, including changes in surface vegetation, arthropod communities, and the interactions between them [6,43,44]. Some arthropod groups may be more adaptable to the changing environment than others [21], with omnivores potentially able to adapt more quickly than generalist herbivores [8,9]. Our study found that, in a montane system, the immediate post-wildfire restructuring of the arthropod community was characterized by a higher proportion of flies, which may serve as primary high-elevation pollinators [45]. While there were minor differences between our study and others that examined total arthropod abundance in Mediterranean ecosystems [35,46], the larger proportion of flies in our large landscape study may have been due to differences in timeline and ecosystem functional requirements. Flies serve as detritivores and scavengers as well as high-elevation pollinators [45], and their mobility as adults allows them to escape fire or recolonize immediately post-fire. Our data also showed beetles and ants to be among the most abundant groups after a small, localized fire, indicating potential common succession in arthropods or permanent changes to community structure under a changing climate [22,47]. While we are unsure of the mechanism to date (i.e., survival, temporary relocation, invasion, etc.), beetles of nearly a dozen families returned to high-severity-fire areas within a year of the fire in research sites throughout AZ and CO [48]. However, because of their life history and habitat requirements, some groups of beetles may be slower to return to a large fire footprint than others, and the timing of trapping efforts is a significant factor in understanding the entire arthropod community throughout the year, not just the monsoon season. More work is needed to fully understand the interactions among arthropod taxa, vegetation, and other invertebrate and vertebrate taxa in rapidly changing post-fire environments.
Our findings suggest that omnivorous beetles, especially members of the family Tenebrionidae, are abundant in the post-fire environment for both large-scale wildfire and small-scale management-induced fire footprints. This suggests that some omnivores may be able to adapt quickly to changing post-fire environments. However, our results also indicate that the response of this group to different mulch types varied across sites, with site B (mixed) showing a stronger response compared to sites C or D (xylem). This could have been due to the availability of unburned refugia preferred by these species at site B, allowing the non-flying adult Tenebrionidae (specifically, Coelocnemis spp. and Eleodes spp.) to access the transect earlier than the other sites, as suggested by Antunes et al. [49]. Similarly, in the small Centennial Forest plots, abundance was not strongly linked to mulch type, and the stronger effect may have been because of the intact forest matrix surrounding each plot, allowing for significant migration into burned areas. Further research over a longer period of time could help determine the relative importance of mulch versus the impact of total footprint area and edge distance on the response of these beetles. Overall, our study highlights the importance of considering both fire size and arthropod functional groups in post-fire ecosystems and their potential to adapt to changing conditions.
Interestingly, the predator response to mulch varied between the two study sites. This could have been due to differences in habitat and resource availability between the two ecosystems. For example, the Centennial Forest experimental plots may have had more unburned refugia or alternative prey sources for predators, while the Museum Fire site may have had a greater reduction in prey availability due to the fire, making the mulched areas more attractive to predators. However, this is speculative and further research is required to confirm this hypothesis. Additionally, the presence of specific predator species, like Carabus and Trachypachus, in the higher-elevation mixed conifer forest may have important implications for post-fire ecosystems, as they can play a significant role in controlling populations of smaller prey species and influencing the overall community structure [11,50]. Therefore, they may be attracted to the xylem mulch over other mulch or untreated areas, as it would provide an ample habitat and ample cover for them to hunt smaller prey near the soil surface [51]. In addition, local climate data indicate that the two years surrounding the initial fire and study time period (2019 and 2020) were two of the driest monsoon years on record, with 2.08 and 1.78 inches (5.28 and 4.52 cm) of precipitation [52]. The second year