Showing papers on "Vegetation (pathology) published in 2001"

Environment and Crime in the Inner City Does Vegetation Reduce Crime

Abstract: Although vegetation has been positively linked to fear of crime and crime in a number of settings, recent findings in urban residential areas have hinted at a possible negative relationship: Reside...

The effect of grazing on the spatial heterogeneity of vegetation

Abstract: Grazing can alter the spatial heterogeneity of vegetation, influencing ecosystem processes and biodiversity. Our objective was to identify why grazing causes increases in the spatial heterogeneity of vegetation in some cases, but decreases in others. The immediate effect of grazing on heterogeneity depends on the interaction between the spatial pattern of grazing and the pre-existing spatial pattern of vegetation. Depending on the scale of observation and on the factors that determine animal distribution, grazing patterns may be stronger or weaker than vegetation patterns, or may mirror the spatial structure of vegetation. For each possible interaction between these patterns, we make a prediction about resulting changes in the spatial heterogeneity of vegetation. Case studies from the literature support our predictions, although ecosystems characterized by strong plant-soil interactions present important exceptions. While the processes by which grazing causes increases in heterogeneity are clear, how grazing leads to decreases in heterogeneity is less so. To explore how grazing can consistently dampen the fine-scale spatial patterns of competing plant species, we built a cell-based simulation model that features two competing plant species, different grazing patterns, and different sources of vegetation pattern. Only the simulations that included neighborhood interactions as a source of vegetation pattern produced results consistent with the predictions we derived from the literature review.

Diversity of Vegetation Patterns and Desertification

Abstract: A new model for vegetation patterns is introduced. The model reproduces a wide range of patterns observed in water-limited regions, including drifting bands, spots, and labyrinths. It predicts transitions from bare soil at low precipitation to homogeneous vegetation at high precipitation, through intermediate states of spot, stripe, and hole patterns. It also predicts wide precipitation ranges where different stable states coexist. Using these predictions we propose a novel explanation of desertification phenomena and a new approach to classifying aridity.

On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation

Abstract: The volume of shade within vegetation canopies is reduced by more than an order of magnitude on cloudy and/or very hazy days compared to clear sunny days because of an increase in the diffuse fraction of the solar radiance. Here we show that vegetation is directly sensitive to changes in the diffuse fraction and we conclude that the productivity and structure of vegetation is strongly influenced by clouds and other atmospheric particles. We also propose that the unexpected decline in atmospheric [CO2] which was observed following the Mt. Pinatubo eruption was in part caused by increased vegetation uptake following an anomalous enhancement of the diffuse fraction by volcanic aerosols that would have reduced the volume of shade within vegetation canopies. These results have important implications for both understanding and modelling the productivity and structure of terrestrial vegetation as well as the global carbon cycle and the climate system.

Riparian vegetation controls on braided stream dynamics

Abstract: Riparian vegetation can significantly influence the morphology of a river, affecting channel geometry and flow dynamics. To examine the effects of riparian vegetation on gravel bed braided streams, we conducted a series of physical experiments at the St. Anthony Falls Laboratory with varying densities of bar and bank vegetation. Water discharge, sediment discharge, and grain size were held constant between runs. For each run, we allowed a braided system to develop, then seeded the flume with alfalfa (Medicago sativa), allowed the seeds to grow, and then continued the run. We collected data on water depth, surface velocity, and bed elevation throughout each run using image-based techniques designed to collect data over a large spatial area with minimal disturbance to the flow. Our results show that the influence of vegetation on overall river patterns varied systematically with the spatial density of plant stems. Vegetation reduced the number of active channels and increased bank stability, leading to lower lateral migration rates, narrower and deeper channels, and increased channel relief. These effects increased with vegetation density. Vegetation influenced flow dynamics, increasing the variance of flow direction in vegetated runs and increasing scour depths through strong downwelling where the flow collided with relatively resistant banks. This oblique bank collision also provides a new mechanism for producing secondary flows. We found it to be more important than the classical curvature-driven mechanism in vegetated runs.

