L. Stroosnijder

Effects of fire and herbivory on the stability of savanna ecosystems

Savanna ecosystems are characterized by the co-occurrence of trees and grass- es. In this paper, we argue that the balance between trees and grasses is, to a large extent, determined by the indirect interactive effects of herbivory and fire. These effects are based on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in the level of grazing leads to reduced fuel load, which makes fire less intense and, thus, less damaging to trees and, consequently, results in an increase in woody vegetation. The system then switches from a state with trees and grasses to a state with solely trees. Similarly, browsers may enhance the effect of fire on trees because they reduce woody biomass, thus indirectly stimulating grass growth. This consequent increase in fuel load results in more intense fire and increased decline of biomass. The system then switches from a state with solely trees to a state with trees and grasses. We maintain that the interaction between fire and herbivory provides a mechanistic explanation for observed discontinuous changes in woody and grass biomass. This is an alternative for the soil degradation mechanism, in which there is a positive feedback between the amount of grass biomass and the amount of water that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous chang- es, such as bush encroachment, on sandy soils. Such changes, however, are frequently ob- served. Therefore, the interactive effects of fire and herbivory provide a more plausible explanation for the occurrence of discontinuous changes in savanna ecosystems.

Spatial Heterogeneity and Irreversible Vegetation Change in Semiarid Grazing Systems

Recent theoretical studies have shown that spatial redistribution of surface water may explain the occurrence of patterns of alternating vegetated and degraded patches in semiarid grasslands. These results implied, however, that spatial redistribution processes cannot explain the collapse of production on coarser scales observed in these systems. We present a spatially explicit vegetation model to investigate possible mechanisms explaining irreversible vegetation collapse on coarse spatial scales. The model results indicate that the dynamics of vegetation on coarse scales are determined by the interaction of two spatial feedback processes. Loss of plant cover in a certain area results in increased availability of water in remaining vegetated patches through run‐on of surface water, promoting within‐patch plant production. Hence, spatial redistribution of surface water creates negative feedback between reduced plant cover and increased plant growth in remaining vegetation. Reduced plant cover, however, results in focusing of herbivore grazing in the remaining vegetation. Hence, redistribution of herbivores creates positive feedback between reduced plant cover and increased losses due to grazing in remaining vegetated patches, leading to collapse of the entire vegetation. This may explain irreversible vegetation shifts in semiarid grasslands on coarse spatial scales.

Effects of agroecological land use succession on soil properties in Chemoga watershed, Blue Nile basin, Ethiopia

This study appraises the effects of land use on soil properties in a typical watershed in the northwestern highland of Ethiopia. Soil samples were collected from major land use types in the watershed: natural forests, cultivated lands, grazing lands and Eucalyptus plantations. The natural forests served as a control against which to assess changes in soil properties resulting from the establishment of the other land use types. Samples were taken at two depths (0–15 and 15–30 cm) in the upstream and downstream areas of the watershed and analyzed for a range of soil properties. The soils in the cultivated fields, grazing lands and Eucalyptus plantations showed significantly higher sand content, but lower Ca2+ and Mg2+ contents and cation exchange capacity (CEC) compared to soils under natural forests. Eucalyptus soils had a statistically significant higher bulk density (BD) than soils under the other three land use types. The forest and Eucalyptus soils also differed significantly from each other in their soil organic matter (SOM) and total N contents. A significant difference in available P among soils of the four land use types was caused by the difference between cultivated and Eucalyptus soils. In contrast, the distribution of soil silt fraction Na+, K+ and pH values did not differ among the four land use types. Significant differences in many of the soil properties were also observed between soils in the two sampled villages. The study underscores the need for policies and strategies for sustainable land use that will attune objectives of economic development to environmental management at the regional and local levels.

Sahelian rangeland development: a catastrophe?

This paper sets out that the dynamics of the Sahelian rangeland vegetation can be interpreted as a cusp catastrophe and that this interpretation offers a promising basis for the description and analysis of this ecosystem. Firstly, an existing scheme of the dynamics of Sahelian herbaceous vegetation is translated into the state-and-transition formulation. Secondly, the application of the cusp catastrophe is explored by studying the behaviour of the Sahelian rangeland ecosystem under changing effective rainfall and grazing intensity, using the transitions from the state-and-transition formulation as vectors along the cusp manifold. This conceptual cusp catastrophe model subsequently results in the identification of hypotheses and the detection of 5 catastrophic properties of this ecosystem (bimodality, inaccessibility, sudden jumps, divergence and hysteresis) that have important management implications. The continuous and the discontinuous processes occurring in the Sahelian rangeland ecosystem can both be captured in a unified conceptual model by applying the cusp catastrophe theory. Testing the hypotheses generated by the conceptual model and searching for additional catastrophic properties, such as divergence of linear response and critical slowing down, is a useful direction for future research.

