Kelley Crews - Academia.edu (original) (raw)

Papers by Kelley Crews

Geomorphology, 2003

Spatially explicit digital technologies are integrated within a geographic information science (G... more Spatially explicit digital technologies are integrated within a geographic information science (GISc) context to map, model, and visualize selected direct and indirect geomorphic processes that influence the spatial organization of the alpine treeline ecotone (ATE) in Glacier National Park (GNP), MT. GISc is used to examine alpine treeline and its biotic and abiotic controls through the application of multi-resolution remote sensing systems, geospatial information and product derivatives, and simulations of treeline spatial organization. Three geomorphic features are examined: relict solifluction terraces, evidence of nonlinearity in the development of a catena, and the locations of isolated boulders. The significance of these features is in constraining subsequent geomorphic and biogeographic processes, thus leading to disequilibrium. Exploration of these features though GISc indicates that visualizations for characterizing the relations of geomorphic patterns and processes within a threedimensional context show promise for improved alpine slope models in the future by defining landscape attributes within a spatially and temporally explicit context.

Savanna ecosystems are geographically extensive and both ecologically and economically important;... more Savanna ecosystems are geographically extensive and both ecologically and economically important; they therefore require monitoring over large spatial extents. There are, in particular, large areas within southern Africa savanna ecosystems that lack consistent geospatial data on vegetation morphological properties, which is a prerequisite for biodiversity conservation and sustainable management of ecological resources. Given the challenges involved in distinguishing and mapping savanna vegetation assemblages using remote sensing, the objective of this study was to develop a vegetation morphology map for the largest protected area in Africa, the central Kalahari. Six vegetation morphology classes were developed and sample training/validation pixels were selected for each class by analyzing extensive in situ data on vegetation structural and functional properties, in combination with existing ancillary data and coarse scale land cover products. The classification feature set consisted of annual and intra annual matrices derived from 14 years of satellite-derived vegetation indices images, and final classification was achieved using an ensemble tree based classifier. All vegetation morphology classes were mapped with high accuracy and the overall classification accuracy was 91.9%. Besides filling the geospatial data gap for the central Kalahari area, this vegetation morphology map is expected to serve as a critical input to ecological studies focusing on habitat use by wildlife and the efficacy of game fencing, as well as contributing to sustainable ecosystem management in the central Kalahari.

International Journal of Remote Sensing, 2014

GeoJournal, 2020

Environmental perceptions are inherently based on an individual’s existing knowledge, experiences... more Environmental perceptions are inherently based on an individual’s existing knowledge, experiences, and future expectations. Methods for measuring environmental perception, therefore, must capture a range of experiences while also being flexible enough to integrate these experiences into a coherent unit for analysis. Many research topics require cross-cultural comparisons in order to corroborate findings; however, assessments of environmental perception are often place- and context-specific. We propose here post-survey Likert constructions (PSLCs), using semi-structured interviews to construct a five-point scale system from multiple household responses after the completion of interviews. This method is able to capture the natural variability in the population using the respondents’ own language and characterizations of phenomena. We applied this method to measure the perceived environmental variability of residents living in a dynamic flooding landscape in the Okavango Delta, Botswana. The PSLC method captures the differences in environmental perception in a location with different settlement and cultural histories, multiple language groups, and different environmental conditions. The method easily transfers to other environments and populations, allowing for potential cross-cultural comparisons of perceived environmental variability. This publication responds to calls for increased transparency in reporting the development, execution, advantages, and disadvantages of methods related to environmental change.

Landscape Ecology, 2019

Context Potential woody cover, the upper bound of woody plant cover in savanna ecosystems, repres... more Context Potential woody cover, the upper bound of woody plant cover in savanna ecosystems, represents the end-point of woody plant encroachment and is highly relevant to the dynamics of savanna ecosystems. Objectives This study aims to identify the appropriate spatial scale for potential woody cover observation in the savanna of central Texas, USA. Methods The upper bound of woody plant cover was modeled over the east–west precipitation gradient of Texas savanna, at four different spatial scales respectively (30 m, 100 m, 250 m, and 500 m). Results The estimated upper bound of woody plant cover demonstrates a three-segment pattern across the precipitation gradient at all the four observation scales. The pattern begins with a low stable level and ends at a high stable level, with a linear transitional level in between. The magnitude of the upper bound under given precipitation conditions decreases with spatial scale, but stabilizes by 250 m scale. Conclusions A spatial scale between 250 and 500 m is recommended for potential woody cover observation. Water availability plays a more important role in limiting woody plant cover at larger spatial scales in savanna ecosystems. In addition, the scale dependency of upper bound woody plant cover is more pronounced in the arid region.

