Showing papers on "Vegetation (pathology) published in 2005"

Recent trends in vegetation dynamics in the African Sahel and their relationship to climate

Abstract: Contrary to assertions of widespread irreversible desertification in the African Sahel, a recent increase in seasonal greenness over large areas of the Sahel has been observed, which has been interpreted as a recovery from the great Sahelian droughts. This research investigates temporal and spatial patterns of vegetation greenness and rainfall variability in the African Sahel and their interrelationships based on analyses of Normalized Difference Vegetation Index (NDVI) time series for the period 1982–2003 and gridded satellite rainfall estimates. While rainfall emerges as the dominant causative factor for the increase in vegetation greenness, there is evidence of another causative factor, hypothetically a human-induced change superimposed on the climate trend. r 2005 Elsevier Ltd. All rights reserved.

Impact of vegetation on flow routing and sedimentation patterns: Three-dimensional modeling for a tidal marsh

Abstract: [1] A three-dimensional hydrodynamic and sediment transport model was used to study the relative impact of (1) vegetation, (2) micro-topography, and (3) water level fluctuations on the spatial flow and sedimentation patterns in a tidal marsh landscape during single inundation events. The model incorporates three-dimensional (3-D) effects of vegetation on the flow (drag and turbulence). After extensive calibration and validation against field data, the model showed that the 3-D vegetation structure is determinant for the flow and sedimentation patterns. As long as the water level is below the top of the vegetation, differences in flow resistance between vegetated and unvegetated areas result in faster flow routing over unvegetated areas, so that vegetated areas are flooded from unvegetated areas, with flow directions more or less perpendicular to the vegetation edge. At the vegetation edge, flow velocities are reduced and sediments are rapidly trapped. In contrast, in between vegetated areas, flow velocities are enhanced, resulting in reduced sedimentation or erosion. As the water level overtops the vegetation, the flow paths described above change to more large-scale sheet flow crossing both vegetated and unvegetated areas. As a result, sedimentation patterns are then spatially more homogeneous. Our results suggest that the presence of a vegetation cover is the key factor controlling the long-term geomorphic development of tidal marsh landforms, leading to the formation of (1) unvegetated tidal channels and (2) vegetated platforms with a levee-basin topography in between these channels.

Monitoring the response of vegetation phenology to precipitation in Africa by coupling MODIS and TRMM instruments

Abstract: [1] While temperature controls on vegetation phenology in humid temperate climates have been widely investigated, water availability is the primary limit on vegetation growth in arid and semiarid ecosystems at continental and global scales. This paper explores the response of vegetation phenology to precipitation across Africa from 2000 to 2003 using vegetation index data from the Moderate-Resolution Imaging Spectroradiometer (MODIS) and daily rainfall data obtained from the Tropical Rainfall Measuring Mission (TRMM). The results indicate that well-defined thresholds exist in cumulative rainfall that stimulate vegetation green-up in arid and semiarid regions of Africa. Therefore cumulative rainfall provides an appropriate criterion for determining the onset of the rainy season, which can be used to predict the onset of vegetation green-up. Following the end of the rainy season, the onset of vegetation dormancy occurs with a lag of about 54 and 84 days in the Sahelian and sub-Sahelian region and in southern Africa, respectively. Further, spatial patterns in vegetation phenology can be divided into three distinct regions in arid and semiarid ecosystems of Africa because of well-defined patterns in rainfall seasonality. In response to rainfall patterns controlled by the Intertropical Convergence Zone in the Sahelian and sub-Sahelian region, the timing of vegetation phenology shifts gradually in a north-south direction at a rate of 0.12 d/km for green-up onset and 0.05 d/km for dormancy onset. In contrast, patterns in vegetation phenology and rainfall seasonality are much more complex in southern Africa. The shift rates and trends in this region are consistent locally but vary abruptly across different ecosystems or climate regimes. Multiple annual cycles of vegetation growth closely follow rainy seasons in parts of the Greater Horn of Africa.

Large parts of the world are brown or black: A different view on the ‘Green World’ hypothesis

Abstract: Climate sets the limits to plant growth but does climate determine the global distribution of major biomes? I suggest methods for evaluating whether vegetation is largely climate or consumer-controlled, focusing on large mammal herbivores and fire as influential consumers. Large parts of the world appear not to be at equilibrium with climate. Consumer-controlled ecosystems are ancient and diverse. Their distinctive ecology warrants special attention. Abbreviations: DGVM = Dynamic Global Vegetation Model; HSS = Hanston et al. (1960).

