Field Exclosure Research Articles

The contribution of pollinators varies among soybean cultivar traits

Soybean is one of the most widely cultivated species worldwide. Empirical studies have shown that animal pollination can contribute between 0 to 50% to soybean yields. However, the role of animal pollination in soybean production is often overlooked in management decisions. Understanding the factors driving variability in pollinator contribution can aid in developing effective management strategies. In this study, we experimentally assessed the contribution of both autonomous and animal pollination across nine widely cultivated soybean cultivars in the Chaco region of Argentina. Additionally, we explored whether specific traits of these cultivars could explain the observed variability in pollination contribution. We used field exclosure experiments to study cultivars that differed in flower color, genetic modifications, and maturity groups, and analyzed the variability in pollinators´ contributions across years and locations. We found that the overall reduction in production between open and bagged plants was, on average, 40% (CI 25-51%). The contribution of pollinators varied depending on flower color, maturity groups, and locations, but not across different years or genetic modifications. Cultivars with purple flowers showed greater differences between open and bagged plants compared to those with white flowers, indicating that flower color may influence the attractiveness of flowers to pollinators. Additionally, pollinators’ contribution varied across maturity groups, potentially due to the differential timing of the flowering affecting the local abundance of pollinators within the crop. Notably, the variable used to estimate pollinators’ contribution (i.e., seeds, pods, or yield) conditioned the results. Pollinators´ contribution can be highly variable, and traits associated with cultivars can help improve our understanding of such heterogeneity. Our results showed that the contribution of pollinators to soybean cultivars in the Chaco region of Argentina ranges from modest to high. This suggests that conserving pollinators and managing agricultural fields at plot and landscape scales can significantly impact soybean production.

Using path analysis to determine interacting effects of biotic and abiotic factors on patch-scale biogeochemical rates in a prairie stream

Biogeochemical rates within streams vary with ecosystem properties including the distribution of fishes. While many studies investigate the singular effect of fishes on ecosystem components, there is a limited understanding of how fish presence interacts with other ecosystem properties to affect ecosystem structure and function. Here, we used path analyses to elucidate direct and indirect effects of fish presence, and other ecosystem properties on ecosystem respiration (ER), gross primary production (GPP), and ammonium uptake. Experimental responses of fish removal on patch scale (300 cm2) benthic rates of ER, GPP, and ammonium uptake were measured at two sites in a prairie stream. The effect of fish was determined by comparing substrata from field exclosures with fish absent to substrata exposed to fish. Total path model-explained variance was greatest for ER (R2 = 0.55) and least for ammonium uptake (R2 = 0.36) and GPP (R2 = 0.34). Fish decreased algal biomass and directly increased all biogeochemical rates. The relative importance of the different abiotic ecosystem properties varied by process; however, FBOM and substrata size were important for most rates. This study provides evidence for predominantly direct effects of fish on both stream structure and function in a prairie stream. Our results emphasize that interactions between biotic and abiotic factors should be considered when determining drivers of biogeochemical activity. We suggest that simple linear food webs that consist of top–down or bottom–up control are not always sufficient to describe animal effects on ecosystem rates.

Assessing the relative importance of earthworms as granivores under field conditions

Plant communities are often seed-limited, and while rodents and birds are considered to be the primary contributors to surface seed removal, there is growing appreciation for the role of soil invertebrates, including earthworms. There have been very few studies assessing the relative importance of earthworms as granivores under field conditions compared to other taxa. We conducted a small-scale experiment in which we used earthworm inoculations and field exclosures to control access to surface-sown grass seed in an open field habitat and monitored impacts on initial grass recruitment. In isolation, earthworms reduced grass recruitment under field conditions (−22% biomass), but their impact was smaller than the reductions that occurred irrespective of earthworm presence when aboveground taxa had access to plots (−80–83%). The results of this small-scale study suggest that although earthworm-seed interactions may be ecologically important overall, in some habitats other taxa may be more influential than earthworms in terms of total surface seed removal.