International Journal of Applied Earth Observation and Geoinformation, 2019

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) Vegetation Continuous Fields (... more Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) Vegetation Continuous Fields (VCF) tree cover (MOD44B) is an annual product that depicts the fractional coverage of trees on the earth’s terrestrial surface at 250m resolution. It has to be noted that the training data of this product is limited to trees above 5 m in height. However, this product has been widely applied in savanna ecosystems where many woody plant species seldom reach 5 m tall. Though the short woody plants in savanna ecosystems could have been reflected in this product due to similar spectral implication as that of tall trees, its applicability has never been rigorously tested. Given so, this study aims to fill this void. It was tested in Texas savanna, which is featured by a wide precipitation gradient and an accompanying woody plant cover gradation. Firstly, we assessed the ability of the MODIS tree cover product in reflecting woody plant cover at pixel level. A per-pixel comparison was conducted between the tree cover values and reference measurement of woody plant cover based on 1-m resolution digital orthophotos. Secondly, we assessed the ability of this tree cover product in reflecting savanna landscape pattern, particularly the potential woody cover pattern over the precipitation gradient in Texas savanna. The MODIS tree cover product based potential woody cover pattern was assessed against the reference pattern of the study area. Thirdly, we assessed the year-to-year consistency of this annual product, under the assumption that tree cover changes very little within one-year period. Reduced major axis regression was conducted between the corresponding tree cover pixels of all successive years from 2000 to 2016. Research results suggest that the MODIS tree cover product has great potential in reflecting woody plant cover at pixel level by empirical calibration. Moreover, this tree cover product demonstrates the ability of revealing the increasing trend of potential woody cover with mean annual precipitation (MAP) across Texas savanna. Furthermore, the lack of temporal consistency is revealed for this annual product. It is worth noting that the potential woody cover pattern endowed by the precipitation gradient in Texas savanna is in a three-segment shape.

Land, 2019

Texas savanna experienced substantial woody plant encroachment during the past several decades, r... more Texas savanna experienced substantial woody plant encroachment during the past several decades, resulting in habitat fragmentation and species loss. A detailed map of woody plant abundance and distribution in this area is critically needed for management purpose. This study endeavors to map the fractional woody cover of Texas savanna at Landsat scale (30 m) in an affordable way. The top of atmosphere reflectance, thermal bands, and NDVI layer of Web-Enabled Landsat Data (WELD) of 2012 were used as predictors, together with mean annual precipitation. Classification and Regression Trees (CART) were calibrated against training data of a whole range of fractional woody cover, which were derived from 1-m resolution digital orthophotos of 2012. Validation indicates a reasonable pixel level accuracy of the result fractional woody cover map, with a R-squared value of 0.45. Moreover, the result map clearly depicts the distribution of woody plants across the study area, as reflected by the or...

International Journal of Applied Earth Observation and Geoinformation, 2019

Abstract Vegetation cover in savannas is characterized by high spatial heterogeneity driven by na... more Abstract Vegetation cover in savannas is characterized by high spatial heterogeneity driven by natural and anthropogenic drivers acting at multiple spatial and temporal scales. This research article identifies trends in vegetation cover and productivity in response to both land management and variable precipitation through a methodological approach that incorporates vegetation transect fieldwork with satellite imagery time series analysis. The phenological cycles of semi-arid savanna vegetation were analyzed over 27 years for both a protected area and a privately-owned property in order to capture multiple vegetation cover categories within the eastern Edwards Plateau ecoregion of central Texas. Line-intercept vegetation transects were established across these two sites in 2015, with ground cover and structural vegetation measurements collected along each transect. These measurements were classified based upon distinct vegetation cover categories: woodland, intermediate, open, and open disturbed. Time series analysis of vegetation productivity utilized 111 multi-season Landsat TM5 and Landsat 8 images that captured spring, fall, and winter seasonalities across the 27-year time period spanning from 1988 to 2015, with the Soil-Adjusted Vegetation Index (SAVI) calculated as an indicator of vegetative productivity. Harmonic regression was applied to this time series in order to determine the relationship between time of year with respect to growing season and productivity level across vegetation cover types. Fixed mean harmonic amplitudes for calendar year productivity were calculated across the entire time period as well as across three natural breaks-based sub-divisions of the 27-year period. Mean amplitudes per transect were then plotted against field-truthed tree-grass ratios in order to determine the relationship between tree and grass cover and mean harmonic SAVI amplitude. Harmonic regression identified inter- and intra-annual seasonal variability across cover types, with woodland sites exhibiting much lower variability in SAVI levels than open sites. These results provide insight into the phenological response of vegetation cover types typical of semi-arid savannas to spatially and temporally variable precipitation levels and land management schemes, allowing for better informed management and monitoring strategies for these environments.

Human Ecology, 2018

Variability in environmental phenomena such as fire, flooding, and weather-related events can hav... more Variability in environmental phenomena such as fire, flooding, and weather-related events can have significant impacts for social and environmental systems and their coupled interactions. Livelihoods systems reliant on the natural environment can be disrupted or eliminated, while associated governance regimes require negotiation to ensure equitable and sustainable management responses. These patterns can be particularly pronounced within areas prone to flooding, as these sites can experience variability in the location, timing, amount, and duration of flooding events. While research within the social and natural sciences has evaluated these dynamics within flooding regimes, the coupled interactions can be underemphasized even though they are integral in producing livelihood systems and possibilities for environmental management. This paper details research conducted from 2011 to 2016 in five villages located in different locations within the Okavango Delta of Botswana. We report the findings from qualitative interviewing and livelihood mapping activities that are integrated with remote sensing analysis to provide concrete empirical detail on the variability of flooding and resulting variations in perception and livelihood responses. The paper demonstrates that flooding dynamics vary at discrete locations and produce diverse perceptions that are tied to livelihood adjustments in place-specific ways. These patterns are also embedded in regional and global processes that have significant implications for household vulnerability within socio-ecological systems strongly impacted by local and distant climatic and hydrological drivers of change.

