Macroarthropod leaf litter community recovery after prescribed fire in an east texas mixed hardwood-pine forest.
Prescribed burning of the forest floor is a commonly used tool of the forest manager. It reduces competition between crop trees and non-commercial plants for resources such as nutrients, water, and space while reducing the fuel load for potential unintentional fires (Wade 1988). It is also used to enhance forests for recreation by reducing the amount of mid- and under-story vegetation (Wade 1988). While removing the leaf litter, fire also affects the myriad of small and microscopic species that break down the organic litter. These species make up the prey base for many small predatory species such as lizards of the genera Scincella (Slater 1949; Lewis 1951; Hamilton & Pollack 1961; Brooks 1964) and Plestiodon (formerly Eumeces) (Fitch 1954), shrews of the genus Blarina (Ritzi et al. 2005), and arachnids of the orders Aranea (Gertsch 1979) and Scorpionida (Hadley and Williams 1968). Because changes in prey species density should dramatically affect predator populations (Pianka 1994), the recovery of these predator populations after disturbance, such as a fire, is thereby limited by prey availability. Therefore, documenting the effects of burning on leaf litter arthropods is important to understanding the effects of this practice on forest-dwelling species at higher trophic levels.
Leaf litter and deadfall have two primary functions related to small predators: (1) allowing them to acquire prey that live therein while (2) providing escape cover to avoid being preyed upon by species at a higher trophic level, such as snakes, felids, and avian predators. Full recovery or all members of the forest community to the pre-burn state is limited by the restoration of many interrelated biotic factors, including macroarthropod prey base. This study investigates the effects that prescribed burning has on the leaf litter macroarthropods (>0.5 mm) on the floor of an East Texas forest.
MATERIALS AND METHODS
Study area.-Four mixed pine-hardwood stands in northern Smith County in East Texas were chosen for this experiment based upon similarity of floral assembly and topography, proximity to one another, accessibility, and the amount of time that had elapsed since they were burned. Three divisions of Tyler State Park (32 [degrees] 28' 55.12" N, 95 [degrees] 16' 48.81- W, elev: 192m) were chosen as experimental burned sites and Sheff's Woods Preserve (32 [degrees] 29' 45.10" N, 95 [degrees] 15' 37.65" W, elev: 174m) was chosen as the non-burned control. These sites were classified by the number of months that have lapsed since they were last burned (MPB = Months Post-Burn). Site one was burned in January 2002 (30MPB), site two in January 2003 (18MPB), and site three was burned in January 2004 (8MPB) (Sparks, pers. comm.). Before current boundaries were established, these all were once part of a large, continuous mixed pine-hardwood forest habitat that resembled Sheffs Woods Preserve and extended across much of the region (L. Wolford, Nature Conservancy, pers. comm.).
Five samples of leaf litter were collected from each of the four study sites in late July and early August 2004. All leaf litter and organic material in 0.25 m2 of the forest floor was collected and immediately transported to the laboratory. These samples were placed in Tullgren funnels and exposed to a light and heat source for 7 days as outlined by Sutherland (1996) and New (1998). A container of 70% ethyl alcohol was placed beneath the funnels to collect and preserve specimens. While the length and width of the samples remained constant, the depth of substrate, and consequently the volume of the sample, varied. Each sample was inspected under a dissecting microscope and all arthropods over 0.5mm in length were identified to order, using keys by Bland & Jaques (1978) and Chu & Cutkomp (1992).
A MANOVA was used to determine differences in densities among treatments for each macroarthropod order. An ANOVA with a post-hoc Tukey Test was performed on each order that exhibited statistically significant differences to determine the origin of the variance. T-tests were performed in order to compare each treatment to the Sheff's Wood Preserve, the unburned control site.
RESULTS AND DISCUSSION
In total, 18 orders of arthropods were identified within five classes: Arachnida, Diplopoda, Entognatha, Insecta, and Malacostraca. The 6MPB tract of forest contained 8 of these orders, the 18MPB tract contained 13, the 30MPB tract contained 14, and the control tract contained 16. The insect orders Hemiptera, Neuroptera, and Zoraptera, as well as the class Diplopoda, were exclusive to the control site, while Isopoda was exclusive to the 30MPB tract. With the exception of Diplopoda, these groups each occurred in relatively small numbers within their respective areas (Table 1).
