Changes in structure of savanna woodlands in northern Botswana following the impacts of elephants and fire

Fluctuations in densities of woody plant species were monitored in plots within three northern Botswana woodland types subjected to elephant damage and burning. Woodlands dominated by Baikiaea plurijuga and Colophospermum mopane sustained significant changes occurring on an annual basis, whereas Acacia erioloba plots maintained a typical structure. The structure of A. erioloba woodlands appeared to be influenced by factors other than elephants and the occurrence of fire. Woodlands dominated by C. mopane plants were subjected to obtrusive elephant damage, although the densities of tall trees remained largely unchanged. The effects of fire were most prominent in B. plurijuga woodlands. Tree densities declined consistently and plants of lower height classes, such as shrubs and seedlings increased in densities in areas subjected to a high occurrence of fire.     

Observations of tree growth, woodland structure and elephant damage on Colophospermum mopane in Luangwa Valley, Zambia

Six samples of tagged Colophospermum mopane were monitored for five years in locations with varying soil characteristics but with similar elephant densities. Physiognomic variation among the samples was related to soil differences, which also correlated with different browsing habits by elephants. The impact of elephant browsing further influenced both the physiognomy and demography of C. mopane. Results from this study suggest that the influence of soils and elephants on C. mopane alter successional transitions from grassland to woodland. Soils that promote coppicing of C. mopane yield less stable woodlands when associated with elephants than soils promoting woodlands with large bolus, non-coppicing trees. The dynamics of the latter are determined more by tree recruitment as influenced by such agents as other browsers or frequency and seasonality of bush fires. Implications for forest/elephant management are discussed.     

Elephant herbivory in Majete Wildlife Reserve,Malawi

Elephants have a major influence on vegetation structure, composition and ecosystem processes, and are primary agents of habitat change in Africa. At moderate‐to‐high population densities, elephants can damage vegetation, especially when enclosed in protected areas. This study examines the effects of elephant browsing on woody trees in Majete Wildlife Reserve (WR), south‐western Malawi. Regression analysis is used to assess the associations of six factors known to drive elephant browsing in other areas and determine which ones have the most influence on browsing at Majete WR. Twenty‐four per cent of tagged trees had been subject to elephant browsing. The model with vegetation type, stem diameter and distance from permanent water correctly predicted browsing for 80% of the observations. Elephants mostly favoured riparian woodlands, followed by Acacia‐dominated woodland and Brachystegia‐dominated woodland. Browsing occurrence was negatively related to distance from permanent water and diameter at breast height(DBH). A larger number of trees, sampled at random and covering a larger portion of the reserve would provide more reliable estimates of browsing and related factors. Knowledge of time‐ and site‐specific factors affecting elephant browsing can be used to forecast future habitat transformations and manipulate the range of the elephants within the reserve.     

Changes in the structure and composition of miombo woodlands mediated by elephants (Loxodonta africana) and fire over a 26-year period in north-western Zimbabwe

Changes in structure and composition of miombo woodlands mediated by elephants and fire were studied in 26-year-old permanent transects established in 1972 in north-western Zimbabwe. Elephants caused 48% decline in proportions of large trees (>11 cm diameter), significant reductions (30.9–90.9%) in tree heights, reductions in stem areas (43.5%) and densities (2.5%) of all trees. There were increases in proportions of small trees (64.8%), shrub canopy volumes (271%) and shrub densities (172%). These increases are attributed to natural recruitment because of longer fire-free periods and reduction of tree suppression effects on lower strata as a result of elephant-induced tree declines. Frequencies of occurrence of most species dropped by 28–89.6%. Brachystegia boehmii was replaced by Pseudolachnostylis maprouneifolia as the most dominant tree, largely because of high elephant preference for Brachystegia boehmii . A new suite of species, dominated by Combretaceae, increased in dominance resulting in local floristic changes. Reductions in old elephant (33.4%), old unknown (89.9%) and new elephant (13.7%) damage suggest that elephant occupancy of miombo woodlands has declined, possibly because of limited availability of preferred browse species. This study clearly shows that elephants and fire have contributed significantly to the changes in miombo woodlands in the area.     

Vegetation changes during a 36-year period in northern Chobe National Park, Botswana

Changes in vegetation cover in northern Chobe National Park (Botswana) were assessed using aerial photographs from 1962, 1985 and 1998, with subsequent ground proofing. In addition, cumulative browsing by elephants and the occurrence of fire scars were recorded on random vegetation sites within shrubland (n = 20) and mixed woodland (n = 20). Coverage of woodland vegetation decreased from 60% to 30% between 1962 and 1998, while shrubland vegetation increased from 5% to 33% during the same period. During the study period, woodland has gradually retreated away from the river front. While riparian forest covered a continuous area along the riverfront in 1962, only fragments were left in 1998. We found a significant decrease in browse use with increasing distance to the Chobe river for Combretum apiculatum, Combretum elaeagnoides, Combretum mossambicense and other woody plants combined (all P < 0.0001). The occurrence of fire (P < 0.0001) and basal area (P < 0.0001) were positively related to distance to the river. Elephant browsing occurred on >70% of available stems within 2 km from the river, while less than 20% of the trees had fire scars in the same zone. Beyond 7 km from the river, elephant browsing was reduced to >50% of available stems, while more than 50% of the trees had fire scars. The density of any of the shrubs was not related to distance to the river neither within shrubland (all P > 0.05) nor within mixed woodlands (all P > 0.05).     

The effects of herbivory by a mega- and mesoherbivore on tree recruitment in sand forest, South Africa

Herbivory by megaherbivores on woody vegetation in general is well documented; however studies focusing on the individual browsing effects of both mega- and mesoherbivore species on recruitment are scarce. We determined these effects for elephant Loxodonta africana and nyala Tragelaphus angasii in the critically endangered Sand Forest, which is restricted to east southern Africa, and is conserved mainly in small reserves with high herbivore densities. Replicated experimental treatments (400 m(2)) in a single forest patch were used to exclude elephant, or both elephant and nyala. In each treatment, all woody individuals were identified to species and number of stems, diameter and height were recorded. Results of changes after two years are presented. Individual tree and stem densities had increased in absence of nyala and elephant. Seedling recruitment (based on height and diameter) was inhibited by nyala, and by elephant and nyala in combination, thereby preventing recruitment into the sapling stage. Neither nyala or elephant significantly reduced sapling densities. Excluding both elephant and nyala in combination enhanced recruitment of woody species, as seedling densities increased, indicating that forest regeneration is impacted by both mega- and mesoherbivores. The Sand Forest tree community approached an inverse J-shaped curve, with the highest abundance in the smaller size classes. However, the larger characteristic tree species in particular, such as Newtonia hildebrandtii, were missing cohorts in the middle size classes. When setting management goals to conserve habitats of key importance, conservation management plans need to consider the total herbivore assemblage present and the resulting browsing effects on vegetation. Especially in Africa, where the broadest suite of megaherbivores still persists, and which is currently dealing with the 'elephant problem', the individual effects of different herbivore species on recruitment and dynamics of forests and woodlands are important issues which need conclusive answers.     

Effect of selective logging on tree and understory regeneration in miombo woodland in western Tanzania

Tree regeneration and understory response to selective cutting of Pterocarpus angolensis DC, and Sterculia quinqueloba Sim, was measured outside Katavi National Park, Tanzania. Contrary to expectations, a selective harvest had no effect on tree recruitment for either species in this miombo woodland. In unlogged plots along transects taken at increasing distance from a main road, the stand densities of young trees declined away from the road. Since anthropogenic activity along the road results in a high fire frequency but low native ungulate densities, the increased rate of tree regeneration near the road may result from either reduced grazing and browsing pressure by native ungulates, or reduced fire intensity and severity along roads. These effects appear to mask any possible effect of selective harvest on tree recruitment. Currently, there is no evidence of compensatory recruitment of trees of any species into the canopy to replace logged trees and this will lead to a gradual thinning in overstory stand density.     

Model highlights likely long‐term influences of mesobrowsers versus those of elephants on woodland dynamics

The potential long‐term influences of mesobrowsers versus those of savannah elephants on woodland dynamics have not been explored. This may be a critical omission especially in southern African savannahs, where efforts to preserve existing woodlands are typically directed at elephant management. We describe a simple browse–browser model, parameterized from an extensive review of the literature and our own data, including quantitative assessment of impala impact, from the study site, iMfolozi Park, South Africa. As there is a paucity of species‐specific demographic data on savannah woody species, we modelled, in a novel approach, functional groups of plant species typical of Acacia woodlands. Outputs suggest that over the long term (100 years), low‐to‐moderate densities of impala will have a similar impact on woodland structure, in terms of density of adult trees, as low‐to‐moderate densities of elephant. Further, the outputs highlight the apparently strong synergistic effect impala and elephant impacts combined have on woodland dynamics, suggesting that reduction or removal of either impala or elephant will radically reduce long‐term destruction of savannah woodlands. Recorded changes in adult tree numbers in iMfolozi broadly supported the model's outputs.     

Elephant (Loxodonta africana) Disturbance to Riparian Woodland: Effects on Tree-Species Richness,Diversity and Functional Redundancy

The substantial increase in elephant populations across many areas in southern Africa over past decades is prompting concerns about the effects on biodiversity. We investigated the outcomes of elephant disturbance on tree-species presence, density, and richness, and on alpha and beta diversity within riparian woodland in Chobe National Park, Botswana. We enumerated all tree species occurring in 32 plots (0.06 ha) along the Chobe riverfront. Plots were stratified by soil type (nutrient-rich alluvium vs. nutrient-poor Kalahari sand covering alluvium) and elephant impact (high vs. low impact on both soil types). We tested four predictions: elephants reduce tree density, richness, and alpha diversity; beta diversity is greater in vegetation subjected to high elephant impact; elephant impact on tree-species composition is greater on nutrient-poor than on nutrient-rich soil; and the loss or decline of abundant tree species on heavily disturbed sites is offset by an increase in abundance of functionally similar species, ones that are minor on lightly disturbed sites. Elephant browsing substantially affected tree-species composition, reducing density, species richness, evenness, and alpha diversity but had no effect on beta diversity. The dominant species on relatively undisturbed areas were partly replaced by functionally similar species on heavily disturbed sites. Soil type influenced species composition on lightly disturbed sites but was less important at higher elephant densities. Our findings are important for areas with extreme dry-season densities of elephants but should not be extrapolated to infer purported effects of elephants on tree diversity at lower densities.     

Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position

Elephant are considered major drivers of ecosystems, but their effects within small-scale landscape features and on other herbivores still remain unclear. Elephant impact on vegetation has been widely studied in areas where elephant have been present for many years. We therefore examined the combined effect of short-term elephant presence (< 4 years) and hillslope position on tree species assemblages, resource availability, browsing intensity and soil properties. Short-term elephant presence did not affect woody species assemblages, but did affect height distribution, with greater sapling densities in elephant access areas. Overall tree and stem densities were also not affected by elephant. By contrast, slope position affected woody species assemblages, but not height distributions and densities. Variation in species assemblages was statistically best explained by levels of total cations, Zinc, sand and clay. Although elephant and mesoherbivore browsing intensities were unaffected by slope position, we found lower mesoherbivore browsing intensity on crests with high elephant browsing intensity. Thus, elephant appear to indirectly facilitate the survival of saplings, via the displacement of mesoherbivores, providing a window of opportunity for saplings to grow into taller trees. In the short-term, effects of elephant can be minor and in the opposite direction of expectation. In addition, such behavioural displacement promotes recruitment of saplings into larger height classes. The interaction between slope position and elephant effect found here is in contrast with other studies, and illustrates the importance of examining ecosystem complexity as a function of variation in species presence and topography. The absence of a direct effect of elephant on vegetation, but the presence of an effect on mesoherbivore browsing, is relevant for conservation areas especially where both herbivore groups are actively managed.     

