摘要:

The height of white—flowered trillium (Trillium grandiflorum) is a useful indicator of deer browsing intensity. In their foraging activities deer select larger plants over smaller plants. Because flowering plants are larger than nonflowering plants, the number of plants in flower decreases with increasing browsing intensity. As browsing intensity increases, the height of the Trillium becomes shorter in successive growing seasons, presumably due to the loss of photosynthetic capacity and reduction in belowground resources. Trillium stem height was positively correlated with reproductive output by perennial herbaceous plants and negatively correlated with the percent of the herbaceous understory that is browsed. This indicates change in stem height is as indication of the general status of the herbaceous flora as influenced by deer browsing. Based on deer population densities associated with study sites supporting Trillium populations with stable stem heights and flowering plants, maintenance of deer densities of 4—6 individuals/km2is recommended for deciduous forests in northeastern Illinois. In eastern United States, research workers who assume they are studying relatively undisturbed sites should be aware that intense deer browsing may have imposed an alteration on their study sites.

ISSN:1051-0761    

DOI:10.2307/1942119

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Studies were performed to determine what types of microhabitat characteristics attract the most common bird species to nest at particular sites on tundra habitats in the Prudhoe Bay oil field. Microhabitat variables of 2 x 2 m plots centered on bird nests were measured and compared with those of plots centered on random points. Results indicated differences in amount of microrelief, graminoid and shrub/forb cover, roughness of topography, and presence of water among species. These differences have implications for management of abandoned gravel sites as oil production declines in the Prudhoe Bay oil field. The amount and variability of microrelief plays an important role in influencing nest—site selection. Birds do not require total coverage by graminoid and shrub/forb plant species at nest sites. Water plays an important role by influencing plant growth at disturbed gravel sites.

ISSN:1051-0761    

DOI:10.2307/1942120

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The transfer of engineered genes (transgenes) from crops to natural populations will depend first on mating between the crop and related weeds and then upon the relative fitness of the weed—crop hybrid. While weed—crop hybridization is known to occur readily under agricultural conditions, almost nothing is known of the fitness of the hybrids produced. Therefore, we measured the relative fitness of weedy radishes and their sibling weed—crop hybrids under field conditions. Specifically, we compared germination success, time to first flowering, fruit production, seed production, and frequency of transmission of the crop allele to seed progeny. Hybrids showed significantly greater fruit and seed production, and equaled weeds in all other measured characters. Thus, in this experiment, the fitness of hybrids exceeded that of their wild siblings. These results suggest that, in at least this system, neutral or advantageous transgenes introduced into natural populations will tend to persist.

ISSN:1051-0761    

DOI:10.2307/1942121

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Fire maintains tallgrass prairies, but fire season is virtually unexplored as a formative influence of prairie floristics. Replicated mid—July burns, simulating the peak of lightning—caused fire in natural grasslands, strongly influenced species composition and cover in experimental restorations in southern Wisconsin. Perennials that flowered before mid—July showed only 17% cover in unburned plots and 6% in plots burned in March, but 46% cover in plots burned in mid—July. Late—flowering species dominating the remnants managed with dormant—season burns accounted for 80% cover in unburned plots and 92% cover in plots burned in March, but only 47% cover in plots burned in July. Management practices that employ dormant—season burns may produce quite different communities than those maintained by midsummer burns that simulate the timing of lightning fires.

