scholarly journals Mapping trends in woody cover throughout Namibian savanna with MODIS seasonal phenological metrics and field inventory data

2019 ◽  
Author(s):  
Vladimir R. Wingate ◽  
Nikolaus J. Kuhn ◽  
Stuart R. Phinn ◽  
Cornelis van der Waal
Keyword(s):  
Environmental Factors ◽  
Field Measurements ◽  
Kendall Test ◽  
Shrub Encroachment ◽  
Spatial Extent ◽  
Woody Vegetation ◽  
Herbaceous Vegetation ◽  
Savanna Vegetation ◽  
Woody Cover ◽  
Phenological Metrics

Abstract. Woody vegetation is an integral component of savannas. Here, two main change processes alter woody vegetation, namely shrub encroachment and deforestation. Both impact a range of ecosystem services and functions across scales. Accurate estimates of change, including spatial extent, rate and drivers are lacking. This is primarily due to savanna vegetation comprising woody and herbaceous vegetation, each of which exhibit divergent phenological characteristics, and vary importantly in their response to climatic and environmental factors. This study uses phenological metrics derived from the MODIS MOD13Q1 NDVI time-series to model woody cover as a function of field measurements, and to map trends across Namibia. These metrics enhance the contrasting phenological characteristics of woody and herbaceous vegetation, and standardizes their annual response to climatic and environmental factors by integrating short term variation. Trends in woody cover are excellent indicators of shrub encroachment and deforestation. Trend significance was computed using the Mann-Kendall test, while change statistics, including the rate and spatial extent of change were derived using the Theil-Sen slope. Change was evaluated in relation to drivers including land-use, population, biomes and precipitation. An overall decrease in woody cover was identified, with the most pronounced decreases found in urban and densely populated areas. Decreases in woody cover were not homogenously distributed; losses predominated in tropical desert and dry forests, but gains were found across shrub lands.

Mapping trends in woody cover throughout Namibian savannah with MODIS seasonal phenological metrics and field inventory data

2021 ◽  
Author(s):  
Vladimir Wingate
Keyword(s):  
Vegetation Cover ◽  
Forest Cover ◽  
Validation Dataset ◽  
Anthropogenic Factors ◽  
Woody Vegetation ◽  
Mean Annual Precipitation ◽  
Herbaceous Vegetation ◽  
Modis Ndvi ◽  
Woody Cover ◽  
Phenological Metrics

<p>Woody vegetation is an integral component of Namibian savannahs and essential to people’s livelihoods. Savannah vegetation varies in response to climatic, environmental and anthropogenic factors, moreover, its constituent plant functional types (woody and herbaceous vegetation) exhibit divergent phenological characteristics. Together, these make accurate estimates of changes in tree and shrub cover densities over time difficult to achieve. Two contrasting land degradation processes affecting woody vegetation cover are widespread: (i) the replacement of the herbaceous layer with hardy shrubs (shrub encroachment) and (ii) the loss of forest cover (deforestation). Both processes impact a range of ecosystem services, from local (i.e. local forage and timber resources) to global scales (i.e. biome carbon sequestration). To map trends in woody cover, field observations from 484 sample plots were used to model percentage woody cover as a function of seasonal phenological metrics derived from the MODIS NDVI time-series. An independent validation dataset found a RMSE of 19.73% and an R2 of 0.93%. Trends in modelled woody cover were assessed in relation to land-use, population density and mean annual precipitation. An overall declining trend was identified, with certain land-uses, including protected areas, revealing a declining trend. Significant negative trends covered 11.80% of the study area, while 9.20% underwent positive trends. Trends in woody vegetation cover are mostly unrelated to those of precipitation, except for certain areas which show high coefficients of determination, and imply the presence of predominantly herbaceous vegetation. As such, this study presents a novel method for the identification of grasslands in Namibia.</p>

A Quantitative Appraisal of Woody Shrub Encroachment in Western New South Wales.