International Journal of Applied Earth Observation and Geoinformation, 2016

While woody plant encroachment has been observed worldwide in savannas and adversely affected the... more While woody plant encroachment has been observed worldwide in savannas and adversely affected the ecosystem structure and function, a thorough understanding of the nature of this phenomenon is urgently required for savanna management and restoration. Among others, potential woody cover (the maximum realizable woody cover that a given site can support), especially its variation over environment has huge implication on the encroachment management in particular, and on tree-grass interactions in general. This project was designed to explore the pattern of potential woody cover in Texas savanna, an ecosystem with a large rainfall gradient in west-east direction. Substantial random pixels were sampled across the study area from MODIS Vegetation Continuous Fields (VCF) tree cover layer (250m). Since potential woody cover is suggested to be limited by water availability, a nonlinear 99 th quantile regression was performed between the observed woody cover and mean annual precipitation (MAP) to model the pattern of potential woody cover. Research result suggests a segmented relationship between potential woody cover and MAP at MODIS scale. Potential biases as well as the practical and theoretical implications were discussed. Through this study, the hypothesis about the primary role of water availability in determining savanna woody cover was further confirmed in a relatively understudied US-located savanna.

Society & Natural Resources, 2017

ABSTRACT The Okavango Delta in Botswana hosts abundant wildlife and a human population with diver... more ABSTRACT The Okavango Delta in Botswana hosts abundant wildlife and a human population with diverse livelihoods. Representing a heterogeneous landscape nuanced by spatial and temporal variability, the region has recently seen an expansion of floodwaters with social impacts ranging from livelihood disruption to human displacement. This article reports on in-depth interviews conducted in 2012 regarding these transitions in Mababe, a community in the eastern Okavango Delta, to evaluate how dynamic environmental processes alter perceptions and livelihood responses. We focus on community members’ variable interactions with wildlife in spaces of human–wildlife overlap also experiencing change. While human–wildlife interactions can have negative effects, we find that perpetuating the common narrative of human–wildlife conflict overlooks how disruptions can usher in new relationships between people and animals. In order to move beyond the conflict narrative, we conclude that spatial and temporal context is essential to evaluate effects of dynamic, uneven, and sometimes unpredictable human–wildlife encounters.

International Journal of Wildland Fire, 2016

The relative importance of various drivers of fire regimes in savanna ecosystems can be location-... more The relative importance of various drivers of fire regimes in savanna ecosystems can be location-specific. We utilised satellite-derived time-series burned area (2001–13) to examine how spatiotemporal variations in burned area and fire frequency were determined by rainfall, vegetation morphology and land use in semiarid savanna. Mean precipitation of the rainy season (Nov–Apr) had a strong and positive relationship with burned area in the following dry season (variance explained 63%), with the relationship being strongest inside protected areas (variance explained 73%). Burned area and fire frequency were higher in vegetation types with higher herbaceous cover, indicating a causal link between herbaceous load and fire. Among land use, fire frequency was highest in protected areas and lowest in farms and ranches. Spatial models (generalised linear models with Poisson and negative binomial distribution) accounting for spatial autocorrelation showed that land-use classes and vegetation...

International Journal of Remote Sensing, 2016

ABSTRACT Fractional cover of photosynthetic vegetation (FPV), non-photosynthetic vegetation (FNPV... more ABSTRACT Fractional cover of photosynthetic vegetation (FPV), non-photosynthetic vegetation (FNPV), and bare soil (FBS) has been retrieved for Australian tropical savannah based on linear unmixing of the two-dimensional response envelope of the normalized difference vegetation index (NDVI) and short wave infrared ratio (SWIR)32 vegetation indices (VI) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance data. The approach assumes that cover fractions are made up of a simple mixture of green leaves, senescent leaves, and bare soil. In this study, we examine retrieval of fractional cover using this approach for a study area in southern Africa with a more complex vegetation structure. Region-specific end-members were defined using Hyperion images from different locations and times of the season. These end-members were applied to a 10-year time series of MODIS-derived NDVI and SWIR32 (from 2002 to 2011) to unmix FPV, FNPV, and FBS. Results of validation with classified high-resolution imagery indicated major bias in estimation of FNPV and FBS, with regression coefficients for predicted versus observed data substantially less than 1.0 and relatively large intercept values. Examination with Hyperion images of the inverse relationship between the MODIS-equivalent SWIR32 index and the Hyperion-derived cellulose absorption index (CAI) to which it nominally approximates revealed: (1) non-compliant positive regression coefficients for certain vegetation types; and (2) shifts in slope and intercept of compliant regression curves related to day of year and geographical location. The results suggest that the NDVI–SWIR32 response cannot be used to approximate the NDVI–CAI response in complex savannah systems like southern Africa that cannot be described as simple mixtures of green leaves, dry herbaceous material high in cellulose, and bare soil. Methods that use a complete set of multispectral channels at higher spatial resolution may be needed for accurate retrieval of fractional cover in Africa.