ISSN:1051-0761    

DOI:10.2307/1942122

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

The relationship of total basal area to soil and topographic factors was studied in 47 undisturbed, mature, compositionally stable (climax) stands. Forward stepwise multiple linear regression was used to develop two predictive models. Effective soil depth, percentage stone, aspect, and slope position were predictor variables in model I. Soil profile available water capacity, slope position, and aspect were predictor variables in model II. Model I is the more practical and easily applied model in stands where soil depth can be rapidly determined; model II requires additional soil measurements but can be used to assess the more direct biological relationship between soil water and basal area. Models I and II accounted for 91 and 93% of the variation in stand basal area, respectively, and were validated with data from seven additional mature stands. Subsequently the models were used to predict potential (maximum) basal area in eight stands where it had been reduced by disturbance; the models also were used to evaluate the effect of farming on the productivity of fields abandoned in the 1930s. Presettlement basal area was estimated to have been between 18 and 26 m2/ha but soil loss due to erosion has reduced the potential basal area to 8—21 m2/ha should hardwood stands redevelop. For comparison with a standard site evaluation technique, 27 undisturbed Quercus alba stands were used to develop another multiple linear regression model to predict site index from site factors. Soil profile available water holding capacity, slope position, and aspect were predictor variables in model III, which accounted for 73% of the variation in site index, a substantial reduction in predictive value from model II, which predicts potential stand basal area using the same three variables. Simple regression indicated a strong positive linear relationship between stand basal area and site index (r = 0.85, P<.01). The strong relationship of stand basal area to soil and topographic factors and to standard site index measurements indicates that models to predict potential basal area may be used to evaluate site productivity in the Illinois Shawnee Hills.

ISSN:1051-0761    

DOI:10.2307/1942123

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Forests dominated by Pinus elliottii var densa have undergone a reduction in area in the Florida Keys (USA). A previous investigation interpreted the presence of halophytic species in a former pine forest in Key Largo as evidence of sea—level rise. We therefore examined aerial photos and field evidence to learn how the 15—cm rise in local sea level over the last 70 yr had affected the distribution of pines on a second island, where intact pine forests still remained in 1991. The distribution of in situ dead pine stems showed that the area occupied by pines on Sugarloaf Key was 88 ha at some time prior to the earliest available aerial photographs, in 1935. The area of pine forest was reduced to 46 ha by 1935, and continued to decrease through 1991, when it covered 30 ha. The pattern of pine mortality was related to topographic position, with the areas where pines died earliest occupying the lowest elevations. Our analysis of current vegetation patterns showed that the areas of earliest pine mortality are now populated by a higher proportion of halophytic plant assemblages than areas of more recent pine mortality. We also compared the physiological responses of pines in two portions of the island: one where pine forest reduction had been most pronounced, and a second where the extent of the forest had changed little over the past 50 yr. Both groundwater and soil water salinity were higher in the area of rapid pine forest reduction, and the pines sampled there exhibited higher physiological stress, as indicated by pre—dawn water potential and stemwood carbon isotope ratios. These results suggest that the salinization of ground— and soil water that occurs as sea level rises is a major factor in the reduction of pine forests of Sugarloaf Key. If sea level continues to increase, the Florida Keys will experience a decline in both landscape and species diversity, as species—rich upland communities are replaced by simpler mangrove communities. This pattern may also occur in other low—lying island ecosystems with limited freshwater resources.

ISSN:1051-0761    

DOI:10.2307/1942124

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

As genetically engineered crop varieties near widespread cultivation, both agronomic and environmental concerns mandate the development of effective strategies for isolating transgenic varieties from related non—transgenic varieties or cross—fertile weeds. We present the results of the first field experiment designed to test the effectiveness of two containment strategies that are commonly used in field trials of transgenic crops: (1) an isolation zone devoid of vegetation to discourage emigration of insect pollinators from transgenic plots; and (2) trap crops (non—transgenic varieties of the same crop planted adjacent to the transgenic plot that can "cleanse" emigrating pollinators of transgenic pollen). In conjunction with field trials of genetically engineered canola (Brassica napus) conducted by Calgene, Inc., in California and Georgia, we varied both the width of the barren zone and the presence or absence of a trap crop, and measured the effects on gene escape. Escape was easily detected since the genetic construct inserted into the transgenic canola contained a gene that rendered seedlings resistant to the normally lethal antibiotic kanamycin. Our results suggest that barren zones 4—8 m in width may actually increase seed contamination over what would be expected if the intervening ground were instead planted entirely with a trap crop. When trap crops occupied a limited portion of the isolation zone separating transgenic and non—transgenic varieties, the effectiveness of the trap depended on the width of the isolation zone: they reduced gene escape when the two varieties were separated by 8 m, but increased escape across a 4—m isolation zone. We conclude that, for the relatively short isolation distances we examined, the most effective strategy for reducing the escape of transgenic pollen is to devote the entire region between transgenic and non—transgenic varieties to a trap crop.