of the study had a more normal monsoon pattern with 10.9 inches (27.69 cm) of precipitation, but it was also one of the hottest seasons on record with a high temperature of 96 °F (35.5 °C [53]) in July 2021, while the average high temperature in July is 81 °F (27.2 °C) [52]. Under changing climate, this is likely to be a more common pattern in the higher elevations and fire seasons. With loss of canopy and hot, dry conditions, the larger and more robust xylem mulch pieces may provide adequate shading at the soil surface for the survival and success of these predatory beetles and their prey. Overall, our findings suggest that post-fire mulching can have complex and varied effects on arthropod community structure, with responses varying by functional group, species, ecosystem type, and fire size. Our results suggest that it may be possible to treat small fire scars within an intact forest matrix using common restoration techniques [54], protecting the soil surface without detracting from overall arthropod community recovery. However, treatments of large-scale wildfires may have more significant impacts on returning arthropod community composition. Further research is needed to fully understand these effects and their implications for ecosystem function and resilience.
One of the most interesting outcomes of this study was that there was no difference in fungivore abundance across all treatment types at either study site. There was a very significant effect of time period for the fungivores, most likely due to their life history, but no effect of mulch type on these animals. For post-fire ecosystems, this may be one of the more positive outcomes of our research, as managers consider ecological restoration and the critical components of forest recovery. Fungivores play a significant role in soil nutrient cycling and aid the breakdown of organic matter in fire-damaged soils [55]. In addition, specimens of the mycorrhizal associate Thalycra, which is common throughout northern Arizona [56], were abundant even in the untreated areas of the Museum Fire and the Centennial Forest mulch plot. As a known associate of Rhizopogon fungi, these animals may be critical for reforestation in ponderosa pine forests [56]. We also noted larger proportions of the genus Epurea and the silken fungus beetle family Cryptophagidae, both small beetles often found under bark and in decaying logs, in the Museum Fire site than the Centennial Forest site. Interestingly, Cryptophagidae were more abundant in the xylem-treated areas of the Museum Fire than at any other site, leading us to believe that the xylem did, in fact, create some critical habitats or harbor more fungal growth than the other transects.
In contrast to longer-term studies, our data showed that total herbivores among the beetles were among the most abundant groups in the early stages of post-fire colonization. According to Bieber et al. [8], herbivores tend to be the least abundant after 5 years in a mulched landscape. One reason for this, as shown in the separate herbivore and bark beetle analyses, may be the immediate response of bark beetles seeking habitats in fire-damaged trees. While the abundance of generalist herbivores was low, likely due to a lack of understory vegetation resources, the more specialist bark beetles showed a rapid response to habitat availability in this post-fire ecosystem.
The bark beetles in the Museum Fire footprint were all native to the area and have been observed across all our study sites in northern Arizona on smaller fire scars [48]. They have often appeared in unburned forests in our research focused on ground-dwelling arthropods and likely migrated into the area due to an increase in available habitats. The majority of these animals captured (93 total over three trap periods) were characterized as secondary bark beetles and not as primary agents of tree mortality, and they were likely responding to habitat availability due to the burned overstory. However, they are also important for nutrient cycling during the post-fire period. The beetles of the genus Hylastes are root bark beetles, meaning early breakdown of dead and dying trees is performed by small larvae, with both arthropod and vertebrate predators further breaking down the wood in search of larvae as the trees begin to fall. The small Gnathotrichus species found at the lower elevations were oak twig beetles and were most likely responding to similar habitat availability and resource use in the aftermath of the fire and dying trees. By studying the responses of these bark beetles to a post-fire ecosystem, we can gain a better understanding of how different species contribute to the overall recovery process.

