COMMUNITYAND ECOSYSTEMECOLOCY Influence of Prescribed Fire on Carabid Beetle (Carabidae) and Spider .(Araneae) Assemblages in Forest Litter in Southwestern Oregon CHRISTINE G. NrWA t ^ND ROBERT W. PECK Z Forestry Seienees Laboratory, Pacific Northwest Researeh Station, USDK Forest Selwtee, 3200 SW Jefferson Way, Corvallis, OR 973~1 rn ...., ::2" Environ. Entomol. 31 (5): 785-796 (2002) CO3 ooO~ ABSTRACT The objective of this study was to determine if prescribed fire affects spider (Araneae) and carabid beetle (Carabidae) abundance, and whether the magnitude of this effect varies with time since fire. Within mixed conifer stands; nine understory fuels-reduction burns, ranging fi'om <1 to 15 yr old, were compared with adjacent unburned sites. Pitfall trapswere used to compare macroar- thropod abundance over 5 me. In total, 3,441 spiders in 24 families and >120 species, and 14,793 carabid -~o- beetles fi'om 17 species, were identified fi'om the samples. Seven spider ftmailies and five species of t'D earabid beeries were abundant enough to be analyzed statistically. Four spider families were more abundant in unburned sites (Antrodiaetidae, Cybaeidae, Thomisidae mad Linyphiidae) while three families were more numerous in bm'ned sites (Lycosidae, Gnsphosidae arid Dictynidae). Four of five carabid beetle species were more abundant in unburned sites [Pte~sttct,us herculanens Manne,'heim, P. setosus Hatch, Scaphinott~ rugiceps rugiceps (Horn) and Zacotus matthewsii LeConte]. There was no difference found for Omus caziert van den Berghe. No differences in species rtehness or diversity (Simpson, Shannon-Wiener and Berger-Parker indices) were found for spiders or earabid beetles. Overall, the relationship between abundance mad time since burning was weak, with marginal significance found only for Dictynidae and Cnaphosidae. We suggest that changes in foraging substrate, prey availability or mieroelimatic conditions since fire may have interacted with llfe history characteristics to influence the abund:mce of these organisms. Differences in fire intensities mnong years may have masked patterns in arthropod abundance associated with time since burning. KEY WORDS preselibed fire, forest litter arthropods, spiders, Carabidae, Oregon, pitfall traps IS ^ major factor influeneing the composition ,and on many forests. Although prescribed ~qre can be ef- strueture of forests in the Pacific Northwest (Agee fective atobtaining this goal, it etudes the possibility 1993). In southwestern Oregon, where summers are of greatly modifying the structure of the habitat for typically hot and dry, wildfires historically are fl'e- understor,] arthropod communities. The effect of pre- quent; average fire return intervals as short as 20 yr scribed fire on tu'thropod communities, and tile eco- have oecun'ed over much inland coniferous forest in logical processes they influence, is poorly understood. this region (Atzet and Wheeler 1982). These wildfires A variety of studies have investigated the effect of were generally of light intensity, and occasionally fires fire on forest arthropod co,nmunities. Most have would bm'n large areas, destroying much timber. To looked at either the influence of stand-replacing wild- prevent this loss of wood resources, fire suppression fire (Harris and Whiteomb 1974, Richardson and Hol- was initiated in the early 1900s. In the absence of fire, liday 1982, Holliday 1992) or prescribed fire as a site however, major changes in the structure of forests preparation measure (e.g., slash burning) following oecurred in many areas. These changes include an timber harvest (I-Iuhta 1971, Neumann 1991, Beaudi7 increase in the density of tree species less tolerant of et al. 1997). Fewer studies have looked at the effect of fire (McNeil and Zobel 1980) and unnaturally high prescribed underburning on litter arthropod commu- aeeumulations of both downed and standing dead nities. In the JatTah forests of western Australia, both wood (Parsons and DeBenedettl 1979). As a result of Majer (1984) and Abbott (1984) found no difference this greater fuel load, the chance of high intensiW, in total arthropod abundance between bunaed and eatastrophic wildfire has increased. To minimize this unburned sites immediately after prescribed fire, al- risk, controlled understory burning has been intro- though abundances were lower in'burned sites for duced as a management strategy to reduce fuel loads some taxa. Among South Carolina longleaf pine stands burned ~1, 2, and 3 yr before smnpling, New mad ' E-mall:eniwa@fs,fed.us. Hanula (1998) also found no difference in total litter 786 ENVIHONMENTAL ENTOMOLOGY Vol. 31, no. 5 at'thropod abundance, but spiders were more numer- gins-fir (>50% canopy cover); (2) the site had not ous in stands bm'ned 3 yr previously than in those experienced confounding events, such as thinning, burned 2 yr prior. Results from these studies