Banded vegetation patterning in arid and semiarid environments : ecological processes and consequences for management

Abstract: Banded vegetation patterns and related structures * Theories on the origins, maintenance, dynamics and functioning of banded landscapes * Specific methods of study * Runoff and Erosion Processes * Soil Water balance * Soil Biota in Banded Landscapes * Vegetation dynamics: recruitment and regeneration in two-phase mosaics * Multiscale Modelling of Vegetation Bands * Landscape Models for Banded Vegetation Genesis * Productivity of patterned vegetation * Towards Improved Management of Arid and Semi-Arid Banded Landscapes * Banded landscapes: Ecological Developments and Management Consequences

The Original US Public Land Survey Records: Their Use and Limitations in Reconstructing Presettlement Vegetation

Abstract: Their Use and Limitations in Reconstructing Presettlement Vegetation Above: European settlement introduced new disturbances to the northern Lake States, including broadscale removal of forest vegetation and tilling of the soil. This photograph was taken in Chippewa County, Wisconsin, in 1895, at the height of the lumbering era in this region. C ou rt es y of S ta te H is to ric al S oc ie ty o f W is co ns in divided into 36 one-square-mile sections (Stewart 1935) (fig. 1). Surveyors traversed the boundaries between all sections and, in so doing, marked the intersection of section lines (section corners) and the midpoint between section corners (quarter corners), using a wooden post set into the ground, a mound of earth, or stones. Surveyors also marked the locations where section lines crossed navigable rivers, bayous, or lakes (meander corners) (fig. 2). At section, quarter, and meander corners, surveyors “blazed” two to four nearby trees as “witness” or “bearing” trees. Blazing consisted of inscribing the corner identification and coordinates on the tree, so that given locations could subsequently be relocated. Tree species, diameter, compass bearing, and distance from the corner were then recorded in surveyor notebooks. Other data recorded by surveyors, which are frequently used in studies of historical vegetation, include (1) the location of abrupt boundaries between distinct vegetation types (e.g., swamp versus upland forest); (2) incidences of visible fire or wind disturbance; and (3) line and township descriptions of dominant overand understory species, agricultural suitability of the soils, topography, and Native American and early Euro-American settlements. Thorough reviews of the survey technique, changes over time in survey instructions, and consistency between the actual survey and the instructions provided can be found in Bourdo (1956). Data Quality The intention of the PLS was legal rather than scientific; although surveyors were provided general instructions on how to conduct the survey, lack of expertise and care meant they often did not apply methods consistently. Preference for or discrimination against certain tree species, sizes, and locations has been documented, as well as cases of fraud, which contemporary surveyors usually uncovered and resurveyed (Bourdo 1956). Despite inconsistencies, data collection was largely systematic, quantitative, and statistically representative. PLS records also provide the broadest coverage (Ohio to the 6 Journal of Forestry • October 2001 Figure 1. Government Land Office surveyors first surveyed township and range lines, which together divided the land into townships, measuring 36 square miles. Townships were divided into one-square-mile units, called sections, and survey data were collected along section boundaries. Surveyors marked and recorded witness trees at section corners, quarter corners, and meander corners. Figure 2. Page of notes recorded by a surveyor of the original US Public Land Survey. A schematic of data collected runs right to left along the bottom.

Restoration success of low-production plant communities on former agricultural soils after top-soil removal

Abstract: The success in restoring seven low-production vegetation types on former agricultural soil after top-soil removal was investigated. The colonization and establishment of target species in permanent plots was recorded during the first nine years after restoration measures were taken. For each permanent plot abiotic site conditions were used to determine which of the vegetation types could persist there. A comparison of the actual vegetation in the permanent plots with reference releves of the selected vegetation type revealed a gradually increasing similarity during consecutive years for five vegetation types. This was due to the occurrence of an increasing number of target species and the number of permanent plots they occurred in. However, nine years after top-soil removal a large number of the target species were still lacking from the vegetation in the permanent plots, although most were present in the local species pool. Seed dispersal therefore seems to be a major limiting factor for restoration of these low-production vegetation types on formerly agricultural soils.

