Fire Return Period Research Articles

A laissez-faire management approach in a grassland landscape results in a fine-scale, spatio-temporally heterogeneous fire pattern

What are the spatio-temporal characteristics of a fire pattern that is allowed to develop over time across a species- rich, predominantly grassy landscape? More than 1 300 fires were documented over 13 years in the 48 000 ha Songimvelo Game Reserve in the Barberton Mountainland, South Africa. Most fires were set by pastoralists and were uncontrolled. Lightning fires and prescribed burns made up <2% and <8% respectively of the total area burnt. Median and mean burn areas in the 31 700 ha game-fenced section were 35 and 187 ha respectively with a fire return period of 2.9 years. In the less-grazed 14 000 ha section that receives a higher annual rainfall, median and mean fire sizes were 124 and 501 ha and the return period was 1.5 years. These fire return periods are in line with generally accepted management prescriptions for grasslands. The annual number of fires, individual patches and different fire histories occurring in a moving window across the landscape were consistently higher, compared with those resulting from a small-scale or large-scale fixed fire regime. This laissez-faire approach thus resulted in a fine-scale, dynamic fire pattern with high landscape indices that could be conducive to maintaining biodiversity.

Spatiotemporal distribution dynamics of elephants in response to density, rainfall, rivers and fire in Kruger National Park, South Africa

AbstractAimAfrican elephants are ecosystem engineers. As such, their long‐term patterns of distribution and abundance (i.e., space‐use intensity) will influence ecosystem structure and function. We elucidate these patterns for bull versus herd groups, by analysing the spatiotemporal dynamics of an increasing elephant population in relation to key ecological drivers: rainfall, distance to major rivers and time since last fire. Significant changes to the long‐term patterns of elephant density and group‐type probabilities are identified and explained.LocationKruger National Park, South Africa.MethodsUsing almost three decades of census records (1985–2012), we applied Multiple Point Process Models to assess the influence of rainfall, rivers and fire in shaping elephant space‐use. Significant changes to the long‐term patterns of elephant density and group type were also identified using kernel density estimates and the spatially varying probability of encountering either bull or herd group.ResultsBull and herd groups are no longer clearly segregated as available empty space becomes more limited. Bull and herd groups have dichotomous resource selection functions, in that bulls concentrate in areas receiving lower rainfall but more frequent fires while herds concentrate in higher rainfall areas experiencing less frequent fires. Both bull and herd groups concentrate closer to major rivers, emphasizing rivers as important spatial drivers. Overall, densities increased most significantly closer to rivers and in areas experiencing fewer fires. Fire was also a strong agent of group‐type change, as the probability of finding bulls, contrary to herds, significantly increased as fire return periods shortened.Main conclusionsElephant distribution and abundance patterns have homogenized in response to increased space limitations, with group‐specific, fire‐driven distribution patterns emerging overtime. Results herein should be used to help manage elephant space‐use through the establishment of possible refuge areas and the development of more empirical research into elephant impacts in future.

Assessing the Spatial Drivers of Land Use and Land Cover Change in the Protected and Communal Areas of the Zambezi Region, Namibia

Understanding the patterns and drivers of land use and land cover (LULC) changes is fundamental for rational and specific planning for sustainable land management. Using remote sensing techniques, geographic information systems (GIS) and statistical modeling via multinomial logistic regression, we sought to identify spatial variables that determine LULC change and their extent over time in the protected and communal areas of the Zambezi Region, Namibia. Multi-temporal satellite imagery of the Landsat series was used to map changes over a period of twenty-six years, divided into three stages (1984–1991, 1991–2000 and 2000–2010). Post classification change detection methodologies were used to determine conversions between LULC classes. Additionally, socio-economic characteristics of the area were used to identify drivers of changes. Four spatial drivers of LULC change that we identified included the distance to the nearest road, distance to settlements, population density and fire return periods. Population density, distance to settlements and fire return period were significantly associated with conversion from crop/grass land to crop/grass land and forest land to crop/grass, forest land to bare land and forest land to forest land in the protected area. In communal areas, distance to the road was found to significantly influence conversion from crop/grass land to crop/grass land. The study concluded that the influence of these drivers is attributable to distinct political and agro-demographical differences during the study period. Policy makers and planners need to take these drivers into consideration together with their subsidiaries to respond and make sound decisions regarding undesirable changes in LULC.

