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--------------------------------------------· Estimating Pre-treatment Variation in the Oak Leaf-chewing Insect Fauna of the Missouri Ozark Forest Ecosystem Project (MOFEP) Robert J. Marquis and J osiane Le Corffl Abstract.-We describe spatial and temporal variation in the insect herbivore communities associated with the MOFEP, prior to applica­ tion of contrasting cutting regimes. No pre-treatment differences were found in total insect density on either black (Quercus velutina) or white oak (Q. alba) during 1993-1995. There was great seasonal variation in insect abundance on both host species as well as high variation across years for white oak. White oak on north- and east­ facing slopes tended to have more insects than white oak on south­ and west-facing slopes. Sites under the auspices of the Missouri Department of Conservation for a longer time had higher insect densities than more recently acquired sites. The goal of the Missouri Forest Ecosystem levels can have negative impacts on growth and Project (MOFEP) is to determine the effects of survivorship of trees (Rexrode 1971, Coffelt et altemative forest management schemes on al. 1993) and can potentially influence ecosys­ long-term forest productivity, genetic structure tem processes such as nutrient ~ycling (Parker of plant populations, and biodiversity of the and Patton 1975). Th~ resultant economic communities under management. Treatments impact could be substantial because timber (even-aged versus uneven-aged management, harvesting and processing in Missouri generate plus controls; see Brookshire et al. 1997 for $3 billion in economic activity annually (T. explanation of the MOFEP treatments and Robison, personal communication). Oaks are design) were begun in 1996. During 1993- also important as food for wildlife (Brezner 1995, sampling was carried out to quantify any 1972, White 1995), but their economic contri­ differences that might exist among replicates, so bution in this form has yet to be estimated. that potential true effects produced by the treatments could be distinguished from condi­ From a conservation standpoint, Lepidoptera, tions existing prior to treatment application. and butterflies in particular, have come under increasing scrutiny as the abundance of many One of the major components of these Ozark species declines (e.g., Thomas and Mallorie forested communities, in terms of their ecologi­ 1985, Hill1995, Legge et al. 1996). Lepidoptera cal, economic, and conservation role, are the can be used as indicator species for changes in insect herbivores that feed on oaks. The major­ habitat quality because their populations are ity of leaf-feeding insects on Missouri oaks are intimately tied to the abundance of their host larvae of Lepidoptera (moths and butterflies) plant, and host plant decimation is often (Marquis and Whelan 1994). As herbivores, synonomous with habitat destruction (e.g., they have potential gro\\1:h impact on their Eberhardt and Thomas 1991, Sparrow et al. hosts. Non-outbreak population levels cause 1994). In Missouri, these insects represent a enough damage to reduce growth of saplings major component of the State's biodiversity and (Marquis and Whelan 1994), while outbreak natural heritage: at least 300 species have been documented on oaks in the Eastem United States and 200 on white oak alone in Missouri (Tietz 1972, Covell 1984). Additionally, these 1Associate Professor of Biology and Postdoctoral insects are important because they serve as Research Associate, respectively, Department of hosts for a number of parasitoids, both insects Biology, University of Missouri-St. Louis, 8001 and nematodes. In so doing, they could serve to Natural Bridge Road, St. Louis, MO 63121- maintain a reservoir population of parasitoids 4499. that might be used to control invading exotic 332 insect pests such as the European gypsy moth, pattems. Results of analyses of herbivore Lymantria dispar. community composition and canopy sampling will be presented separately. The goal of this project was to describe variation in the abundance of the leaf-chewing insect METHODS fauna among replicate sites of the MOFEP project during the 3 years before ha.IVesting General Census Methods treatments were applied. Two host plant spe­ cies were chosen for study, black (Quercus To determine insect abundance, leaves were veZutina) and white (Q. alba) oaks. These two searched both top and bottom, as were associ­ species dominate the canopy of most MOFEP ated branches and the main stem of the tree. upland stands. Limited resources prevented us Chewing insects encountered were classified to from sampling more host species. We chose to morpho-species. Each census person was given include all leaf-chewing insect species, and not a training period in identification prior to actual a subset of that fauna, because preliminary sampling, and had a list of descriptions for all sampling in 1989-1992 suggested that species' morpho-species known to be encountered. At abundances of leaf-chewing insects fluctuate