ISSN:1051-0761    

DOI:10.2307/1942125

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Recent accretion was measured by the feldspar marker horizon method in two gravity—drained, managed, marsh impoundments and unmanaged reference marshes located on the rapidly subsiding coast of Louisiana. Water level management was designed to limit hydrologic exchange to the managed marsh by regulating the direction and rate of water flows. During a drawdown—flooding water management cycle, the unmanaged reference marshes had significantly higher vertical accretion rates, higher soil bulk density and soil mineral matter content, lower soil organic matter content, and higher rates of organic matter accumulation than the managed marsh. The rate of mineral matter accumulation was higher in both reference marshes, but was significantly higher in only one. Spatial variability in accumulation rates was low when analyzed in one managed marsh site, suggesting a primarily autochthonous source of matter. In contrast, the associated reference marsh apparently received allochthonous material that settled out in a distinct spatial pattern as water velocity decreased. The impoundment marshes experienced an accretion deficit of one full order of magnitude (0.1 vs. 1,0 cm/yr) based on comparison of accretion and sea level rise data, while the unmanaged reference marshes experienced a five—fold smaller deficit or no deficit. These data suggest that the gravity—drained impoundments likely have a shorter life expectancy than the reference marshes in the rapidly subsiding Louisiana coast.

ISSN:1051-0761    

DOI:10.2307/1942126

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We manipulated the fire regime and soil disturbance in four grass—dominated Carolina bay wetlands during a prolonged drought period and examined vegetation composition and cover within dominant vegetation types prior to and after treatments. We used the seedling emergence technique to determine the role of the seed bank in the recovery process. Burning did not affect richness, evenness, or diversity (all vegetation types combined); however, soil tillage increased diversity, including both evenness and richness. Percent similarity of the vegetation before and after disturbance was greater in the burning treatment than in the tillage treatment, probably due to greater disruption of the rhizomes of the perennial vegetation by tillage. Vegetation types varied in degree of recovery, although dominance was not altered by either treatment. Several native fugitive species increased following disturbance, indicating that species coexistence in these Carolina bay wetlands depends on the life history characteristics of residual vegetation, as well as that of seed bank species. The seed bank (72 600 seedlings/m2) was larger and species richness (108 species) was greater than reported for most other freshwater wetlands. No differences in species richness, evenness, or diversity were detectable among the seed banks associated with different vegetation types in the bays. The floristic composition of the seed bank did not resemble the standing vegetation in patches dominated by large perennial grasses (Panicum hemitomon, Leersia hexandra, and Andropogon virginicus). In contrast, seed bank samples associated with vegetation dominated by an annual forb (Iva microcephala) or the small perennial grasses Panicum wrightianum or P. acuminatum var. unciphyllum more closely reflected the standing vegetation. Species appear to persist with recurring and multiple disturbances because of seed banks, rhizomes, morphological plasticity during inundation, sexual reproduction following inundation, and perenniality coupled with early sexual maturity. Conservation management of intermittently inundated wetlands should incorporate techniques to foster maintenance of endemic species richness and the recruitment of rare species. During periods of drought, it may be desirable to disturb the aggressive perennial vegetation to allow recruitment of less common species.

ISSN:1051-0761    

DOI:10.2307/1942127

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Data on hydrogeology, soils, and vegetation collected on four transects across the 48—km wetland—to—upland transition zone of the Great Dismal Swamp of Virginia/North Carolina, USA, were used to analyze changes along the moisture/elevation gradient, to characterize the wetland—upland ecotone, and to select tentative wetland—upland boundaries based on these three parameters. Transition zone vegetation was dominated by three facultative hydrophytes: Acer rubrum, Liquidambar styraciflua, and Nyssa sylvatica. On the basis of ordination performed on consecutive 25—m transect increments, each transect was divided into three zones: wetland, ecotone, and upland. The water table was within the root zone (0—30 cm below the ground surface) an average of 25—100% of the growing season at all well sites in wetland,<25—100% for most well sites in the ecotone, and100 m horizontal distance and 0.4 m vertical distance. Only a vegetation boundary was established on the fourth transect.

ISSN:1051-0761    

DOI:10.2307/1942128

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY