Differences In Plant Cover Research Articles

Early Dynamics of Plant Communities on Revegetated Motorway Slopes from Southern Spain: Is Hydroseeding Always Needed?

Abstract The increasing global rate of road construction is leading to a parallel increase of areas of degraded soil conditions and steep slopes that need revegetation. Hydroseeding with commercial seeds of fast‐growing grasses and legumes is a common practice in revegetation of motorway slopes. We carried out 3 years of monitoring of vegetation dynamics on hydroseeded and nonhydroseeded motorway slopes (48 slopes) in a maritime Mediterranean zone in Málaga (southern Spain). Our main objectives were to test whether hydroseeding significantly increases species richness and plant cover and whether hydroseeded species act as starters, facilitating the establishment of the vegetation and quickly disappearing once the communities are established. A hydroseeding success index (HSI, ranging from 0 to 1) was used to assess the relative abundance over time of the 14 species from the hydroseeding mixture. Species richness and cover was significantly higher on embankments (50–70 species per embankment, 80–90% cover) than on roadcuts (6–10 species per roadcut, 18–30% cover). Performance of hydroseeded species was poor from the very beginning (HSI, 0.2–0.3). On embankments, either presence or abundance of hydroseeded species did not significantly vary throughout the study. Both hydroseeded and nonhydroseeded communities exhibited a significant decrease in species richness, a significant increase in plant cover, and a highly dynamic species composition over time, with Sorensen index of 0.3–0.5 between years. There were no significant differences in plant cover, species richness, and aboveground biomass between hydroseeded and nonhydroseeded plots on embankments throughout the study. Our results demonstrate that there are situations in which the use of hydroseeding for revegetation is not needed. Further research should focus on understanding the establishment of autochthonous species and identifying environmental conditions under which the addition of commercial seeds may not be needed, or indeed situations where it may be harmful in suppressing autochthonous species.

Effects of livestock grazing on rangeland grasshopper (Orthoptera: Acrididae) abundance

Livestock may impact habitat quality for grasshoppers by reducing food availability and by altering microclimate and potential oviposition sites. A 5-year study was conducted to create consistent grazing impacts on replicated plots and measure their effects on plant cover, microclimate, and grasshopper abundance. Cattle were used to produce two levels of grazing intensity that were compared to ungrazed controls. Differences in plant cover were greatest immediately after grazing each summer, grasshopper microhabitats tending to be shadier, cooler, less windy, and more humid in the ungrazed plots. The grasshopper assemblage included five of the worst pest grasshopper species in North America: Ageneotettix deorum, Aulocara elliotti, Melanoplus sanguinipes, M. packardii, and Camnula pellucida. Most species had greater abundance on ungrazed pastures, particularly during the 4–6 weeks after grazing each year. However, A. elliotti was often more abundant in heavily grazed areas early in the year when early instars were present and in late summer when adults were predominant. There was no strong evidence that the effect of grazing on grasshopper abundance increased over the 5-year study. At this time, all changes in grasshopper numbers cannot be directly attributed to particular habitat characteristics that changed after grazing, but the results suggest that grazing management could be used to reduce pest grasshopper densities.

The role of arbuscular mycorrhizal fungi and glomalin in soil aggregation: comparing effects of five plant species

Soil aggregation and soil structure are fundamental properties of natural and managed ecosystems. However, most of our knowledge on the role of plant species in soil aggregation is derived from work in agroecosystems or with agriculturally important plants. Here we examined the effects of five plant species on soil aggregate water stability. The five species (three grasses, one forb, and a legume) were from the same natural grassland, and were grown in monoculture plots in the field. Our first goal was to test if productivity-related or species-specific factors would prevail in determining soil aggregation. We also tested what the relative importance of the soil protein glomalin (produced by arbuscular mycorrhizal fungi, AMF) in soil aggregation is, compared to other factors, including AMF hyphal and root length and percent plant cover. We found significant differences in soil aggregate water stability (1–2 mm size class) for the five plant species examined, and corresponding differences in plant cover, root weight and length, AMF soil hyphal length, and glomalin concentrations. A structural equation modeling approach (path analysis) was used to distinguish direct from indirect effects of factors on soil aggregation based on covariance structures. Root length, soil glomalin, and percent cover contributed equally strong paths to water-stable aggregation. The direct effect of glomalin was much stronger than the direct effect of AMF hyphae themselves, suggesting that this protein is involved in a very important hypha-mediated mechanism of soil aggregate stabilization, at least for the 1–2-mm size class of aggregates.

