Intraspecific competition between shrubs in a semi-arid savanna

Tree-on-tree competitive interactions may be more important in affecting the distribution of the tree components of savannas than inter-specific competition with grasses. The presence of intraspecific competition is expected to negatively affect inter-tree spacing, individual size distributions and plant physiology as well as survival/mortality. In this field removal experiment on Acacia mellifera, one of South Africa’s most common encroachers on nutrient-poor soils, the growth, water relations and mortalities of shrubs where all neighbouring woody competitors were removed (target) were monitored three times during each of three growing seasons. After 3 years, the nitrogen and carbon isotopic ratios of the study plants were analysed. Target shrubs benefitted from removal of neighbours, resulting in greater growth, less water stress, a relatively small degree of canopy dieback and reduced reliance on N₂ fixation. Target shrubs grew by 25 ± 4% in height relative to 7 ± 4% for controls, with the targets suffering a maximum of <15% canopy dieback compared to up to 60% in the controls. Severe environmental stress is known to affect neighbour interactions among shrubs and competition may constrain shrub sizes and avoid density-dependent mortality. In contrast, release from competition in our study may have allowed greater growth of target plants, increasing their total evapo-transpirational leaf surface areas and leaving them vulnerable to drought and water stress. Intratree competition on shallow nutrient-poor soils in savannas may thus aid the persistence of bush encroachment by regulating the sizes of individual shrubs below the threshold of drought vulnerability.     

Grass bud responses to fire in a semiarid savanna system

Increasingly, land managers have attempted to use extreme prescribed fire as a method to address woody plant encroachment in savanna ecosystems. The effect that these fires have on herbaceous vegetation is poorly understood. We experimentally examined immediate (<24 hr) bud response of two dominant graminoids, a C₃ caespitose grass, Nassella leucotricha, and a C₄ stoloniferous grass, Hilaria belangeri, following fires of varying energy (J/m²) in a semiarid savanna in the Edwards Plateau ecoregion of Texas. Treatments included high‐ and low‐energy fires determined by contrasting fuel loading and a no burn (control) treatment. Belowground axillary buds were counted and their activities classified to determine immediate effects of fire energy on bud activity, dormancy, and mortality. High‐energy burns resulted in immediate mortality of N. leucotricha and H. belangeri buds (p < .05). Active buds decreased following high‐energy and low‐energy burns for both species (p < .05). In contrast, bud activity, dormancy, and mortality remained constant in the control. In the high‐energy treatment, 100% (n = 24) of N. leucotricha individuals resprouted while only 25% (n = 24) of H. belangeri individuals resprouted (p < .0001) 3 weeks following treatment application. Bud depths differed between species and may account for this divergence, with average bud depths for N. leucotricha 1.3 cm deeper than H. belangeri (p < .0001). Synthesis and applications: Our results suggest that fire energy directly affects bud activity and mortality through soil heating for these two species. It is imperative to understand how fire energy impacts the bud banks of grasses to better predict grass response to increased use of extreme prescribed fire in land management.     

Biocomplexity in large tree mortality: interactions between elephant,fire and landscape in an African savanna

The vegetation dynamics of the savanna ecosystem are driven by complex interactions between biotic and abiotic factors, and thus are expected to exhibit emergent properties of biocomplexity. We explore the relative importance of static and dynamic drivers in explaining the patterns of mortality of large trees in the Kruger National Park, South Africa. Data on large trees were collected from 22 transects in April 2006, and these transects were re‐sampled in November 2008. Of the 2546 individually‐identified trees that were re‐sampled, 290 (11.4%) died in the interim. We tested several competing hypotheses with varying levels of complexity, and found that mortality of large trees was affected mainly by both static (geophysical and landscape characteristics) and dynamic (elephant damage and fire) factors that were either additive or interactive in their effects. Elephant damage was the main predictor of tree mortality, but fire also played an important role depending on the landscape type. Other static variables such as position‐on‐slope, height below canopy, and altitude had weak effects in explaining tree mortality. These results indicate that keystone features such as large trees, show differential vulnerability to mortality that is landscape‐specific. For conservation managers, this implies that the dynamic drivers (elephant and fire) of tree mortality have to be managed at the specific landscape‐level. We suggest that this emergent biocomplexity in the spatial and temporal patterns of large tree mortality is not unique to the African savannas, but is likely widespread across heterogeneous landscapes.     

Grass production and decomposition in Southern Guinea savanna,Nigeria

Summary Annual grass production in ungrazed plots was 2,731 kg ha^-1, litter production was 1,619 kg ha^-1 and decomposition was 1,789 kg ha^-1. In grazed plots the corresponding figures were 3,157 kg ha^-1, 1,440 kg ha^-1, and 1,475 kg ha^-1 respectively; cattle consumed 1,405 kg ha^-1. Litter disappearance was greatest in the dry season: 1,226 kg ha^-1 (69% of the annual total) disappearing in the 4 months of December to March in the ungrazed plots, largely due to consumption (790 kg ha^-1 in December to March) by fungus-growing termites (Macrotermitinae). A positive linear relationship was found between maximum grass biomass and annual rainfall in West Africa.     

