Exclosure Plots Research Articles

Effects of grazing exclusion on species composition in high-altitude grasslands of the Central Alps

Our study examines the effects of grazing exclusion on low-productive subalpine and alpine grasslands of the Central Alps (UNESCO Biosphere Park Gurgler Kamm, Obergurgl, Austria). A long-term exclusion experiment was established in 2000 in the subalpine, the lower, and the upper alpine zone. With exception of the subalpine zone, domestic herbivores have been grazing during the whole growing season. In grazed and exclosure plots species frequencies were recorded for 7 years. We analysed exclosure effects on species number, community composition, life forms, and functional groups. Species richness did not decrease significantly within the exclosures, but changes in species composition occurred in each zone, although some were transitory in nature. The dynamic trends of the plots were significantly explained by the ‘treatment×year’ effect along the whole altitudinal gradient, but the effects decreased considerably with altitude. In the subalpine and upper alpine exclosures, stress-tolerators, species of low or no nutritive value, and mosses showed a decreasing trend, whereas tall grasses (subalpine exclosures), competitors, and species with high or medium nutritive values (lower alpine exclosures) tended to increase. Overall, our 7-year study revealed that several functional groups reacted to grazing, according to our main expectations. We suggest that these effects will intensify in the long term. Die Auswirkungen eines Beweidungsausschlusses wurden seit dem Jahr 2000 auf nährstoffarmen subalpinen und alpinen Weiden in den Zentralalpen (UNESCO Biosphärenpark Gurgler Kamm, Obergurgl, Österreich) untersucht. Mit Ausnahme der subalpinen Stufe hielten sich die Weidetiere während der gesamten Vegetationsperiode auf den Flächen auf. In beweideten und nicht beweideten Dauerflächen wurden die Frequenzen der Pflanzenarten über sieben Jahre hinweg erhoben. Wir analysierten die Effekte des Beweidungsausschlusses auf Artenzahl, Artenzusammensetzung der Gesellschaft, Lebensformen und funktionelle Gruppen. Es konnte zwar keine signifikante Abnahme der Artenvielfalt in den Ausschlussflächen festgestellt werden, doch änderte sich die Zusammensetzung der Bestände mit der Zeit sehr deutlich, wenn auch manche dieser Veränderungen Übergangscharakter hatten. Signifikant unterschiedliche Entwicklungen zwischen den beweideten und nicht beweideten Dauerflächen konnten auf jeder Höhenstufe nachgewiesen werden, sie fielen jedoch mit zunehmender Höhe deutlich geringer aus. Unter Beweidungsausschluss nahmen in der subalpinen und oberen alpinen Stufe Stresstolerante, Arten mit geringem oder ohne Nährstoffwert und Moose tendenziell ab, während hochwüchsige Gräser (subalpine Stufe), konkurrenzkräftige Arten und Arten mit hohem oder mittlerem Nährstoffwert (untere alpine Stufe) tendenziell zunahmen. Insgesamt zeigte unsere 7-jährige Studie, dass mehrere funktionelle Gruppen auf den Beweidungsausschluss reagierten. Damit wurden unsere zentralen Erwartungen bestätigt. Langfristig wird diese Entwicklung sicherlich noch deutlicher zum Ausdruck kommen.

Soil properties changes after short-term livestock exclusion in “degraded” communally managed rangelands in the western Bophirima District, South Africa

Livestock grazing and detrimental overgrazing are commonly associated with severe rangeland degradation particularly in communally managed pastoral systems in South Africa. Effects of grazing exclusion on vegetation are well documented, but there is a dearth of quantitative research conducted in communal managed range-lands to investigate the extent of associated soil properties changes. This study compared soil characteristics between grazed and 6 years adjacent exclosure plots at three communally managed sites namely Austrey, Southey/Eska and Tseoge in the western Bophirima District. A slow rate of change in soil properties between the grazed and exclosure plots was observed and differences were site-specific where detected. Phosphorus was significantly different at Tseoge (p = 0.001) and nitrate at Southey/Eska site (p<0.0001). Soil organic carbon was not affected by grazing exclusion at all sites, but the activity of ß-glucosidase activity was significantly different between exclosure and grazed plots. The activity of dehydrogenase was significantly different at Southey/ Eska site only (p = 0.002), whereas soil microbial biomass showed significant difference only at Tseoge site (p = 0.03). These results suggest that short-term livestock removal does not lead to a significant improvement of soil condition, as reflected by changes in soil properties. Caution is recommended when interpreting these results beyond the constraints imposed by the design of the study and the lack of information about livestock management practices and regimes at the sites.