Remote Sensing of Environment, 2015

MODIS time-series Seasonal trend analysis Semi-arid savanna Vegetation morphology Kalahari Rainfa... more MODIS time-series Seasonal trend analysis Semi-arid savanna Vegetation morphology Kalahari Rainfall Woody plant encroachment Fire frequency Land use change Google Earth In African savanna, spatio-temporal variability in moisture availability, fire regime and land transformation related to exploitative land uses are the main drivers of changing vegetation greenness patterns. Deconstructing the role of these drivers at local scale is critical for managing the impact of projected climate and land use changes on savanna ecosystems. Focusing on an area encompassing Africa's largest terrestrial protected area, this study utilized time-series MODIS NDVI (2000-2014) and employed a robust trend analysis technique to detect significant temporal trend in key greenness metrics including overall greenness, peak and timing of the peak of annual greenness and examined how spatial variability in vegetation morphology and land use impacted the distribution of these greenness parameters. To access causation of change, we linked detected greenness trends to precipitation trends derived from Tropical Rainfall Measurement Mission (TRMM), annual burnt area extent and fire frequency and compared multi-temporal imagery in Google Earth. Results show that distribution of both overall and peak annual greenness was vegetation morphology specific as with decreasing woody cover the proportion of pixels showing negative overall and peak annual greenness trend increased consistently. Majority of the area with significant trend in the timing of the peak of annual greenness showed late greenup, implying changing phenological patterns. Trend analysis of TRMM derived mean annual precipitation showed no significant change in precipitation over the study period. At local scale, increase in overall and peak annual greenness was associated with woody plant encroachment due to increased moisture availability and reduced fire frequency due to construction of fire breaks in areas under farm/ranch land use. In protected areas, negative overall and peak annual greenness was driven by unmanaged high intensity mega fire events and outside due to anthropogenic land clearing for pastoral agriculture. These results further our understanding of the complex interaction of multiple ecological factors and resultant spatial variability in savanna inter-annual vegetation greenness patterns and are useful for implementing local scale management strategies.

The SAGE Handbook of Remote Sensing

... level models represent processes across hierarchical spatial scales, for example, the effects... more ... level models represent processes across hierarchical spatial scales, for example, the effects of commu ... ephemeral processes or entities are delin-eated; happily, at least, archival remote sensing does not do the same problems of accurate memory recall that social scientists face ...

Human Ecology, 2008

Pastoralists of the high Andes Mountains raise mixed herds of camelids and sheep. This study eval... more Pastoralists of the high Andes Mountains raise mixed herds of camelids and sheep. This study evaluates the land use of herdsmen who are confronted by both socioeconomic and climate changes in Huancavelica, central Peru. Land use/ land cover change (LULCC) was measured through satellite imagery, and pastoralists' capacity to adapt to socioenvironmental changes was evaluated through interviews and archival research. The most dynamic LULCCs between 1990 and 2000 were large increases in wetlands and a loss of permanent ice. We conclude that the people's responses to these changes will depend on availability of institutions to manage pastures, other household resources, and perceptions of these biophysical changes. Socioenvironmental change is not new in the study area, but current shifts will likely force this community to alter its rules of access to pastures, its economic rationales in regards to commodities produced, and the degree of dependence on seasonal wage labor. In this scenario, households with a greater amount of livestock will fare better in terms of assets and capital that will allow them to benefit from the increasing presence of a market economy in a landscape undergoing climate change.

Landscape Series, 2009

... Page 2. 206 KA Crews and A. Moffett 9.1 Introduction ... For a period in the scientific liter... more ... Page 2. 206 KA Crews and A. Moffett 9.1 Introduction ... For a period in the scientific literature, it appeared that, particularly in tropical forests, there was a consensus as to the presence of deforestation (whether complete or thinning) (eg, Skole and Tucker 1993; Turner et al. ...

Background/Question/Methods Understanding, monitoring and managing savanna ecosystems requires ch... more Background/Question/Methods Understanding, monitoring and managing savanna ecosystems requires characterizing both functional and structural properties of vegetation. From a functional perspective, in savannas, quantitative estimation of fractional cover of photosynthetic vegetation (fPV), non-photosynthetic vegetation (fNPV) and bare soil (fBS) is important as it relates to carbon dynamics and ecosystem function. On the other hand, vegetation morphology classes describe the structural properties of the ecosystem. Due to high functional diversity and structural heterogeneity in savannas, accurately characterizing both these properties using remote sensing is methodologically challenging. While mapping both fractional cover and vegetation morphology classes are important research themes within savanna remote sensing, very few studies have considered systematic investigation of their spatial association across different spatial resolutions. Focusing on the semi-arid savanna ecosystem ...

Land, 2015

Savanna ecosystems are geographically extensive and both ecologically and economically important;... more Savanna ecosystems are geographically extensive and both ecologically and economically important; they therefore require monitoring over large spatial extents. There are, in particular, large areas within southern Africa savanna ecosystems that lack consistent geospatial data on vegetation morphological properties, which is a prerequisite for biodiversity conservation and sustainable management of ecological resources. Given the challenges involved in distinguishing and mapping savanna vegetation assemblages using remote sensing, the objective of this study was to develop a vegetation morphology map for the largest protected area in Africa, the central Kalahari. Six vegetation morphology classes were developed and sample training/validation pixels were selected for each class by analyzing extensive in situ data on vegetation structural and functional properties, in combination with existing ancillary data and coarse scale land cover products. The classification feature set consisted of annual and intra annual matrices derived from 14 years of satellite-derived vegetation indices images, and final classification was achieved using an ensemble tree based classifier. All vegetation morphology classes were mapped with high accuracy and the overall classification accuracy was 91.9%. Besides filling the geospatial data gap for the central Kalahari area, this vegetation morphology map is expected to serve as a critical input to ecological studies focusing on habitat use by wildlife and the efficacy of game fencing, as well as contributing to sustainable ecosystem management in the central Kalahari.