Responses of belowground communities to large aboveground herbivores: Meta-analysis reveals biome-dependent patterns and critical research gaps.

The importance of herbivore-plant and soil biota-plant interactions in terrestrial ecosystems is amply recognized, but the effects of aboveground herbivores on soil biota remain challenging to predict. To find global patterns in belowground responses to vertebrate herbivores, we performed a meta-analysis of studies that had measured abundance or activity of soil organisms inside and outside field exclosures (areas that excluded herbivores). Responses were often controlled by climate, ecosystem type, and dominant herbivore identity. Soil microfauna and especially root-feeding nematodes were negatively affected by herbivores in subarctic sites. In arid ecosystems, herbivore presence tended to reduce microbial biomass and nitrogen mineralization. Herbivores decreased soil respiration in subarctic ecosystems and increased it in temperate ecosystems, but had no net effect on microbial biomass or nitrogen mineralization in those ecosystems. Responses of soil fauna, microbial biomass, and nitrogen mineralization shifted from neutral to negative with increasing herbivore body size. Responses of animal decomposers tended to switch from negative to positive with increasing precipitation, but also differed among taxa, for instance Oribatida responded negatively to herbivores, whereas Collembola did not. Our findings imply that losses and gains of aboveground herbivores will interact with climate and land use changes, inducing functional shifts in soil communities. To conceptualize the mechanisms behind our findings and link them with previous theoretical frameworks, we propose two complementary approaches to predict soil biological responses to vertebrate herbivores, one focused on an herbivore body size gradient, and the other on a climate severity gradient. Major research gaps were revealed, with tropical biomes, protists, and soil macrofauna being especially overlooked.

Disturbance-dependent invasion of the woody weed, Calotropis procera, in Australian rangelands

Plant invasions are threats to biodiversity and ecosystem processes that have far reaching ecological and economic impacts. Understanding the mechanisms of invasion essentially helps in developing effective management strategies. Rubber bush (Calotropis procera) is an introduced milkweed that invades Australian beef production rangelands. Its establishment is often associated with disturbances caused by pastoral management practices. We examined whether or not rubber bush (1) outcompetes native grasses, (2) can invade intact rangeland, and (3) if disturbance facilitates rubber bush establishment and spread in grassy rangelands. We measured the competitive response of different densities of Mitchell grass (Astrebla pectinata) individuals and the competitive effects of associate rubber bush seedlings in an additive common garden experiment. Replicated field exclosure experiments, under grass-dominated and tropical savanna woodland conditions examined the effect of increasing levels of disturbance on rubber bush seedling emergence. The dominant native Mitchell grass was a stronger competitor than rubber bush when grown together under greenhouse conditions, whereby root and shoot biomass yields were more restricted in rubber bush compared with Mitchell grass. This finding was corroborated in the field exclosure experiments at both sites, where seedling emergence increased 5-fold in seeded and highly disturbed plots where superficial soils were turned over by treatments simulating heavy grazing and trampling by cattle or machinery. Emergence of rubber bush seedlings in seeded plots that were undisturbed, clipped and grazed was minimal and rubber bush seedlings did not survive the seedling stage in these plots. These results demonstrate that disturbance to the superficial soil stratum affects the ability of rubber bush seeds to successfully establish in a microsite, and high levels of soil disturbance substantially increase establishment. Thus, rubber bush is a poor competitor of Mitchell grass and does not invade intact grassland. Consequently, rubber bush invasion is disturbance-dependent in the vast Australian rangelands. The spread of this weed may be arrested by management practices that minimise disturbances to grass cover.

Can seed caching enhance seedling survival of Indian ricegrass (Achnatherum hymenoides) through intraspecific facilitation?