High white-tailed deer densities benefit graminoids and contribute to biotic homogenization of forest ground-layer vegetation

Biotic homogenization, with its emphasis on invasions, extinctions, and convergence in taxonomic similarity, provides an important framework for investigating changes in biodiversity across scales. Through their selective foraging, large populations of white-tailed deer are altering population sizes, driving extirpations, and facilitating invasions of plants throughout the eastern United States. I hypothesize that deer can drive biotic homogenization in forest understory communities by shifting species composition to one dominated by grasses, sedges, and ferns (all wind-pollinated plants). I report the effects of 16 years of deer exclusion in a hemlock-northern hardwood stand in N Wisconsin using a block design. Species composition showed greater convergence in control plots than exclosure plots, indicating deer can drive biotic homogenization at the stand level. Total percent cover is nearly 4 times greater in exclosure plots. Percent cover by woody plants, broadleaf herbs, and ferns is 150, 63, and 20 times greater in exclosure plots, respectively, while cover by sedges and grasses is 3.8 and 2.2 times greater in control plots. Cover by species with showy, insect-pollinated flowers is 79 times greater in exclosures. Graminoid-dominated control plots represent a novel state not observed fifty years ago, and could reflect the emergence of a grazing lawn. The increase in graminoids at this study area and throughout the region could under some global change scenarios be an early stage of conversion from forest to savanna or wood pasture.

Indirect Effects of a Keystone Herbivore Elevate Local Animal Diversity

Abstract: We quantified indirect effects of white‐tailed deer (Odocoileus virginianus) on ground‐dwelling herpetofauna and invertebrates in Cuyahoga Valley National Park, Ohio, USA. We placed cover boards at 12 sites, each consisting of a 10 × 10‐m fenced (exclosure) plot and an unfenced (control) plot. Periodically, during May‐December 2004 and May‐September 2005, we counted salamanders, snakes, and a variety of invertebrate taxa. Salamander, snake, and gastropod abundance as well as invertebrate richness (no. of species or higher level taxa) were higher in control than exclosure plots. Our findings suggest that management actions taken to regulate deer densities could have the unintended effect of reducing local animal diversity.

Effects of long‐term grazing on the morphological and functional traits of Leymus chinensis in the semiarid grassland of Inner Mongolia, China

AbstractLivestock grazing represents a major human disturbance to grasslands throughout the world. We evaluated the effects of long‐term grazing (&gt;20 years) on a dominant perennial grass species, Leymus chinensis (Trin.) Tzvel., in the semiarid grassland of Inner Mongolia, China, in terms of its morphological and functional responses. L. chinensis, one of the most abundant species in semiarid grassland, had significantly smaller leaf area, fewer vegetative tillers and shorter shoot internodes length, but significantly greater specific leaf area for the individuals in the grazing plot than those in the exclosure (grazing‐free) plot. Long‐term grazing also altered the relative distribution of biomass to leaves, roots and rhizomes. The biomass, coverage and the number of species were lower in the grazing plot by 50–70% in comparison with those in the grazing‐free plot. In addition, the long‐term grazing substantively decreased the physiological capacities of this grass species, including significantly lower net photosynthetic rate, apparent quantum efficiency, photochemical efficiency of PSII and water use efficiency. Significantly higher stable oxygen isotope ratios (δ18O) of the soil water in the grazing plot than those in the grazing‐free plot indicated a much higher soil evaporation in the grazing plot because of less litter coverage. Seasonal patterns in the δ18O values of the stem water of L. chinensis and three other common species in the grazing and grazing‐free plots indicated that plants in the grazing and grazing‐free plots might shift their water sources differently from a dry month (June) to a wet month (August). This study illustrated the importance of using different approaches to study the possible responses of grass species in arid regions to human disturbances, such as long‐term grazing.