Geomorphology, 2003

Spatially explicit digital technologies are integrated within a geographic information science (G... more Spatially explicit digital technologies are integrated within a geographic information science (GISc) context to map, model, and visualize selected direct and indirect geomorphic processes that influence the spatial organization of the alpine treeline ecotone (ATE) in Glacier National Park (GNP), MT. GISc is used to examine alpine treeline and its biotic and abiotic controls through the application of multi-resolution remote sensing systems, geospatial information and product derivatives, and simulations of treeline spatial organization. Three geomorphic features are examined: relict solifluction terraces, evidence of nonlinearity in the development of a catena, and the locations of isolated boulders. The significance of these features is in constraining subsequent geomorphic and biogeographic processes, thus leading to disequilibrium. Exploration of these features though GISc indicates that visualizations for characterizing the relations of geomorphic patterns and processes within a threedimensional context show promise for improved alpine slope models in the future by defining landscape attributes within a spatially and temporally explicit context.

Savanna ecosystems are geographically extensive and both ecologically and economically important;... more Savanna ecosystems are geographically extensive and both ecologically and economically important; they therefore require monitoring over large spatial extents. There are, in particular, large areas within southern Africa savanna ecosystems that lack consistent geospatial data on vegetation morphological properties, which is a prerequisite for biodiversity conservation and sustainable management of ecological resources. Given the challenges involved in distinguishing and mapping savanna vegetation assemblages using remote sensing, the objective of this study was to develop a vegetation morphology map for the largest protected area in Africa, the central Kalahari. Six vegetation morphology classes were developed and sample training/validation pixels were selected for each class by analyzing extensive in situ data on vegetation structural and functional properties, in combination with existing ancillary data and coarse scale land cover products. The classification feature set consisted of annual and intra annual matrices derived from 14 years of satellite-derived vegetation indices images, and final classification was achieved using an ensemble tree based classifier. All vegetation morphology classes were mapped with high accuracy and the overall classification accuracy was 91.9%. Besides filling the geospatial data gap for the central Kalahari area, this vegetation morphology map is expected to serve as a critical input to ecological studies focusing on habitat use by wildlife and the efficacy of game fencing, as well as contributing to sustainable ecosystem management in the central Kalahari.

International Journal of Remote Sensing, 2014

GeoJournal, 2020

Environmental perceptions are inherently based on an individual’s existing knowledge, experiences... more Environmental perceptions are inherently based on an individual’s existing knowledge, experiences, and future expectations. Methods for measuring environmental perception, therefore, must capture a range of experiences while also being flexible enough to integrate these experiences into a coherent unit for analysis. Many research topics require cross-cultural comparisons in order to corroborate findings; however, assessments of environmental perception are often place- and context-specific. We propose here post-survey Likert constructions (PSLCs), using semi-structured interviews to construct a five-point scale system from multiple household responses after the completion of interviews. This method is able to capture the natural variability in the population using the respondents’ own language and characterizations of phenomena. We applied this method to measure the perceived environmental variability of residents living in a dynamic flooding landscape in the Okavango Delta, Botswana. The PSLC method captures the differences in environmental perception in a location with different settlement and cultural histories, multiple language groups, and different environmental conditions. The method easily transfers to other environments and populations, allowing for potential cross-cultural comparisons of perceived environmental variability. This publication responds to calls for increased transparency in reporting the development, execution, advantages, and disadvantages of methods related to environmental change.

Landscape Ecology, 2019

Context Potential woody cover, the upper bound of woody plant cover in savanna ecosystems, repres... more Context Potential woody cover, the upper bound of woody plant cover in savanna ecosystems, represents the end-point of woody plant encroachment and is highly relevant to the dynamics of savanna ecosystems. Objectives This study aims to identify the appropriate spatial scale for potential woody cover observation in the savanna of central Texas, USA. Methods The upper bound of woody plant cover was modeled over the east–west precipitation gradient of Texas savanna, at four different spatial scales respectively (30 m, 100 m, 250 m, and 500 m). Results The estimated upper bound of woody plant cover demonstrates a three-segment pattern across the precipitation gradient at all the four observation scales. The pattern begins with a low stable level and ends at a high stable level, with a linear transitional level in between. The magnitude of the upper bound under given precipitation conditions decreases with spatial scale, but stabilizes by 250 m scale. Conclusions A spatial scale between 250 and 500 m is recommended for potential woody cover observation. Water availability plays a more important role in limiting woody plant cover at larger spatial scales in savanna ecosystems. In addition, the scale dependency of upper bound woody plant cover is more pronounced in the arid region.