A simple model to predict soil moisture: Bridging Event and Continuous Hydrological (BEACH) modelling

This paper introduces a simple two-layer soil water balance model developed to Bridge Event And Continuous Hydrological (BEACH) modelling. BEACH is a spatially distributed daily basis hydrological model formulated to predict the initial condition of soil moisture for event-based soil erosion and rainfall-runoff models. The latter models usually require the spatially distributed values of antecedent soil moisture content and other input parameters at the onset of an event. BEACH uses daily meteorological records, soil physical properties, basic crop characteristics and topographical data. The basic processes incorporated in the model are precipitation, infiltration, transpiration, evaporation, lateral flow, vertical flow and plant growth. The principal advantage of this model lies in its ability to provide timely information on the spatially distributed soil moisture content over a given area without the need for repeated field visits. The application of this model to the CATSOP experimental catchment showed that it has the capability to estimate soil moisture content with acceptable accuracy. The root mean squared error of the predicted soil moisture content for 6 monitored locations within the catchment ranged from 0.011 to 0.065cm^3cm^-^3. The predicted daily discharge at the outlet of the study area agreed well with the observed data. The coefficient of determination and Nash-Sutcliffe efficiency of the predicted discharge were 0.824 and 0.786, respectively. BEACH has been developed within freely available GIS and programming language, PCRaster. It is a useful teaching tool for learning about distributed water balance modelling and land use scenario analysis.

Biomass production of Eucalyptus boundary plantations and their effect on crop productivity on Ethiopian highland Vertisols

In recent years, Eucalyptus globulus planted along field boundaries has come to dominate the central highland landscape of Ethiopia. Although evidence is scanty, there is a perception that this practice adversely affects crop productivity. An on-farm trial was conducted on Pellic Vertisol at Ginchi to determine the production potential of eucalypt boundaries and their effect on the productivity of adjacent crops of tef (Eragrostis tef) and wheat (Triticum sp.). The experiment comprised three stand ages, four field aspects and six distances from the tree-crop interface, using a split-split plot design with three replicates. Wood production rates ranged between 168 kg ha−1 y−1 (four years old) and 2901 kg ha−1 y−1 (twelve years). Thus eucalypt boundaries planted on a hectare of land would satisfy 50 to 75% of the annual biomass energy requirement of a rural household of five persons. Significant depression of tef and wheat yields occurred over the first 12m from the tree line: the reduction was 20 to 73% for tef and 20 to 51% for wheat, equivalent to yield losses of 4.4 to 26% and 4.5 to 10% per hectare respectively. Nevertheless, in financial terms, the tree component adequately compensated for crop yield reduction and even generated additional income. Therefore, eucalypt boundaries have great potential to satisfy the rising demand for wood, without requiring a major change in land use on the highland Vertisols. The greater availability of wood will reduce the demand for dung and crop residues for fuel, and thus may contribute to improved soil management on croplands while relieving the increasing pressure on indigenous forest and woodlands.

Relationship between vegetation growth rates at the onset of the wet season and soil type in the Sahel of Burkina Faso: implications for resource utilisation at large scales

In the Sahel, poor soil quality and rainfall levels have a great influence on pasture production and hence on secondary output. In areas where rainfall is the limiting factor for primary production, recovery of primary and secondary production after the dry season depends on soil type. On sandy soils a large fraction of rainfall infiltrates and becomes available for plant growth, stimulating fast herbage growth, while on clayey and loamy soils low infiltration rates generate runoff, leading to slower herbage growth rates. The very different moisture retention characteristic of sands and clays is another possible cause for the observed differences in growth rates. In this paper we investigate the herbage growth rate from the onset of the rainy season. We hypothesise that, in areas where rainfall is the limiting factor for primary production, the vegetation growth rate on clayey soils is lower than that on sandy soils. We will test this hypothesis using long-term rainfall, soil types and satellite derived normalised difference vegetation index data. This research shows that the growth rates on sandy soil are significantly greater than that on clayey soils during the early part of the rainy season. We also show that these differences can be detected at large scales using satellite imagery. We also conclude that, at this scale, movement strategies of pastoralists would be intrinsically linked to not only rainfall patterns and distribution, but also to the underlying soil types in the region as this affects the quality and quantity of fodder available.

