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Plant species, plant community diversity and microbial interactions can significantly impact soil microbial communities, yet there are few data on the interactive effects of plant species and plant community diversity on soil bacterial communities. We hypothesized that plant species and plant community diversity affect soil bacterial communities by setting the context in which bacterial interactions occur. Specifically, we examined soil bacterial community composition and diversity in relation to plant "host" species, plant community richness, bacterial antagonists, and soil edaphic characteristics. Soil bacterial communities associated with four different prairie plant species (Andropogon gerardii, Schizachyrium scoparium, Lespedeza capitata, and' Lupinus perennis) grown in plant communities of increasing species richness (1, 4, 8, and 16 species) were sequenced. Additionally, soils were evaluated for populations of antagonistic bacteria and edaphic characteristics. Plant species effects on soil bacterial community composition were small and depended on plant community richness. In contrast, increasing plant community richness significantly altered soil bacterial community composition and was negatively correlated with bacterial diversity. Concentrations of soil carbon, organic matter, nitrogen, phosphorus, and potassium were similarly negatively correlated with bacterial diversity, whereas the proportion of antagonistic bacteria was positively correlated with soil bacterial diversity. Results suggest that plant species influences on soil bacterial communities depend on plant community diversity and are mediated through the effects of plant-derived resources on antagonistic soil microbes.

Resource abundance and plant diversity are two predominant factors hypothesized to influence the amount of damage plants receive from natural enemies. Many impacts of these environmental variables on plant damage are likely indirect and result because both resource availability and diversity can influence plant traits associated with attractiveness to herbivores or susceptibility to pathogens. We used a long-term, manipulative field experiment to investigate how carbon dioxide (CO2) enrichment, nitrogen (N) fertilization, and plant community diversity affect plant traits and the amount of herbivore and pathogen damage experienced by the common prairie legume Lespedeza capitata .W e detected little evidence that CO2 or N affected plant traits; however, plants growing in high- diversity treatments (polycultures) were taller, were less pubescent, and produced thinner leaves (higher specific leaf area). Interestingly, we also detected little evidence that CO2 or N affect damage. Plants growing in polycultures compared to monocultures, however, experienced a fivefold increase in damage from generalist herbivores, 64% less damage from specialist herbivores, and 91% less damage from pathogens. Moreover, within diversity treatments, damage by generalist herbivores was negatively correlated with pubescence and often was positively correlated with plant height, while damage by specialist herbivores typically was positively correlated with pubescence and negatively associated with height. These patterns are consistent with changes in plant traits driving differences in herbivory between diversity treatments. In contrast, changes in measured plant traits did not explain the difference in disease incidence between monocultures and polycultures. In summary, our data provide little evidence that CO2 or N supply alter damage from natural enemies. By contrast, plants grown in monocultures experienced greater specialist herbivore and pathogen damage but less generalist herbivore damage than plants grown in diverse communities. Part of this diversity effect was mediated by changes in plant traits, many of which likely are plastic responses to diversity treatments, but some of which may be the result of evolutionary changes in response to these long-term experimental manipulations.

Invasive species are a global threat to biodiversity and the functioning of natural ecosystems. Here, we report on a two-year experiment aimed at elucidating the combined and relative effects of three key controls on plant invasions: propagule supply, soil nitrogen (N) availability, and herbivory by native insects. We focus on the exotic species Lespedeza cuneata, a Rank 1 invasive species. Propagule supply and soil N-availability interacted to control the density and foliar cover of L. cuneata. In low N plots, density and foliar cover of L. cuneata were higher in the propagule addition plots than in the plots to which propagules were not added. Surprisingly, this interaction was significant only when the abundance of herbivores was experimentally reduced. This experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species.

We measured legume abundance following 13- and 5-yr experiments testing for the effects of mammalian herbivores, nutrients, and climate on plant communities in an old field and a savanna in east central Minnesota. Total legume abundance was signif- icantly greater in plots with herbivores excluded. Within herbivore exclosures, legumes were more abundant in plots to which P, K, S, Mg, Mn, Ca, Na, and trace minerals had been added. Legumes were significantly more abundant in the savanna than in the old field. Lathyrus venosus, a rapidly growing early-maturing species, was largely responsible for these results. Two other common legumes at our study site, Amnorpha canescens and Les- pedeza capitata, were not significantly affected by herbivore exclosures. However, within herbivore exclosures, addition of nutrients significantly reduced Lespedeza. Late summer total legume biomass within herbivore exclosures increased strongly following exclosure establishment in 1982, declined dramatically following a severe drought in 1988, and then increased again following the drought. This trend suggested that herbivore effects we measured in 1994 restUlted from long-term accumulation of legumes following the estab- lishment of exclosures. Our results suggest that herbivores and nutrients other than nitrogen can dramatically limit the abundance of some legume species that might otherwise dominate grassland plant communities on nitrogen-poor soils. Limitation of legumes by colonization and drought may also be important. Thus, herbivores and nutrients other than nitrogen may be critical in structuring grassland plant communities and influencing succession, even on nitrogen-poor soils.

Habitat corridors, a common management strategy for increasing connectivity in fragmented landscapes, have experimentally validated positive influences on species movement and diversity. However, long-standing concerns that corridors could negatively impact native species by spreading antagonists, such as disease, remain largely untested. Using a large-scale, replicated experiment, we evaluated whether corridors increase the incidence of plant parasites. We found that corridor impacts varied with parasite dispersal mode. Connectivity provided by corridors increased incidence of biotically dispersed parasites (galls on Solidago odora) but not of abiotically dispersed parasites (foliar fungi on S. odora and three Lespedeza spp.). Both biotically and abiotically dispersed parasites responded to edge effects, but the direction of responses varied across species. Although our results require additional tests for generality to other species and landscapes, they suggest that, when establishing conservation corridors, managers should focus on mitigating two potential negative effects: the indirect effects of narrow corridors in creating edges and direct effects of corridors in enhancing connectivity of biotically dispersed parasites.