Changes in miombo woodland cover in and around Sengwa Wildlife Research Area, Zimbabwe, in relation to elephants and fire

One of the consequences of impacts of elephants and fire on woodlands is a change in woody cover, which often results in major challenges for wildlife managers. Changes in miombo woodland cover in and around Sengwa Wildlife Research Area (SWRA) between 1958 and 1996 were quantified by analyzing aerial photographs. Woody cover in SWRA decreazed from 95.2% in 1958 to 68.2% in 1996, with a lowest mean of 62.9% in 1983. The annual absolute rate of woody cover change in SWRA increazed from ?1.1% per annum between 1958 and 1964 to a recovery of 1.6% per annum between 1993 and 1996, while the annual relative rate increazed from ?1.1% per annum between 1958 and 1964 to 3.3% per annum between 1993 and 1996. There was a strong negative correlation between elephant densities and woody cover in SWRA, suggesting that loss of woody cover was mainly due to elephants. Woodland recovery after 1983 was due to reductions in elephant populations through legal and illegal off‐take and reductions in fire frequency. Surrounding areas experienced less woody cover losses than SWRA, mainly due to tree removal by locals whose densities increazed after the eradication of tsetse fly in the 1970s.     

Humans and elephants as treefall drivers in African savannas

Humans have played a major role in altering savanna structure and function, and growing land‐use pressure will only increase their influence on woody cover. Yet humans are often overlooked as ecological components. Both humans and the African elephant Loxodonta africana alter woody vegetation in savannas through removal of large trees and activities that may increase shrub cover. Interactive effects of both humans and elephants with fire may also alter vegetation structure and composition. Here we capitalize on a macroscale experimental opportunity – brought about by the juxtaposition of an elephant‐mediated landscape, human‐utilized communal harvesting lands and a nature reserve fenced off from both humans and elephants – to investigate the influence of humans and elephants on height‐specific treefall dynamics. We surveyed 6812 ha using repeat, airborne high resolution Light Detection and Ranging (LiDAR) to track the fate of 453 685 tree canopies over two years. Human‐mediated biennial treefall rates were 2–3.5 fold higher than the background treefall rate of 1.5% treefall ha^–1, while elephant‐mediated treefall rates were 5 times higher at 7.6% treefall ha^–1 than the control site. Model predictors of treefall revealed that human or elephant presence was the most important variable, followed by the interaction between geology and fire frequency. Treefall patterns were spatially heterogeneous with elephant‐driven treefall associated with geology and surface water, while human patterns were related to perceived ease of access to wood harvesting areas and settlement expansion. Our results show humans and elephants utilize all height classes of woody vegetation, and that large tree shortages in a heavily utilized communal land has transferred treefall occurrence to shorter vegetation. Elephant‐ and human‐dominated landscapes are tied to interactive effects that may hinder tree seedling survival which, combined with tree loss in the landscape, may compromise woodland sustainability.     

Effect of low rainfall and browsing by large herbivores on an enclosed savannah habitat in Kenya

Savannah ecosystems in East Africa are rarely stable and can experience rapid local changes from dense woodlands to open plains. In this 3‐year study there was a reduction of 16.3% in a height‐stratified sample of nearly 1000 individually marked Acacia drepanolobium trees. The study was carried out in an enclosed fire‐free wooded grassland habitat in the Laikipia region of Kenya. The trees were monitored from 1998 to 2001, a period that included 12 months when rainfall was 60% below average. Elephants were responsible for the loss of 40% of the trees, black rhinos 33% and 27% died from the effects of the drought. Low rainfall was correlated with increased damage as elephants switched diet from grass to trees. Heavy browsing by giraffes reduced tree growth rates and increased their susceptibility to drought. Hence the combination of low rainfall and heavy browsing by elephants, black rhinos and giraffes led to the rapid tree loss. These findings have implications for research into the causes of instability in savannah ecosystems and the management of enclosed reserves.     