suggest compaction, or having been burned on more than one that the abundance of some forest litter arthropods occasion, and; (3) the site could be matched with an may be redueed following prescribed fire, but the adjacent or nearby unbm'ned control site, with similar longer-term response is unclear. aspect, elevation, forest structm-e mad reoent history. The primary goal of this resem'eh w~ to determine Locating acceptable control sites was critical due to the short- and longer-term effects of prescribed un- differences in forest structure inherent to the varied derstory fuels reduction burning on litter macro,ar- landscape. Fires were located using GIS maps pro- thropod communities within a managed forest eco- vided by personnel at the Rogue River NF, Ashland, system in southwestern Oregon, We hypothesized OR. The perimeter of each fire was determined by that fire would have an initial impact on the abun- locating fire sore's on tree trunks. All nine prescribed dance of many ,'u~hropod tax,a, but that the m'thropod fires were conducted dm'ing spring months. The pre- community in burned areas would become increas- scribed burn that took place duling the year of the ingly similar to those in unburned areas as tlnae since study (1998) was performed over two time periods; fire increased, We focused on em'abid beetles (Cara- 8-9 June and 19 June 1998. The second burn was bidae) and spiders (Araneae) for the following rea- required due to poor initialburn conditions and con- sons: (1) they ale abundant trod diverse in forest cerns for air quality in the nearby town of Ashland. ecosystems; (2) they have a long history of use in Arthropod Sampling. During 1998, m~hropods were ecological studies, many of which used pitfall traps to collected umng pitfall traps. PitfMl traps consisted of a quantify relative activity or abundance (Thtele 1977, g-liter plastic bucket (14-cm-dimneter opening) fitted Wise 1993); (3) most common species are relatively with a metal funnel and covered with a 25 by gS-em easy to identify; (4) many are known to be sensitive plastic rain cover suspended with nails 5 cm above the to changes in habitat structure (Thiele 1977, Uetz trap. Trap buckets were buried in the ground with 1970); (5) they use a variety of feeding modes, with their tops placed flush with the level of the forest floor, some being specialists and some generalists (Thlele A 250-ml glass canning jar partially filled with ethy.lene 1977, Wise 1993); and (6) both have been cited as taxa glycol (50% solution) was placed beneath each funnel potentially useful as indicators of environmental dis- to catch and preserve arthropods falling into the traps. turbanee (Melver et el. 1992, Beaudry et el. 1907). Within each of the 18 sites, 19. pitfall traps were placed at 30-m intervals alone transects. Transects ran along Materials and Methods terrain contours and were placed no closer than 30 m from a site boundary~ Study )xrea. This study took place within the Ash- Pitfall traps at all sites except the 1998 and 1995 and watershed, ~6.4 km south of Ashland, OR, in the burns were run for seven 9.-wk (approximately) in- Ashland Ranger District, Rogue River National Forest. tervals: one hlterval in the late spring (3 June-17 Ranging from 900 to 1,500 m elevation, this ,area of the June); two intervals in the late spring/early summer Klmnath Mountains of southwest Oregon is repre- (17 June-8 July mad eight July-21 July), two in mid- sented by white fir [Abies concolor (Gord. & Clend) summer (4 August-18 August and 18 August-1 Sep- Lindl. ex Hildebr. ] and Douglas-fir [Pseudotsuga men- tember), and two in the late summer/early fall (15 ziesll (Mirb.) Franeo] climax forest zones. The distri- September-29 September and 29 September-13 Oc- bution and elevational range of forest zones in this tober). The gap between sanapling periods was de- area m'e greatly int2uenced by local geomo~-ohologieal signed to reduce trapping pressm'e on the litter ar- and climatic conditions; however, in general the white thropod community. Because it was not known on fir zone averages 1,300 m in elevation and the Dou- what date burning would take place, stunpling before glas-fir zone averages 850 m in elevation (Atzet et el. the bum was not possible; traps in the 1998 bum site 1996). Under natural fire regimes, these forests would were installed on 18 June in the portion of the site that consist of a mixed assemblage of conifers, dominated burned 8-9 June. Traps were not placed in the 1995 by ponderosa pine (Pinus ponderosa Dougl. ex Laws.) bm'n until 1 July because of uncertainty of the avail- and Douglas-fir. They also would include white fir and ability of the site. For traps in the 1998 and 1995 sites, sugar pine (Pinus lambertiana Dougl.), and would be retrieval of catches began on the first collection date multistoried and relatively open, containing little following their placement.