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Publications - Crop Science

Brüser, K., et al., (2014), Discrimination and characterization of management systems in semi-arid rangelands of South Africa using RapidEye time series, International Journal of Remote Sensing, 35 (5): 1653-1673

In South African grasslands, rangeland management is strongly related to land tenure. Communal farms are reported to exhibit less desirable vegetation conditions for livestock than commercial farms. Time series of high spatial and temporal resolution imagery may be useful for improved evaluation of these rangelands as they provide information at a spatial scale similar to the typical scale of field assessments and may thus overcome the limited spatio-temporal representativeness of field measurements. A time series of 13 RapidEye images over one growing season (2010–2011) was used to explore spectral differences between and within two management systems (commercial vs. communal). Isomap ordination was applied to map continuous spectral dissimilarities of sample plots. Using regression with simultaneous autoregressive models (SAR), dissimilarities were subsequently related to ecological variables of plant and soil, including indicators for grazing effects. The largest differences were found between sample plots of communal and commercial farms. Vegetation attributes were significantly related to dissimilarities in reflectance, both from the growing season and the dormant period. However, these relationships did not suggest vegetation degradation on communal farms. They further suggest that a management-related pattern of grazing disturbance in the summer months led to spectral differences between farms but could have impaired the detailed characterization of spectral dissimilarities related to differences in vegetation composition.

Garcia, M., et al., (2014), Response of community-aggregated plant functional traits along grazing gradients: insights from African semi-arid grasslands, Applied Vegetation Science, 17 (3): 470-481

The grassland biome of South Africa is a major resource for livestock farming; yet the soils of these rangelands are stressed differently by various management systems. The aim of this study was to investigate how basic soil properties respond to different management systems. For this purpose we sampled rangeland management systems under communal (continuous grazing), commercial (rotational grazing) and land reform (mixture of grazing systems) farming. Within each of these systems a grazing gradient was identified with decreasing grazing pressure with increasing distance to the water points. Results showed that communal farms with continuous grazing were generally depleted in the respective nutrient stocks. The depletion increased with rising grazing pressure. Along that line there was a breakdown of macroaggregates with losses of the C and N stored therein. However, the commercial farms also exhibited a decline of macroaggregates and their associated C content nearby the water points. Aggregate fractionation is a sensitive indicator for detecting the beginning of soil degradation in this biome; yet, degradation was less pronounced under the rotational grazing of the commercial farms than under communal property right conditions. Hence, soil analyses confirm that fences and appropriate grazing periods are needed to manage these rangelands sustainably.

Changwony,  K.D., et al., (2014),  Biomass and quality changes of forages along land use and soil type gradients in the riparian zone of Lake Naivasha, Kenya, Ecological Indicators 49:169–177

 The recession of the water level of Lake Naivasha has incrementally exposed land surfaces creating a chronosequential transect representing durations of 1–30 years of exposure to grazing. This chronosequence provides a unique model to study the effects of land use duration on resource availability and resource base quality. Particularly, pasture quality changes in the riparian land of tropical fresh water lakes have so far not been studied. We assessed the effect of the duration of exposure to grazing on the biomass production, crude protein content and energy quality of pastures in a 4 × 4 latin square design (4 chronosequence positions × 4 soil types). Species composition was recorded and biomass was sampled at monthly intervals from February to August 2011. Soil moisture was recorded using frequency domain reflectometry sensors. Vegetation samples were analyzed for dry matter, nitrogen and metabolizable energy. Increased land use duration favored a shift in species dominance from Pennisetum clandestinum to Cynodon plectostachyus, which was associated with a reduction in dry matter yield and increased plant nitrogen content. All measured variables tended to be higher in soils formed on alluvial than in those formed on lacustrine deposits. Increased soil N and gravimetric moisture content stimulated biomass accumulation. The crude protein yield and metabolizable energy changed with phenological stages of the pasture and declined significantly towards maturity (seed setting of grasses). Continuous grazing and reduced soil moisture content, both during low rainfall and increased distance from the lake shore, affected the composition of pasture grasses as well as forage yield and quality. This may thus differentially affect the suitability of the riparian land as pasture ground and feed resource area for grazing animals.

