Savanna System Research Articles

Leaf green-up in a semi-arid African savanna –separating tree and grass responses to environmental cues

Abstract Question: Can satellite time series be used to identify tree and grass green-up dates in a semi-arid savanna system, and are there predictable environmental cues for green-up for each life form? Location: Acacia nigrescens/Combretum apiculatum savanna, Kruger National Park, South Africa (25° S, 31° E). Methods: Remotely-sensed data from the MODIS sensor were used to provide a five year record of greenness (NDVI) between 2000 and 2005. The seasonal and inter-annual patterns of leaf display of trees and grasses were described, using additional ecological information to separate the greening signal of each life form from the satellite time series. Linking this data to daily meteorological and soil moisture data allowed the cues responsible for leaf flush in trees and grasses to be identified and a predictive model of savanna leaf-out was developed. This was tested on a 22-year NDVI dataset from the Advanced Very High Resolution Radiometer. A day length cue for tree green-up predicted 86% of the gree...

MATRIPHAGY IN THE NEOTROPICAL PSEUDOSCORPION PARATEMNOIDES NIDIFICATOR (BALZAN 1888) (ATEMNIDAE)

Abstract We studied the natural history and social behavior of Paratemnoides nidificator (Balzan 1888) in a tropical savanna system. Females were responsible for all nymphal care. We observed, for the first time in pseudoscorpions, the occurrence of matriphagy behavior by the offspring. During conditions of food deprivation, the mother went out of the nest and passively awaited the protonymphs' attack, not reacting to the capture nor to the nymphs feeding on her body. We suggest that this extreme form of parental care, matriphagy, can reduce cannibalism among protonymphs and facilitate the evolution of social behavior in pseudoscorpions.

The suitability of the degradation gradient method in arid Namibia

AbstractThe Degradation Gradient Method (DGM) is a sophisticated technique for the assessment of range condition. It applies multivariate analyses of herbaceous species data to detect subtle degrees of overgrazing. The suitability of this multivariate method was tested in the central Highland Savanna of Namibia by comparing its results against a univariate analysis of herbaceous data in a simple but robust Range‐Unit Model. Despite aridity and topographical heterogeneity, the DGM performed unexpectedly well under these conditions. The relative instability of this dry savanna system favoured the applicability of the DGM by promoting a clear grazing gradient. Using species density data only resulted in an incorrect outcome of the multivariate analysis. The sensitivity of the DGM could be improved by combining density and cover data.

Response of African savanna ants to long‐term fire regimes

Summary Despite the fact that fire is considered an important disturbance in savannas across the world and is used widely as a management tool in conservation areas, little is known about the effects of burning on their insect communities. This study made use of a 50‐year fire experiment to investigate the responses of ant assemblages to long‐term burning regimes. The effects of fire frequency, season and time since fire (fuel age) were tested on epigaeic ants across three savanna vegetation types (Mopane woodland, Acacia savanna and Terminalia woodland) in Kruger National Park, South Africa. There was no significant effect of burning on mean ant species richness and abundance between treatments, although there were significant differences in ant assemblage composition between burned (treatment) and unburned (control) plots. The effects of season, frequency of burn and plot age on assemblage structure were weak and often not significant. Epigaeic ant assemblages in this savanna system appeared to be highly resistant and resilient to burning. The response of ants to fire was linked to changes in habitat cover and structure: the effect of fire on vegetation and ants was less pronounced in lower rainfall areas, where differences in vegetation structure between burnt and unburnt plots were less pronounced than in higher rainfall areas. Synthesis and applications. The effect of fire on ant assemblages in the mid‐ to northern Kruger National Park depends on whether a patch burns or not, rather than the specifics of the burning treatment. Thus, conservation managers can focus concerns regarding the subtleties of fire regimes on other taxa or areas of particular concern.