International Journal of Applied Earth Observation and Geoinformation, 2019

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) Vegetation Continuous Fields (... more Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) Vegetation Continuous Fields (VCF) tree cover (MOD44B) is an annual product that depicts the fractional coverage of trees on the earth’s terrestrial surface at 250m resolution. It has to be noted that the training data of this product is limited to trees above 5 m in height. However, this product has been widely applied in savanna ecosystems where many woody plant species seldom reach 5 m tall. Though the short woody plants in savanna ecosystems could have been reflected in this product due to similar spectral implication as that of tall trees, its applicability has never been rigorously tested. Given so, this study aims to fill this void. It was tested in Texas savanna, which is featured by a wide precipitation gradient and an accompanying woody plant cover gradation. Firstly, we assessed the ability of the MODIS tree cover product in reflecting woody plant cover at pixel level. A per-pixel comparison was conducted between the tree cover values and reference measurement of woody plant cover based on 1-m resolution digital orthophotos. Secondly, we assessed the ability of this tree cover product in reflecting savanna landscape pattern, particularly the potential woody cover pattern over the precipitation gradient in Texas savanna. The MODIS tree cover product based potential woody cover pattern was assessed against the reference pattern of the study area. Thirdly, we assessed the year-to-year consistency of this annual product, under the assumption that tree cover changes very little within one-year period. Reduced major axis regression was conducted between the corresponding tree cover pixels of all successive years from 2000 to 2016. Research results suggest that the MODIS tree cover product has great potential in reflecting woody plant cover at pixel level by empirical calibration. Moreover, this tree cover product demonstrates the ability of revealing the increasing trend of potential woody cover with mean annual precipitation (MAP) across Texas savanna. Furthermore, the lack of temporal consistency is revealed for this annual product. It is worth noting that the potential woody cover pattern endowed by the precipitation gradient in Texas savanna is in a three-segment shape.

Land, 2019

Texas savanna experienced substantial woody plant encroachment during the past several decades, r... more Texas savanna experienced substantial woody plant encroachment during the past several decades, resulting in habitat fragmentation and species loss. A detailed map of woody plant abundance and distribution in this area is critically needed for management purpose. This study endeavors to map the fractional woody cover of Texas savanna at Landsat scale (30 m) in an affordable way. The top of atmosphere reflectance, thermal bands, and NDVI layer of Web-Enabled Landsat Data (WELD) of 2012 were used as predictors, together with mean annual precipitation. Classification and Regression Trees (CART) were calibrated against training data of a whole range of fractional woody cover, which were derived from 1-m resolution digital orthophotos of 2012. Validation indicates a reasonable pixel level accuracy of the result fractional woody cover map, with a R-squared value of 0.45. Moreover, the result map clearly depicts the distribution of woody plants across the study area, as reflected by the or...

International Journal of Applied Earth Observation and Geoinformation, 2019

Abstract Vegetation cover in savannas is characterized by high spatial heterogeneity driven by na... more Abstract Vegetation cover in savannas is characterized by high spatial heterogeneity driven by natural and anthropogenic drivers acting at multiple spatial and temporal scales. This research article identifies trends in vegetation cover and productivity in response to both land management and variable precipitation through a methodological approach that incorporates vegetation transect fieldwork with satellite imagery time series analysis. The phenological cycles of semi-arid savanna vegetation were analyzed over 27 years for both a protected area and a privately-owned property in order to capture multiple vegetation cover categories within the eastern Edwards Plateau ecoregion of central Texas. Line-intercept vegetation transects were established across these two sites in 2015, with ground cover and structural vegetation measurements collected along each transect. These measurements were classified based upon distinct vegetation cover categories: woodland, intermediate, open, and open disturbed. Time series analysis of vegetation productivity utilized 111 multi-season Landsat TM5 and Landsat 8 images that captured spring, fall, and winter seasonalities across the 27-year time period spanning from 1988 to 2015, with the Soil-Adjusted Vegetation Index (SAVI) calculated as an indicator of vegetative productivity. Harmonic regression was applied to this time series in order to determine the relationship between time of year with respect to growing season and productivity level across vegetation cover types. Fixed mean harmonic amplitudes for calendar year productivity were calculated across the entire time period as well as across three natural breaks-based sub-divisions of the 27-year period. Mean amplitudes per transect were then plotted against field-truthed tree-grass ratios in order to determine the relationship between tree and grass cover and mean harmonic SAVI amplitude. Harmonic regression identified inter- and intra-annual seasonal variability across cover types, with woodland sites exhibiting much lower variability in SAVI levels than open sites. These results provide insight into the phenological response of vegetation cover types typical of semi-arid savannas to spatially and temporally variable precipitation levels and land management schemes, allowing for better informed management and monitoring strategies for these environments.

Human Ecology, 2018

Variability in environmental phenomena such as fire, flooding, and weather-related events can hav... more Variability in environmental phenomena such as fire, flooding, and weather-related events can have significant impacts for social and environmental systems and their coupled interactions. Livelihoods systems reliant on the natural environment can be disrupted or eliminated, while associated governance regimes require negotiation to ensure equitable and sustainable management responses. These patterns can be particularly pronounced within areas prone to flooding, as these sites can experience variability in the location, timing, amount, and duration of flooding events. While research within the social and natural sciences has evaluated these dynamics within flooding regimes, the coupled interactions can be underemphasized even though they are integral in producing livelihood systems and possibilities for environmental management. This paper details research conducted from 2011 to 2016 in five villages located in different locations within the Okavango Delta of Botswana. We report the findings from qualitative interviewing and livelihood mapping activities that are integrated with remote sensing analysis to provide concrete empirical detail on the variability of flooding and resulting variations in perception and livelihood responses. The paper demonstrates that flooding dynamics vary at discrete locations and produce diverse perceptions that are tied to livelihood adjustments in place-specific ways. These patterns are also embedded in regional and global processes that have significant implications for household vulnerability within socio-ecological systems strongly impacted by local and distant climatic and hydrological drivers of change.