Abrupt vegetation changes in the Segura Mountains of southern Spain throughout the Holocene

Abstract: Summary 1 The fossil pollen record of Canada de la Cruz in the Segura mountains of southern Spain yields insights into high-elevation vegetational dynamics over the last c. 8320 years. Phases of xerophytic grassland alternate with high-mountain open pine forests and expansion of deciduous forests and Mediterranean scrub at lower altitudes. 2 Longer-term stable vegetation patterns are interrupted by multidecadal to century-scale shifts at about 7770, 3370, 2630, 1525 and 790 years BP. 3 Some of the vegetation types have no modern analogues and represent high-altitude remnants of widespread last-glacial xerophytic communities. Other species patterns, characteristic of current scrub associations, appeared only within the last 800 years. 4 The sequence fits within the regional context of a generally wet mid-Holocene (c. 7700–3300 years BP) characterized by spread of mesophilous vegetation, between drier conditions characterized by greater abundance of xerophytes. 5 The pollen record and current ecological studies on high-elevation vegetation of Mediterranean Spain suggest that control of vegetation is primarily climatic although grazing pressure, which would have pushed vegetation over a threshold for change, cannot be discounted.

Banded Vegetation Patterning in Arid and Semiarid Environments

Abstract: Banded vegetation patterns and related structures * Theories on the origins, maintenance, dynamics and functioning of banded landscapes * Specific methods of study * Runoff and Erosion Processes * Soil Water balance * Soil Biota in Banded Landscapes * Vegetation dynamics: recruitment and regeneration in two-phase mosaics * Multiscale Modelling of Vegetation Bands * Landscape Models for Banded Vegetation Genesis * Productivity of patterned vegetation * Towards Improved Management of Arid and Semi-Arid Banded Landscapes * Banded landscapes: Ecological Developments and Management Consequences

Competitive abilities of introduced and native grasses

Abstract: Differences in competitive ability may explain the maintenance of existing plant populations and the invasion of new areas by plant species. We used field experiments to examine the competitive responses of Agropyron cristatum (L.) Gaertn., an introduced C3 grass, and Bouteloua gracilis (HBK.) Lag., a native C4 grass, and the competitive effects of Agropyron-dominated vegetation and successional prairie. We also tested whether the outcome of competitive interactions varied with water availability. In each vegetation type, transplants of each species were grown under two levels of competition (presence or absence of neighboring vegetation) and three levels of water availability (high, medium, or low). Transplant survival, growth, and biomass allocation patterns were measured. Water availability had no effect on the measured variables, suggesting that both species were limited by another resource. Growth rates were affected more by competition, while survival and root: shoot ratio were affected more by transplant species identity. In the successional prairie, neighboring vegetation suppressed the growth of Agropyron transplants less than that of Bouteloua transplants, suggesting that Agropyron has a stronger ability to resist competitive suppression in that vegetation type. The spread of Agropyron into surrounding vegetation may relate to its ability to resist competitive suppression. In the Agropyron-dominated vegetation, neighboring vegetation suppressed the growth of both species by the same extent. However, competition accounted for more variation in transplant growth in Agropyron-dominated vegetation than in successional prairie, suggesting that Agropyron has strong competitive effects which hinder plant growth and prevent other species from establishing in Agropyron fields.

The distribution and abundance of ground-dwelling mammals in relation to time since wildfire and vegetation structure in south-eastern Australia

Abstract: Vegetation undergoes a natural succession after wildfire. Following an initial flush of vegetation, when light and other resources become limiting, the stand structure rapidly reaches a maximum. As a result, vegetation structure does not form an even distribution over the landscape, but rather a patchwork pattern. The position and characteristics of a patch of habitat in the landscape may be critical in determining the faunal composition. In this paper a sequence of ‘habitat complexity scores’ (which describe vegetation structure independently of plant species) collected over 20 years following a wildfire was utilised to estimate vegetation structure in relation to time since fire. This information was compared with data collected over the same period on medium-sized and large grounddwelling mammals to examine the response of mammals to changes in vegetation structure. Models are presented of the response of ground-dwelling mammals to time since wildfire and to changes in habitat complexity scores, with predictions up to 25 years after wildfire.