Patterns of elephant impact on woody plants in the Hluhluwe-Imfolozi park, Kwazulu-Natal, South Africa

This study identifies patterns of elephant Loxodonta africana africana impacts upon tree species and woody plant communities in Hluhluwe-Imfolozi Park, a South African savannahs/woodlands area. Elephants were reintroduced there from 1981, following more than 80 years of absence. Data were collected in 2003 on elephant impact on woodland in the Park. Different vegetation types were susceptible to different types and levels of damage by elephants, suggesting that elephants will not homogenize the vegetation. Elephants targeted larger stems for all types of damage, with a strong preference for some of the less abundant species such as Albizia versicolor (breaking and toppling) and Cordia caffra and Schotia brachypetala (debarking). Elephant impacts tended to be distributed evenly across the park landscape, irrespective of stem density or proximity to permanent water. Overall, elephants have little impact on slowing or reversing the spread of undesirable woody species, but are having a marked impact on certain less common tree species and larger tree size-classes in the Hluhluwe-Imfolozi Park.     

Recruitment of Quercus agrifolia in central California: the importance of shrub-dominated patches

Abstract. Many perennial plants strongly enhance the survival of seedlings of other species. We studied patterns of long-term recruitment of Quercus agrifolia (Coastal live oak) associated with shrub-dominated communities by counting Q. agrifolia recruits on a time sequence of historical aerial photographs and comparing recruitment among mapped patches of coastal sage scrub, chaparral, and grassland in an 1120-ha landscape. Because we could not identify new recruits in existing woodlands with aerial photographs, we studied the recruitment of Q. agrifolia in this vegetation type indirectly by comparing population size structures and the spatial relationships between shrubs and recruits among woodlands that varied in understory community type. At the landscape scale, recruitment was higher in coastal sage scrub vegetation than predicted by the extent of its coverage, commensurate with the spatial coverage of chaparral, and very low in grassland. Recruitment within woodland communities also varied considerably. In woodland communities on sheltered, north-oriented topography with understories dominated by shrubs, there were large numbers of small Q. agrifolia, and recruits were not significantly spatially associated with shrubs within plots. In woodlands with herbaceous understories there were few individuals in the small size classes, and recruits were strongly spatially associated with shrubs within plots. Woodlands with shrub-dominated understories have population structures that appear to be stable, but woodlands with herbaceous understories exhibit size structures associated with declining populations. Quercus recruitment into shrub-dominated patches corresponds with previous documentation of facilitative relationships between shrubs and oak seedlings, and suggests the occurrence of an unusual form of patch dynamics in these landscapes.     

The savanna ecology of Kidepo Valley National Park, Uganda

Censuses indicated similar elephant densities in some habitats to those in certain other East African Parks where woodland has been almost completely destroyed. Giraffe densities were low. Herd structure indicated a healthy elephant population; but poor recruitment in the giraffe population. Policy for the national parks is discussed. It is suggested that it is not in the interests of Uganda or science to allow Kidepo Valley Park to become dominated by elephant in the same way as Kabalega and Rwenzori Park. Evidence that this is happening under current elephant use is strong. Cropping of two thirds of the elephant in the Narus valley is recommended. The revenue from cropping should be used for essential fire control measures and for a monitoring programme to ascertain optimum elephant numbers. Elephant should not be cropped in the absence of fire control.     

Understorey foraging and habitat selection by sheep in mixed Atlantic woodland

Abstract. The main objectives of this study were to investigate sheep foraging behaviour in mixed Atlantic woodland and to assess its impact on the forest understorey. We established 89 plots along four forest types: Fagus woodland, Quercus woodland, riparian gallery forest and conifer plantations. The presence of plant species in the forest understorey and their foraging damage was surveyed bimonthly from July 1996 to June 1997. In addition, we estimated the selection of woodland types by sheep through the pellet‐group count technique. The intensity of foraging by sheep was negligible for most of the plant species, however several species showed substantial damage in some woodland types. Among the species that were abundant and widespread in the entire study area, Rubus ulmifolius, graminoids and Ilex aquifolium were consumed most. Sheep selected only larch plantations, where grasses and Rubus were very abundant. This grazing behaviour reduced browsing damage of the understorey of woodland stands with higher conservation value, such as Quercus and Fagus woodlands.     