Microarthropod population diversity, distribution, and species composition were studied in three South Carolina old fields with the use of fiberglass—mesh litterbags. Mean total arthropod densities were highest in a 1st—year successional field (abandoned cornfield), lowest in a 12—year broomsedge field, and intermediate in a 12—year lespedeza field. Densities differed significantly with season, field, and litter type within field. Litter moisture content also differed significantly with these factors and changes in moisture were correlated with changes in density or microarthropods. Both species diversity and species composition differed significantly between fields, but litter type had no significant effect. Conclusions were (1) the litter fauna is a transient subset of a larger and possibly more diverse soil fauna, present in the litter only during favorable moisture regimes; (2) species diversity and composition of the litter fauna are determined primarily by the successional stage of the field and are affected little by the introduction of allochthonous litter in the small amounts contained in the decimeter—square litterbags used in this study.

Aboveground vegetative production was measured for two consecutive years in three South Carolina old fields beginning with a 1-yr mixed forb field, a 12-yr broomsedge (Anidropogon) field, and a 12-yr Lespedeza field. Vegetative production was periodically har- vested and corrected for mortality between sampling periods. Total aboveground production by the vegetation was mixed forb, 1,321 and 1,447 kcal/m2; broomsedge, 898 and 1,297 kcal/m2; and lespedeza, 4,467 and 4,490 kcal/m2 for the 2 yr, respectively. Close approxi- mations of the annual vegetative production by each field were obtained by multiplying the average annual standing crop of detritus by the mean annual detritus disappearance rate. Summing the maximum standing crops of each species, a practice often used to avoid under- estimation of the annual production, may in some circumstances lead to serious overestimation of production. Comparisons of specific productivity parameters determined for the three South Carolina fields with those determined for a 30-yr-old field in Michigan differed widely between all four fields except for the ratios of total vegetative production/maximum standing crops of green materials.

Measurements of the web height and counts of the number of webs on randomly selected plots in lespedeza fields reveal that immature Argiope aurantia and Argiope trifasciata, very large araneid spiders, place their webs at different heights. In late summer this vertical stratification disappears, while the numbers of the two species on plots become positively correlated. Coexistence of the two spider species may depend in part upon the usual occurrence of high mortality during the immature stages. Invasion of webs by araneids is reported as possible competitive interference. The two largest species of European araneids also show vertical stratification only as immatures. However, the niches of most Polish Araneus species in fields seem differentiated by parameters related to prey size: season of breeding and size differences between species.

In a recent investigation (Drew, '42) of the revegetation of abandoned cropland in Boone and Callaway Counties, Missouri, attention was called to the frequent occurrence of various legumes in eroded old fields. As an outgrowth of these observations, the present study is an attempt to determine differences in the establishment and growth of several species of legumes on eroded areas of a given soil type 2 and to evaluate some of the edaphic factors related to these differences. Two native legumes, Cassia fasciculata Michx., an annual, and Lespedeza virginica (L.) Britt., a perennial, both frequently found in eroded old fields, were selected along with two introduced legumes, Lespedeza stipulacea Maxim., an annual, and L. service Benth., a perennial, for the experimentation. L. stipulacea at present is widely planted in this area as a cover and forage plant on poor soils, and L. sericea is being used increasingly for the same purpose. The two particular species of native legumes were selected principally because of an adequate supply of excellent nursery-grown seed furnished by the Soil Conservation Service nursery at Elsberry, Missouri. It was hoped that the study would indicate: (1) the effect of the various chemical and physical factors of the soil which are changed by erosion, upon the establishment, growth, and composition of these legumes; (2) the relative value of the legumes as to their ability to compete with other vegetation under adverse conditions; (3) their relative value as producers of soil cover; and (4) their relative value as efficient users of soils of low nutrient levels and poor physical conditions. Except for a recent paper by Allard ('42), who has evaluated soil factors responsible for the dearth of natural vegetation in certain bare areas of old fields on Bull Run Mountain, Virginia, very few ecologists have attacked directly the problem of the factors involved in the diminution of growth of plants on eroded or very poor soils. However, closely related aspects of this 1 The writers wish to acknowledge gratefully the assistance rendered them during this investigation by the following persons: Mr. H. A. Steavenson and Mr. C. J. Coukos of the U. S. Soil Conservation Service, for suggestions and generous contribution of materials; Professor C. A. Helm, Department of Field Crops, for seed of Lespedeza stipulacea; Mr. D. E. Wade of the University of Missouri Wildlife Area and Arboretum, for his excellent cooperation in facilitating the study; Dr. W. A. Albrecht and Dr. E. R. Graham, of the Soils Department, for most helpful criticism and suggestions in connection with soils analyses; and members of the Botany Department who have aided in the field work and have offered valuable suggestions. 2 Although the soil type (Union silt loam) on which these studies were conducted differs in various respects from those (Lindley series) on which the revegetation process was investigated, observations, repeatedly verified, indicate little substantial difference in this general area in the vegetation on these two soil types.

A comparison was made of several cumulative species/cumulative individuals indices using a statistical distribution corresponding to the species of insects in a sample from the herb stratum of a lespedeza field. Criteria used in comparison were that the indices be intensive for a given universe regardless of sample size and that the indices differentiate between universes having different numbers of species for a given number of individuals. Species/log individuals, species–1/natural log of individuals, log species/log individuals, and species/square root of individuals were examined. Only the index of species/square root of individual met both of the criteria.