969 Response of ground-dwelling spider assemblages to prescribed fire following stand structure manipulation in the southern Cascade Range1 Nancy E. Gillette, Richard S. Vetter, Sylvia R. Mori, Carline R. Rudolph, and Dessa R. Welty Abstract: We assessed spider (Arachnida: Araneae) responses to prescribed fire following stand structure treatments in pon- derosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) stands in the Cascade Range of California. Stands were logged or left untreated to create three levels of structural diversity. We logged one treatment to minimize old-growth characteristics (low diversity) and one to enhance old-growth characteristics (high diversity) and we used unlogged Research Natural Areas (RNAs) as old-growth, highest-diversity reference stands. We conducted low-intensity prescribed fire on half of each plot fol- lowing harvest. Spider assemblages in unburned, logged stands were similar to one another but diverged from those in RNAs, with increased abundance, species richness, and diversity in more structurally diverse stands. Prescribed fire, which altered habitat in the organic soil layer where many spiders forage, resulted in altered spider assemblages and population declines in most plots. Fire generally reduced spider species richness, evenness, and diversity. Several taxa were potential indicators of fire and old-growth structure, and we discovered one species and one genus that were previously unknown. There was evi- dence that old-growth characteristics intensified the effects of fire on spider abundance. This outcome probably results from the deep litter layers in high-diversity stands and RNAs, which constituted greater fuel loads than low-diversity stands. Re´sume´ : Nous avons e´value´ la re´action des araigne´es (Arachnida : Araneae) au bruˆlage dirige´ a` la suite de traitements vi- sant a` modifier la structure du peuplement dans des peuplements de pin ponderosa (Pinus ponderosa Dougl. ex P. & C. Laws.) situe´s dans la chaıˆne des Cascades en Californie. Les peuplements furent re´colte´s ou ne furent pas traite´s afin de cre´er trois niveaux de diversite´ structurale. Un traitement visait a` re´duire au minimum les caracte´ristiques de vieille foreˆt (fa- ible diversite´), un autre visait a` accentuer les caracte´ristiques de vieille foreˆt (forte diversite´) et des zones naturelles de re- cherche non exploite´es qui ont e´te´ utilise´es comme peuplements te´moins de vieille foreˆt avaient la plus forte diversite´. Nous avons effectue´ un bruˆlage dirige´ de faible intensite´ dans la moitie´ de chacune des parcelles apre`slare´colte. Les assemblages d’araigne´es dans les peuplements re´colte´s non bruˆle´se´taient semblables mais diffe´raient de ceux des peuplements te´moins. L’abondance ainsi que la richesse et la diversite´ en espe`ces e´taient plus grandes dans les peuplements avec la plus forte diversite´ structurale. Le bruˆlage dirige´, qui a alte´re´ l’habitat dans l’horizon organique du sol ou` plusieurs araigne´es se nour- rissent, a modifie´ les assemblages d’araigne´es dont la population a diminue´ dans la plupart des parcelles. Le feu a ge´ne´rale- ment re´duit la richesse, l’e´quitabilite´ et la diversite´ des espe`ces. Plusieurs taxons e´taient des indicateurs potentiels du feu et d’une structure de vieille foreˆt et nous avons de´couvert une espe`ce et un genre jusqu’a` maintenant inconnus. Il y avait des indices que les caracte´ristiques de vieille foreˆt accentuent les effets du feu sur l’abondance des araigne´es. Ceci est probable- ment le re´sultat de l’e´paisse couche de litie`re qui constitue une charge de combustibles plus importante dans les peuplements avec une forte diversite´ et les peuplements te´moins que dans les peuplements avec une faible diversite´. [Traduit par la Re´daction] Introduction Land management agencies have responded by applying stand structure modifications (thinning) and prescribed fire Spatial and stand structural patterns of forests in the to create more fire- and insect-resistant stands and to ap- northwestern United States have been drastically modified proximate the more open and structurally diverse stands that since presettlement times (Hessburg et al. 2000; Taylor and existed when European immigrants first colonized the Pa- Skinner 2003), rendering them more susceptible to wide- cific Northwest. Little is known, however, about the impact spread wildfire and insect outbreaks (Hessburg et al. 2005). of such treatments on the biodiversity and function of im- Received 15 May 2007. Accepted 10 December 2007. Published on the NRC Research Press Web site at cjfr.nrc.ca on 21 April 2008. N.E. Gillette2 and S.R. Mori. USDA Forest Service Pacific Southwest Research Station, P.O. Box 245, Berkeley, CA 94701, USA. R.S. Vetter. Department of Entomology, University of California, Riverside, CA 92521, USA. C.R. Rudolph. USDA Forest Service Institute of Forest Genetics, 2480 Carson Road, Placerville, CA 95667, USA. D.R. Welty. USDA Forest Service Placerville Nursery, 2375 Fruitridge Road, Camino, CA 95709, USA. 1This article is one of a selection of papers from the Special Forum on Ecological Studies in Interior Ponderosa Pine — First Findings from Blacks Mountain Interdisciplinary Research. 