Linstädter, A., et al., (2014), Are there consistent grazing indicators in drylands? Testing plant functional types of various complexity in South Africa’s grassland and savanna biomes, PLOS ONE, 9, e104672

Despite our growing knowledge on plants’ functional responses to grazing, there is no consensus if an optimum level of functional aggregation exists for detecting grazing effects in drylands. With a comparative approach we searched for plant functional types (PFTs) with a consistent response to grazing across two areas differing in climatic aridity, situated in South Africa’s grassland and savanna biomes. We aggregated herbaceous species into PFTs, using hierarchical combinations of traits (from single- to three-trait PFTs). Traits relate to life history, growth form and leaf width. We first confirmed that soil and grazing gradients were largely independent from each other, and then searched in each biome for PFTs with a sensitive response to grazing, avoiding confounding with soil conditions. We found no response consistency, but biome-specific optimum aggregation levels. Three-trait PFTs (e.g. broad-leaved perennial grasses) and two-trait PFTs (e.g. perennial grasses) performed best as indicators of grazing effects in the semi-arid grassland and in the arid savanna biome, respectively. Some PFTs increased with grazing pressure in the grassland, but decreased in the savanna. We applied biome-specific grazing indicators to evaluate if differences in grazing management related to land tenure (communal versus freehold) had effects on vegetation. Tenure effects were small, which we mainly attributed to large variability in grazing pressure across farms. We conclude that the striking lack of generalizable PFT responses to grazing is due to a convergence of aridity and grazing effects, and unlikely to be overcome by more refined classification approaches. Hence, PFTs with an opposite response to grazing in the two biomes rather have a unimodal response along a gradient of additive forces of aridity and grazing. The study advocates for hierarchical trait combinations to identify localized indicator sets for grazing effects. Its methodological approach may also be useful for identifying ecological indicators in other ecosystems

Dold, C. and Becker, M. (2015), Soil attributes and plant production changes in a  tropical littoral Wetland, Journal of Plant Nutrition and Soil Science. 178(4): 609-621

 Lake Naivasha is a freshwater lake in the East African Rift Valley. With continued lake level declines between 1980 and 2011, the newly exposed land areas were gradually taken for agricultural use. The resulting chronosequences allow for an analysis of the effects of land use duration on nutrient dynamics and agricultural production. Transects representing land use durations of 0–30 (cropland) and 15–30 years (pasture) were established on soils formed on alluvial deposits and lacustrine sediments. We assessed changes in topsoil nitrogen (N) stocks (t ha?1), ammonium mineralization potential (N-supplying capacity), and plant-available P with increasing durations of land use. An additional greenhouse experiment studied the responses of kikuyu grass (Cenchrus clandestinus) and maize (Zea mays) in potted topsoil collected from differnt land-use types and chronosequence positions. With increasing duration of land use we noted a significant decline (P < 5%) in soil N contents under both pasture and cropland uses, following a model of exponential decay. The N stocks decreased at 84?kg?ha?1 a?1 and a decay rate constant of 0.019 a?1 in pasture soil within 15 years, and at 75?kg?ha?1 a?1 with a decay rate-constant of 0.013 a?1 in cropland soil within 30 years. While the ammonium-N mineralization potential also decreased with land use duration, the trends were significant only in lacustrine pasture soils. Plant-available P did not show any trends that were related to the duration of land use. Kikuyu grass and maize accumulated less dry matter and N as the duration of use increased. This biomass accumulation was significantly related to soil N. A continued mineralization of soil organic matter has possibly contributed to the observed soil N depletion over time. The continuous agricultural use of the littoral wetland zone of Lake Naivasha is likely to entail declining production potentials for both pastures and food crops.