Grazer movements: spatial and temporal responses to burning in a tall-grass African savanna

The consequences of burning on grazer distributions in the landscape were investigated in a savanna system in South Africa. Distribution and extent of fires can be highly variable within and between years, but how this influences patterns of grazing is not well understood. Three years of fire and grazer distribution data from Hluhluwe-Umfolozi Park, KwaZulu Natal were used to describe how animals respond to different burn situations. Animals were attracted to the burnt areas after fires and this resulted in a decrease in grazer densities in unburnt areas. This effect was more pronounced when a larger area of the park had burnt, and increased over time as area burnt increased. Thus, grazing pressure in an area will depend on the size, timing and distribution of fires in a larger spatial context. Fires also seem to promote a more uniform grazing environment: although grazer density overall is higher in areas that have burnt, the grazers are more dispersed in these areas, and grazing at any one point may be less intense than in the unburnt areas (which are grazed patchily). Therefore at the beginning of the wet season—a key time of year for grass growth—fires are acting to decrease grazing pressure at any one point in both burnt and unburnt areas. The presence of fire in grazing systems could affect grass community composition because fire alters the intensity and frequency of utilisation of the grass sward.

GRASS RESPONSE TO CLIPPING IN AN AFRICAN SAVANNA: TESTING THE GRAZING OPTIMIZATION HYPOTHESIS

It has been suggested that grazing could stimulate the aboveground dry mass and/or nitrogen yields available to grazers (i.e., the grazing optimization hypothesis, GOH), but the actual importance of this effect is still controversial. The GOH has only been tested for a few grasslands and one savanna ecosystem, but not for the moistest and warmest grasslands and savannas of the world. The objectives of this study were to test the GOH in the humid savanna of Lamto (West Africa) by analyzing the growth of grasses in response to a field trial with three levels of clipping × two levels of fertilization. We quantified the effects of clipping and fertilization on the dry matter and nitrogen yields to grazers (i.e., mass or nitrogen amount in clipped‐off tissues during the experiment) and on the remaining yield (i.e., mass or nitrogen amount in residual phytomass at the end of the experiment) over a three‐month period. Total phytomass yield, the sum of yield to grazers, and the remaining yield was maintained under moderate clipping frequency and fertilization as compared to control conditions. Both clipping frequencies decreased the remaining phytomass yield as compared to control plots. Clipping frequency significantly increased nitrogen concentrations in the total yield, in the remaining yield, and in the yield to grazers. Total nitrogen yield and nitrogen yield to grazers were 65% and 91% higher on the plots experiencing moderate clipping frequency with fertilization as compared to control plots. The study shows that grazers in this humid savanna system could potentially modify ecosystem processes in such a way as to partly alleviate nutritional deficiencies, but only in the presence of increased nitrogen availability.

Natural mortality amoung four common ungulate species on Letaba Ranch, Limpopo Province, South Africa

Five years of mortality data of impala Aepyceros melampus, blue wildebeest Connochaetes taurinus, buffalo Syncerus caffer and kudu Tragelaphus strepsiceros have been used to describe the minimum mortality profiles of the respective species in an open savanna system with the full compliment of predators. Predation is the principle cause of natural mortalities on the study site, Letaba Ranch, which is adjacent to the Kruger National Park. The principle cause of impala mortality are leopards Panthera pardus and wild dogs Lycaon pictus. Lion Panthera leo were major contributors to the mortality of wildebeest and buffalo. Anthrax Bacillus anthracis was the main cause of kudu mortality.

An improved cellular automaton model for simulating fire in a spatially heterogeneous Savanna system

Developments in and around game reserves and ranches in South Africa have led to controlled burning becoming a necessary and regular activity. The management objectives of these fires are well-defined, and thus predicting the duration and extent of a burn is vitally important. Testing scenarios via computer simulation is desirable since this removes the potential risks associated with fire, whilst at the same time ensuring that management policies are attained. There are various approaches to developing a spatial simulation fire model. In this article we present a cellular automaton (CA) model that is capable of predicting fire spread in spatially heterogeneous Savanna systems. The physical basis of Rothermel's fire spread model (1972) was modified to a spatial context and used to improve the CA model introduced by Karafyllidis and Thanailakis (1997). The proposed fire model was verified using data for three human-induced fires in the Mkuze Game Reserve, South Africa, and was found to satisfactorily predict spatial fire behaviour.