1998 ◽  
Vol 20 (1) ◽  
pp. 26 ◽  
Author(s):  
DB Gardiner ◽  
GJ Tupper ◽  
GS Dundeon
Keyword(s):  
Information System ◽  
Geographic Information System ◽  
Vegetation Cover ◽  
New South ◽  
Geographic Information ◽  
Shrub Encroachment ◽  
Woody Vegetation ◽  
Shrub Cover ◽  
South Wales ◽  
Woody Cover

Landsat Multispectral Scanner (MSS) digital imagery was used to estimate the distribution, density and change in woody shrub cover over time in western New South Wales. The purpose of the project was to derive maps of woody cover which can be used as a basis for regional planning and property planning. Assessment of woody vegetation cover using satellite imagery enables regions which are more susceptible to shrub encroachment to be targeted for control strategies. Dry season images which had minimal green vegetation were used, because the spectral signatures of scrubby ground cover interfered with the proper classification of woody vegetation. For each region, multidate imagery was classified using a pixel unmixing algorithm to derive data sets which showed woody canopy cover. These data were then rescaled to percentage values using aerial photography sampled throughout each region. A geographic information system (GIS) was used to derive changes in woody cover between both dates and to present the data in map form. Most current woody cover in the study area occurs at less than 20% cover, whilst higher levels (40 to 80%) occur in the eastern parts of the Louth and Barnato regions. At least 20,3 10 km2 of the 120,000 km2 study area is already affected by woody vegetation cover levels of greater than 40%, which significantly reduces carrying capacity and pastoral productivity. Changes in woody cover over a 10 to 20 year period were varied. Approximately 24% (26,041 km2) was relatively stable, whilst 20% of the Barnato region had moderate decreases (1 1 to 30%) due to wildfires, and increases of 11 to 30% cover occurred on 'hard red' soils in the east. Emerging woody vegetation of less than 10% cover occurred over 1816 km2 of Sandplains and Stony Lowlands in the Louth and Barnato regions, whilst woody vegetation levels of more than 40% cover occurred in the Barnato region. Considerable 'infilling' of previously unwooded areas was noted for regions which already had high levels of woody cover. A minimal amount of prescribed clearing was apparent from the change data, which suggests that effective control of shrubs is difficult to achieve and that future scenarios will see continued encroachment. The findings suggest that the southern Louth and Barnato regions are most at risk of further shrub encroachment, and that these areas need to be targeted for shrub control. The data provide a quantitative estimate of woody shrub cover which is useful for economic assessments, as well as providing an information base upon which woody shrub management strategies can be developed. Key words: Landsat Multispectral Scanner, remote sensing, geographic information system, change detection, rangeland, monitoring, land cover.

Effect of photoscale, interpreter bias and land type on woody crown-cover estimates from aerial photography

2007 ◽  
Vol 55 (4) ◽  
pp. 457 ◽  
Author(s):  
R. J. Fensham ◽  
R. J. Fairfax
Keyword(s):  
Aerial Photography ◽  
Vegetation Cover ◽  
Field Measurements ◽  
Woody Vegetation ◽  
Vegetation Types ◽  
Land Type ◽  
Crown Cover ◽  
Woody Cover ◽  
Land Types ◽  
Residual Deviance