The influence of mistletoes on nutrient cycling in a semi-arid savanna, southwest Zimbabwe

Parasitic plants are increasingly becoming the focus of research in many ecosystems. They have been shown to alter litterfall properties and decomposition rates in environments where they occur. Despite this recognition, the role of mistletoes in nutrient cycling in semi-arid savanna remains poorly understood. We investigated the litter input, element returns, and associated below-canopy soil nutrient concentrations of three mistletoe species (Erianthemum ngamicum, Plicosepalus kalachariensis, and Viscum verrucosum) that parasitize Acacia karroo trees in a semi-arid savanna, southwest Zimbabwe. Element concentrations in mistletoe leaf litter were enriched relative to the host. Litterfall from mistletoes significantly increased overall litterfall by up to 173 %, with E. ngamicum and P. kalachariensis having greater litterfall than their host trees. Associated with these changes in litterfall was an increase in element returns and the below-canopy soil nutrient concentrations. The increase in nutrient returns was due to both the effect of enriched mistletoe litter and increased volumes of litterfall beneath host trees. Litterfall, element returns, and the below-canopy soil nutrient concentrations were significantly influenced by mistletoe density, with higher values at high mistletoe density. Overall, E. ngamicum and P. kalachariensis had greater influence on litterfall, element returns, and soil nutrient concentrations than V. verrucosum. These findings are consistent with current understanding of enhanced nutrient cycling in the presence of parasitic plants particularly in nutrient-poor ecosystems. We conclude that the introduction of nutrients and associated increase in resource heterogeneity play an important role in determining ecosystem structure and function in semi-arid savannas.     

Tree-grass co-existence in savanna: Interactions of rain and fire

The mechanisms permitting the co-existence of tree and grass in savannas have been a source of contention for many years. The two main classes of explanations involve either competition for resources, or differential sensitivity to disturbances. Published models focus principally on one or the other of these mechanisms. Here we introduce a simple ecohydrologic model of savanna vegetation involving both competition for water, and differential sensitivity of trees and grasses to fire disturbances. We show how the co-existence of trees and grasses in savannas can be simultaneously controlled by rainfall and fire, and how the relative importance of the two factors distinguishes between dry and moist savannas. The stability map allows to predict the changes in vegetation structure along gradients of rainfall and fire disturbances realistically, and to clarify the distinction between climate- and disturbance-dependent ecosystems.     

Grass competition is more important than fire for suppressing encroachment of Acacia sieberiana seedlings

Plant Ecology - Understanding the factors controlling tree seedling recruitment is an integral research priority for savanna ecosystems, particularly for the management of woody plant encroachment....     

The abundance of large wild herbivores in a semi-arid savanna in relation to seasons, pans and livestock

Large herbivores, including livestock and ostrich, were counted along a 200-km long track in south-western Kalahari, Botswana. Altogether, 37 counts were made during different seasons. The number of animals seen and group size were recorded. These variables were compared with monthly and accumulated rainfall (number of animals and group size) and distance to pans (number of animals). Observations of game and livestock were also related to distance to villages. In four of the seven studied species, most animals were seen during the wet season. Group size also varied between seasons apart from the non-gregarious steenbok and duiker. Number of animals and group size were in some cases correlated with rainfall during the month of observation or with accumulated rainfall during the preceding months. The various species were more often observed close to pans than further away from the pans. Compared to livestock, game was on average observed >10 km further away from the villages. Few observations of game were made between village centres and the livestock observations most distant from the village.     

The influence of mistletoes on the litter-layer arthropod abundance and diversity in a semi-arid savanna,Southwest Zimbabwe

Aims

The below-canopy soil moisture content and litter-layer arthropod abundance and diversity of Acacia karroo trees parasitized by each of three mistletoe species (Erianthemum ngamicum, Plicosepalus kalachariensis, and Viscum verrucosum) and uninfected A. karroo trees were investigated in semi-arid savanna, southwest Zimbabwe.

Results

The soils below the canopies of mistletoe-infected trees were significantly low in moisture content compared to those beneath uninfected A. karroo trees. Nevertheless, arthropod species diversity was greater by up to 34 % below the canopies of mistletoe-infected trees than beneath uninfected A. karroo trees, with greater abundances beneath trees infected by E. ngamicum and P. kalachariensis. In addition, the majority of the arthropod species associated with mistletoe-infected trees had litter as their dominant foraging substrate.

Conclusions

Our findings show that mistletoes increase the abundance and diversity of litter-dwelling and –foraging arthropods due to increase in the quality and quantity of litterfall beneath mistletoe-infected trees. By altering the below-canopy arthropod communities and soil moisture content, mistletoes have potential to modify ecosystem processes such as decomposition, soil process rates, and nutrient cycling. Therefore, we suggest that the resulting increase in resource heterogeneity plays an important role in determining the structure and functioning of semi-arid savanna ecosystems.     