Tewes, A., et al., (2015), Using RapidEye and MODIS Data Fusion to Monitor Vegetation Dynamics in Semi-Arid Rangelands in South Africa, Remote Sensing, 7 (6): 6510-6534

Image time series of high temporal and spatial resolution capture land surface dynamics of heterogeneous landscapes. We applied the ESTARFM (Enhanced Spatial and Temporal Adaptive Reflectance Fusion Model) algorithm to multi-spectral images covering two semi-arid heterogeneous rangeland study sites located in South Africa. MODIS 250 m resolution and RapidEye 5 m resolution images were fused to produce synthetic RapidEye images, from June 2011 to July 2012. We evaluated the performance of the algorithm by comparing predicted surface reflectance values to real RapidEye images. Our results show that ESTARFM predictions are accurate, with a coefficient of determination for the red band 0.80 < R2 < 0.92, and for the near-infrared band 0.83 < R2 < 0.93, a mean relative bias between 6% and 12% for the red band and 4% to 9% in the near-infrared band. Heterogeneous vegetation at sub-MODIS resolution is captured adequately: A comparison of NDVI time series derived from RapidEye and ESTARFM data shows that the characteristic phenological dynamics of different vegetation types are reproduced well. We conclude that the ESTARFM algorithm allows us to produce synthetic remote sensing images at high spatial combined with high temporal resolution and so provides valuable information on vegetation dynamics in semi-arid, heterogeneous rangeland landscapes.

Changwony,  K.D., et al.,  (2015),  Feed intake and digestibility by sheep of natural vegetation in the riparian land of lake Naivasha, Kenya, Small Ruminant Research 123:75-82

Riparian lands are key dry season feed resource areas in Kenya. The feed quality of pasturesof Lake Naivasha riparian has previously not been studied. Chronosequence positions cor-responding to 30, 25, 20, and 15 years of land use and four transects perpendicular to thechronosequence representing observed soil differences were selected, intersections form-ing forage sampling plots. A 4 x 4 Graeco-Latin square design experiment using sheep wasconducted to determine voluntary intake, digestibility, and effects of land use duration andsoil type. The sheep were housed in metabolic cages and fed for ad libitum intake. Feedintake and faecal output were recorded and samples taken for analysis. Land use dura-tion did not affect dry matter (DM) and organic matter (OM) intake (DMI and OMI). Thedigestibility of DM and OM and of fibre fractions decreased with increased land use dura-tion. Increased duration of exposure from lake recession and land use leads to increasedforage fibre, reduced digestibility of DM and fibre fractions and reductions in Ca, K and P con-tent but did not affect DM intake. Lake Naivasha riparian pastures would provide sufficientenergy and can, with nitrogen supplementation, be a good feed resource for ruminants. Useof chronosequence approach provides an analytical framework to study effects of land useduration on feed quality

Becker, M., et al., (2016), Land-Use Changes and the Invasion Dynamics of Shrubs in Baringo, Journal of Eastern African Studies, 10(1): 111-129

In the semi-arid savannahs around Lake Baringo, Kenya, the recent spread of bush encroachment by the invasive alien species Prosopis juliflora and the native Dodonaea viscosa has changed human–environment interactions. This article suggests how the spread dynamics of Prosopis and Dodonaea have operated. It also describes the strategies Baringo's peoples have adopted in the face of this dramatic bush invasion, relates these dynamics to current invasion theory, and analyses possible implications for Baringo's social–ecological systems. It is suggested that recent increased climate variability has triggered changes in land management and livelihoods around Lake Baringo, paving the way for bush encroachment and species invasion. The extent and speed of these changes has exceeded the capacity of local communities to adapt their productive systems, destabilizing the socio-ecology of the dryland savannahs around Lake Baringo and placing them in imminent danger of collapse.