Wildfire Occurrence in Siberia and Seasonal Variations in Heat and Moisture Supply

Subregional occurrence of fires in Siberian forests and seasonal variations in heat and moisture supply are analyzed. Instrumental data on wildfires registered through satellite monitoring data for the period of 1996–2016 are used. The dynamics of the weather fire danger index (PV-1) and intraseasonal anomalies of the Selyaninov’s hydrothermal coefficient (HTC) defining fire occurrence variation are revealed using meteorological data series for the Siberian subregions. The statistical regularities of the dynamics of the weather fire danger index are summarized for subregions. The separated scenarios are formalized by model functions. The projections of the probability of the scenarios’ implementation, the fire return period, and the respective relative burned area are determined.

Management impacts on fire occurrence: A comparison of fire regimes of African and South American tropical savannas in different protected areas

Humans can alter fire dynamics in grassland systems by changing fire frequency, fire seasonality and fuel conditions. These changes have effects on vegetation structure and recovery, species composition, and ecosystem function. Understanding how human management can affect fire regimes is vital to detect potential changes in the resilience of plant communities, and to predict vegetation responses to human interventions. We evaluated the fire regimes of two recently protected areas in Madagascar (Ibity and Itremo NPA) and one in Brazil (Serra do Cipó NP) before and after livestock exclusion and fire suppression policies. We compare the pre- and post-management fire history in these areas and analyze differences in terms of total annual burned area, density of ignitions, burn scar size distribution, fire return period and seasonal fire distribution. More than 90% of total park areas were burned at least once during the studied period, for all parks. We observed a significant reduction in the number of ignitions for Ibity NPA and Serra do Cipó NP after livestock exclusion and active fire suppression, but no significant change in total burned area for each protected area. We also observed a seasonal shift in burning, with fires happening later in the fire season (October–November) after management intervention. However, the protected areas in Madagascar had shorter fire return intervals (3.23 and 1.82 years) than those in Brazil (7.91 years). Our results demonstrate that fire exclusion is unattainable, and probably unwarranted in tropical grassland conservation areas, but show how human intervention in fire and vegetation patterns can alter various aspects of the fire regimes. This information can help with formulating realistic and effective fire management policies in these valuable conservation areas.

Mid- and Late Holocene vegetation dynamics and fire history in the boreal forest of European Russia: A case study from Meshchera Lowlands

Climate, fire, and human activities strongly affected the development of vegetation communities during the Holocene, yet the relative importance of these individual factors remains unclear in many areas. This paper presents new multi-proxy records of environmental change for the Meshchera Lowlands (the central part of the East European Plain) during the Holocene. Changes in regional vegetation during the Mid- and Late Holocene were influenced by climate, fire regime and human impact, as indicated by pollen, plant macrofossil, charcoal and testate amoebae analysis from several peat cores, along with reconstruction of tree cover from pollen assemblages. Since 8500calyrBP, the vegetation history represented a series of consecutive phases of birch, birch-pine and pine-broadleaf forests, with introduction of spruce after 2500calyrBP. Maximal abundance of broadleaf tree species was detected from 4700 to 2000calyrBP. Vegetation dynamics were strongly influenced by human activity since 1400calyrBP. High fire frequency was recorded for the periods 8500–4500calyrBP and 3500–2000calyrBP, when the fire return interval varied from 40 to 80years. Since 2000calyrBP, the fire return period exceeded 500years suggesting a significant decline in fire frequency during the last two millennia.

Biomass burning fuel consumption dynamics in the tropics and subtropics assessed from satellite