Diversity of zonal páramo plant communities in Ecuador

Abstract.Patterns of vascular plant species diversity in high‐altitude Ecuadorian ecosystems (‘páramos’) are examined. Data from two independent surveys were used: the first from 12 different locations and 192 samples, the other from 18 locations and 243 samples. These surveys included 348 and 284 species, respectively. The data confirmed the occurrence of two main zones in terms of vascular plant species diversity. The grass páramo and superpáramo were distinguished by differences in plant cover, species richness, α‐diversity and β‐diversity. The transition between these two zones begins at around 4000 m. Grass páramo samples comprised more species but the strong dominance of tussock grasses resulted in low equitability compared with the superpáramo, where safe sites for plant survival are limited and the environment does not permit continuous grass cover. Turnover of species across the altitudinal gradient is higher in the grass páramo than in the superpáramo. This is due largely to agricultural fires at lower altitudes, which create a fine‐scale mosaic of burned patches that enhances variability at this scale. Despite the loss of equitability, intermediate levels of fire disturbance appear to promote species richness within the samples. It is suggested that the complex patterns of páramo diversity in the Ecuadorian Andes are largely the outcome of three interrelated factors: altitude, disturbance and the availability of safe sites at the highest altitudes.

The influence of herbivory on plant cover and species composition in the Pryor Mountain Wild Horse Range, USA

We investigated the effects of short- and long-term ungulate grazing on plant species cover and composition in arid lowland and more mesic upland communities of the Pryor Mountain Wild Horse Range (PMWHR). Measurements were taken over two years which differed significantly in growing season precipitation. Interannual differences in plant cover were significantly greater than differences between grazed and ungrazed communities. In the arid lowlands total plant cover decreased from 47% in 1993, a relatively wet year, to 29% in 1994, a relatively dry year. In the more mesic uplands total plant cover decreased from 107% in 1993 to 56% in 1994. The magnitude of change in cover was greatest in the grasses, especially for Pseudoroegneria spicata, the most abundant species in the lowland communities, which decreased from 21% cover in 1993 to 11% in 1994. There was not a consistent effect of herbivory on plant cover across sites, but its effects, particularly on the dominant perennial grasses, were conspicuous at some sites. For instance, in the lowlands cover of P. spicata was 3–12% in long-term grazed sites and 9–28% in short- to long-term ungrazed sites. Our study indicates that abiotic factors (e.g., precipitation) are more likely than grazing to affect abundances of key plant species, and hence ecosystem dynamics, in the PMWHR, and that the effects of herbivory are more localized and more prevalent in the lowland grasses than in the other plant functional groups.

Assessing Grazing Impacts by Remote Sensing in Hyper-Arid Environments

Assessing vegetation status via remote sensing techniques using various vegetation indices has been successfully applied to semi-arid and arid environments. We tested the feasibility of applying such techniques for assessing grazing impact in hyper-arid environments with a high variance in soil type over space. An anticlinal erosional cirque called Makhtesh Ramon in the Negev desert, Israel, was selected for the study. The cirque is typified by low rainfall (40‐90 mm per year), a variety of soil substrates and is subject to grazing by a herd of Asiatic wild asses (Equus hemionus) reintroduced into the cirque between 1984 and 1987. As a control, we used an ungrazed dry riverbed south of the cirque that runs parallel to the riverbed draining the cirque. We used 5 common vegetation indices derived from Landsat 5 satellite Thematic Mapper (TM). Four images were used, representing dry and wet seasons in above- and below-average rainfall years (1995 and 1987, respectively). To test whether we can detect changes in plant community structure via satellite data we correlated vegetation indices from the TM to ground measurements made along the altitudinal gradient of the cirque. To test whether differences in plant cover could be detected, we correlated the vegetation indices with ground measurements of cover in and out of the cirque (grazed and ungrazed areas). Although ground measurements showed that community structure changed following grazing with altitudinal gradient and ground cover was 30% lower inside the cirque than outside the cirque, none of the 5 vegetation indices correlated with the ground measurements. Transformed Soil Adjusted Vegetation Index (TSAVI) and Normalized Difference Vegetation Index (NDVI) produced the best results. We hypothesize that the low vegetation cover that typifies hyper-arid environments increases the noise to signal ratio. Thus, a 30% decline in vegetation cover in this case is only an absolute decline of 4% from 15.8 to 11.2%. Because TM is sensitive to absolute cover rather than the relative differences, it is difficult to demonstrate differences among TM images. Using ANOVA to test the effect of season and grazing status on TSAVI and NDVI, we found a significant interaction between season and grazing status in 1995 with indices declining more from wet to dry season inside the cirque than outside the cirque. No such pattern was found in 1987. These data suggest that satellite imagery may detect changes in plant cover over time but can not serve as a direct index of plant cover in hyper-arid conditions.