Table 1. Mean density of macroarthropods ([m.sub.-2]) for four mixed hardwood-pine tracts representing different stages of recovery following a burn. Time Since Burn CLASS: Order 6 Months 18 Months 30 Months Control ARACHN IDA: Acarina 34.4 137.6 115.4 239.2 Aranea 5.6 14.4 22.4 28 Chelonethida 0 2.4 0.8 0.8 Phalangida 0 0.8 0.8 0 DI PLOPODA: Julida 0 0 0 24.8 ENTOGNATFIA: Collembola 6.4 59.2 100.8 76 INSECTA: Coleoptera 6.4 24 47.2 92 Diptera 6.4 4.8 12 6.4 Hymenoptera 8 14.4 17.6 31.2 Hemiptera 0 0 0 0.8 Isoptera 26.4 24.8 20 20.8 Lepidoptera 0 4 2.4 23.2 Neuropiera 0 0 0 0.8 Orthoptera 0 1.6 4.8 0.8 Psocoptera 0 1.6 12 1.6 Thysanoptera 4.8 113.6 65.6 78.4 Zoraptera 0 0 0 0.8 Isopoda 0 0 1.6 0 MALACOSTRACA: Isopoda 0 0 1.6 0 Total: 98.4 403.2 423.4 625.6
The total density of macroinvertebrates exhibited significant variation among sites ([F.sub.2,12] = 10.86, P = 0.005; Tukey Test: 6MPB x 18MPB, P < 0.05; 6MPB x 30MPB, P < 0.01; Fig. 1). Within these burned sites, significant differences were identified among four individual groups: Coleoptera (F2,12= 7.72, P = 0.014; Tukey Test: 6MPB x 30MPB, P < 0.05), Collembola (F2.12 = 4.95, P = 0.040; Tukey Test: 6MPB x 18MPB, P < 0.05), Thysanoptera (F2,12 = 6.26, P = 0.023; Tukey Test: 6MPB x 30MPB, P < 0.05), and Acarina (F2,12= 12.83, P = 0.003; Tukey Test: 6MPB x 18MPB, P < 0.01; 6MPB x 30MPB, P < 0.05).
[FIGURE 1 OMITTED]
There were no significant differences between the 30MPB tract and the control for any group. Those significantly higher in density relative to the 6MPB tract were insects of the orders Collembola ([t.sub.(4)] = 3.70, P = 0.021), Coleoptera ([t.sub.(4)] = 2.88, P = 0.045). Lepidoptera ([t.sub.(4)] = 9.13, P = <0.001), and Hymenoptera ([t.sub.(4)] = 2.89, P = 0.045), and the Arachnid orders Acarina (1(4) = 3.15, P = 0.035) and Aranea ([t.sub.(4)] = 2.97, P = 0.041). Only Hymenoptera density remained significantly lower in the 18MPB tract (44) = 3.02, P = 0.039). While an effort was made to collect all specimens within each sample, highly mobile or flying species, such as large spiders, roaches, grasshoppers and crickets may have evaded capture by fleeing the sample as the collector approached the site, thereby incorporating some bias toward smaller, immobile species.
The effects of a burn on refugia and prey-base resources are most apparent in the months immediately following a burn. There is little structural organic material available as refugia, and macroinvertebrate density is depleted. Individually, there was no significant variation among many orders in the 1 8MPB and 30MPB tracts, but when all orders were included in the analysis, significant differences were detected. These findings indicate that some individual populations of leaf litter macroinvertebrates do not recover fully until well after the second year post-burn. This might be due to lack of food from decomposing leaf litter and lack of a noticeable fungal element. Furthermore, many arthropods are very mobile, so absence of some orders in the years following a burn is potentially due more to the absence of the yet-recovered niche that they normally occupy rather than their inability to disperse from surrounding habitat.