Abstract. Landscape fires occur on a large scale in (sub)tropical savannas and grasslands, affecting ecosystem dynamics, regional air quality and concentrations of atmospheric trace gasses. Fuel consumption per unit of area burned is an important but poorly constrained parameter in fire emission modelling. We combined satellite-derived burned area with fire radiative power (FRP) data to derive fuel consumption estimates for land cover types with low tree cover in South America, Sub-Saharan Africa, and Australia. We developed a new approach to estimate fuel consumption, based on FRP data from the polar-orbiting Moderate Resolution Imaging Spectroradiometer (MODIS) and the geostationary Spinning Enhanced Visible and Infrared Imager (SEVIRI) in combination with MODIS burned-area estimates. The fuel consumption estimates based on the geostationary and polar-orbiting instruments showed good agreement in terms of spatial patterns. We used field measurements of fuel consumption to constrain our results, but the large variation in fuel consumption in both space and time complicated this comparison and absolute fuel consumption estimates remained more uncertain. Spatial patterns in fuel consumption could be partly explained by vegetation productivity and fire return periods. In South America, most fires occurred in savannas with relatively long fire return periods, resulting in comparatively high fuel consumption as opposed to the more frequently burning savannas in Sub-Saharan Africa. Strikingly, we found the infrequently burning interior of Australia to have higher fuel consumption than the more productive but frequently burning savannas in northern Australia. Vegetation type also played an important role in explaining the distribution of fuel consumption, by affecting both fuel build-up rates and fire return periods. Hummock grasslands, which were responsible for a large share of Australian biomass burning, showed larger fuel build-up rates than equally productive grasslands in Africa, although this effect might have been partially driven by the presence of grazers in Africa or differences in landscape management. Finally, land management in the form of deforestation and agriculture also considerably affected fuel consumption regionally. We conclude that combining FRP and burned-area estimates, calibrated against field measurements, is a promising approach in deriving quantitative estimates of fuel consumption. Satellite-derived fuel consumption estimates may both challenge our current understanding of spatiotemporal fuel consumption dynamics and serve as reference datasets to improve biogeochemical modelling approaches. Future field studies especially designed to validate satellite-based products, or airborne remote sensing, may further improve confidence in the absolute fuel consumption estimates which are quickly becoming the weakest link in fire emission estimates.

Fire policy optimization to maximize suitable habitat for locally rare species under different climatic conditions: A case study of antelopes in the Kruger National Park

Fire is a key ecosystem driver in savannahs and it can have large impacts on species distribution and density. A re-examination of fire management in Kruger National Park is currently under review with the objective to maintain natural ecosystem dynamics and favour tourists' ability to observe animals. We used data on location, intensity and frequency of fires and census data on three species considered as rare and of conservation concern in the park, tsessebe, roan and sable antelope to estimate the relationship between fire occurrence and species occurrence and density. We also investigated the impacts of different environmental predictors on antelope populations. The model predictors that most affected the density and presence of antelopes were mean fire return period, the type of geological substrate and the presence of water-points. We then used our models to evaluate different fire management scenarios and make recommendations for an optimal fire management strategy for the conservation of these rare antelopes. We also tested our scenarios under different precipitation conditions, in order to investigate the likely response of species to climate change. Roan antelope is the most sensitive species to climatic variations, while sable seems to be the most resilient. The approach described here can also be used to improve the conservation of locally rare species in other regions and habitats.

Modelling the shrub encroachment in a grassland with a Cellular Automata Model

Abstract. Arid and semi-arid grasslands of southwestern North America have changed dramatically over the last 150 years as a result of shrub encroachment, i.e. the increase in density, cover and biomass of indigenous shrubby plants in grasslands. Numerous studies have documented the expansion of shrublands in the southwestern American grasslands; in particular shrub encroachment has occurred strongly in part of the northern Chihuahuan desert since 1860. This encroachment has been simulated using an ecohydrological Cellular Automata model, CATGraSS. It is a spatially distributed model driven by spatially explicit irradiance and runs on a fine-resolution gridded domain. Plant competition is modelled by keeping track of mortality and establishment of plants; both are calculated probabilistically based on soil moisture stress. For this study CATGraSS has been improved with a stochastic fire module and a grazing function. The model has been implemented in a small area in Sevilleta National Wildlife Refuge (SNWR), characterized by two vegetation types (grass savanna and creosote bush shrub), considering as encroachment causes the fire return period increase, the grazing increase, the seed dispersal caused by animals, the role of wind direction and plant type competition. The model is able to reproduce the encroachment that has occurred in SNWR, simulating an increase of the shrub from 2% in 1860 to the current shrub percentage, 42%, and highlighting among the most influential factors the reduced fire frequency and the increased grazing intensity.

The ongoing development of a pragmatic and adaptive fire management policy in a large African savanna protected area

This paper describes recent changes to the fire management policy of the 1.9 million ha Kruger National Park in South Africa. It provides a real-life example of adaptive learning in an environment where understanding is incomplete, but where management nonetheless has to proceed. The previous policy called for the application of fire to meet burnt area targets that were set for administrative subdivisions, and that were assessed at the scale of the entire park. This was problematic because the park is large and heterogeneous, and because sound ecological motivations that could link burning prescriptions to ecological objectives were missing. The new policy divides the park into five fire management zones on the basis of differences in mean annual rainfall, historic fire return periods, and geology. In addition, it proposes fire management actions designed to achieve specified ecological objectives in each zone, and includes fire-regime related thresholds and associated ecological outcomes against which to assess the effectiveness of management. The new policy is an improvement over previous iterations, but several challenges remain. Most important among these would be to continually improve the understanding of the effects of fire, and to develop frameworks for assessing the impacts of fire together with other ecosystem drivers that interact strongly with fire to influence the attainment of ecological objectives.