Positive interactions among individual plants (facilitation) may often enhance seedling survival in stressful environments. Many granivorous small mammal species cache groups of seeds for future consumption in shallowly buried scatterhoards, and seeds of many plant species germinate and establish aggregated clusters of seedlings from such caches. Scatterhoards made by desert heteromyid rodents provide a major source of seedling recruitment for Indian ricegrass (Achnatherum hymenoides), a perennial bunchgrass species occurring widely across arid southwestern deserts of North America. However, effects of the resulting clumping of seedlings on subsequent survival have not been quantified under field conditions, which include extended periods of excessive summer heat and little precipitation. We monitored Indian ricegrass seedlings transplanted into field exclosures at two western Nevada study sites as either single seedlings or clumps of 25 or 35 seedlings and compared their survival. Survival was positively correlated with the number of seedlings growing together, and survival of whole clumps of 25 or 35 seedlings was significantly greater than that of seedlings growing singly. Moreover, individual seedlings within clumps of 35 seedlings had significantly higher survival than seedlings growing singly. No single seedlings survived through the hot and very dry summer following their planting, but a small proportion (1.8–2.8 %) of individual seedlings within clumps survived. Results of this field experiment suggest that facilitative benefits accruing to Indian ricegrass due to seed-caching desert rodents can extend past seedling establishment and into the long-term survival of the plant.

Invasive earthworms as seed predators of temperate forest plants

Soil seedbanks play a key role in forest plant communities, contributing to regeneration and acting as a refuge from seed predators. This study provides evidence that seeds entering the soil seedbank are vulnerable to granivory by invasive earthworms in temperate forests. Overall, 73 % of seeds of 6 ecologically important forest species were removed from the soil surface over 2 weeks in a Lumbricus terrestris microcosm experiment; 30 % vanished entirely, and presumably were destroyed. The invasive garlic mustard, Alliaria petiolata, was subject to the highest rates of removal. In contrast, results from a field exclosure experiment using 23 species of seed indicate that while seed predation by worms is still detectable, predation by rodents often may mask impacts of earthworms under natural conditions. Worms and rodents preferred different sizes of seeds: while seed predation by rodents was high in mid- to large-seeded species, earthworms tended to prefer smaller seeds. These findings suggest that although rodents are the main driver of seed predation, invasive earthworms may act as an additional ecological filter, and potentially may further influence the species composition of forest plant communities.

Expected and unexpected effects of waterbirds on Mediterranean aquatic plants

The mute swan Cygnus olor Gmelin and the greater flamingo Phoenicopterus roseus Pallas potentially affect plants, directly through grazing and indirectly through mechanical disturbance during their foraging activities. We studied in a field exclosure experiment, from May 2010 to July 2011, their respective impact on macrophytes in two Mediterranean lagoons (Camargue, South of France) with different plant communities. In one lagoon, greater flamingo negatively affected Ruppia cirrhosa Petagna cover from June to July, and dry biomass in July. In the other lagoon, greater flamingo and mute swan had a detrimental effect on Zostera noltii Hornemann cover at the beginning of the growing season (April). They also reduced Chaetomorpha sp. cover from April to July and dry biomass in July. Combined activities of waterbirds decreased the dominance of Chaetomorpha sp. bed, thus favouring later (July) Z. noltii cover and partially dry biomass in areas available for greater flamingo and mute swan. However, the impact of waterbirds was not entirely beneficial for Z. noltii, as its abundance during July was indeed higher in ungrazed areas with low Chaetomorpha sp. abundance in previous months. The combined effect of waterbirds on Z. noltii is thus dual, mainly favouring its development by reducing competing macroalgae but conversely mitigating it through direct impact at both ends of the growing season. Depending on the ecological context, waterbirds can thus affect in two opposite ways the growth of plant species such as Z. noltii or R. cirrhosa which are of importance with regard to conservation. Large waterbird species can be considered as ecosystem engineers in Mediterranean wetlands.

