Canopy Arthropod Assemblages in Four Overstory and Three Understory Plant Species in a Mixed-Conifer Old-Growth Forest in California Timothy D. Schowalter and Yanli Zhang Abstract: We compared canopy arthropod assemblages among overstory conifer and understory angiosperm species at Teakettle Experimental Forest in the Sierra Nevada in California during 1998–2000. Arthropods were sampled from upper, middle, and lower crown levels of one overstory tree of each of the four dominant conifer species (Jeffrey pine, sugar pine, white fir, and incense cedar), and from three understory plants of each of the major understory species (California black oak, manzanita, and white-thorn ceanothus) in each of five replicate plots during June and Aug. in each of the 3 years to represent seasonal and annual variation in abundances. Many taxa differed significantly in abundance among plant species, with one to five taxa being significant indicators for each plant species. Five to eight taxa on each plant species showed significant differences in abundance among years. Aphids and scale insects, predaceous mirids, and some detritivores showed peak abundances in 1999, a particularly dry year, whereas most other taxa showed lowest abundances during 1999 or declining abundances during this period, suggesting association with wetter conditions. Nonmetric multidimensional scaling (NMS), supported by multi-response permutation procedure (MRPP), showed that arthropod assem- blages differed significantly among the seven plant species, especially between overstory conifers and understory angiosperms, and among the 3 years. These data indicate that the diversity and structure of arthropod communities depend on vegetation structure and/or condition, perhaps as modified by annual variation in weather conditions. FOR.SCI. 51(3):233–242. Key Words: Insect, spider, mite, disturbance. OREST CANOPIES REPRESENT major ecological func- and Turchin 1993, Dudt and Shure 1994, Schowalter 1995, tions (e.g., photosynthesis, evapotranspiration, and Ozanne et al. 1997, Roland and Taylor 1997, Schowalter F interception of light, water, wind, and nutrients) that and Ganio 1999). Because insects are small, have short control biomass accumulation and environmental conditions generation times, and rapid reproductive rates, their popu- (Ruangpanit 1985, Chen et al. 1995, Lewis 1998). Arthro- lation sizes can change rapidly in response to changes in pods can dramatically alter canopy structure and function in environmental conditions such as those resulting from ways that can stabilize primary production or interfere with changes in weather or in host abundance or biochemical management goals (e.g., Mattson and Addy 1975, Romme condition (Schowalter 1985). et al. 1986, Schowalter 2000). For example, low intensity of This study was designed to compare canopy arthropod feeding on foliage by insects can stimulate nutrient turnover abundances among four dominant overstory conifer species and increase tree growth (Alfaro and Shepherd 1991), and three dominant understory angiosperm species in a whereas high intensity of feeding on foliage can reduce tree mixed-conifer forest before thinning and burning treatment. growth and lead to tree mortality and opening of the canopy We expected significant differences in arthropod abun- (Schowalter et al. 1986). dances and assemblage structure among plant species that Factors affecting arthropod abundances and assemblage would be selected for posttreatment study. structure are not well understood (Barker and Pinard 2001, Foggo et al. 2001). Individual plant species typically have Methods relatively distinct arthropod assemblages, due to character- istic biochemical interactions between host and associated We studied canopy arthropod assemblages on five rep- species. Arthropod species can respond positively, nega- licate plots at the Teakettle Experimental Forest, 80 km east tively, or not at all to particular environmental changes, of Fresno, in Sierra County, CA, during 1998–2000. This depending on their adaptations to temperature, relative hu- site is located at 2,000–2,500 m elevation and is dominated midity, changes in plant growth, chemistry, or abundance, by old-growth (Ͼ300 years old), mixed-conifer forest. An- etc. (Shure and Phillips 1991, Schowalter 1985, Schowalter nual precipitation averages 110 cm at 2,100 m and falls T.D. Schowalter, Entomology Department, Louisiana State University, Baton Rouge, LA 70803—Phone: (225) 578-1628; [email protected]. Yanli Zhang, Department of Entomology, Oregon State University, Corvallis, OR 97331; present address: 2744 SW Pickford Street, Corvallis, OR 97333. Acknowledgments: We thank three anonymous reviewers and an associate editor for helpful comments on the manuscript. This study