Determinants of spatial variation in fire return period in a semiarid African savanna

Fire pattern is predicted to vary across an African savanna in accordance with spatial variation in rainfall through its effects on fuel production, vegetation type (on account of differences in fuel load and in flammability), and distribution of herbivores (because of their effects on fuel load). These predictions were examined for the 23 651-ha Mkuzi Game Reserve, KwaZulu-Natal, based on a 37-year data set. Fire return period varied from no occurrence to a fire every 1.76 years. Approximately 75% of the reserve experienced a fire approximately every 5 years, 25% every 4.1–2.2 years and less than 1% every 2 years on average. Fire return period decreased in relation to an increase in mean annual rainfall. For terrestrial vegetation types, median fire return periods decreased with increasing herbaceous biomass, from forest that did not burn to grasslands that burnt every 2.64 years. Fire was absent from some permanent wetlands but seasonal wetlands burnt every 5.29 years. Grazer biomass above 0.5 animal units ha–1 had a limiting influence on the maximum fire frequency of fire-prone vegetation types. The primary determinant of long-term spatial fire patterns is thus fuel load as determined by mean rainfall, vegetation type, and the effects of grazing herbivores.

Changing the fire management regime in the renosterveld and lowland fynbos of the Bontebok National Park

This paper evaluates the history of fire management in the Bontebok National Park (3435 ha) over a period of almost four decades. A GIS database was compiled of all fires between 1972 and 2009 and the fire regime was analysed in terms of the frequency, season, size and cause of fires. Since the early 1970s, short interval burning was implemented to promote grazing for bontebok, but from 2004 the fire interval was lengthened to favour plant species diversity, an increasingly urgent conservation priority for the park. In total, 43 fires were recorded, ranging in size from 9 to 1007 ha, collectively spanning 14 013 ha. The majority of fires were large (100–500 ha), with fires of >100 ha accounting for 96% of the area burnt. The overall mean fire return period (FRP) for the park was 7.2 years, which is short judged by fynbos standards. FRPs under the old and new management regimes were 6.7 and 10.9 years respectively. Under the old regime, FRPs in renosterveld and fynbos were 5.8 and 8.0 years respectively. Large parts of the park repeatedly experienced fires at immature vegetation ages resulting in the elimination of slow-maturing seed-regenerating plant species such as Protea repens. Post-fire age distribution was highly skewed towards young vegetation, with 75% of fire-prone vegetation burning at post-fire ages of ≤7 years, and <10% of fire-prone vegetation surviving beyond 10 years of age. Prescribed and accidental fires respectively accounted for 70% and 30% of the total area burnt. Prescribed burning was mostly done in March–April, and only 8% of the total area burnt, burnt outside of the ecologically acceptable fire season. This study identified areas which have been subject to ecologically appropriate and inappropriate fire return intervals, providing a basis for informed future management and research.

Woody cover in African savannas: the role of resources, fire and herbivory

ABSTRACTAim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas.Location Savanna grasslands of East, West and Southern Africa.Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique.Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall‐determined levels. Woody cover showed a complex, non‐linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N‐deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold.Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N‐enrichment and elevated levels of CO2 are likely to have opposing, and potentially interacting, influences on the tree–grass balance in savannas.

The effect of long-term fire treatments on invertebrates: results from experimental plots at Cathedral Peak, South Africa

This study examined the impacts of different burning regimes (fire frequency and season) and fire history (time since last burn) on invertebrate morphospecies richness and abundance. The study was carried out in the Brotherton experimental plots at Cathedral Peak in the KwaZulu-Natal Drakensberg. Pitfall traps targeting epigaeic invertebrates were set for one week, and soil cores were collected for extraction of hypogaeic invertebrates in 21 plots. A total of 2585 individual specimens from 219 morphospecies were identified. Mean morphospecies richness was consistently highest in blocks burnt in autumn, and higher for biennial burns or two years since last burn for epigaeic, winged and wingless invertebrates, but this was not the case for soil invertebrates which showed little real response to fire. Fire season and frequency significantly influenced epigaeic and winged invertebrate species richness and abundance, with autumn burning and a fire return period of two years favouring richness and abundance. However, the small size of the plots and their proximity and a runaway fire in 2000 could have influenced results and could also result in relatively homogenous communities.