Above-ground vs. below-ground interactive effects of mammalian herbivory on tallgrass prairie plant and soil characteristics

To better understand how mammalian herbivory affects tallgrass prairie, I set up field exclosures to test effects of above-ground herbivory (AGH) (Bison bison L. is a potential above-ground herbivore), below-ground herbivory (BGH) (Geomys bursarius and Spermophilus tridecemlineatus are potential below-ground herbivores), and the interaction of the exclusion of both, on tallgrass prairie plant species, plant community parameters, and soil parameters in the Flint Hills of Northeastern Kansas. I found that (1) AGH reduced the cover of some forbs but increased the cover of others while both AGH and BGH decreased the cover of dominant C4 grasses, (2) no treatments significantly affected species richness but ABH reduced total cover and maximum height of the vegetation while BGH reduced species evenness and maximum height, and (3) no treatments affected soil pH or soil nitrogen but BGH reduced soil organic matter, soil phosphorus, and soil potassium. Whereas above-ground mammalian herbivory has significant effects on individual plant populations and plant community structure, it is BGH that mainly affects soil parameters. Results suggest that for prairie patches where both kinds of herbivory are present, these effects may be additive leading to cumulative effects on tallgrass prairie plants both directly, through herbivory, and indirectly, through soil changes.

Soil Phosphorus Concentration under the Canopy of Perennial Grasses of Different Successional Stages Exposed to Water Stress

In this field study, the effects of water‐stress, rainfed, or irrigated conditions on soil phosphorus (P) concentration under the canopy of the native grasses of semi‐arid Argentina (late seral Stipa clarazii Ball., early serals S. tenuis Phil., and S. gynerioides Wild. Ex Steud) at different developmental stages and sampling dates during 1996 and 1997 were determined. Plants grew in a uniform matrix such that each plant of S. clarazii and S. tenuis was surrounded by four plants of S. gynerioides. In September 1996, Bray and Kurtz extracted soil P concentrations were greater under plants of S. clarazii if water stress was imposed during the vegetative stage than under rainfed conditions. Bray and Kurtz extracted soil P concentrations were either similar or between 8.7 to 31% greater under plants of S. clarazii than under those of the other two species. The higher nutrient concentration under S. clarazii than under S. gynerioides and S. tenuis may explain previously measured increases in density, frequency, and cover throughout a long‐term span (years) in the late over the earlier seral perennial grasses under field exclosures to domestic livestock or appropriate grazing management conditions.

Effect of Season on the Impact of Ecosystem Engineers in the New River, NC

The importance of biotic interactions in structuring communities can depend on seasonal changes in abiotic context. The crayfish and benthic-feeding fish of the headwaters of the New River have been shown to be ecosystem engineers due to their ability to reduce the amount of sediment accumulation in cobble beds during the summer. However, the temperature-driven seasonal reduction in the activity level of these organisms may reduce their impacts during the colder months. I measured the impact of benthic-feeding fish and crayfish on sediment accrual in the headwaters of the New River during the winter using a field exclosure experiment. I found that during the winter there was no effect of benthic-feeding fish and crayfish on the amount of sediment accumulating in cobble beds in the headwaters of the New River. These findings suggest that seasonal changes in the effect of ecosystem engineers need to be quantified for a complete understanding of the effect of these organisms.

Population Dynamics of <I>Aphis glycines</I> (Homoptera: Aphididae) and Its Natural Enemies in Soybean in Northern China

Abstract A field survey of soybean aphid, Aphis glycines Matsumura, and its natural enemies was conducted during summer 2002 in Langfang, northern China (116.4° E, 39.3° N). Aphids colonized soybean when plants were still small in early July. After a lag of 2 wk, aphid density increased rapidly in late July, reaching a peak of 114 ± 46 aphids per five soybean plants on 1 August. The population declined to a plateau immediately after this peak and then declined again starting in mid-August, although a second small peak occurred in late August. The finite rate of increase varied from zero- to five-fold, and the aphid seemed to be limited by natural enemies. The main species of natural enemy were the aphid parasitoid Lysiphlebus sp., the aphid predators Propylaea japonica (Thunberg), Scymnus (Neopullus) babai Sasaji, and Paragus tibialis (Fallen). In a field exclosure experiment, A. glycines density in small-mesh cages peaked three-fold higher than in large-mesh cages and 12-fold higher than on uncaged plant...