Evaluation of an exclusion plot design for determining the impacts of native and exotic herbivores on forest understoreys.

To study the effects of grazing and browsing by Sambar deer (Cervus unicolor), swamp wallaby (Wallabia bicolor) and wombats (Vombatus ursinus) exclosure plots measuring 10 m x 10 m were erected in the Upper Yarra and O'Shannassy water catchments near Melbourne, Victoria. Total exclusion fences and partial exclusion fences were erected. Design details and costs are provided. Operational problems are discussed.

Interactions between sheep, rodents, graminoids, and bryophytes in an oceanic alpine ecosystem of low productivity

Large herbivores directly affect plant communities in alpine ecosystems. In addition, they may compete with, or facilitate foraging by, small herbivores and also cause strong indirect effects on plants. We used an exclosure experiment to examine short-term (5-y) effects of cessation of sheep grazing on rodent grazing and plant communities in an oceanic alpine environment of low productivity with a long history of heavy sheep grazing. Exclusion of sheep significantly impacted plant communities. Vascular plant height increased, but Deschampsia flexuosa was the only vascular plant species that increased in sheep exclosures. Changes in the frequency of graminoids, herbs, and dwarf shrubs in exclosed plots were not related to cessation of grazing, but 6 bryophyte species significantly increased or decreased in response to exclusion of sheep. The absence of large grazers thus brought about a change in the species composition in favour of successional bryophytes and the preferred fodder plant. Neither vascular plant nor bryophyte species richness, nor the total cover of bryophytes and lichens, were affected. Cessation of sheep grazing reduced the level of rodent grazing. Rodent grazing correlated with changes in plant communities that led to reduced height and cover of vascular plants, reduced cryptogam cover, and reduced frequencies of 3 bryophyte species. A strong correlation between sheep fodder value index and rodent grazing indirectly indicated additive herbivory. In addition, some of the rodent effects were compensatory; e.g., Nardus stricta, which is not grazed by sheep, was significantly reduced by rodents. Our study points to a more central role of facilitation in structuring herbivore assemblages in the short term, with direct implications for the joint effects of large and small herbivores on the cover and frequency of graminoids and bryophytes.

An experimental test of density- and distant-dependent recruitment of mahogany (Swietenia macrophylla) in southeastern Amazonia

According to the Janzen-Connell model, high mortality of seeds and seedlings in proximity to conspecific adults can help maintain species diversity in tropical forests. Using a natural population of big-leaf mahogany (Swietenia macrophylla King), we tested the model's mechanism by examining seed predation and juvenile recruitment in the forest understory and in treefall gaps in the vicinity of both isolated and clumped adults. We used tethered seeds placed in three types of exclosure plots: (1) complete access to seeds, (2) semi-access (access by small-sized seed predators) and (3) no access (all mammals excluded). Exclosure treatments were applied within the understory (both near and far from adults) and in gaps at eight fruiting adults in the late dry season (2001) and scored ten months later. Significantly more seeds were removed in canopy gaps near clumped adults than at isolated adults; otherwise, none of the treatment factors significantly influenced seed predation. In contrast, understory juvenile recruitment was significantly enhanced by distance from adults and was twice as high at isolated than clumped adults, providing novel support for the Janzen-Connell mechanism. No-access exclosures protected significantly more seeds than semi- and full-access exclosures, implicating small mammals in seed losses. Across the eight trees, juvenile recruitment in the no-access exclosures decreased significantly with conspecific adult densities, implicating non-mammalian density-responsive factor(s) in mortality following germination; likely a known specialist invertebrate herbivore. When all treatments were combined, conspecific adult basal area and total DBH explained 72 and 90% of variation in overall juvenile recruitment, respectively. Collectively, these results indicate that Janzen-Connell effects can operate in S. macrophylla, especially during the seed-to-seedling transition, and will likely reduce recruitment in areas of high conspecific densities. They also suggest that further research into the causes of density-dependence in tropical trees should investigate mortality agents following germination.