TEMPORAL DYNAMICS IN SOIL CARBON AND NITROGEN RESOURCES AT A GRASSLAND–SHRUBLAND ECOTONE

Plant communities of large portions of the southwestern United States have changed from grassland to desert shrubland. Previous studies have demonstrated that soil nutrient resources become spatially more heterogeneous and are redistributed into islands of fertility with the shift in vegetation. The research presented here addressed the question of whether soil resources become more temporally heterogeneous as well as more spatially heterogeneous when grassland undergoes desertification to form shrubland. Within adjacent grassland and creosotebush sites, soil profiles were described at three soil pits, and samples were collected for description of nutrient resources within the profile. Relative abundance of plant cover and bare soil was determined within each site using line transects. Surface samples (0-20 cm depth) of bare soil and soil beneath the canopies of grasses and creo- sotebush were collected 17 times during 1992-1994. Soil samples were analyzed for mois- ture, extractable ammonium and nitrate, nitrogen mineralization potential, microbial bio- mass carbon, total organic carbon, microbial respiration, dehydrogenase activity, the ratio of microbial C to total organic C (Cmic/Corg), and the ratio of microbial respiration to biomass carbon (metabolic quotient). The major differences in the structure of soils between sites were the apparent loss of 3-5 cm depth of sandy surface soil at the creosotebush site and an associated increase in calcium carbonate content at a more shallow depth. Soils under plants at both sites had greater total and available nutrient resources, with higher concen- trations under creosotebush than under grasses. Greatest temporal variation in available soil resources was observed in soils under creosotebush. When expressed on the basis of area, available soil resources were higher in the grassland than in the creosotebush shrub- land, primarily due to the difference in plant cover (45% in grassland, 8% in creosotebush shrubland).

Tracer-based studies of soil water movement in semi-arid forests of New Mexico

The related issues of water movement and contaminant transport in arid and semi-arid environments have generated considerable interest and concern in the last few decades. Essential to understanding these issues is knowledge of how water moves through the soils that form the uppermost part of the vadose zone. The use of tracers, both natural and artificially introduced, is proving to be an effective method for gaining such knowledge in dry regions, where investigation by other means is difficult. In this study, natural stable-isotope and chloride tracers were used to investigate water movement in the soils of a piñon–juniper woodland and of a ponderosa pine forest on the Pajarito Plateau in northern New Mexico. The objectives were to (1) estimate and compare near-surface flux rates and evaluate the importance of evaporation in the two communities, and (2) determine to what extent differences in flux rates and evaporation are due to differences in plant cover and/or soil hydraulic properties. The results of this study will aid in evaluating the potential for contaminant mobility in semi-arid systems such as the Pajarito Plateau and, in addition, will increase understanding of nutrient distributions and plant water use in semi-arid environments. The stable-isotope data indicate a similarity between the piñon–juniper and ponderosa communities with respect to evaporation: in both, it is restricted mainly to the upper 10 cm of soil. Chloride profiles from the two communities, on the other hand, show a distinct difference with respect to downward fluxes: in the ponderosa pine forest, these fluxes (≈0.02 cm year −1) are an order of magnitude lower than those in the piñon–juniper woodland (≈0.2 cm year −1), even though total precipitation is about 4 cm year −1 higher in the ponderosa pine forest. This difference, however, appears to be related not to plant cover, but to differences in soil hydraulic properties. The soils of the ponderosa pine forest contain clay-rich B horizons that appear to restrict downward movement of water through the soil matrix, whereas the soils of the piñon–juniper community have B horizons much lower in clay content. The effect of differing soil properties on water movement suggests that contaminant distributions will vary across the Pajarito Plateau. The data on soil water ages support this hypothesis: they indicate that water (and, thus, contaminants) moves through the soil matrix in less than a decade in some areas, whereas in other areas, water takes hundreds of years to pass through the entire soil profile.