Direct assessment of competitive ability and defoliation tolerance in perennial grasses

Stipa clarazii is a dominant, late-seral perennial grass species under exclosure or lightly grazed rangelands in semiarid Argentina, whereas S. tenuis and S. ambigua are earlier-seral perennial tussock grasses. Recent studies have demonstrated that late-seral species are more competitive and can have either similar or greater herbivory tolerance than earlier-seral species. We hyphothesized that (1) tolerance to defoliation is greater on defoliated plants of S. clarazii than on those of S. tenuis and S. ambigua, (2) competitive ability is greater in S. clarazii than in S. tenuis and S. ambigua when plants remain undefoliated or are exposed to a comparable defoliation intensity, and (3) competitive ability decreases in S. clarazii when this species is selectively defoliated within a non-defoliated, nearby neighbourhood of S. tenuis or S. ambigua. The study objectives included obtaining a direct measure of competitive ability and defoliation tolerance in the three perennial grasses when they grew either in isolation or in different neighbourhoods and were exposed to various defoliation patterns in the field. In 1998, two parallel studies were conducted within a 2-yr-old field exclosure during one growing season. In one study, responses were evaluated on either defoliated or undefoliated plants of all three species without neighbours. In a second experiment, four types of neighborhoods were selected such that a central (target) plant of one species was surrounded by five neighboring plants of a different species. Three different defoliation patterns were imposed on each neighbourhood type. Results led to acceptance of the first and second and rejection of the third hypothesis. When plants grew without nearby neighbours, greater growth rates for height, dry matter production and end-of-season daughter tiller production per parent tiller in S. clarazii relative to S. tenuis and S. ambigua contributed to greater regrowth in S. clarazii. Amounts of 15N atom excess were always greater in S. clarazii than in S. tenuis and S. ambigua independently of neighbourhood type and pattern of defoliation. However, uptake of 15N was similar in S. clarazii to that in S. tenuis or S. ambigua when S. clarazii was selectively defoliated within a non-defoliated, nearby neighbourhood of any of the other two species. Selective herbivory of S. clarazii, rather than factors associated with its competitive ability and defoliation tolerance, were found to be the major determinant driving its replacement by earlier-seral, less desirable perennial grasses in rangelands of central Argentina. Key words: 15N, Argentina rangelands, biomass, nitrogen uptake, species replacement, Stipa

A plant functional approach to the prediction of changes in Australian rangeland vegetation under grazing and fire

Abstract. Grazing by domestic livestock and changed fire regimes by humans have caused major changes in the productivity and composition of rangelands in Australia and other continents. Of particular concern are the commonly observed loss of perennial forage species and the increasing abundance of woody plants. Grazing and fire‐induced changes are difficult to predict from current process knowledge and are often too costly or time‐consuming to investigate experimentally. We describe the development and use of ARENA, a new simulation model. A plant functional approach is used in which the relative growth rate, competitive ability and life cycle of the plant types are mainly defined by the plant's morphology and allocation pattern, plus its water‐use efficiency and nitrogen concentration. The soil and plant types can be parameterized to a large extent with information from the literature, facilitating application in a broad range of dryland environments. The model has been tested for two soils and pasture communities in the seasonally dry tropics of the Victoria River District, N Australia. Predictions of pasture production and perennial grass fraction under undisturbed conditions agreed with observations in field exclosures. Predictions of maximum tree density also coincided with observations along a rainfall gradient. Simulation experiments were conducted to explore the effect of different stocking levels and fire management regimes on pasture productivity and composition. Responses of pastures on red loam and grey clay soils were generally consistent with regional field experience, but the model did not reproduce the expected changes in the abundance of woody plants.

Prudent Protomognathus and despotic Leptothorax duloticus: differential costs of ant slavery.