Divergent vegetation growth responses to the 2003 heat wave in the

Abstract: [1] In 2003, Europe experienced its hottest summer in >500 years. Satellite-derived photosynthetic activity estimates across the Alps revealed a pattern of high elevation growth enhancement and low elevation growth suppression in response to these extreme summer temperatures. Surface weather-derived effective growing season lengths were shorter in 2003 by an average of 9% and 5% for colline and montane areas respectively and were 2%, 12% and 64% longer for subalpine, alpine and nival areas respectively. In situ forest growth measurements of 244 trees at 15 sites across Switzerland verified this pattern and revealed that this divergent response was consistent between species. We suggest that warmer summer temperatures lengthened the snow-free growing season at high elevations while they increased summertime evaporative demand at lower elevations. Our investigation demonstrates that climatic changes are affecting plants beyond simply shifting the elevation belts upwards.

Biomarker records of late Neogene changes in northeast African vegetation

Abstract: Open savannah grasslands (dominated by C4 plants) became a significant component of northeast African vegetation during the late Neogene. We present molecule-specific carbon isotopic mea- surements of terrestrial plant biomarkers preserved in marine sed- iments off northeast Africa that allow reconstruction of orbital- scale vegetation changes in short time windows over the past 9.4 m.y. The biomarker data show large-amplitude vegetation vari- ability as early as 3.8 Ma, with the greatest C4 expansion occurring after 3.4 Ma. We sampled orbital-scale oscillations of up to 5‰, almost as large as the observed late Neogene range of 7‰, sug- gesting that large and repeated oscillations between more open and more closed landscapes were an important aspect of northeast Af- rican vegetation change during the past 4 m.y.

Vegetation class dependent errors in lidar ground elevation and canopy height estimates in a boreal wetland environment

Abstract: An airborne scanning light detection and ranging (lidar) survey using a discrete pulse return airborne laser terrain mapper (ALTM) was conducted over the Utikuma boreal wetland area of northern Alberta in August 2002. These data were analysed to quantify vegetation class dependent errors in lidar ground surface elevation and vegetation canopy surface height. The sensitivity of lidar-derived land-cover frictional parameters to these height errors was also investigated. Aquatic vegetation was associated with the largest error in lidar ground surface definition (+0.15 m, SD = 0.22, probability of no difference in height P < 0.01), likely a result of saturated ground conditions. The largest absolute errors in lidar canopy surface height were associated with tall vegetation classes; however, the largest relative errors were associated with low shrub (63%, –0.52 m, P < 0.01) and aquatic vegetation (54%, –0.24 m, P < 0.01) classes. The openness and orientation of vegetation foliage (i.e., minimal projection of h...

Notions on dynamic-catenal phytosociology as a basis of landscape science

Abstract: Short essential notions of dynamic-catenal phytosociology are defined as the basis of landscape vegetation science. The most important units – vegetation series, geoseries, permaseries and geopermaseries (sigmetum, geosigmetum, permasigmetum and permageosigmetum) – are discussed and synthesized in several figures.

Synergy between small- and large-scale feedbacks of vegetation on the water cycle

Abstract: Predictions of the effects of climate change on the extent of forests, savannas and deserts are usually based on simple response models derived from actual vegetation distributions. In this review, we show two major problems with the implicitly assumed straightforward cause–effect relationship. Firstly, several studies suggest that vegetation itself may have considerable effects on regional climate implying a positive feedback, which can potentially lead to large-scale hysteresis. Secondly, vegetation ecologists have found that effects of plants on microclimate and soils can cause a microscale positive feedback, implying that critical precipitation conditions for colonization of a site may differ from those for disappearance from that site. We argue that it is important to integrate these nonlinearities at disparate scales in models to produce more realistic predictions of potential effects of climate change and deforestation.