Woody vegetation cover interpreted from aerial photography requires assessment against field data as the signature of woody vegetation cover may differ between photoscales, vegetation types and photo-interpreters. Measurements of aerial woody cover taken from aerial photography of four different photoscales were compared with a field dataset from Eucalyptus- and Acacia-dominated landscapes of semi-arid Queensland. Two interpreters employed a method that utilises a stereoscope and sample-point graticule for manual quantified measurements of aerial woody cover. Both interpreters generated highly significant models accounting for 77 and 78% of deviance. Photoscale appears to have a consistent effect whereby the signature of woody cover increases as the photoscale decreases from 1 : 25 000 to 1 : 80 000, although the magnitude of this effect was different between interpreters. The results suggest no substantial differences in the shape of models predicting crown cover between Acacia- and Eucalyptus-dominated land types, although the precision of the models was greater for the Acacia (90–91% of residual deviance) than for the Eucalyptus (50–56% of residual deviance) land type. The reduced accuracy in the Eucalyptus land type probably reflects the relatively diffuse crowns of the dominant trees. The models generated for this dataset are within the range of those from other calibration studies employing photography of a range of scales and methodologies. The effect of photoscale is verified between the available studies, but there may also be variations arising from methodological differences or image properties. The present study highlights the influence of photoscale and interpreter bias for assessing woody crown cover from aerial photography. Studies that employ aerial photography should carefully consider potential biases and cater for them by calibrating assessments with field measurements.

scholarly journals Spatially explicit statistical modeling of random and systematic land cover transitions in the main grassland plain of Nech Sar National Park, Ethiopia

2019 ◽  
Vol 8 (1) ◽  
Author(s):  
Kiros Tsegay Deribew
Keyword(s):  
Land Cover ◽  
Land Cover Change ◽  
Statistical Modeling ◽  
National Park ◽  
Spatial Extent ◽  
Aerial Photographs ◽  
Woody Vegetation ◽  
Spatially Explicit ◽  
Strong Signal ◽  
The Past

AbstractThe main grassland plain of Nech Sar National Park (NSNP) is a federally managed protected area in Ethiopia designated to protect endemic and endangered species. However, like other national parks in Ethiopia, the park has experienced significant land cover change over the past few decades. Indeed, the livelihoods of local populations in such developing countries are entirely dependent upon natural resources and, as a result, both direct and indirect anthropogenic pressures have been placed on natural parks. While previous research has looked at land cover change in the region, these studies have not been spatially explicit and, as a result, knowledge gaps in identifying systematic transitions continue to exist. This study seeks to quantify the spatial extent and land cover change trends in NSNP, identify the strong signal transitions, and identify and quantify the location of determinants of change. To this end, the author classifies panchromatic aerial photographs in 1986, multispectral SPOT imagery in 2005, and Sentinel imagery in 2019. The spatial extent and trends of land cover change analysis between these time periods were conducted. The strong signal transitions were systematically identified and quantified. Then, the basic driving forces of the change were identified. The locations of these transitions were also identified and quantified using the spatially explicit statistical model. The analysis revealed that over the past three decades (1986–2019), nearly 52% of the study area experienced clear landscape change, out of which the net change and swap change attributed to 39% and 13%, respectively. The conversion of woody vegetation to grassland (~ 5%), subsequently grassland-to-open-overgrazed land (28.26%), and restoration of woody vegetation (0.76%) and grassland (0.72%) from riverine forest and open-overgrazed land, respectively, were found to be the fully systematic transitions whereas the rest transitions were recorded either partly systematic or random transitions. The location of these most systematic land cover transitions identified through the spatially explicit statistical modeling showed drivers due to biophysical conditions, accessibility, and urban/market expansions, coupled with successive government policies for biodiversity management, geo-politics, demographic, and socioeconomic factors. These findings provide important insights into biodiversity loss, land degradation, and ecosystem disruption. Therefore, the model for predicted probability generally suggests a 0.75 km and 0.72 km buffers which are likely to protect forest and grassland from conversion to grassland and open-overgrazed land, respectively.

scholarly journals Monitoring scrub weed change in the Canterbury region using satellite imagery

2007 ◽  
Vol 60 ◽  
pp. 137-140 ◽  
Author(s):  
J.D. Shepherd ◽  
J.R. Dymond ◽  
J.R.I. Cuff
Keyword(s):  
Satellite Imagery ◽  
Vegetation Cover ◽  
Satellite Image ◽  
Visual Assessment ◽  
Woody Vegetation ◽  
Spatial Change ◽  
Valid Data ◽  
Woody Cover ◽  
Landsat Satellite ◽  
Thematic Data