Ants,fire, and bark traits affect how African savanna trees recover following damage

Bark damage resulting from elephant feeding is common in African savanna trees with subsequent interactions with fire, insects, and other pathogens often resulting in tree mortality. Yet, surprisingly little is known about how savanna trees respond to bark damage. We addressed this by investigating how the inner bark of marula (Sclerocarya birrea), a widespread tree species favoured by elephants, recovers after bark damage. We used a long‐term fire experiment in the Kruger National Park to measure bark recovery with and without fire. At 24 months post‐damage, mean wound closure was 98, 92, and 72%, respectively, in annual and biennial burns and fire‐exclusion treatments. Fire exclusion resulted in higher rates of ant colonization of bark wounds, and such ant colonization resulted in significantly lower bark recovery. We also investigated how ten common savanna tree species respond to bark damage and tested for relationships between bark damage, bark recovery, and bark traits while accounting for phylogeny. We found phylogenetic signal in bark dry matter content, bark N and bark P, but not in bark thickness. Bark recovery and damage was highest in species which had thick moist inner bark and low wood densities (Anacardiaceae), intermediate in species which had moderate inner bark thickness and wood densities (Fabaceae) and lowest in species which had thin inner bark and high wood densities (Combretaceae). Elephants prefer species with thick, moist inner bark, traits that also appear to result in faster recovery rates.     

Disturbance and forest dynamics along a transect from Andean rain forest to Patagonian shrubland

Abstract. At ca. 40° S in northern Patagonia, Andean rain forests are replaced eastwards by woodlands and shrublands and eventually by steppe. Along this gradient we examined stand dynamics by analyzing tree population age structures and tree growth patterns. We also examined spatial and temporal characteristics of disturbance regimes by dating disturbances and mapping stands of differing disturbance history. From west to east, the ecological importance of earthquake-related disturbance decreases, whereas that of fire, logging, and livestock increases. Abrupt changes in rates of tree growth correspond with earthquakes in 1837, 1939 and 1960. In the mesic western forests earthquakes can result in massive new tree establishment on landslide-affected sites and increased rates of treefall. Fire, however, is the more pervasive disturbance over most of the gradient and creates extensive even-aged patches dominated by the regionally dominant trees, Nothofagus and Austrocedrus. Although some lightning-ignited and aboriginal-set fires occurred in these forests prior to European settlement, much of the present forest structure may be attributed to the massive burning associated with European settlement of this area near the turn of the present century. In contrast to the settlement-related increase in fire frequency in the western forested district, at the woodland/steppe ecotone the demise of the native American population resulted in a decrease in fire frequency. Heavy browsing and grazing following fire can seriously impede post-fire tree regeneration. These preliminary results document the important influences of varying disturbance regimes along a major environmental gradient in creating landscape-scale vegetation patterns.     

Elephant herbivory,frost damage and topkill in Kalahari sand woodland savanna trees

Questions: Which factors best predict the probability of elephant and frost damage in Kalahari sand woodland savanna? What is the association between tree mortality and the disturbance regime? Location: Western Zimbabwe. Methods: Elephant and frost damage, topkill, and whole‐plant mortality were quantified in ten common tree species in a Kalahari sand savanna in Zimbabwe. Individual trees were tagged in 20 plots and monitored over a two‐year period. A model selection approach was used to test the association between the probability of damage and size, prior damage, and neighbourhood effects, and to investigate the effect of damage on mortality. Results: Elephant damage differed strongly among species, and was not influenced by neighbourhood effects or prior disturbance. Frost damage also varied across species, and declined as a function of stem size and neighbourhood tree cover, and ‐ against expectations ‐ prior disturbance. Topkill increased as a function of elephant and frost damage, but was lower in previously damaged than in undamaged trees. Conclusions: Frost and elephant damage are influenced by community composition, and frost damage is also correlated with community structure and prior disturbance. Frost is an important and generally overlooked disturbance agent in southern African woodlands, where it may play a key role in association with other disturbance factors ‐ such as elephant herbivory ‐ that reduce woodland canopy cover.