After the first leaf-fall, leaves remained whole for some time before decomposing. Therefore, there is increased stratification of the leaf litter over time, from minute leaf particles to whole leaves. With this increased depth and stratification, the volume of each sample increases because the samples are standardized along a two-dimensional plane. More leaf litter at varying stages of decomposition is present per unit area over time. Therefore, the same area of the forest floor theoretically increases its carrying capacity of organisms over time because there is a larger volume of substrate. The relative abundance of Diplopods, which are saprophagous, in Sheff's Woods Preserve, and complete lack of specimens in all burned sites indicates that there are some major groups that may not return until specific elements of the leaf litter layer are re-established. Springette (1979) recognized that fungivorous species, in particular, are reduced in number and density following a fire. This re-establishment of decomposing matter and fungal activity increases the diversity of the leaf litter layer by opening trophic niches for more specialized decomposers.
The total number of macroinvertebrate orders was similar for all sites. This is contrary to the findings of Springette (1976), who found that both population densities and species-level diversity were reduced by fire in a jarrah forest of Western Australia. However, Abbott et al. (1980), within the same system, found that the number of litter- and soil-dwelling species just following a burn is comparable to unburned numbers, while the densities of these species are lower. Heyward & Tissot (1936) had similar results within a Longleaf Pine forest in the southwestern United States. These reports mirror the present findings and indicate that fire does not necessarily affect diversity within the invertebrate fauna, at least at the level of order, but it does reduce the density. While this may be a function of a reduced volume of litter per unit area, the density of macroarthropods is nonetheless significantly reduced by fire.
By reducing the amount of macroarthropods in the forest floor substrate, burning decreases prey resources for many primary predators within the forest floor habitat. Watson (2004) indicated that these resource deficits affect the recovery of Scincella lateralis, a small skink that directly depends on both leaf litter and associated invertebrate fauna. This species is limited by the reduction in prey-base and the potential of predation by birds and other predators. Whelan et al. (1980) noted increased foraging behavior by birds following a burn. This exacerbates the problems for small predators related to reduced refuge coupled with a decreased prey base. A leaf-litter dwelling predator must utilize a larger area to find the same amount of prey, exposing itself to a greater risk of becoming prey due to increased activity and the lack of cover. The cost of obtaining prey is reduced with increased refuge availability, allowing for predator species to begin recovery before the macroarthropod fauna is fully replenished. Notably, total macroarthropod density partially recovers within the relatively short period of two to three years. This is not surprising given that this system evolved in forest habitats where fire was historically a natural occurrence. As long as the fire is not too intense and a mosaic of undisturbed habitat remains among the burned sites, these fire-managed areas should be able to recover fully from the perimeter in and from the ground up.
These findings provide a baseline evaluation of macroarthropod richness and density that is integral to the recovery of the next trophic level. The reaction and recovery of many primary predators within the forest habitat is directly related to the recovery of the macroarthropod community as a prey resource. Following a low-intensity fire within a mixed hardwood-pine forest, the leaf litter and associated macroarthropod community exhibited trends consistent with recovery within a couple of years, thereby forming a basis for recovery for the entire forest floor community.
This work was conducted under Texas Parks and Wildlife State Park Scientific Study Permit No. 30-03. James F. Bergan, Director of Science and Stewardship for the Texas Chapter of The Nature Conservancy, provided permission to survey Sheff's Woods Preserve. Neil Ford, Daniel Formanowicz, Sophia Passy, and Jeanette Boylan served as valuable advisors during this study. Laura Gough, Christian Cox, Rebbekah Watson, Cathy Shaw, R. Michael Burger and William Gehrmann provided editorial assistance throughout the evolution of this manuscript.
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CMW at: email@example.com
Charles M. Watson (1), (2), (3), and Melisssa L. Nieholson (3)
(1.) Department of Biology, Midwestern State University 3410 Taft Blvd., Wichita Palls, Texas, 7630S (2.) Department Biology, University of Texas at Arlington, Arlington. Texas 76019 (3.) Department of Biology, University of Texas at Tyler, Tyler, Texas 75799