Grazing Optimization and Nutrient Cycling: Potential Impact of Large Herbivores in a Savanna System

Grazing Optimization and Nutrient Cycling: Potential Impact of Large Herbivores in a Savanna System

A phytogeographic analysis of the woody elements of New World savannas

An analysis was made of the floristic composition of 45 savanna sites located throughout the southern neoarctic and neotropics. A total of 533 woody species were recorded from published and reliable unpublished floristic lists; of these, 234 species (44%) were from 10 Brazilian cerrado sites, with 187 (80%) of them restricted to the cerrado biome. The cerrados were clearly shown to be the most diverse New World savanna system with a high degree of endemism. The data were analysed using two multivariate techniques: TWINSPAN and DCA. Four phytogeographic zones were identified: Central Brazil and Bolivia extending to Southern Amazonia; north of Amazonia extending across the isthmus of Central America and including the Caribbean; Belize, Guatemala and Southern Mexico; and north of the Mexican Plateau. The analyses revealed gradients of floristic variation associated with latitude and longitude, and showed the great heterogeneity of savanna vegetation.

Detecting Process from Snapshot Pattern: Lessons from Tree Spacing in the Southern Kalahari

The spatial distribution of plants is often thought to be an indicator of underlying biotic and abiotic processes. However, there are relatively few examples of spatial patterns being analysed to detect an underlying ecological process. Using the spacing patterns being analysed to detect an underlying ecological process. Using the spacing of savanna trees in the southern Kalahari as an example, we applied methods of computer simulation modelling and point pattern analysis in an evaluation of their potential for identifying relevant pattern generating processes from snapshot pattern. We compared real tree patterns from the southern Kalahari, derived from aerial photographs, with patterns produced from computer simulation experiments in an investigation of the following questions: does the present pattern of tree distributions allow us to characterize (1) the relative importance of the major driving forces (e.g., competition for moisture, grass fire, herbivory), (2) the spatial dimensions and structures of the underlying processes, and (3) the actual dynamic status of the ecological system (a phase of decline, increase or constancy with respect to tree abundance)? The simulation experiments are based on a well established, spatially explicit, grid-based model that simulates the vegetation dynamics of the major life forms under a realistic rainfall scenario of the southern Kalahari and under the impact of grass fires, herbivory and the formation of localized clumps with increased tree seed availability. For a realistic range of parameters the simulation model produces long-term coexistence of trees and grasses with tree densities that correspond with long-term coexistence of trees and grasses with tree densities that correspond with densities observed in the field. Both real tree distributions derived from acrial photographs and tree pattern produced by the model are characterized by a tendency towards even spacing at small scales, clumping at intermediate scales and randomness or clumping at large scales. However, increasing the spatio-temporal correlation in the formation of seed patches in the model caused an increase in the tendency towards clumping in the tree distribution whereas an increase in seed patch numbers led to a decrease in clumping. Within single simulation runs the tree pattern could change in response to the variable rainfall sequences and the corresponding differences in grass fire frequency: periods of slightly increasing tree numbers caused by higher precipitation were characterized by an increase in tree clumping whereas periods of slightly decreasing tree numbers showed a tendency towards random or even tree spacing. Simulating the transition of an open savanna to a savanna woodland showed that the tree pattern in the transitional phase can be diagnostic of the underlying process: If the transition was caused by improved moisture conditions the transitional phase was characterized by increased clumping in the tree pattern. In contrast, a transition caused by an increase in the number of localized tree seed patches led to a characteristic even spacing of trees. Even though the simulated savanna clearly showed non-equilibrium dynamics, simulation results indicate that the tree population in the simulated area of the southern Kalahari is in a state of long-term tree-grass coexistence with the persisting structure of an open savanna system.

Remains of woody plants from Saouga, a medieval west African village

Charcoal, fruits and seeds of woody plants have been studied from a settlement mound in the Sahel of Burkina Faso. The archaeobotanical results provide information on economy and environmental conditions at the periphery of the medieval kingdom of Songhai at around 1000 A.D. Millet (Penniselum americanum) was the basic crop, cultivated in fields in which also grew useful trees (park savannas). Besides millet, fruits of the park savanna trees and other wild woody plants were an important part of the diet. Stratigraphical changes in the charcoal diagram indicate that millet production was intensified and the park savanna system established in the course of mound formation. The charcoal results show that the vegetation around 1000 A.D. was more diverse than today, containing many Sahelo-Sudanian elements which cannot be found in the area any more. This indicates slightly higher precipitation than today but also less severe human impact.