5. Conclusions

Arthropods return rapidly to fire-damaged ecosystems, but the extent of the changes in community composition is not well-known. Although our study was limited in size and scope, we demonstrated potential habitat drivers of arthropod return to fire-damaged ecosystems. In contrast to our initial assumption that mixed mulch would create more habitats owing to increased surface area and nutrient input, it seems that the xylem treatment was the most favorable for several beetle families. We suspect that the larger size of individual pieces may create more open crevices and gaps for habitats and corridors through the burned areas or for ambushing small prey near the soil surface, as well as improved soil surface shading after significant canopy loss. We anticipated a higher number of fungivores in the mulched areas but found that fungivores simply returned to any available habitat in northern Arizona post-fire ecosystems. We believe that mulch also drives other interactions with insects that use it for habitat as well as food over a longer period as it decays and returns nutrients to fire-damaged soils, which warrants further investigation into long-term community composition and structure over time.
While Ferrenberg et al. [21] suggested that duff, litter, and forbs affect arthropod community composition, mulch can also affect a variety of outcomes related to early vegetation in landscape-scale treatments [31,32,57]. Our limited vegetation survey found that some native and invasive plants (most notably native Mahonia (Oregon grape) and Chenopodium (goosefoot) in the Museum Fire site and invasive Lactuca serriola (prickly lettuce) and Bromus tectorum (cheatgrass) in Centennial Forest, similar to the findings of Jonas et al. [29] were established within the xylem treatments, but we were unable to evaluate the interactions between arthropods and vegetation over this short time period. Further studies on post-fire arthropod communities and erosion treatments will provide more information on the importance of interactions between arthropods and the recovery or invasion of understory plants. Our on-site observations also confirmed that logging slash mulch may be a positive long-term input for erosion control into severely damaged ecosystems on these high slopes. The unexpected trap losses due to erosion in this study occurred at lower unmulched or mixed-mulch elevations, whereas the high-elevation traps in the xylem treatments remained in place and undamaged for the duration of the study. We believe that this also demonstrates not only an increased potential habitat for arthropods but also an erosion protection effect for a longer period of time, giving soil and vegetation more opportunity to recover.
In summary, forest fires have significant impacts on soil and vegetation, leading to water-repellent soil surfaces that can cause flash flooding and debris flows. Fires can also have ecological consequences, including changes to arthropod communities through direct mortality and indirect changes due to loss of habitat and surface vegetation. Some arthropod groups may be more adaptable to changing environments than others, as demonstrated by the greater proportion of flies at higher elevations, where they serve as primary pollinators, and a wide variety of highly mobile beetle taxa. The responses of arthropod groups to different mulch types varied across sites, highlighting the importance of considering the functional groups of arthropods in post-fire ecosystems and their potential to adapt to changing conditions. Overall, post-fire mulching can have complex and varied effects on arthropod community structure, with responses varying by functional group, species, and ecosystem type. Fungivores were abundant across all treatment types at both study sites, which may be a positive outcome for managers considering ecological restoration and the critical components of forest recovery. Further research is needed to fully understand the interactions among arthropod taxa, vegetation, and other invertebrate and vertebrate taxa in rapidly changing post-fire environments.

Author Contributions

Conceptualization, C.M. and R.H.; Methodology, C.M. and R.H.; Formal Analysis, C.M. and A.A.; Investigation, C.M.; Resources, R.H. and A.A.; Data Curation, C.M.; Writing—Original Draft Preparation, C.M.; Writing—Review and Editing, C.M., A.A. and R.H.; Visualization, C.M. and A.A.; Supervision, R.H.; Project Administration, R.H.; Funding Acquisition, C.M. and R.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Joint Fire Science Program (JFSP) as part of a Graduate Research Innovation (GRIN) award, project ID 20-1-01-28, with R.H. as the principal investigator and C.M. as the investigating graduate student.

Data Availability Statement

The data presented in this study will be openly available in the Forest Service Research Data Archive at http://www.fs.usda.gov/rds/archive/ (accessed on 1 June 2023) upon completion of the grant agreement and/or final report requirements for the JFSP.

Conflicts of Interest

The authors declare no conflict of interest. A.A. was guest editor for this special issue, but the manuscript was handled by an outside party.