Population structure, persistence, and seasonality of autochthonous Escherichia coli in temperate, coastal forest soil from a Great Lakes watershed

The common occurrence of Escherichia coli in temperate soils has previously been reported, however, there are few studies to date to characterize its source, distribution, persistent capability and genetic diversity. In this study, undisturbed, forest soils within six randomly selected 0.5 m2 exclosure plots (covered by netting of 2.3 mm2 mesh size) were monitored from March to October 2003 for E. coli in order to describe its numerical and population characteristics. Culturable E. coli occurred in 88% of the samples collected, with overall mean counts of 16 MPN g(-1), ranging from < 1 to 1657 (n = 66). Escherichia coli counts did not correlate with substrate moisture content, air, or soil temperatures, suggesting that seasonality were not a strong factor in population density control. Mean E. coli counts in soil samples (n = 60) were significantly higher inside than immediately outside the exclosures; E. coli distribution within the exclosures was patchy. Repetitive extragenic palindromic polymerase chain reaction (Rep-PCR) demonstrated genetic heterogeneity of E. coli within and among exclosure sites, and the soil strains were genetically distinct from animal (E. coli) strains tested (i.e. gulls, terns, deer and most geese). These results suggest that E. coli can occur and persist for extended periods in undisturbed temperate forest soils independent of recent allochthonous input and season, and that the soil E. coli populations formed a cohesive phylogenetic group in comparison to the set of fecal strains with which they were compared. Thus, in assessing E. coli sources within a stream, it is important to differentiate background soil loadings from inputs derived from animal and human fecal contamination.

A measurable effect of feral grazing on pasture accumulation rate

Feral animals such as possums are known to utilise pasture as a substantial part of their diet, with individual animal intake rates well quantified. The objective of this study was to quantify this effect in terms of pasture accumulation rates, in areas where these animals are likely to occur in high densities; i.e. the boundaries between native forest and pastoral farms. Pasture accumulation rate was measured in small plots open to feral grazing and plots excluded from grazing with electrified flexinets, at six sites throughout the Waikato. Three further sites, within possum control schemes, were established as controls. Pasture accumulation rates were significantly greater within the exclosure plots at all six uncontrolled sites, by ~3 kg DM/ha/d in late-winter and ~7 kg DM/ ha/d in late spring. In contrast there were no significant differences between open and exclosure plots at the three sites where there was active possum control. This effect is quite substantial in the context of livestock consumption, though is not entirely reconcilable with predictions based on possum intake and diet studies. It nevertheless represents a source of loss which is easily countered, with additional benefits in terms of lowered Tb risk and improvement of native vegetation condition. Keywords: feral grazing, grazing exclosure, pasture accumulation rate, possum diet

The Impacts of Nutria on Vegetation in Oregon

Nutria have been present in the Pacific Northwest for more than 70 years, and though their dramatic impacts on wetlands in the southeastern U.S. is well documented, the northwestern populations have been little studied. Using paired exclosure plots, nutria herbivory pressure on the native vegetation is shown to be considerable but dependent on species type and disturbance history. In coastal wetland habitats, nutria selectively feed on forbs compared to grasses, lowering their aboveground biomass. This study also shows opposing responses to nutria herbivory for disturbed and undisturbed plots, with nutria lowering total biomass in areas that have not experienced a biomass-clearing disturbance event and thus have diminished competition intensity.