The spatial change of woody vegetation in the Canterbury region was automatically mapped between 1990 and 2001 using Landsat satellite image mosaics The intersection of valid data from these mosaics gave coverage of 84 of the Canterbury region Changes in woody cover greater than 5 ha were identified Of the 5 ha areas of woody change only those that were likely to have been a scrub change were selected using ancillary thematic data for current vegetation cover (eg afforestation and deforestation were excluded) This resulted in 2466 polygons of potential scrub change These polygons were rapidly checked by visual assessment of the satellite imagery and assigned to exotic or indigenous scrub change categories Between 1990 and 2001 the total scrub weed area in the Canterbury region increased by 3600 400 ha and indigenous scrub increased by 2300 400 ha

scholarly journals Trends in Woody and Herbaceous Vegetation in the Savannas of West Africa

2019 ◽  
Vol 11 (5) ◽  
pp. 576 ◽  
Author(s):  
Julius Y. Anchang ◽  
Lara Prihodko ◽  
Armel T. Kaptué ◽  
Christopher W. Ross ◽  
Wenjie Ji ◽  
...  
Keyword(s):  
Vegetation Change ◽  
Positive Association ◽  
Information Criterion ◽  
West African ◽  
Woody Vegetation ◽  
The West ◽  
Herbaceous Vegetation ◽  
Discernible Effect ◽  
Use Efficiency ◽  
Rain Use Efficiency

We assess 32 years of vegetation change in the West African Sudano-Sahelian region following the drought events of the 1970s and 1980s. Change in decadal mean rain use efficiency is used to diagnose trends in woody vegetation that is expected to respond more slowly to post-drought rainfall gains, while change in the slope of the productivity–rainfall relationship is used to infer changing herbaceous conditions between early and late periods of the time series. The linearity/non-linearity of the productivity–rainfall relationship and its impact on the interpretation of overall greening trends, and specific woody and herbaceous vegetation trends, is also examined. Our results show a mostly positive association between productivity and rainfall (69% of pixels), which can be best described as linear (32%) or saturating (37%). Choosing the ‘best’ model at a specific location using Akaike Information Criterion has no discernible effect on the interpretation of overall greening or herbaceous trends, but does influence the detection of trends in woody vegetation. We conclude that widespread recovery in woody vegetation is responsible for the post-drought greening phenomenon reported elsewhere for the Sahel and Sudanian sub-regions. Meanwhile, trends in herbaceous vegetation are less pronounced, with no consistent indication towards either herbaceous degradation or recovery.

scholarly journals Patterns in woody vegetation structure across African savannas

2017 ◽  
Vol 14 (13) ◽  
pp. 3239-3252 ◽  
Author(s):  
Christoffer R. Axelsson ◽  
Niall P. Hanan
Keyword(s):  
Vegetation Structure ◽  
Woody Plants ◽  
Woody Plant ◽  
Annual Precipitation ◽  
Woody Vegetation ◽  
Mean Annual Precipitation ◽  
Crown Density ◽  
Woody Cover ◽  
Ecohydrological Processes ◽  
Crown Size