Transition rule complexity in grid-based automata models

Grid-based automata models have been widely applied in simulating ecological process and spatial patterns at all spatial scales. In this paper, we present methods for calculating the effects of number of states, size of the neighborhood, means of tallying neighborhood states, and choice of deterministic or stochastic rules on the complexity and tractability of spatial automata models. We use as examples Conway's Game of Life and models for successional dynamics in a mesquite savanna landscape in south Texas. The number of possible neighborhood state configurations largely determines the complexity of automata models. The number of different configurations in Life, a two-state, deterministic, voting-rule model with an eight-cell Moore neighborhood is 18. A similar model for the seven-state savanna system would have 21,021 different neighborhood configurations. For stochastic models, the number of possible state transitions is the number of neighborhood configurations times the number of possible cell states. A stochastic, unique neighbor model for the savanna system with a Moore neighborhood and seven possible states would have 282,475,249 possible neighborhood-based state transitions. Stochastic models with an eight-cell Moore neighborhood are probably most appropriate for ecological applications. The best options for minimizing the complexity of ecological models are using voting rather than unique neighbor transition rules, reducing the number of possible states, and implementing ecologically-based heuristics to simplify the transition rule table.

The southern limit of the Mediterranean vegetation in the Sahara during the Holocene

Holocene records from the southern Sahara in Niger allow a reconstruction of the vegetation history and inform us about the former extension of the Mediterranean. Both pollen and charcoal analyses evidenced the direct contact of Sudanian and Saharan savannas during the middle Holocene at about 19°N, whereas at 20°N the transition from the Saharan savanna to the desert was found. In southwestern Libya (26°N) a combination of a Saharan desert vegetation and a semi‐desert Artemisia shrub on the plateaus demonstrated the contact with Mediterranean influenced formations. Regular ash and charcoal layers in middle‐Holocene sediments of the northern Niger prove an early interference of man with the vegetation development. One has to imagine that, in combination with the cattle‐keeping and the later metal production, man could have changed the former northern Sudanian vegetation into the present Sahelian savanna system from the middle Holocene on.

Ecosystem structure and growth of the African savanna

Ecosystems are highly structured, interlocking sets of organismic and environmental interactions. Particular ecosystems reappear in the geological record, structured similarly regardless of their evolutionary history. Ulanowicz has hypothesized that energy throughput and compartmentalization are an important part of the evolutionary growth and development of ecosystems, and that redundancy within compartments can shield an ecosystem from perturbation. The compartments studied in this paper are mammalian guilds and redundancy has been interpreted as guild depth, defined as the number of genera within a particular guild. The fossil history and paleocology of the African savanna ecosystem, from potential precursor systems in the Early Miocene, through the protosavanna, the flowering of the savanna, and the severe depletion of this system about 1.5 Ma, show that redundancy and compartmentalization in this ecosystem did increase with time, and that redundancy did, at 1.5 Ma, protect the system's survival. A similarly redundant savanna system in North America destructure and restructured to prairie at about 5 Ma, apparently due to even more extreme climatic perturbations.

Changes in bulk precipitation reactivity throughout the vegetation/soil continuum in a Trachpogon savanna (Venezuela)

Abstract The reactivity of bulk precipitation moving into the soil of a bush island Trachypogon savanna throughout a forest grove and the herbaceous layer was analyzed during four consecutive years. In the forest grove, there was a significant decrease of annual hydrogen (H+) content (meq/ha) of 44–75% as rainfall leached through the canopy, whereas the bulk throughfall pH in the grass layer was similar to the bulk precipitation pH. As throughfall passed throughout the soil profile of the grove, the H+ content decreased 87–70%. Results evidenced a buffering capacity of the system when bulk precipitation was acid during the last measured year. The savanna system retained 92–93% of H+ input from the bulk precipitation indicating that a redistribution of H+ ocurred instead of a net input or net loss. Canopy neutralization of bulk precipitation appears to take place in the grove canopy mainly by organic and bicarbonate salts.