Seed predation of Virola bicuhyba (Schott) Warb. (Myristicaceae) in the Atlantic forest of south-eastern Brazil

Seed predation of Virola bicuhyba (Myristicaceae) was studied in an area of Atlantic forest in south-eastern Brazil, with the objective of testing the Janzen-Connell model. The predation of seeds was evaluated at three different distances from the parent tree for two classes of predators: invertebrates and vertebrates. The method of exclosure plots (closed plots) and open control plots was used, distributed at 5, 15 and 25 m from the trunk of each adult fruiting tree of V. bicuhyba. In Experiment 1, 1,200 seeds were used and, in Experiment 2, 1,440 seeds. Both experiments did not agree with Janzen-Connell model, as seed predation by invertebrates and vertebrates was independent of the distance from the parent tree. Seed predation rate was high, however the impact of predation by vertebrates was higher than by invertebrates, indicating that it is the main cause of seed mortality.

Long-term response of spring flora to chronic herbivory and deer exclusion in Great Smoky Mountains National Park, USA

Chronic herbivory by white-tailed deer ( Odocoileus virginianus Zimmerman) can have profound impacts on the function and structure of forest ecosystems. We examined the combined influence of intense herbivory associated with a deer population eruption and chronic herbivory by the post eruption population on the spring flora of Cades Cove, Great Smoky Mountains National Park. During the 1970s the deer population reached a peak of 43 deer per km 2, from which it has slowly declined in recent decades. To examine the influence of intense herbivory, we compared the abundance and flowering rates of early flowering plants in Cades Cove to a nearby reference site with similar bedrock geology, vegetation, and disturbance history but contrasting history of deer abundance. Our results suggest that significant changes ( p ⩽ 0.05) in the diversity, evenness, and species richness of the spring flora occurred during the eruptive phase. Trillium spp. and other liliaceous species appeared to be disproportionately impacted. Comparisons between control and exclosure plots established after the deer population eruption indicate that recovery has been largely restricted to species that were able to persist under intense herbivory. These species have increased in number in exclosures, suggesting continued impacts by deer on the plant community outside the exclosures. Little to no recolonization by browse sensitive species was observed. Consequently, to restore the natural diversity of early flowering plants once present in Cades Cove, active restoration may be necessary in addition to maintaining deer densities below current levels.

High-intensity deer culling increases growth of mountain beech seedlings in New Zealand

Browsing of mountain beech seedlings by introduced deer in the central North Island of New Zealand appears to have inhibited canopy regeneration over large areas. In 1998, a trial of high-, medium- and low-intensity deer-culling treatments was initiated in Kaimanawa and Kaweka Forest Parks to test whether mountain beech (Nothofagus solandri var. cliffortioides) forest regeneration could be restored by removing deer. Paired exclosure plots (one plot fenced to exclude deer and the other unfenced) were established within a high-intensity culling area, to monitor the benefits of recreational, commercial and aerial deer culling. Paired plots were also established within low- and medium-intensity culling areas. Medium-intensity culling was allowed through recreational and commercial deer culling. In two low-intensity culling treatment areas, deer management remained substantially unaltered. Annual relative growth rates of tagged seedlings from spring 1998 to spring 2001 from low-, medium- and high-intensity culling areas provide strong evidence that mountain beech seedling growth increases once browsing by deer is removed through fencing. Faecal pellet data indicated that high-intensity deer culling reduced deer abundance by 67% in comparison to medium- and low-intensity culling areas. This apparent reduction in deer abundance appears to have led to a doubling in mountain beech seedling growth in the high-intensity culling area outside fences, in comparison to low- and medium-intensity deer culling areas where there was little or no evidence of benefits for seedling growth.