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Forests 14 01421 g001 550
Figure 1. Soil burn severity map of the Museum Fire footprint showing four transects (labeled A, B, C, and D) containing pitfall traps against high (red), moderate (yellow), low (dark green), and very low (light green) SBS levels. GIS data/layers provided by K. Paffett, USFS.
Figure 1. Soil burn severity map of the Museum Fire footprint showing four transects (labeled A, B, C, and D) containing pitfall traps against high (red), moderate (yellow), low (dark green), and very low (light green) SBS levels. GIS data/layers provided by K. Paffett, USFS.
Forests 14 01421 g001
Forests 14 01421 g002 550
Figure 2. NMS ordination for total Museum Fire beetle community by function with treatment as the variable (type: 0 = no treatment, 1 = mixed mulch, 2 = xylem only).
Figure 2. NMS ordination for total Museum Fire beetle community by function with treatment as the variable (type: 0 = no treatment, 1 = mixed mulch, 2 = xylem only).
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Forests 14 01421 g003 550
Figure 3. NMS ordination for total Museum Fire beetle community by function with a significantly different community in time period two (period: 1 = late summer 2020, 2 = early summer 2021, 3 = late summer 2021).
Figure 3. NMS ordination for total Museum Fire beetle community by function with a significantly different community in time period two (period: 1 = late summer 2020, 2 = early summer 2021, 3 = late summer 2021).
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Forests 14 01421 g004a 550Forests 14 01421 g004b 550
Figure 4. Average individuals per trap by significant family or functional group from Museum Fire transects: (a) Carabidae, significant in the xylem-only treatment; (b) Cryptophagidae, significant in the xylem-only treatment; (c) Curculionidae, significant in the untreated transect; (d) Tenebrionidae, significant in the mixed-mulch treatment; (e) predators, significant presence in the xylem-only treatment; (f) omnivores, significant in the mixed-mulch treatment; (g) bark beetles, significant only in the untreated transect. Average values marked with the same letter indicate no statistical significance for the difference between treatment groups.
Figure 4. Average individuals per trap by significant family or functional group from Museum Fire transects: (a) Carabidae, significant in the xylem-only treatment; (b) Cryptophagidae, significant in the xylem-only treatment; (c) Curculionidae, significant in the untreated transect; (d) Tenebrionidae, significant in the mixed-mulch treatment; (e) predators, significant presence in the xylem-only treatment; (f) omnivores, significant in the mixed-mulch treatment; (g) bark beetles, significant only in the untreated transect. Average values marked with the same letter indicate no statistical significance for the difference between treatment groups.
Forests 14 01421 g004aForests 14 01421 g004b
Table
Table 1. Central GPS points for Museum Fire transects and Centennial Forest site.
Table 1. Central GPS points for Museum Fire transects and Centennial Forest site.
Study SiteCenter of Transect Elevation
Museum Fire A335°15′31″ N 111°37′26″ W7720 feet (2353 m)
Museum Fire B335°16′2″ N 111°36′36″ W8400 feet (2560 m)
Museum Fire C335°15′39″ N 111°36′9″ W8860 feet (2700 m)
Museum Fire D335°15′27″ N 111°36′26″ W8760 feet (2670 m)
Centennial Forest (all plots)35°11′6″ N 111°46′1″ W7200 feet (2195 m)
Table
Table 2. General abundance by percent capture for Museum Fire and Centennial Forest sites.