2Corresponding author (e-mail: [email protected]). Can. J. For. Res. 38: 969–980 (2008) doi:10.1139/X07-245 # 2008 NRC Canada 970 Can. J. For. Res. Vol. 38, 2008 portant forest biotic communities, particularly the soil and tices. Such indicators are best derived from randomized, re- litter fauna (Freckman 1994; Moldenke et al. 1999). There plicated, and controlled prospective studies, which are rare. is growing evidence that, collectively, spiders (Arachnida: Most past studies have necessarily relied on retrospective Araneae) are important in stabilizing populations of other ar- and (or) chronosequence studies because of the expense and thropods, including pest insects (Coyle 1981), so forest man- logistical difficulty of large-scale forest management manip- agement practices may have unintended consequences if ulations, with a few noteworthy exceptions such as the impacts for ground-dwelling arthropods are not taken into EMEND study in northern Alberta, Canada (Work et al. consideration. There is, therefore, an acute need for repli- 2004), and the EVO project in Finland (Vanha-Majamaa cated, controlled experiments to elucidate the consequences et al. 2007). of habitat modification for structurally complex faunal as- The study of spider response to silvicultural treatments semblages, particularly those of spiders (Wise 1993). such as thinning and prescribed fire is in its infancy, partic- Spiders are generalist predators with a diet limited almost ularly with respect to interior ponderosa pine (Pinus ponder- exclusively to arthropods, principally insects but including osa Dougl. ex P. & C. Laws.) forests. For example, it may other spiders (Wise 1993; Foelix 1996). As a group, spiders seem intuitively obvious that retention patches might pro- are megadiverse and have worldwide distribution, occupying vide refugia for recolonization of treated plots following almost all terrestrial habitats (Coddington and Levi 1991); harvesting or prescribed fire, but results have been ambigu- most individual species, however, are locally adapted to mi- ous in some cases (Matveinen-Huju et al. 2006) and re- crohabitats (Foelix 1996). The habitat requirements of spider sponses from different geographic areas have yielded species are thought to be driven both by microclimate and contradictory results. We know of no published results, in by the spatial characteristics of the habitat because of the in- fact, for biological indicators of effects of forest manage- fluence of habitat architecture on the development of speci- ment on the spider fauna of interior ponderosa pine forests, alized predatory adaptations by spiders (McIver et al. 1992). and such indicators of disturbance are highly desirable be- For example, bolas spiders (Araneidae), which lure and cause they can serve as surrogate measures of long-term sus- snare male moths by synthesizing the female moths’ sex tainability of forest management (Brennan et al. 2006). As pheromones and then swinging a baited line of silk from such, they may help land managers predict the outcome of understorey vegetation, occupy an extremely narrow stratum treatments such as prescribed fire and various tree-retention of vegetation (Yeargan 1994). Spitting spiders (Scytodidae) harvesting plans for soil arthropods. Interior ponderosa pine are slow-moving nocturnal hunters that catch their prey by forests (SAF forest type 237) (Barrett et al. 1980), which are spraying adhesive silk from modified venom glands in a zig- characterized by short growing seasons with limited summer zag pattern to immobilize their prey (Li et al. 1999). Such precipitation, represent the most extensive forest type in highly specialized predatory adaptations are myriad among North America, ranging from west-central Mexico to south- Araneae, so spider species are expected to reflect habitat ar- ern British Columbia (Oliver and Powers 1998). Considering chitecture more reliably than many other arthropod groups. the extent and value of this forest type, it is prudent to as- Indeed, spiders have been reported to be very sensitive to sess impacts and determine reliable indicator species that habitat structure, in particular the understorey vegetation are specific to this geographic region before initiating large- that provides the architectural framework of their habitats