Abstract. Vegetation structure in water-limited systems is to a large degree controlled by ecohydrological processes, including mean annual precipitation (MAP) modulated by the characteristics of precipitation and geomorphology that collectively determine how rainfall is distributed vertically into soils or horizontally in the landscape. We anticipate that woody canopy cover, crown density, crown size, and the level of spatial aggregation among woody plants in the landscape will vary across environmental gradients. A high level of woody plant aggregation is most distinct in periodic vegetation patterns (PVPs), which emerge as a result of ecohydrological processes such as runoff generation and increased infiltration close to plants. Similar, albeit weaker, forces may influence the spatial distribution of woody plants elsewhere in savannas. Exploring these trends can extend our knowledge of how semi-arid vegetation structure is constrained by rainfall regime, soil type, topography, and disturbance processes such as fire. Using high-spatial-resolution imagery, a flexible classification framework, and a crown delineation method, we extracted woody vegetation properties from 876 sites spread over African savannas. At each site, we estimated woody cover, mean crown size, crown density, and the degree of aggregation among woody plants. This enabled us to elucidate the effects of rainfall regimes (MAP and seasonality), soil texture, slope, and fire frequency on woody vegetation properties. We found that previously documented increases in woody cover with rainfall is more consistently a result of increasing crown size than increasing density of woody plants. Along a gradient of mean annual precipitation from the driest (< 200 mm yr−1) to the wettest (1200–1400 mm yr−1) end, mean estimates of crown size, crown density, and woody cover increased by 233, 73, and 491 % respectively. We also found a unimodal relationship between mean crown size and sand content suggesting that maximal savanna tree sizes do not occur in either coarse sands or heavy clays. When examining the occurrence of PVPs, we found that the same factors that contribute to the formation of PVPs also correlate with higher levels of woody plant aggregation elsewhere in savannas and that rainfall seasonality plays a key role for the underlying processes.

Scale relationships and linkages between woody vegetation communities along a large tropical floodplain river, north Australia

2009 ◽  
Vol 26 (1) ◽  
pp. 79-92 ◽  
Author(s):  
Aaron M. Petty ◽  
Michael M. Douglas
Keyword(s):  
Riparian Vegetation ◽  
Flood Pulse ◽  
Woody Vegetation ◽  
Stream Channels ◽  
River Continuum ◽  
Forest Zone ◽  
Savanna Vegetation ◽  
Riparian Woodlands ◽  
Closed Forest ◽  
Patterns And Processes

Abstract:Riparian vegetation varies according to hydrogeomorphic processes operating across different scales over two didmensions: transversely (across-stream) and longitudinally (parallel to stream). We tested the hypothesis that vegetation patterns reveal the scale and direction of underlying processes. We correlated patterns of dominant woody vegetation with environmental variables at 28 sites located within four geomorphologically distinct regions along the length of the South Alligator River catchment of Kakadu National Park, northern Australia. Across the catchment there existed a strong transverse boundary between upland savanna vegetation and two zones of riparian vegetation: Melaleuca-spp.-dominated closed-forest vegetation along stream channels and mixed open-woodland vegetation adjacent to closed forest. We surmise that there is hierarchic constraint on smaller-scale catchment processes due to fire incursion into the riparian zone and access to water during the dry season. Within the closed-forest zone, vegetation did not vary transversely, but did longitudinally. Riparian woodlands also varied longitudinally, but in the upper reaches varied independently of stream variables. By contrast, in the lower reaches woodland was strongly correlated with stream variables. The observed pattern of weak transverse linkages in headwaters but strong linkages in lower reaches is analogous to models developed for in-stream patterns and processes, particularly the river continuum and flood-pulse concepts.

scholarly journals Ramet demography of Aechmea distichantha (Bromeliaceae) in two contrasting years in the understory and open areas of a South American xerophytic forest

Rodriguésia ◽  
2020 ◽  
Vol 71 ◽  
Author(s):  
Ignacio Martín Barberis ◽  
Graciela Klekailo ◽  
Juliana Albertengo ◽  
Juan Ignacio Cárcamo ◽  
José María Cárcamo ◽  
...  
Keyword(s):  
Low Temperatures ◽  
Temporal Variations ◽  
Spatial And Temporal Variations ◽  
Woody Vegetation ◽  
South American ◽  
Extreme Temperatures ◽  
Herbaceous Vegetation ◽  
Winter Temperatures ◽  
Schinopsis Balansae ◽  
Sun And Shade