International Journal of Applied Earth Observation and Geoinformation, 2016

While woody plant encroachment has been observed worldwide in savannas and adversely affected the... more While woody plant encroachment has been observed worldwide in savannas and adversely affected the ecosystem structure and function, a thorough understanding of the nature of this phenomenon is urgently required for savanna management and restoration. Among others, potential woody cover (the maximum realizable woody cover that a given site can support), especially its variation over environment has huge implication on the encroachment management in particular, and on tree-grass interactions in general. This project was designed to explore the pattern of potential woody cover in Texas savanna, an ecosystem with a large rainfall gradient in west-east direction. Substantial random pixels were sampled across the study area from MODIS Vegetation Continuous Fields (VCF) tree cover layer (250m). Since potential woody cover is suggested to be limited by water availability, a nonlinear 99 th quantile regression was performed between the observed woody cover and mean annual precipitation (MAP) to model the pattern of potential woody cover. Research result suggests a segmented relationship between potential woody cover and MAP at MODIS scale. Potential biases as well as the practical and theoretical implications were discussed. Through this study, the hypothesis about the primary role of water availability in determining savanna woody cover was further confirmed in a relatively understudied US-located savanna.

Society & Natural Resources, 2017

ABSTRACT The Okavango Delta in Botswana hosts abundant wildlife and a human population with diver... more ABSTRACT The Okavango Delta in Botswana hosts abundant wildlife and a human population with diverse livelihoods. Representing a heterogeneous landscape nuanced by spatial and temporal variability, the region has recently seen an expansion of floodwaters with social impacts ranging from livelihood disruption to human displacement. This article reports on in-depth interviews conducted in 2012 regarding these transitions in Mababe, a community in the eastern Okavango Delta, to evaluate how dynamic environmental processes alter perceptions and livelihood responses. We focus on community members’ variable interactions with wildlife in spaces of human–wildlife overlap also experiencing change. While human–wildlife interactions can have negative effects, we find that perpetuating the common narrative of human–wildlife conflict overlooks how disruptions can usher in new relationships between people and animals. In order to move beyond the conflict narrative, we conclude that spatial and temporal context is essential to evaluate effects of dynamic, uneven, and sometimes unpredictable human–wildlife encounters.

International Journal of Wildland Fire, 2016

The relative importance of various drivers of fire regimes in savanna ecosystems can be location-... more The relative importance of various drivers of fire regimes in savanna ecosystems can be location-specific. We utilised satellite-derived time-series burned area (2001–13) to examine how spatiotemporal variations in burned area and fire frequency were determined by rainfall, vegetation morphology and land use in semiarid savanna. Mean precipitation of the rainy season (Nov–Apr) had a strong and positive relationship with burned area in the following dry season (variance explained 63%), with the relationship being strongest inside protected areas (variance explained 73%). Burned area and fire frequency were higher in vegetation types with higher herbaceous cover, indicating a causal link between herbaceous load and fire. Among land use, fire frequency was highest in protected areas and lowest in farms and ranches. Spatial models (generalised linear models with Poisson and negative binomial distribution) accounting for spatial autocorrelation showed that land-use classes and vegetation...

International Journal of Remote Sensing, 2016

ABSTRACT Fractional cover of photosynthetic vegetation (FPV), non-photosynthetic vegetation (FNPV... more ABSTRACT Fractional cover of photosynthetic vegetation (FPV), non-photosynthetic vegetation (FNPV), and bare soil (FBS) has been retrieved for Australian tropical savannah based on linear unmixing of the two-dimensional response envelope of the normalized difference vegetation index (NDVI) and short wave infrared ratio (SWIR)32 vegetation indices (VI) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance data. The approach assumes that cover fractions are made up of a simple mixture of green leaves, senescent leaves, and bare soil. In this study, we examine retrieval of fractional cover using this approach for a study area in southern Africa with a more complex vegetation structure. Region-specific end-members were defined using Hyperion images from different locations and times of the season. These end-members were applied to a 10-year time series of MODIS-derived NDVI and SWIR32 (from 2002 to 2011) to unmix FPV, FNPV, and FBS. Results of validation with classified high-resolution imagery indicated major bias in estimation of FNPV and FBS, with regression coefficients for predicted versus observed data substantially less than 1.0 and relatively large intercept values. Examination with Hyperion images of the inverse relationship between the MODIS-equivalent SWIR32 index and the Hyperion-derived cellulose absorption index (CAI) to which it nominally approximates revealed: (1) non-compliant positive regression coefficients for certain vegetation types; and (2) shifts in slope and intercept of compliant regression curves related to day of year and geographical location. The results suggest that the NDVI–SWIR32 response cannot be used to approximate the NDVI–CAI response in complex savannah systems like southern Africa that cannot be described as simple mixtures of green leaves, dry herbaceous material high in cellulose, and bare soil. Methods that use a complete set of multispectral channels at higher spatial resolution may be needed for accurate retrieval of fractional cover in Africa.