Insect Associates of Pistia stratiotes (Arales: Araceae) in Southeastern Florida

Emergence traps and plant quadrat samples were used to survey insects associated with monocultures of Pistia stratiotes (L.) in an abandoned aquaculture pond and a roadside drainage ditch in St. Lucie County, Florida, USA. From 12-14 months of biweekly or monthly sampling, 47,251 specimens representing 13 orders, 90 families, and more than 300 species were identified. Of the 20,221 individuals from emergence traps, 96.5% were Diptera, of which 87.1% belonged to the Chironomidae. Plant quadrats yielded 27,030 specimens of which 55.0% were aquatic Diptera, 22.3% Odonata, 13.3% Hemiptera, and 8.7% Coleoptera. Mosquito larvae of the genus Mansonia accounted for 86.9% of the aquatic Diptera. Mean numbers of individuals from both trapping techniques were highest in the fall and lowest in the winter and spring. The more protected aquaculture pond had consistently more species of aquatic insects, but the emergence traps in the drainage ditch caught more species of aerial insects. These results are interpreted on the basis of differences in plant cover at the two sites. The two trapping techniques are regarded as complementary for surveying the insects of P. stratiotes, although the fauna from quadrat samples are more likely to be associated directly with the plant. Surveys of the insect faunas of water lettuce in Ghana, Argentina, and in Florida show many similarities in the relative abundances of representatives of the same orders and families, suggesting comparable community structures. Many of the abundant Diptera (Ceratopogonidae, Chironomidae, Culicidae) and Odonata (Coenagrionidae, Libellulidae) associated with water lettuce in southeastern Florida and the Chaco of Argentina belong to the same genera and are probable ecological homologs.

Detection and Classification of Muskox Habitat on Banks Island, Northwest Territories, Canada, Using Landsat Thematic Mapper Data

The feasibility of using Landsat Thematic Mapper data for mapping muskox summer habitat was tested on northern Banks Island, Northwest Territories. Digital image enhancement and classification techniques were examined to determine if summer foraging habitats could be detected and mapped using Thematic Mapper imagery. Interpretations of the satellite data were verified in the field during the summers of 1988 and 1989. The most important summer foraging habitats for muskoxen included the wet sedge meadow, graminoid tundra and graminoid/dwarf shrub tundra cover types. These lowland habitats were generally distinguishable on enhanced colour images and were easily differentiated from upland areas. The most suitable colour composite for differentiating muskox summer habitats was the near-infrared (band 4), shortwave infrared (band 5) and red (band 3) spectral bands displayed in red, green and blue respectively. Upland cover types, including dwarf shrub tundra, hummocky tundra and dwarf shrub/lichen barrens, were more difficult to differentiate because of spectral variability resulting from differences in plant cover and site characteristics. The classified image had an overall accuracy of 88%. The summer habitats of particular importance to muskoxen had classification accuracies of 84-89%. Detection of important foraging habitats on Thematic Mapper imagery is attributable to the spectral distinctiveness of wet graminoid communities and the high spectral sensitivity and spatial resolution of the infrared sensors, which allow detection of differences in surface moisture and vegetation physiognomy.Key words: bison habitat, boreal forest, digital image analysis, supervised classification, Landsat Thematic Mapper data, habitat mapping, Bison bison athubuscue, remote sensing

AN EXPERIMENT ON THE APPLICATIONS OF GEOBOTANICAL GUIDES IN DISTINGUISHING BETWEEN LITHOLOGICALLY SIMILAR STRATA OF DIFFERENT ORIGIN

This article will present the techniques and the various steps followed in the utilization of geobotanical indicators for geologic mapping of areas east of and adjacent to the Mugodzharskiye mountains. It was found that despite the apparent homogeneity of the plant cover on lithologically similar rocks, a geobotanical analysis reveals a number of fundamental differences. Even when the floristic composition of associations is the same, other differences are seen in quantitative ratios of species, in the occurrence of some species in the plant cover, and finally, in the development cycle of an association as a whole, or of its individual species. In many instances also considerable floristic differences are observed in the plant cover found on lithologically similar rocks. Differences in plant cover on lithologically similar formations result from the physical chemical properties of rocks, which in turn result from the origin of rocks, the conditions of their formation, and the depths of their occurrence. I...