Classification of veldfire risk in South Africa for the administration of the legislation regarding fire management

The risk associated with vegetation fires (veldfires) in South Africa is substantial, and veldfires cause severe losses to life, property and the environment in most areas of the country. However, this risk has two parts: first, that arising from wildfires (i.e. unwanted veldfires) that cause damage to assets, and, second, the risk arising from ecologically inappropriate fire regimes in environments where fire plays an ecological role. South Africa has adopted the National Veld and Forest Fire Act, No. 101 of 1998 as a major instrument for improved management of veldfires.This paper provides a classification of veldfire risk in South Africa, based in the first instance on ecological information, as a contribution to a strategy for the implementation of the new legislation. We assigned each of the major vegetation types in South Africa to one of 13 different fire-ecology types. A fire-ecology type comprises one or more vegeatation types which have a similar frequency and nature of veldfires. South Africa is divided into six metropolitan and 231 local municipalities and each of these was assigned a fire- ecology type. Wildfire risk was assessed as the combination of the likelihood and consequences of veldfires in each fire-ecology type. The likelihood of veldfire was related to the average fire return period for that vegetation type, determined from the literature. Likelihood ratings were (fires 1 in 2 years), likely (fires 1 in 5 years), possible (fires 1 in 10 years), unlikely (fires 1 in 20 years) and rare (fires 1 in 100 years). The consequences of veldfires in each fire-ecology type were estimated from a range of sources including fire reports and newspaper accounts of veldfires. The consequences of veldfires were grouped into the following categories: catastrophic (regular loss of life and significant economic consequences); major (extensive injuries and serious economic consequences); moderate (localised damage and economic losses); minor (minor financial losses and damage); and insignificant (damage inconsequential). Risk categories were defined as low, medium, high or extreme, depending on the combinations of likelihood and consequences. For example, if veldfires almost certain and catastrophic, risk was extreme; and if they were rare, but had moderate consequences, the risk was low. Grassland and savanna areas, which cover almost 40% of the eastern half of the country, were at highest risk. Mediterranean shrublands, which are well known for their veldfires were rated only as high, because most of the area is mountainous, infertile, and unoccupied, or sparsely occupied.

Effects of Data Uncertainties on Estimated Soil Organic Carbon in the Sudan

Estimating the rate of soil carbon sequestration in degraded semiarid agroecosystems is of great interest due to the beneficial effects on soil properties, soil fertility, and the potential atmospheric CO2 mitigation. One of several applied methodologies, biogeochemical modeling, offers possibilities to distribute estimates of soil organic carbon (SOC) over regions, estimate the effects of changes in climate and ecosystem management on SOC, and quantify changes of soil properties over time or space. In this study, the sensitivity of the Century model was assessed in regards to uncertainties for soil texture, natural fire return periods, grazing intensities, and climate. The study area was situated in central Sudan and dominated by subsistence agroecosystems. Uncertainties in the modeling of historical SOC levels, prior to known human interactions, can be reflected in the estimation of the current or future SOC levels, as some soil processes take many years to occur. The relationship between these differences in historical and current SOC levels was calculated. Soil texture, derived from a number of different sources, had the greatest impact on modeled SOC. Overall, data uncertainties for the five parameters tested resulted in SOC variations of up to 160 g C/m2 (1.6 t C/ha) for the estimation of the current level (year 2002), which corresponds to a difference of approximately 80% of the average current level.

Spatial and temporal variation in a mesic savanna fire regime: responses to variation in annual rainfall

This paper analyses the fire regime over 41 years (1956 to 1996) in Hluhluwe-Umfolozi Park, a mesic savanna area in South Africa. The study focuses at the landscape scale of tens of kilometres and at the medium term temporal scale of decades. Variation in fire regime was analysed in relation to variation in annual rainfall and the influence of management. The average annual area of the park that burnt was 26% and the average area of individual fires was 9.1 km2. Statistics relating to the percentage of area burnt annually, the average size of an individual burn, the mean and median fire return periods and the influence of management philosophy relative to the spatial and temporal variation in rainfall conditions are presented. Sources of ignition and implications for management are discussed.