Invasion of the noxious weed Nicotiana glauca R. Graham after an episodic flooding event in the arid zone of Australia

During November 1997, an extreme rainfall event caused flooding in the Olary Creek and created a terminal lake within mallee vegetation on Nagaela Station, far western New South Wales. To assess the effect of this rare event on native flora, six 25×25 m 2 grazing exclusion plots were established on flooded (4), and un-flooded control (2) areas. We investigated (i) the distribution of Nicotiana glauca in relation to the 1997 flooding event (ii) the variation in density of N. glauca soil-stored seed between soil in control and exclosure plots (iii) the allelopathic effects of N. glauca on germination of Lactuca sativa seeds. Analysis of N. glauca stem sections revealed a maximum of five growth rings, confirming an origin following the 1997 flood event. At a September 1998 assessment, no N. glauca were found across flooded and control plots. In October 2000 N. glauca occurred at densities of 24 and 12 N. glauca seedlings ha −1 in the flooded unfenced and fenced plots respectively. No N. glauca individuals were recorded either from control fenced or unfenced plots. The soil-stored seed study revealed that larger numbers of N. glauca seeds were recovered from the flooded open plot (1936±968) than in flooded fenced plots (856±428.12) but none was found in the control plots. Leachates obtained from dry leaves and twigs had a significantly greater negative impact ( p>0.001) on germination of L. sativa seeds than leachates obtained from fresh leaves and bark. This study showed that the density of N. glauca is high in areas where grazing had eliminated competition from native shrubs and grasses. N. glauca is potentially a serious weed along the Olary Creek and ephemeral lake boundaries. Management strategies such as reducing grazing pressure from native and introduced animals and/or effective eradication programs should be adopted, to minimize further infestation.

Potential impacts of fire and grazing in an endangered ecological community: plant composition and shrub and eucalypt regeneration in Cumberland Plain Woodland

Exclosure plots were used to determine the effect of fire and grazing on the structure of a grassy-woodland community. Eighteen months after fire and fence treatments were applied, the species richness, cover and composition of shrubs, trees, herbs and grasses were assessed and compared to pre-treatment censuses. Unburned plots had fewer shrub species and a lower abundance of shrubs, indicating the importance of fire in promoting regeneration of shrub species. Eucalypt species were more abundant and richer following the wildfire burn in summer, suggesting timing of fires is an important aspect in the establishment of the canopy species. Interactions between fire and grazing were found for the abundance of eucalypts (although weak) and resprouting eucalypts, suggesting a subtle interaction between fire and grazing shortly after fire. There was no effect of grazing and no interaction effect between fire and grazing on shrub species richness and abundance or tree species richness and seedling abundance.All plots showed a change in species composition despite treatment, and 46 species (32% of total richness) were recorded only in the final survey. The high rainfall during the 18-month study is likely to be an important factor in facilitating the establishment of species following all disturbances. This may have ameliorated the impact of grazing as abundant food was available throughout the woodland. The interaction between fire and grazing may be more important in structuring these grassy communities during periods of lower rainfall.

Sacred Forests in Tibet

The treeless desertlike environments of southern Tibet are assumed to be naturally unsuitable for forests. Yet, climatic conditions do allow for the growth of indigenous trees in Lhasa and many parts of southern Tibet, even where there is no high groundwater table or irrigation. This was discovered and proven in a Sino–German research project launched in 1997. The project made an inventory of forest relics, correlated residual tree stands with climatic data, and successfully cultivated nonirrigated indigenous junipers and cypresses. The eroded semidesert landscape of southern Tibet appears to have a huge potential for reforestation. The area with a potential for tree growth was investigated using the Geographical Information System known as GRASS (Geographical Resource Analysis Support System). Reforestation measures could meet the heavy demand for timber and firewood, help combat erosion on overgrazed slopes, and restore the degraded pastures. Grazing must be excluded on reforestation plots. Simultaneously, rangelands may regenerate after overgrazing. The optimum duration of the ungrazed period varies with altitude, humidity, soil conditions, and the degree of degradation. Successional trends observed on exclosure plots suggest that the drier the climate and the lower the initial degree of herbaceous vegetation cover, the longer the ungrazed period will be beneficial for pasture regeneration. Challenges in research and practice resulting from these preliminary results are highlighted.