no time were leaves collected. All insects were unpredictably (RJM, unpublished data). Thus, left intact on the plant unless individuals were a study that focused on only common species in unknown. In that case, unknowns were taken one year would have no estimates of abun­ back to the laboratory for rearing and photo­ dances of previously rare species that had graphing. Each unknown was given its own become common. High diversity of the leaf­ unique sample number and description, and chewing guild (at least 200 species) prevented this information was entered into a database. us from sampling other herbivore guilds associ­ Photographs were taken, and the insect was ated with black and white oak. However, num­ observed through development until it could be bers of galling and sap-sucking insects in the verified as a previously-recognized species or MOFEP region, and throughout Missouri, classified as a species new to our inventory. appear to be low (Marquis and Whelan 1994). Photos were used in the field to help with identification when necessary. Our overall objective here is to describe pre­ treatment variation (years 1993-1995) in the Preliminary Sampling to Establish abundance of the leaf-chewing insect herbivores Adequate Sample Size associated with Quercus alba and Q. veZutina at the MOFEP sites. In this paper, we first de­ Preliminary sampling was conducted in early scribe the initial sampling undertaken to estab­ September 1992 to determine the sample size lish the sample size chosen for the 3 years of necessary to adequately estimate the mean and pre-treatment sampling. Second, we analyze variance in total numbers of insects per tree per the pre-treatment data to determine whether stand and per site, and species composition per sites assigned to different treatments actually site. Ten trees per stand were sampled in site 6 demonstrated differences before treatments for each of three north-facing slopes and three were applied. Third, we present analysis of the south-facing slopes for both black and white spatial variation in herbivore abundances oak. The mean and variance in total numbers irrespective of the original sampling design to of insects per leaf area per tree were plotted for understand possible sources of background each stand and across stands for each tree heterogeneity among the study sites, including species. The number of new species encoun­ potential biases inherent in our sampling. tered with increasing number of stands sampled Fourth, we discuss whether we are currently was plotted to determine how the number of undersampling or oversampling in light of the insects species found in a site increased with pattems observed. To do so, we use power increasing number of stands sampled. The analysis to predict the minimum difference total number of species per site was compared produced by treatments that we would be able with species accumulation curves for each to establish as statistically significant. Finally, stand and each host plant. This preliminary we make recommendations about post-treat­ sampling then served to determine sample size ment sampling. We present results for total for all subsequent sampling in years 1993- insect numbers sampled in the understory and 1995, described below. For this analysis and all analyze abundances of a subset of species to subsequent analyses, insect abundance was illustrate the variety of individual species expressed on a leaf area basis. Average leaf 333 /~ • ~CCJ)JFJEIP area per tree species was estimated based on a Number of Samples sample of 200 undamaged, fully expanded leaves per species, with a maximum of five Due to changes in herbivore abundance and leaves per tree. composition through the year (Marquis and Whelan 1994), trees were censused four times 1993-1995 Censuses per year: April-May, late June, late July, and August-September. Each census required 2 Overall MOFEP Design weeks to complete. At each census, sites were always censused in the same order for each of The MOFEP design (see figure 2 in Brookshire the 3 years: 6, 5, 4, 3, 2, 1, 7, 8, 9. et al. 1997) includes nine sites of approximately 400 ha each. Each of these sites has been Statistical Analysis assigned to either a control, even-aged, and uneven-aged treatment. Sites are blocked by For data collected in years 1993-1995, repeated geographic proximity and other general charac­ measures analysis of variance (AN OVA) (Littell teristics (Sheriff and He 1997). et al. 1991, p. 130, 282, von Ende 1993) was first used to test for pre-treatment differences Tree Attributes among sites as they were assigned to treat­ ments, for block effects as designated in the Ground-level censusing (0.5 to 2.5 m) was original design (Brookshire et al. 1997), and for conducted from a mixture of saplings and low­ the relative importance of ELT, census, and hanging branches of sub-canopy to canopy year on insect density (number of insects per trees. leaf area per tree). All but block, treated as a random effect, were considered to be fixed.

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