Global and regional modelling of Arctic-boreal vegetation distribution and its sensitivity to altered forcing.

Understanding the distribution and function of Arctic and boreal ecosystems under current conditions and their vulnerability to altered forcing is crucial to our assessment of future global environmental change. Such efforts can be facilitated by the development and application of ecological models that simulate realistic patterns of vegetation change at high latitudes. This paper reviews three classes of ecological models that have been implemented to extrapolate vegetation information in space (e.g. across the Arctic and adjacent domains) and over historical and future periods (e.g. under altered climate and other forcings). These are: (i) equilibrium biogeographical models; (ii) frame-based transient ecosystem models, and (iii) dynamic global vegetation models (DGVMs). The equilibrium response of high-latitude vegetation to scenarios of increased surface air temperatures projected by equilibrium biogeographical models is for tundra to be replaced by a northward shift of boreal woodland and forests. A frame-based model (ALFRESCO) indicates the same directional changes, but illustrates how response time depends on rate of temperature increase and concomitant changes in moisture regime and fire disturbance return period. Key disadvantages of the equilibrium models are that they do not simulate time-dependent responses of vegetation and the role of disturbance is omitted or highly generalized. Disadvantages of the frame-based models are that vegetation type is modelled as a set unit as opposed to an association of individually simulated plant functional types and that the role of ecosystem biogeochemistry in succession is not explicitly considered. DGVMs explicitly model disturbance (e.g. fire), operate on plant functional types, and incorporate constraints of nutrient availability on biomass production in the simulation of vegetation dynamics. Under changing climate, DGVMs detail conversion of tundra to tree-dominated boreal landscapes along with time-dependent responses of biomass, net primary production, and soil organic matter turnover--which all increase with warming. Key improvements to DGVMs that are needed to portray behaviour of arctic and boreal ecosystems adequately are the inclusion of anaerobic soil processes for inundated landscapes, permafrost dynamics, and moss-lichen layer biogeochemistry, as well as broader explicit accounting of disturbance regimes (including insect outbreaks and land management). Transient simulation of these landscapes can be further tailored to high-latitude processes and issues by spatially interactive, gridded application of arctic/boreal frame-based models and development of dynamic regional vegetation models (DRVMs) utilizing plant functional type schemes that capture the variety of high-latitude environments.

Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998

Changes in the cover and density of shrubs and trees were assessed from aerial photographs (1940, 1974 and 1998) as well as from fixed‐point photographs taken in 1984 and 1996 in the Kruger National Park, South Africa. Woody cover (trees and shrubs combined) increased by 12% on granite substrates but decreased by 64% on basalt substrates over the past 58 years. Both these figures are expressed in terms of the initial values, respectively. The density of the large tree component of woody vegetation decreased on both substrates. Woody vegetation cover declined as fire return periods became shorter, but the relationship was weak. The increases in woody plant density and cover on granite are thought to be the result of decreased competition from grasses, which in turn is a result of overgrazing by wild herbivores whose numbers have been kept high through the provision of surface water. These effects were not seen on the relatively nutrient‐rich basalts, where grasses can recover rapidly even after heavy grazing. The decline in overall woody cover on basalts is interpreted as a result of regular, short‐interval prescribed burning over the past 40 years, while the universal decline in large trees seems to result from an interaction between regular, frequent fires and utilization by elephants. The implications for management are discussed.

A history of Kaiparoro clearing and the limits ofNothofagusin the northern Tararua Range, New Zealand

Abstract Kaiparoro clearing is a 40 ha enclave of tussock grassland within forest, straddling a rounded peneplain remnant in the northern Tararua Range. The grassland occurs at an unusually low altitude compared with other North Island red tussock grasslands. Charcoal and pollen from a peat bog suggests that fire created the clearing and helped maintain open vegetation throughout its 3500+ year history. There appear to be no edaphic constraints on forest occupation of most of the site. Before the clearing was created, tussock grasses were probably confined to small riparian peat bogs within low forest on the peneplain surface. Rates of shrub invasion of the tussock clearing by Dracophyllum longifolium, Leptospermum scoparium, and Olearia colensoi are slow. A fire return-period of <400 years would be sufficient to perpetuate the open or non-forest vegetation. Kaiparoro coincides with the modern-day distributional limits of Nothofagus in the northern Tararua Range. The pollen record indicates that Nothofagu...