Isotopic evidence for changes in residue decomposition and N-cycling in winter flooded rice fields by foraging waterfowl

Winter flooded rice fields can serve as substitute habitat for migratory waterfowl. Not much is known about the effects of the foraging waterfowl on nutrient cycling in rice production systems. This study quantifies the effect of foraging waterfowl on decomposition of rice residue and N-cycling in a winter flooded rice field in the Sacramento Valley, California. Along two transects in a field, pairs of control plots and exclosure plots, which excluded waterfowl, were set up. The original straw residue within the inner 2 m 2 of the 3 m×3 m plots was replaced by 15 N labeled straw residue. The labeled residue was subsequently followed into the light fraction and mineral fraction of the soil through the winter flooding period. Foraging waterfowl increased the loss of 15 N from the residue from 83 to 89%, but did not affect the mass decomposition and loss of total N of the residue. No significant effect of the waterfowl was seen in the recovery of the residue N in the total soil N pool and in the light fraction, although there was a low recovery of residue N within these pools in the control treatments. The recovery of residue N in the NH 4 +-N pool after winter flooding was significantly lower in the presence of waterfowl. The increased loss of residue N and lower recovery of residue N in the presence of waterfowl may indicate that the rate of N-cycling was increased. A better understanding of the N-cycle in winter flooded rice fields in the presence of waterfowl is needed to assess the potential benefits of winter flooding for the rice farmers.

MUTE SWAN (CYGNUS OLOR) IMPACT ON SUBMERGED AQUATIC VEGETATION AND MACROINVERTEBRATES IN A RHODE ISLAND COASTAL POND

We followed the annual use of a shallow coastal pond by molting Mute Swans to examine their potential impacts on waterfowl habitat. Effects of Mute Swan herbivory on submerged aquatic vegetation (SAV) and macroinvertebrates were measured by comparing control vs. exclosure plots in varying water depths from 1990 to 1994. Mute Swans fed in all water depths, but appeared to prefer feeding in shallow sites (< 0.5 m) where they were capable of reducing SAV biomass by up to 95%. The impact of Mute Swans on macroinvertebrates could not be demonstrated. Only three species of macroinvertebrates (Grammarus spp., Hydrobia spp., Limnephilus spp.) showed reduced densities on some grazed plots.

Ground Layer Competition and Herbivory Effects on Cherrybark Oak (Quercus pagoda Raf.) Regeneration in Experimental Canopy Gaps

COLLINS, BEVERLY (Savannah River Ecology Laboratory, Aiken, SC 29802). Ground layer competition and herbivory effects on cherrybark oak (Quercus pagoda Raf.) regeneration in experimental canopy gaps. J. Torrey Bot. Soc. 130:147-157. 2003. -Windthrows and small-scale forest harvests generate environmental heterogeneity and dense regrowth. In such sites, interactions among environmental factors, competition with ground layer vegetation, and herbivores attracted to the disturbed area all potentially influence recruitment and early establishment of canopy tree species. I examined the interactive effects of overstory gap size, canopy openness, herb layer competition, and mammalian (deer, swamp rabbits) herbivory on emergence and seedling bank formation of cherrybark oak, Quercus pagoda Raf., in experimental gaps created by mechanized logging in a bottomland hardwood forest. Acorns were planted in exclosures (deer, swamp rabbits, deer + swamp rabbits, neither) in the center of three gaps of each of six gap sizes (7, 10, 14, 20, 29, 40 m radius) and outside the exclosures in center, edge, and forest positions. Mortality was greatest during recruitment (within the first season), and was due primarily to damage to acorns by seed predators and shade effects on seedling establishment. After three seasons, seedlings from damaged acorns in the exclosure plots were shorter than those from undamaged acorns; however, these seedlings had higher relative growth rate, which suggests that acorn damage effects decline over time. Herb layer competition and shade due to position within a gap or gap size were associated with reduced survival or height of seedlings, both in the exclosures and in center, edge, and forest positions. Deer or rabbit herbivory had no measurable effect on seedling survival or growth over the first three seasons. Natural or created canopy gaps ' 14 m radius can be foci for Q. pagoda regeneration if acorns are protected from predators; however, the dense vegetation in regenerating gaps could limit seedling height growth and multiple openings may be necessary for ascent out of the seedling bank.