Climatic variability and vegetation vulnerability in Amazônia

Abstract: [1] Models of climate change predict close coupling between increases in aridity and conversion of Amazonian forests to savanna. Here we assess the vulnerability and resilience of Amazonian vegetation to climate change by analyzing observed climate-vegetation relationships using climate data, observed vegetation distributions, and evapotranspiration rates inferred from eddy flux data. We found that drought frequency is an excellent predictor of the forest-savanna boundary, indicating the key role of extreme climatic events for inducing vegetation change, and highlighting particularly vulnerable regions of Amazonia.

Sources of co2 emission from a northern peatland: root respiration, exudation, and decomposition

Abstract: Northern peatlands are substantial sinks of carbon (C), yet the sources of carbon dioxide (CO2) emitted from peatlands are largely unknown. Since the relationship between roots and peat in C cycling is important, vascular plants growing on the surface of peat deposits should influence CO2 efflux from the peat surface and the overall C balance in peatlands. In our study, 30-cm peat cores were removed from an ombrotrophic bog in boreal, continental western Canada. Surface vegetation in the cores remained intact and included a continuous bryophyte cover dominated by Sphagnum fuscum. In addition, some cores were collected such that either ericaceous shrubs (Ledum groenlandicum) or sedges (Eriophorum vaginatum) were present. We investigated how the presence of each vegetation type influenced soil respiration and the microbial mineralization of root exudates using a pulse 14C labeling of vegetation in the intact peat cores. The role of root biomass and root respiration in CO2 emission and C allocation was quantified for each type of vegetation and compared through both measurement and modeling. Our results show that vascular plants contributed 35-57% of total CO2 efflux from the peat surface, primarily derived from rhizosphere processes, including root respiration as well as microbial mineralization of root exudates. The mineralization of root exudates contributed 14-53 pmol C-CO2.m-2-d-' (17- 24% of total) to CO2 efflux, depending on vegetation type and moisture conditions. The type of vegetation present did not influence the total amount of photosynthetic fixation over the course of the study, but did affect how C was allocated within and between both the aboveground and belowground components of the peat column.

Landscape-level interactions of prefire vegetation, burn severity, and postfire vegetation over a 16-year period in interior Alaska

Abstract: Landsat imagery was used to study the relationship between a remotely sensed burn severity index and prefire vegetation and the postfire vegetation response related to burn severity within a 1986 b...

Influence of vegetation on aeolian sand transport rate from a backshore to a foredune at Hasaki, Japan

Abstract: The influence of vegetation on aeolian sediment transport rate in the region from a backshore to a foredune was investigated at the Hasaki Coast in Japan, where an onshore wind was predominant and the creeping beach grasses Carex kobomugi and Calystegia soldanella were major species. The comparison of cross-shore distributions of the cross-shore component of aeolian sand transport rate with and without vegetation, which were estimated on the basis of the beach profile changes and a mass conservation equation, showed that the creeping grasses influenced the aeolian sand transport rate. The landward aeolian sand transport rate rapidly decreased landward from the seaward limit of vegetation when the grasses grew. The aeolian sand transport rate reduced by 95% with a vegetation cover of 28%. On the other hand, when the grasses were absent, the landward aeolian sand transport rate did not decrease near the seaward vegetation limit, but near the foot of the foredune.

Nitrogen Fixation in the High Arctic: Role of Vegetation and Environmental Conditions

Abstract: The course of nitrogen fixation by moss-associated cyanobacteria in Svalbard (78°N, 16°E), Norway, was studied using the acetylene reduction assay. In situ field measurements of nitrogen fixation activity were conducted in six different types of moss-dominated arctic vegetation from the beginning of the snowmelt in early June to the end of July 1998. Concurrently, the water content of the soil/vegetation layer was determined and correlated with the nitrogen fixation rates. At all sites with diminishing water content during the summer season, nitrogen fixation activity was positively correlated with the amount of available water in the vegetation. At two sites, where water content of the vegetation was constantly higher than 80% (w/w) throughout the season, nitrogen fixation activity was correlated with temperature. Depending on the type of vegetation, nitrogen fixation became limited when the water status fell below a minimum threshold level. The most desiccation-tolerant vegetation for nitrogen ...