Remote Sensing of Environment, 2015

MODIS time-series Seasonal trend analysis Semi-arid savanna Vegetation morphology Kalahari Rainfa... more MODIS time-series Seasonal trend analysis Semi-arid savanna Vegetation morphology Kalahari Rainfall Woody plant encroachment Fire frequency Land use change Google Earth In African savanna, spatio-temporal variability in moisture availability, fire regime and land transformation related to exploitative land uses are the main drivers of changing vegetation greenness patterns. Deconstructing the role of these drivers at local scale is critical for managing the impact of projected climate and land use changes on savanna ecosystems. Focusing on an area encompassing Africa's largest terrestrial protected area, this study utilized time-series MODIS NDVI (2000-2014) and employed a robust trend analysis technique to detect significant temporal trend in key greenness metrics including overall greenness, peak and timing of the peak of annual greenness and examined how spatial variability in vegetation morphology and land use impacted the distribution of these greenness parameters. To access causation of change, we linked detected greenness trends to precipitation trends derived from Tropical Rainfall Measurement Mission (TRMM), annual burnt area extent and fire frequency and compared multi-temporal imagery in Google Earth. Results show that distribution of both overall and peak annual greenness was vegetation morphology specific as with decreasing woody cover the proportion of pixels showing negative overall and peak annual greenness trend increased consistently. Majority of the area with significant trend in the timing of the peak of annual greenness showed late greenup, implying changing phenological patterns. Trend analysis of TRMM derived mean annual precipitation showed no significant change in precipitation over the study period. At local scale, increase in overall and peak annual greenness was associated with woody plant encroachment due to increased moisture availability and reduced fire frequency due to construction of fire breaks in areas under farm/ranch land use. In protected areas, negative overall and peak annual greenness was driven by unmanaged high intensity mega fire events and outside due to anthropogenic land clearing for pastoral agriculture. These results further our understanding of the complex interaction of multiple ecological factors and resultant spatial variability in savanna inter-annual vegetation greenness patterns and are useful for implementing local scale management strategies.

The SAGE Handbook of Remote Sensing

... level models represent processes across hierarchical spatial scales, for example, the effects... more ... level models represent processes across hierarchical spatial scales, for example, the effects of commu ... ephemeral processes or entities are delin-eated; happily, at least, archival remote sensing does not do the same problems of accurate memory recall that social scientists face ...

Human Ecology, 2008

Pastoralists of the high Andes Mountains raise mixed herds of camelids and sheep. This study eval... more Pastoralists of the high Andes Mountains raise mixed herds of camelids and sheep. This study evaluates the land use of herdsmen who are confronted by both socioeconomic and climate changes in Huancavelica, central Peru. Land use/ land cover change (LULCC) was measured through satellite imagery, and pastoralists' capacity to adapt to socioenvironmental changes was evaluated through interviews and archival research. The most dynamic LULCCs between 1990 and 2000 were large increases in wetlands and a loss of permanent ice. We conclude that the people's responses to these changes will depend on availability of institutions to manage pastures, other household resources, and perceptions of these biophysical changes. Socioenvironmental change is not new in the study area, but current shifts will likely force this community to alter its rules of access to pastures, its economic rationales in regards to commodities produced, and the degree of dependence on seasonal wage labor. In this scenario, households with a greater amount of livestock will fare better in terms of assets and capital that will allow them to benefit from the increasing presence of a market economy in a landscape undergoing climate change.

Landscape Series, 2009

... Page 2. 206 KA Crews and A. Moffett 9.1 Introduction ... For a period in the scientific liter... more ... Page 2. 206 KA Crews and A. Moffett 9.1 Introduction ... For a period in the scientific literature, it appeared that, particularly in tropical forests, there was a consensus as to the presence of deforestation (whether complete or thinning) (eg, Skole and Tucker 1993; Turner et al. ...

Background/Question/Methods Understanding, monitoring and managing savanna ecosystems requires ch... more Background/Question/Methods Understanding, monitoring and managing savanna ecosystems requires characterizing both functional and structural properties of vegetation. From a functional perspective, in savannas, quantitative estimation of fractional cover of photosynthetic vegetation (fPV), non-photosynthetic vegetation (fNPV) and bare soil (fBS) is important as it relates to carbon dynamics and ecosystem function. On the other hand, vegetation morphology classes describe the structural properties of the ecosystem. Due to high functional diversity and structural heterogeneity in savannas, accurately characterizing both these properties using remote sensing is methodologically challenging. While mapping both fractional cover and vegetation morphology classes are important research themes within savanna remote sensing, very few studies have considered systematic investigation of their spatial association across different spatial resolutions. Focusing on the semi-arid savanna ecosystem ...

Land, 2015

Savanna ecosystems are geographically extensive and both ecologically and economically important;... more Savanna ecosystems are geographically extensive and both ecologically and economically important; they therefore require monitoring over large spatial extents. There are, in particular, large areas within southern Africa savanna ecosystems that lack consistent geospatial data on vegetation morphological properties, which is a prerequisite for biodiversity conservation and sustainable management of ecological resources. Given the challenges involved in distinguishing and mapping savanna vegetation assemblages using remote sensing, the objective of this study was to develop a vegetation morphology map for the largest protected area in Africa, the central Kalahari. Six vegetation morphology classes were developed and sample training/validation pixels were selected for each class by analyzing extensive in situ data on vegetation structural and functional properties, in combination with existing ancillary data and coarse scale land cover products. The classification feature set consisted of annual and intra annual matrices derived from 14 years of satellite-derived vegetation indices images, and final classification was achieved using an ensemble tree based classifier. All vegetation morphology classes were mapped with high accuracy and the overall classification accuracy was 91.9%. Besides filling the geospatial data gap for the central Kalahari area, this vegetation morphology map is expected to serve as a critical input to ecological studies focusing on habitat use by wildlife and the efficacy of game fencing, as well as contributing to sustainable ecosystem management in the central Kalahari.