Relationship between vegetation and soil seed banks in an arctic coastal marsh

Abstract: Summary 1 The effects of habitat degradation on the soil seed bank at La Perouse Bay, Manitoba are described. Foraging by lesser snow geese leads to loss of vegetation, coupled with changes in soil abiotic conditions and an increase in salinity. 2 The density of seeds and the relative abundance in the seed bank of species characteristic of undisturbed sites decrease following degradation, while the relative abundance of invasive species increases. Vegetation loss had the greatest impact on seed banks of stress-tolerant species and the least impact on species with many widely dispersed seeds. 3 The above-ground vegetation and below-ground seed bank were less similar in un damaged plots than in disturbed plots. In spite of the low degree of similarity, redundancy analysis of the data indicated that approximately half of the variation in the soil seed bank could be explained by the vegetation data and vice versa. 4 More recently degraded soils had richer soil seed banks than those from older disturbances. Site-specific factors not only influenced the species present but also the time lag between loss of vegetation and loss of the seed bank. Seed banks in these impacted and fragmented sites do not recover quickly. 5 Seed banks in sandy beach-ridges were less affected by degradation due to the greater proportion of ruderals present in the original vegetation and the absence of the high soil salinities that are characteristic of degraded salt-marsh soils.

Vegetation response to the ∼14.7 and ∼11.5 ka cal. BP climate transitions: is vegetation lagging climate?

Abstract: The transition from the Last Glacial towards the Holocene is marked by two rapid increases in temperature. These climate transitions are recorded in detail in the Greenland ice-cores, and these records reveal that the climate shifts around 14.7 and 11.5 ka cal. BP seem to have occurred within a few years only. In this paper it is argued that vegetation did respond directly to the climatic warming round 14.7 and 11.5 ka cal. BP. The response of vegetation must, however, not be regarded as an immediate establishment of forests. It is most likely to assume that vegetation present at the moment of climatic improvement, responded by increased flowering, which thus should be reflected in the palynological record as an increased presence. These palynological events occur at approximately 14.8 and 11.7 ka cal. BP, based on calibrated radiocarbon ages. From the palynological evidence, which is available from numerous terrestrial records, the vegetation response to the climate changes seems to be quite different for both transitions. As climate remained relatively warm since the climatic improvement, forests could develop. In the early Lateglacial, it took some time for a dense forest to develop, due to both migrational lags and the delayed response of permafrost melting. For the Early Holocene, forest stands were relatively near, relict permafrost did not occur, and hence vegetation could recover quickly after the climatic improvement.

Environmental, historical, and contextual determinants of vegetation cover: a landscape perspective

Abstract: We formulated and tested models of relationships among determinants of vegetation cover in two agroforested landscapes of eastern North America (Haut Saint-Laurent, Quebec, Canada) that differed by the spatial arrangement of their geomorphic features and intensity of agricultural activities. Our landscape model compared the woody plots of each landscape in terms of the relative influence of environmental attributes, land use history (1958 – 1997), and spatial context (i.e., proximity of similar or contrasting land cover). Our vegetation model evaluated the relative contribution of the same sets of variables to the distributions of herbs, trees, and shrubs. Relationships were assessed using partial Mantel tests and path analyses. Significant environmental and contextual differences were found between the vegetation plots of the two landscapes, but disturbance history was similar. Our vegetation model confirms the dominant effect of historical factors on vegetation patterns. Whereas land-use history overrides environmental and contextual control for trees, herbaceous and shrub species are more sensitive to environmental conditions. Context is determinant only for understory species in older, less-disturbed plots. Results are discussed in relevance to vegetation dynamics in a landscape perspective that integrates interactions between environmental and human influences.