was supported by the USDA Forest Service, Pacific Southwest Research Station (PSW-98-010-RJVA), and the Oregon Agricultural Experiment Station and Forest Research Laboratory at Oregon State University. Manuscript received June 11, 2003, accepted January 14, 2005 Copyright © 2005 by the Society of American Foresters Forest Science 51(3) 2005 233 mostly as snow between Nov. and Apr. Mean, maximum, This sampling technique emphasizes the sedentary fauna and minimum July temperatures are 17, 30, and 3°C, re- that is present on foliage and twig surfaces at any given spectively (Berg 1990). Soils are generally Xerumbrepts sampling time (e.g., aphids, caterpillars, spiders, mites) and and Xeropsamments typical of the southwestern slopes of tends to underrepresent highly mobile species that could be the Sierra Nevada (USDA Forest Service 1993). alarmed and escape (Schowalter 1995, Schowalter and El Nin˜o conditions prevailed during 1998, bringing Ganio 1999). Other sampling techniques have different bi- above-average precipitation (186 cm), especially winter ases, e.g., interception trapping emphasizes flying adult snow, restricting access until mid July; La Nin˜a conditions insects that may or may not be associated with a particular prevailed during 1999, with less than average precipitation plant or even a particular treatment unit; canopy fogging (101 cm); intermediate conditions prevailed during 2000 emphasizes unattached arthropods that would reach collec- (136 cm). Temperatures varied somewhat among years. An tors on the ground and would exclude many small arthro- outbreak of Douglas-fir tussock moth (Orgyia pseudo- pods that would be intercepted before reaching the ground tsugata [McDunnogh]) began in 1997 and collapsed during (e.g., Majer and Recher 1988, Blanton 1990). 1999; extensive fir mortality occurred in areas of severe Samples were sorted in the laboratory by inspecting bags defoliation east of Teakettle, but was negligible at Teakettle for mobile arthropods, then scanning samples under a dis- (M. North, USDA Forest Service Sierra Nevada Research secting microscope (10ϫ) and collecting, identifying, and Ctr., Davis, CA., personal communication, Aug. 2000). tabulating all arthropods. Plant material was dried at 50°C Historically, this forest was co-dominated by large (Ͼ1 and weighed. Arthropod abundances were standardized as m diameter), widely spaced Jeffrey pine (Pinus jeffreyi number per kilogram of plant sample. Grev. and Balf.), sugar pine (Pinus lambertiana Douglas), white fir (Abies concolor Gord. and Glend.), and incense Data Analyses cedar (Calocedrus decurrens [Torrey]) in approximately equal proportions. However, fire exclusion during the past Arthropod taxa often are categorized into functional century has promoted recruitment of white fir, resulting in groups to reflect compartmentalization of natural commu- large areas of closed canopy forest dominated by young nities and to focus on similar resource utilization strategies white fir (Ͻ100 years old, Ͻ70 cm diameter). Understory (Romoser and Stoffolano 1998). We followed previous vegetation is primarily composed of patches of manzanita functional group designations, i.e., folivores (all foliage- (Arctostaphylos patula Greene), and white-thorn ceanothus chewing caterpillars, sawflies, beetles, etc.), sap-suckers (Ceanothus cordulatus Kellogg) under canopy openings. (aphids, scale insects, leafhoppers, etc.), gall-formers (pri- California black oak (Quercus kelloggi Newb.) also is abun- marily gall midges), predators (especially spiders, mites, dant, primarily along outcrops (Barbour et al. 1988, Mc- beetles, snakeflies, etc.) and detritivores (primarily bark Kelvey and Busse 1996). lice, springtails, and oribatid mites), for comparative pur- During 1997, 18 200-m2 plots in a 1,300 ha area were poses (see Schowalter et al. 1981, Schowalter 1995, Scho- established for the purpose of studying effects of thinning walter and Ganio 1999). However, species within functional and burning on these forests. Work described here charac- groups may respond to environmental changes in different terizes the pretreatment condition of the forest. Following ways (Schowalter et al. 1999). Therefore, we analyzed data treatment, additional data will be collected from all plots to for sufficiently abundant species as well as for functional evaluate canopy arthropod responses to the treatments. groups. We also calculated total arthropods and Simpson For this study, we sampled one overstory tree of each of index (Magurran 1988) for each sample. the four dominant conifer species and three individuals of Sample data for three canopy levels (upper, middle, and each of three understory species in each of five plots dis- lower) were pooled for each treatment and plant