The concept of ant slavery rests on the untested assumption that slave-making ants impose fitness costs on colonies of the species they raid. We tested that assumption by comparing the summertime seasonal productivity of Leptothorax spp. colonies in field exclosures without slavemakers, with a colony of the obligatory slave-making ant Protomognathus americanus, or with a colony of the obligatory slavemaker Leptothorax duloticus. Leptothorax longispinosus colonies placed in exclosures with P. americanus colonies did not differ significantly in any demographic attribute from colonies in exclosures without slavemakers. By contrast, Leptothorax curvispinosus colonies exposed to L. duloticus experienced significant reductions in dealate queens, workers, and larvae relative to control colonies exclosed without slavemakers. The pronounced difference in the impact of these slavemakers on their host-species populations correlates with differences in the behavior of the slavemakers observed in the laboratory and likely explains why P. americanus is more abundant than L. duloticus in nature. It seems that more advanced social parasites, like anatomical parasites, evolve to minimize their impact on their hosts, and thus can be regarded as "prudent social parasites."

Suppression of nematophagous fungi by enchytraeid worms: a field exclosure experiment.

The feeding biology of Enchytraeus crypticus and other enchytraeids is poorly understood as is their effect on nematophagous fungi. Because enchytraeids had been associated with nematophagous fungi in the field and had suppressed these fungi in soil microcosms, we tested the hypothesis that exclusion of enchytraeids, largely E. crypticus, would improve establishment of certain nematophagous fungi in field plots. The fungi, Hirsutella rhossiliensis and Monacrosporium gephyropagum, are being studied as potential control agents of plant-parasitic nematodes and were formulated as hyphae in alginate pellets. The pellets were mixed into soil without enchytraeids and placed in cages (PVC pipe, 80 cm3 volume) with fine (20 μm) or coarse (480 μm) mesh; cages were buried 15 cm deep in field plots and then recovered after 6-52 days. When fine mesh was used, enchytraeids were excluded and the fungi increased to large numbers. When coarse mesh was used, enchytraeid numbers in cages increased rapidly and the fungi did poorly. Although mesh also affected other potential fungivores, including collembolans and large dorylaimid nematodes, we suspect that enchytraeids were more important because large numbers were consistently found in cages with coarse mesh soon after the cages were placed in soil. Organisms smaller than enchytraeids (bacteria, fungi, and protozoa) also appeared to be important because the fungi did better in heat-treated soil than in non-heat-treated soil, regardless of mesh size. The rapid increase in enchytraeid numbers in cages with hyphal pellets and coarse mesh was probably caused by movement of enchytraeids toward the pellets with hyphae: increase in enchytraeid numbers was minimal when movement into cages was blocked (or when cages contained pellets without hyphae). Overall, the data were consistent with the hypothesis that enchytraeids, or other meso- or macrofauna, contributed to suppression of nematophagous fungi in our field plots.

Natural Enemy Impact on the Abundance of Diuraphis noxia (Homoptera: Aphididae) in Wheat in Southern France

Natural enemy impact on Diuraphis noxia (Mordvilko) abundance was measured with field exclosure experiments in wheat near Montpellier, France, during three springs. Plants were artificially infested with D. noxia; predators, parasitoids, and aphid–home pathogens were excluded from some plants but not others; and subsequent changes in aphid densities were compared by destructive plant sampling. Each year, D. noxia density in all treatments increased and then declined as plant quality deteriorated, but peak densities were 10– to 18–fold higher on closed–caged plants than on open–caged or uncaged plants. Differences in microclimate, emigration, and aphid competition were eliminated as explanations for differences in D. noxia densities. The mortality required to explain differences in dynamics between closed–caged and open–caged/uncaged D. noxia populations was calculated. Natural enemies were sampled in the surrounding wheat to assess their capacity to inflict this required mortality. The predatory coccinellid Coccinellaseptempunctata L. reached densities of 4–16 per square meter and the predatory syrphids Episyrphus balteatus De Geer, Metasyrphus corollae F., and SphaerophOria scripta L. together reached one per square meter. The parasitoids Aphellinus asychis walker, Aphelinus varipes (Foerster), Diaeretiella rapae (McIntosh), and Aphidius matricariae were also abundant. Given predator search capacities and observed parasitism rates, mortality from natural enemies could easily account for the differences in D. noxia densities between closed–caged and open–caged/uncaged plants.