Abstract The Schinopsis balansae forests of the Wet Chaco are characterized by convex areas with woody vegetation and plain areas with herbaceous vegetation. In the Wet Chaco, Aechmea distichantha is a terrestrial bromeliad that forms dense colonies in the understory and open areas of these forests. The aim of this study was to analyze the spatial and temporal variations in population dynamics of this bromeliad species. We monitored ramets growing in sun and shade conditions during two contrasting years. We analyzed the spatial and temporal variations in survival, flowering, and ramet production. Variations in survival, flowering, and ramet production were more marked between years than between habitats. During the year with wetter and milder temperature conditions, survival and ramet production were higher than during the drier year with more extreme temperatures. Survival of vegetative ramets was less variable than survival of young and reproductive ramets. In the colder year, lower winter temperatures reduced the populations in all stages, being more important in the open areas. Our results highlight the importance of low temperatures on A. distichantha demography at this xerophytic forest located at the southernmost distribution range of this bromeliad species.

scholarly journals Assessing vegetation structure and ANPP dynamics in a grassland-shrubland Chihuahuan ecotone using NDVI-rainfall relationships

2015 ◽  
Vol 12 (1) ◽  
pp. 51-92 ◽  
Author(s):  
M. Moreno-de las Heras ◽  
R. Diaz-Sierra ◽  
L. Turnbull ◽  
J. Wainwright
Keyword(s):  
Primary Production ◽  
Vegetation Change ◽  
Chihuahuan Desert ◽  
Plant Biomass ◽  
Net Primary Production ◽  
Larrea Tridentata ◽  
Shrub Encroachment ◽  
Grass Species ◽  
Natural Habitats ◽  
Herbaceous Vegetation

Abstract. Climate change and the widespread alteration of natural habitats are major drivers of vegetation change in drylands. A classic case of vegetation change is the shrub-encroachment process that has been taking place over the last 150 years in the Chihuahuan Desert, where large areas of grasslands dominated by perennial grass species (black grama, Bouteloua eriopoda, and blue grama, B. gracilis) have transitioned to shrublands dominated by woody species (creosotebush, Larrea tridentata, and mesquite, Prosopis glandulosa), accompanied by accelerated water and wind erosion. Multiple mechanisms drive the shrub-encroachment process, including exogenous triggering factors such as precipitation variations and land-use change, and endogenous amplifying mechanisms brought about by soil erosion-vegetation feedbacks. In this study, simulations of plant biomass dynamics with a simple modelling framework indicate that herbaceous (grasses and forbs) and shrub vegetation in drylands have different responses to antecedent precipitation due to functional differences in plant growth and water-use patterns, and therefore shrub encroachment may be reflected in the analysis of landscape-scale vegetation–rainfall relationships. We analyze the structure and dynamics of vegetation at an 18 km2 grassland-shrubland ecotone in the northern edge of the Chihuahuan Desert (McKenzie Flats, Sevilleta National Wildlife Refuge, NM, USA) by investigating the relationship between decade-scale (2000–2013) records of medium-resolution remote sensing of vegetation greenness (MODIS NDVI) and precipitation. Spatial evaluation of NDVI-rainfall relationship at the studied ecotone indicates that herbaceous vegetation shows quick growth pulses associated with short-term (previous 2 months) precipitation, while shrubs show a slow response to medium-term (previous 5 months) precipitation. We use these relationships to (a) classify landscape types as a function of the spatial distribution of dominant vegetation, and to (b) decompose the NDVI signal into partial primary production components for herbaceous vegetation and shrubs across the study site. We further apply remote-sensed annual net primary production (ANPP) estimations and landscape type classification to explore the influence of inter-annual variations in seasonal precipitation on the production of herbaceous and shrub vegetation. Our results suggest that changes in the amount and temporal pattern of precipitation comprising reductions in monsoonal summer rainfall and/or increases in winter precipitation may enhance the shrub-encroachment process in desert grasslands of the American Southwest.

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