Effects of vegetation canopy and climate on seedling establishment in Mediterranean shrubland

Abstract: Question: Does the influence of plant canopy on seedling establishment interact with climate conditions, and particularly, do intensified drought conditions, enhance a positive effect of the vegetation canopy on seedlings in Mediterranean-type ecosystems. Location: Mediterranean shrubland near Barcelona, Spain at 210 m a.s.l. Methods: Over the course of four years we recorded seedling emergence and survival in open areas and below vegetation under control, drier and warmer experimental climatic conditions. Results: Seedling emergence is more sensitive to climate conditions than later stages of growth. When considering the whole set of species, the total number of established seedlings at the end of the experiment was lower in the drought and warming stands than in control ones, and vegetation canopy increased the number of these seedlings in the drought stands. Drought reduced seedling emergence but not warming, while the interaction between climate treatments and vegetation canopy was not signif...

Evaluation of MODIS and NOAA AVHRR vegetation indices with in situ measurements in a semi‐arid environment

Abstract: Much effort has been made in recent years to improve the spectral and spatial resolution of satellite sensors to develop improved vegetation indices reflecting surface conditions. In this study satellite vegetation indices from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Very High Resolution Radiometer (AVHRR) are evaluated against two years of in situ measurements of vegetation indices in Senegal. The in situ measurements are obtained using four masts equipped with self‐registrating multispectral radiometers designed for the same wavelengths as the satellite sensor channels. In situ measurements of the MODIS Normalized Difference Vegetation Index (NDVI) and AVHRR NDVI are equally sensitive to vegetation; however, the MODIS NDVI is consistently higher than the AVHRR NDVI. The MODIS Enhanced Vegetation Index (EVI) proved more sensitive to dense vegetation than both AVHRR NDVI and MODIS NDVI. EVI and NDVI based on the MODIS 16‐day constrained view angle maximum value composite...

The use of climatic parameters and indices in vegetation distribution. A case study in the Spanish Sistema Central

Abstract: In this study, over 100 phytoclimatic indices and other climatic parameters were calculated using the climatic data from 260 meteorological stations in a Mediterranean territory located in the centre of the Iberian Peninsula. The nature of these indices was very different; some of them expressed general climatic features (e.g. continentality), while others were formulated for different Mediterranean territories and included particular limits of those indices that expressed differences in vegetation distribution. We wanted to know whether all of these indices were able to explain changes in vegetation on a spatial scale, and whether their boundaries worked similarly to the original territory. As they were so numerous, we investigated whether any of them were redundant. To relate vegetation to climate parameters we preferred to use its hierarchical nature, in discrete units (characterized by one or more dominant or co-dominant species), although it is known to vary continuously. These units give clearer results in this kind of phytoclimatic study. We have therefore used the main communities that represent natural potential vegetation. Multivariate and estimative analyses were used as statistical methods. The classification showed different levels of correlation among climatic parameters, but all of them were over 0.5. One hundred and eleven parameters were grouped into five larger groups: temperature (T), annual pluviothermic indices (PTY), summer pluviothermic indices (SPT), winter potential evapotranspiration (WPET) and thermal continentality indices (K). The remaining parameters showed low correlations with these five groups; some of them revealed obvious spatial changes in vegetation, such as summer hydric parameters that were zero in most vegetation types but not in high mountain vegetation. Others showed no clear results. For example, the Kerner index, an index of thermal continentality, showed lower values than expected for certain particular types of vegetation. Parameters relating to the water balance turned out to be very discriminative for separating vegetation types according to the season or the month when water begins to be scarce. Thus, water availability in soils is a limiting factor for the development of vegetation in spring or autumn as well as in summer. As expected, precipitation and temperature discriminated the altitudinal levels of vegetation. Finally, these index limits only worked in the territories where they were formulated, or in nearby areas.

Radiative forcing due to anthropogenic vegetation change based on MODIS surface albedo data

Abstract: [1] In this study we use the capabilities of the MODerate Resolution Imaging Spectroradiometer (MODIS) land surface product to estimate the radiative forcing due to surface albedo changes caused by anthropogenic vegetation changes. We improve the representation of the present surface albedo by using data retrieved from MODIS. The change in surface albedo is based on the current vegetation land cover from MODIS, the MODIS surface albedos for those vegetation types, and a data set for potential natural vegetation. We arrive at a radiative forcing due to anthropogenic vegetation changes of −0.09 Wm−2 since pre-agriculture times to present, weaker than most earlier published results for this climate forcing mechanism. This is mainly due to a lower surface albedo associated with cropland and further with the use of MODIS data to allow us to constrain the surface albedo change.