Long-term effects of mineral and organic fertilization on soil organic matter fractions and sorghum yield under Sudano-Sahelian conditions

Knowledge of changes in soil organic matter (SOM) fractions resulting from agricultural practice is important for decision-making at farm level because of the contrasting effects of different SOM fractions on soils. A long-term trial sited under Sudano-Sahelian conditions was used to assess the effect of organic and inorganic fertilization on SOM fractions and sorghum performance. Sorghum straw and kraal manure were applied annually at 10 t ha(-1), with and without urea at 60 kg N ha(-1). The other treatments included fallowing, a control (no fertilization), and inorganic fertilization only (urea, 60 kg N ha(-1)). Fallowing gave significantly larger soil organic carbon and nitrogen (N) levels than any other treatment. Total soil SOM and N concentrations increased in the following order: urea only 0.053 mm (particulate organic matter, POM). The POM concentrations in the control, straw and urea-only treatments were about one-half of the POM concentrations in the fallow treatment. POM concentrations increased in the following order: urea only <control <straw with or without urea <manure with or without urea <fallow. The fraction of SOM <0.053 mm (fine organic matter, FOM) was greater than POM in all plots except in fallow and manure+urea plots. Total N concentration followed the same trend as SOM, but cultivation led to a decline in both POM-N and FOM-N. Crop yield was greatest in the manure plots and lowest in the straw, control and urea-only plots. Results indicate that under Sudano-Sahelian conditions, SOM, POM and FOM fractions and crop performance were better maintained using organic materials with a low C/N ratio (manure) than with organic material with a high C/N ratio (straw). Urea improved the effect of straw on crop yield and SOM concentration.

Soil nutrient and sediment loss as affected by erosion barriers and nutrient source in semi-arid Burkina Faso

In semi-arid Sahel, soil erosion by water is one major factor accounting for negative nutrient balances in agricultural systems. A field experiment was conducted on a Ferric Lixisol in Burkina Faso to assess the effects of soil and water conservation barriers (stone rows or grass strips of Andropogon gayanus), the sole use of organic (compost) or mineral (urea) source of nitrogen (N), and the combined use of barriers and compost or urea nitrogen on soil nutrient and sediment erosion. The run-off barriers were put along contours. Organic carbon (OC) and nutrients losses through run-off and sediments were monitored during 2001 and 2002. Carbon and nutrient concentrations of generated sediments were very high, of 14–29 g kg−1 OC, 1.0–3.7 g kg−1 N and 0.32–0.71 g kg−1 total P. concentration in run-off water reached 0.023 g L−1 in stone rows treatments compared to only 0.002 g L−1 with treatments without barriers. Annual losses of organic C, N, P, and K were high and greatly dependant on soil loss magnitude. Compared to the control plots, soil losses were reduced on average by 84% during the 2 years with stone rows treatments, while by 71% with grass strips treatments. Compost application reduced soil loss by 75% in plots without barriers and by 89% in plots with stone rows as compared to control plots. With urea application, soil loss was on average 13% lower than in plots with compost. Integrated water and nutrient management can effectively alleviate soil, carbon, and nutrients losses by water erosion, therefore may contribute to intensifying crop production in Sahelian smallholder farming.

Economic benefits of combining soil and water conservation measures with nutrient management in semiarid Burkina Faso

Nutrient limitation is the main cause of per capita decline in crop production in the Sahel, where water shortage also limits an efficient use of available nutrients. Combining soil and water conservation measures with locally available nutrient inputs may optimize crop production and economic benefit in cereal-based farming systems. A study conducted in 2001 and 2002 at Saria, Burkina Faso (annual rainfall 800 mm, PET of 2000 mm yr−1) assessed the combined effects of two types of semi-permeable barriers (stone rows and grass strips of Andropogon gayanus Kunth cv. Bisquamulatus (Hochst.) Hack.) and the application of compost or urea on sorghum performance and economic benefits. The field experiment was carried out on a Ferric Lixisol, 1.5% slope and comprised 9 treatments in which the barriers were put along contours and combined with compost-N or urea-N. Installation of stone rows or grass strips without addition of nutrient inputs was not cost effective, although it induced sorghum yield increase (12–58%) particularly under poor rainfall conditions. Combining compost with stone rows or grass strips significantly increased sorghum yield that induced positive interaction effects (mean added effects of 185 kg ha−1 for stone rows combined with compost-N and 300 kg ha−1 for grass strips combined with compost-N). Economic benefits were substantial (109 480 to 138 180 FCFA ha−1) when compost-N was added to both stone rows and grass strips, whereas limited economic benefits were observed with the application of urea-N (1120 to 22 120 FCFA ha−1). This may provide farmers with capital to invest in soil management and may also contribute to poverty alleviation in sub-Saharan Africa.