Density‐Dependent Processes in Meadow Voles: An Experimental Approach

Patterns of population dynamics are determined by the interaction between density—dependent and density—independent processes. To examine the responsiveness of various demographic and behavioral processes to population density, we maintained replicate meadow vole (Microtus pennsylvanicus) population at low, medium, and high densities inside field exclosures in southern New York. Density was manipulated by removing subadults during biweekly live—trapping sessions. Under higher density conditions voles exhibited: (1) a truncated breeding season; (2) reduction in growth rates of individuals; (3) increase in age (but not in size) at sexual maturity; and 4) reduction in movement distances of individuals. Extremely large—bodied voles were most frequent in low—density enclosures and least common in high—density ones. Survival rates of adults were delayed density—dependent. Reproductive rates during the middle of the breeding season and survival rates of juveniles and subadults were not affected by vole density. We conclude that vole populations are stabilized by density—dependent reductions in reproductive rate and recruitment, but destabilized by density—independent survival rates of young, and by density—dependent compression on individual movements, which accommodates population growth. The time delay in the effects of density on survival of adults may provide a mechanism for vole cycles in which lagged density—dependent mortality plays a central role.

Effects of the frequency, timing, and intensity of simulated browsing on growth and mortality of tree seedlings

Field exclosure studies have shown that mammalian browsers such as white-tailed deer (Odocoileusvirginianus) can have pervasive effects on forest dynamics in eastern North America. Direct experimental tests of the effects of browsing on growth and survival of a wide range of tree species, however, have yielded conflicting results. This study was designed to assess the effects of variation in the frequency, seasonal timing, and intensity of browsing (simulated by mechanical clipping) on the growth and mortality of three of the major tree species of the Hudson Valley, New York. The clipping treatments were applied to seedlings grown under two different light regimes (full sun and 8% of full sun) to examine seedling responses under different levels of shade-induced carbon stress. Our results demonstrate that even 2 successive years of heavy winter clipping (75% of new shoot growth removed) has little immediate effect on growth or survival of any of the three species. It is possible that winter browsing only has significant negative effects when seedlings are browsed repeatedly over long periods of time. However, comparable levels of summer browsing for only 2 years significantly reduced both growth and survival of all three species. While most natural browsing occurs in the dormant season, our results suggest that it is the less frequent browsing during late spring and early summer that has the greatest immediate effect on tree seedlings. Shading reduced growth and increased mortality in all three species; however, there was only a limited interaction between light level and the simulated browsing treatments. The effects of browsing on survival were similar in all three species; however, the effects of browsing on cumulative height and annual growth varied enough among the species to suggest that browsing could cause significant variation among these species in their rate of invasion in old fields and rights of way, and their rate of regeneration following logging or disturbance of forests.

Blarina brevicauda and Peromyscus leucopus Reduce Overwinter Survivorship of Acorn Weevils in an Indiana Hardwood Forest

We tested the hypothesis that free-ranging white-footed mice (Peromyscus leucopus) and northern short-tailed shrews (Blarina brevicauda) reduce the survivorship of acorn weevils (Coleoptera: Curculionidae) overwintering in forest soils. Field exclosures differentially excluding these and other vertebrate insectivores, in combination with translocation of individual shrews from one site to another, were employed over 2 winters. The presence of small mammals led to a significant reduction in survivorship of weevils, but no effect due to density of shrews or presence of other potential predators (e.g., birds) was detected. These results are consistent with the hypothesis that these small mammals, through their consumption of weevils, have a positive effect on oak populations, and thus can affect forest composition and functioning.