Parplies, A., et al. (2016), Phenomapping of rangelands in South Africa using time series of RapidEye data, International Journal of Applied Earth Observation and Geoinformation, 53: 90-102

Phenomapping is an approach which allows the derivation of spatial patterns of vegetation phenology and rangeland productivity based on time series of vegetation indices. In our study, we propose a new spatial mapping approach which combines phenometrics derived from high resolution (HR) satellite time series with spatial logistic regression modeling to discriminate land management systems in rangelands. From the RapidEye time series for selected rangelands in South Africa, we calculated bi-weekly noise reduced Normalized Difference Vegetation Index (NDVI) images. For the growing season of 2011–2012, we further derived principal phenology metrics such as start, end and length of growing season and related phenological variables such as amplitude, left derivative and small integral of the NDVI curve. We then mapped these phenometrics across two different tenure systems, communal and commercial, at the very detailed spatial resolution of 5 m. The result of a binary logistic regression (BLR) has shown that the amplitude and the left derivative of the NDVI curve were statistically significant. These indicators are useful to discriminate commercial from communal rangeland systems. We conclude that phenomapping combined with spatial modeling is a powerful tool that allows efficient aggregation of phenology and productivity metrics for spatially explicit analysis of the relationships of crop phenology with site conditions and management. This approach has particular potential for disaggregated and patchy environments such as in farming systems in semi-arid South Africa, where phenology varies considerably among and within years. Further, we see a strong perspective for phenomapping to support spatially explicit modelling of vegetation.

Oomen, R., et al., (2016a): Modelling rangeland productivity in response to degradation in a semi-arid climate, Ecological Modelling, 322: 54-70

Modelling rangeland is essential for capturing changes at the large temporal and spatial scales at which these systems respond to climate and institutional changes and increasing population pressure, but rangeland models applicable to data sparse regions are rarely available. We developed and evaluated a novel rangeland model aimed at simulating rangeland at different stages of degradation using limited parameterisation and measurements.

The developed model Linrange is a biophysical simulation model of the aboveground part of a mixed grass sward, combined with sub-models for evapotranspiration, soil water dynamics, and root development. Main processes of the biomass model are growth through a source/sink limited mechanism, reserve storage and remobilisation, basal area dynamics, winter dormancy. The grass sward is simulated based on average species characteristics of the dominating grass community.

We show that a model based on simplified biophysical processes and a single set of parameters for a mixed sward can satisfactorily simulate mixed-species rangeland vegetation. The model also could reproduce year-to-year phytomass dynamics, including for exceptionally wet and dry years. Without calibrating specifically for it, the model was able to reproduce observed water-use efficiency values, indicating a good representation of the relationship between the main limiting factor, water, and productivity. By recalibrating the model using only five parameters associated with degradation, the accuracy of simulated degraded rangeland states was close to that of undegraded rangeland. We therefore consider the Linrange model a good tool for research on rangeland dynamics and degradation resulting from management and climate. We also point to directions of further model improvement, particularly regarding the modelling of parameter changes with degradation states.

Linstädter, A., et al. (2016), Assessing the resilience of a real-world social-ecological system: lessons from a multidisciplinary evaluation of a South African pastoral system, Ecology and Society 21 (3):35