Table 2. General abundance by percent capture for Museum Fire and Centennial Forest sites.
Coleoptera OrthopteraAnts (Hymenoptera)DipteraOther HymenopteraSpidersOther
Museum
Fire		48%6%9%26%0.1%3%8%
Centennial Forest37%20%36%1%0.1%5%1%
Table
Table 3. Indicator species in both study sites. Legend: LS1 = September 2020, ES2 = May 2021, LS2 = September 2021, IV = Indicator Value in PCORD.
Table 3. Indicator species in both study sites. Legend: LS1 = September 2020, ES2 = May 2021, LS2 = September 2021, IV = Indicator Value in PCORD.
CENTENNIAL FOREST
FamilyTrap Period (IV, p)Treated/Untreated (IV, p)Mulch Type (IV, p)
Nitidulidae80.3, p = 0.0002 (ES2)NoneNone
Staphylinidae86.4, p = 0.0002 (LS2)NoneNone
Tenebrionidae83.7, p = 0.0002 (ES2)NoneNone
Functional Group
Fungivore81.4, p = 0.0002 (ES2)NoneNone
Omnivore87.9, p = 0.0002 (LS2)NoneNone
Predator88.4, p = 0.0002 (ES2)NoneNone
MUSEUM FIRE
FamilyTrap Period (IV, p)Mulch Type (IV, p)Period × Mulch (IV, p)
Carabidae--21.8, p = 0.0610 (xylem)22.5, p = 0.0594 (LS2, mixed)
Coccinellidae22.3, p = 0.0046 (LS3)--24.6, p = 0.0452 (LS2, mixed)
Curculionidae--50.6, p = 0.0042 (none)--
Nitidulidae77.2, p = 0.0002 (ES2)--41.1, p = 0.0042 (ES2, none)
Scarabaeidae23.5, p = 0.0292 (LS1)----
Tenebrionidae79.8, p = 0.0002 (ES2)34.2, p = 0.0650 (mixed)71.1, p = 0.0002 (ES2, mixed)
Trachypachidae41.7, p = 0.0002 (ES2)20.3, p = 0.0584 (xylem)61.4, p = 0.0004 (ES2, xylem)
Functional GroupTrap Period (IV, p)Mulch TypePeriod × Mulch
Fungivore79.4, p = 0.0002 (ES2)--39.1, p = 0.0048 (ES2, none)
Herbivore (bark beetle)--49.2, p = 0.0014 (none)--
Omnivore79.8, p = 0.0002 (ES2)34.0, p = 0.0690 (mixed)70.8, p = 0.0002 (ES2, mixed)
Predator35.0, p = 0.0624 (ES2)50.0, p = 0.0018 (xylem)39.2, p = 0.0054 (ES2, xylem)
Unknown 44.2, p = 0.0002 (ES2)28.3, p = 0.0408 (xylem)60.5, p = 0.0008 (ES2, xylem)
Table
Table 4. Beetle families and functional groups that showed a statistically or ecologically significant difference in abundance for mulch type in the Museum Fire study area. There were no significant differences for any family or functional group in the Centennial Forest site.
Table 4. Beetle families and functional groups that showed a statistically or ecologically significant difference in abundance for mulch type in the Museum Fire study area. There were no significant differences for any family or functional group in the Centennial Forest site.
FamilyK-W Test Statistics
Carabidae χ2 = 9.9459, p = 0.0069
Cryptophagidaeχ2 = 7.5643, p = 0.0228
Curculionidae χ2 = 12.1, p = 0.0024
Tenebrionidae χ2 = 5.4865, p = 0.0644
Trachypachidaeχ2 = 5.9248, p = 0.0517
FunctionK-W test statistics
Bark beetles (HERBB)χ2 = 13.991, p = 0.0009
Omnivores (OMNI)χ2 = 4.9417, p = 0.0845
Predators (PRED)χ2 = 8.5874, p = 0.0137
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Mott, C.; Antoninka, A.; Hofstetter, R. Arthropod Recolonization of Soil Surface Habitat in Post-Fire Mulch Treatments. Forests 2023, 14, 1421. https://doi.org/10.3390/f14071421

AMA Style

Mott C, Antoninka A, Hofstetter R. Arthropod Recolonization of Soil Surface Habitat in Post-Fire Mulch Treatments. Forests. 2023; 14(7):1421. https://doi.org/10.3390/f14071421

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Mott, Christine, Anita Antoninka, and Richard Hofstetter. 2023. "Arthropod Recolonization of Soil Surface Habitat in Post-Fire Mulch Treatments" Forests 14, no. 7: 1421. https://doi.org/10.3390/f14071421

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