Urban vegetation monitoring in Hong Kong using high resolution multispectral images

Abstract: Very high resolution (VHR) satellite remote sensing systems are now capable of providing imagery with similar spatial detail to aerial photography, but with superior spectral information. This research investigates the hypothesis that it should be possible to use multispectral IKONOS images to quantify urban vegetation, obtaining similar accuracy to that achieved from false colour aerial photographs. Two parameters, vegetation cover and vegetation density are used to represent biomass in the study area (Kowloon, Hong Kong), for which data is collected for 41 field quadrats. Regression equations relating the field measurements of vegetation density to image wavebands obtained similar high correlations for both image types and lower but significant correlations for vegetation cover. Vegetation density is a quantifiable measure of vegetation in multiple layers above ground, representing the total amount of biomass and is thus well able to indicate the diverse structural types of vegetation found in urban areas. Furthermore it can be accurately measured using the IKONOS green/red ratio (Chlorophyll Index). The superiority of the latter to the more commonly used Normalized Difference Vegetation Index (NDVI), is attributed to the suboptimal timing of the imagery during the dry season, and its greater sensitivity to multiple layering within the vegetation canopy. A time and cost comparison between the two image types suggests that the use of IKONOS images is much more cost effective than aerial photographs for urban vegetation monitoring.

Similarity of vegetation dynamics during interglacial periods

Abstract: The Velay sequence (France) provides a unique, continuous, palynological record spanning the last four climatic cycles. A pollen-based reconstruction of temperature and precipitation displays marked climatic cycles. An analysis of the climate and vegetation changes during the interglacial periods reveals comparable features and identical major vegetation successions. Although Marine Isotope Stage (MIS) 11.3 and the Holocene had similar earth precessional variations, their correspondence in terms of vegetation dynamics is low. MIS 9.5, 7.5, and especially 5.5 display closer correlation to the Holocene than MIS 11.3. Ecological factors, such as the distribution and composition of glacial refugia or postglacial migration patterns, may explain these discrepancies. Comparison of ecosystem dynamics during the past five interglacials suggests that vegetation development in the current interglacial has no analogue from the past 500,000 years.

Meta-analysis of standing crop reduction by Rhinanthus spp. and its effect on vegetation structure

Abstract: We performed a quantitative literature review on the effect of the root hemiparasiteRhinanthus on vegetation standing crop.

New similarity indices for the assignment of relevés to the vegetation units of an existing phytosociological classification

Abstract: Every proposed vegetation classification is sooner or later confronted with an accumulation of new data, which has to be assigned to existing vegetation units. Calculation of similarity indices between new releves (vegetation plots) and constancy columns of established vegetation units is a suitable method for computerised assignment of releves to these units. This paper compares several similarity indices using simulated data set where either randomly distributed or diagnostic species prevail in the species composition of the tested releve. Traditional indices, based only on species composition, produce different results than similarity indices that consider species fidelity. However, both types of indices failed in some situations and thus cannot be widely accepted as suitable methods of additional releve assignment. Therefore a combined Frequency-Positive Fidelity Index (FPFI) is proposed. This new index includes compositional similarity of an assigned releve with vegetation unit and retains the advantages and lacks the disadvantages of tested indices. The calculation of all these indices is available in the JUICE program (http://www.sci.muni.cz/botany/juice.htm).

Biotic controls over spectral reflectance of arctic tundra vegetation

Abstract: In this study, seasonal field measurements of the normalized difference vegetation index (NDVI), using a field spectroradiometer, and leaf area index (LAI), using a LI‐COR LAI‐2000 Plant Canopy Analyzer, were compared with above‐ground phytomass data to investigate relationships between vegetation properties and spectral indices for four distinct tundra vegetation types at Ivotuk, Alaska (68.49° N, 155.74° W). NDVI, LAI and above‐ground phytomass data were collected biweekly from four 100 m×100 m grids, each representative of a different vegetation type, during the 1999 growing season. Shrub phytomass, especially the live foliar deciduous shrub phytomass, was the major factor controlling NDVI across all vegetation types. LAI showed the strongest relationship with the overstorey component (total above‐ground excluding moss and lichen) of phytomass and also showed a significant relationship with NDVI. The results from this study illustrated that time of the growing season in which sampling is conducted, non...