Mapping Weekly Rangeland Vegetation Productivity Using MODIS Algorithms

Abstract: The great spatial extent of rangelands combined with recent emphasis on rangeland health has prompted a need for more efficient and cost effective management tools. The Moderate Resolution Imaging Spectroradiometer (MODIS) sensor of the Earth Observing System (EOS) will offer improved and more timely monitoring of rangeland vegetation, and, unlike any previous satellite sensor, the publicly available MODIS data stream will include estimates of rangeland productivity. These estimations of rangeland productivity can be used regionally for measuring biomass production and will be available every eight-days, with global coverage at 1km^ resolution. MODIS derived estimates of rangeland productivity combine remote sensing information with daily meteorological data as inputs to a mathematical model of photosynthetic conversion of solar radiation into plant carbohydrates. Vegetation productivity is .a measure of rangeland vegetation vigor and growth capacity, which are important components of rangeland management and health assessment. Using MODIS data, it will be possible to characterize rangeland vegetation seasonality, estimate herbage quantity and, monitor the rates and trends of change in primary production. Consistent, objective and frequent productivity estimates will be available for even the most inaccessible rangelands. Potential applications of weekly and annual productivity estimates are demonstrated on the Shoshone BLM Administrative District and a larger portion of the Interior Northwestern United States. Productivity estimates were derived using Advanced Very High-Resolution Radiometer data as a surrogate for the MODIS data stream. Shrub and grassland vegetation seasonality for 1991 was characterized. Herbage quantity was estimated from the 1993 shrub and grassland regional net primary production. A 5-year average productivity from 1990 1994 and departures from that average were calculated for the years 1991 and 1993. The measures of departure indicated that 1991 was regionally less productive and 1993 more productive than the five year average. Collaboration between rangeland scientists and managers is necessary to realize the potential for EOS-derived vegetation productivity as a management tool. Future research will include field calibration of the productivity algorithms and exploration of new techniques for using EOSderived productivity measures for rangeland management. Measures of rangeland productivity could become part of an integrated rangeland system analysis. This may permit differentiation between anthropogenic, biotic, and abiotic factors as the primary cause of declining productivity. Other research may include customization of biome properties for selected regions.

A Comparison of Spectral Mixture Analysis and Ten Vegetation Indices for Estimating Boreal Forest Biophysical Information from Airborne Data

Abstract: L'analyse des spectres mixtes (ASM) permet de deriver le pourcentage de couronnes ensoleillees, de fond et d'ombre a l'interieur d'un pixel d'une image de teledetection. Il a ete demontre que l'inf...

Mapping Canadian boreal forest vegetation using pigment and water absorption features derived from the AVIRIS sensor

Abstract: Using imagery of the Canadian boreal forest, we explored the ability of the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) to map vegetation type by taking advantage of pigment and water absorption features. Two techniques were exploited. In the first classification routine, laboratory-acquired leaf spectra representing different "pigment classes" were used in a spectral unmixing procedure to map the relative abundance of pigments in the landscape. The resulting images were then used in a maximum likelihood routine to map the distribution of vegetation cover types. Accuracies for this method range between 66.6 - 80.1%, when compared to a vegetation map prepared by the Saskatchewan Environment and Resource Management (SERM), Forestry Branch Inventory Unit (FBIU). In the second approach, seven indices of vegetation structure and physiological function were calculated from AVIRIS. Cover types were then derived using the index images as inputs in a maximum likelihood classification. Levels of accuracy for this method were between 56.6 and 73.3%, when compared to the same vegetation map. Both of these complementary techniques were able to differentiate important vegetation types such as fen, deciduous trees, and wet and dry conifers at accuracies superior to other well-established classification methods for this area. This improved vegetation classification can now be used to evaluate regional surface-atmosphere fluxes of carbon and water vapor.