In the past decades, social-ecological systems (SESs) worldwide have undergone dramatic transformations with often detrimental consequences for livelihoods. Although resilience thinking offers promising conceptual frameworks to understand SES transformations, empirical resilience assessments of real-world SESs are still rare because SES complexity requires integrating knowledge, theories, and approaches from different disciplines. Taking up this challenge, we empirically assess the resilience of a South African pastoral SES to drought using various methods from natural and social sciences. In the ecological subsystem, we analyze rangelands’ ability to buffer drought effects on forage provision, using soil and vegetation indicators. In the social subsystem, we assess households’ and communities’ capacities to mitigate drought effects, applying agronomic and institutional indicators and benchmarking against practices and institutions in traditional pastoral SESs. Our results indicate that a decoupling of livelihoods from livestock-generated income was initiated by government interventions in the 1930s. In the post-apartheid phase, minimum-input strategies of herd management were adopted, leading to a recovery of rangeland vegetation due to unintentionally reduced stocking densities. Because current livelihood security is mainly based on external monetary resources (pensions, child grants, and disability grants), household resilience to drought is higher than in historical phases. Our study is one of the first to use a truly multidisciplinary resilience assessment. Conflicting results from partial assessments underline that measuring narrow indicator sets may impede a deeper understanding of SES transformations. The results also imply that the resilience of contemporary, open SESs cannot be explained by an inward-looking approach because essential connections and drivers at other scales have become relevant in the globalized world. Our study thus has helped to identify pitfalls in empirical resilience assessment and to improve the conceptualization of SES dynamics.

Alvarez, M.,et al. (2016), Recovery and germination of Prosopis juliflora seeds after ingestion by goats and cattle, Arid Land Research and Management 30(5):000-000 (in print)

Prosopis juliflora is a perennial shrub introduced in the 1980s to the Baringo District in central Kenya, where it developed into a strongly invasive species since the late 1990s. This period coincides with a shift from cattle to goats in the predominance of the ruminant herds in the area. We hypothesize that endozoochory by goats contributes to the invasive spread of Prosopis. Feeding trials and germination tests assessed the seed recovery in dung and the germination capacity of recovered seeds comparing pod ingestion by goats and cattle. Additionally we compared germination from intact pod sections containing seeds, seeds manually extracted from the pods and seeds scarified with concentrated sulphuric acid for 5 min. After eight days following ingestion only 7% of the seeds ingested by goats and 15% of those ingested by cattle were recovered. While no germination occurred in intact pod segments, germination dynamics were similar in seeds that had been manually extracted from pods with those recovered after intestinal passage. The velocity of germination based on the time required for a 25% of germination (t25) was fastest with acid scarification (1.4 days), followed by the intestinal passage through goats (7.1 days), and the treatments of manual removal and intestinal passage through cattle (11.8 and 15.1 days, respectively). We conclude that in the absence of a scarification of seeds surviving the intestinal passage, the endozoochorous dispersal appears to facilitate the invasive spread of Prosopis by delivering seeds from the pods, distributing them widely and possibly providing nutrients with the dung for establishment and initial growth.

Oomen, R.J., et al., (2016b). Effect of management on rangeland phytomass, cover and condition in two biomes in South Africa. African Journal of Range & Forage Science 33 (3): 185-198.

In rangelands, grazing management is a main driver of rangeland condition. Due to masking effects of seasonal climate fluctuations, little is known about (dis)similarity of management effects on rangeland condition and forage provision across major dryland biomes. Taking a macro-ecological perspective, we analysed if management effects differed  between  South  Africa’s  central  grassland  and  Kalahari  savanna  biomes.  We  recorded  proxies  of  forage provision (phytomass, vegetation cover and their ratio) over five seasons, annual rainfall to account for seasonal climate  fluctuations,  and  rangeland  condition  (through  relative  abundances  of  increaser  and  decreaser  species).mRegarding  forage  provision,  we  found  effects  of  management  for  the  savanna,  where,  irrespective  of  rainfall, rotational grazing management resulted in higher phytomass and phytomass–cover ratios than management with continuous grazing. In the grassland, however, this difference was only discernible for phytomass–cover ratio in two  years  with  above-average  antecedent  rainfall.  This  suggests  that  management  effects  are  biome-dependent and that modulating effects of annual rainfall are stronger in the grassland. In either biome, management effects on the dominance of increaser and decreaser species were negligible, i.e. rangeland condition did not differ across management  types  in  either  biome.  We  conclude  that  investigations  on  management  effects  should  account  for interactions with biome and rainfall.