Topo-edaphic, floristic and physiognomic gradients of woody plants in a semi-arid African savanna woodland

The variation in topo-edaphic conditions, woodland composition and physiognomic structure of a semi-arid savanna woodland, namely the 59 400 ha Klaserie Nature Reserve, was studied with multivariate techniques. Spatial variation in the texture and chemistry of both the A-and B-horizons was substantial. One gradient of the A-horizon was of increasing clay, silt, organic matter, pH and conductivity, while a second gradient was of increasing rockiness, slope and gravel content, associated in part with upper landscape positions. Physiognomic structure of the woodland also displayed substantial spatial variation. Variation in tree density had the greatest effect on total canopy volume, and the density of coppice and dead individuals were closely correlated. Topo-edaphic variation, after the effect of vegetation composition had been accounted for, had a direct effect on woodland structure, as clay content of soils or rockiness influenced shrub density, and catenal position together with silt-or gravel-content of soils influenced coppice density. Ordination analyses identified that the included topo-edaphic variables accounted for a significant amount of floristic variation, but also showed that important environmental variables were omitted. Floristic variation of woody elements was of the nature of a large number of relatively equivalent, independent gradients rather than of a few simple primary gradients. Ten woodland types were identified by TWINSPAN which differed in their topo-edaphic and physiognomic character. Distinctive woodland assemblages were found on rocky outcrops or alluvial areas, on deep, sandy soils or on clay-or silt-rich soils. Colophospermum mopane formed the densest woodlands. Characteristics of the B-horizon were often dissimilar to those of the A-horizon, and seemed important for woodland composition.     

Fire and water: the role of grass competition on juvenile tree growth and survival rates in a mesic savanna

Plant Ecology - Trees and grasses compete for space in savanna landscapes where fire also suppresses trees, maintaining a lower tree/grass ratio than precipitation levels can support. While the...     

Seasonality and facilitation drive tree establishment in a semi-arid floodplain savanna

A popular hypothesis for tree and grass coexistence in savannas is that tree seedlings are limited by competition from grasses. However, competition may be important in favourable climatic conditions when abiotic stress is low, whereas facilitation may be more important under stressful conditions. Seasonal and inter-annual fluctuations in abiotic conditions may alter the outcome of tree–grass interactions in savanna systems and contribute to coexistence. We investigated interactions between coolibah (Eucalyptus coolabah) tree seedlings and perennial C₄ grasses in semi-arid savannas in eastern Australia in contrasting seasonal conditions. In glasshouse and field experiments, we measured survival and growth of tree seedlings with different densities of C₄ grasses across seasons. In warm glasshouse conditions, where water was not limiting, competition from grasses reduced tree seedling growth but did not affect tree survival. In the field, all tree seedlings died in hot dry summer conditions irrespective of grass or shade cover, whereas in winter, facilitation from grasses significantly increased tree seedling survival by ameliorating heat stress and protecting seedlings from herbivory. We demonstrated that interactions between tree seedlings and perennial grasses vary seasonally, and timing of tree germination may determine the importance of facilitation or competition in structuring savanna vegetation because of fluctuations in abiotic stress. Our finding that trees can grow and survive in a dense C₄ grass sward contrasts with the common perception that grass competition limits woody plant recruitment in savannas.     

Woody vegetation spatial patterns in a semi-arid savanna of Burkina Faso,West Africa

Spatial patterns of woody individuals were studied in a semi-arid savanna of West Africa located in Burkina Faso at and around 14° 12 N and 2° 27 W. The study was based upon a 10.24 ha plot within which individuals were mapped. Spatial pattern analysis was carried out using second order characteristics of point processes as K functions and pair correlations. The overall density amounted to 298 individuals ha^-1. The most abundant species were Combretum micranthum G. Don., Grewia bicolor Juss. and Pterocarpus lucens Lepr. Anogeissus leiocarpus (D.C.) G. et Perr. was also an important constituant of this vegetation type, owing to its taller stature. Clumped spatial distributions were identified for all species except for two, for which complete spatial randomness (CSR) was found (including P. lucens, a dominant woody plant). No regular pattern was found even when tall individuals were considered alone. Aggregation dominates interspecific relationships, resulting in multispecific clumps and patches. The overall aggregation pattern was constituted by two different structures. A coarse-grain pattern of ca. 30–40 m was based on edaphic features, and expresses the contrast between sparse stands on petroferric outcrops and denser patches on less shallow soils. A finer-grain pattern made of clumps ca. 5–10 m wide, with no obvious relation to pre-existing soil heterogeneity. There was no overall pattern for saplings (between 0.5 m and 1.5 m in height) irrespective of species, and thus no obvious common facilitation factor. For species with a high recruitment level there was no significant relationship between mature adult and saplings. The only case of clumped saplings with randomly distributed adults was found in P. lucens. However, this cannot be unequivocally interpreted as density dependent regulation since the existence of such a process was not consistent with the spatial distribution of dead P. lucens individuals (victims of the last drought). The mean density around dead P. lucens was lower than around surviving ones, indicating that the last drought tended to reinforce clumping rather than promote a regular pattern of trees. Spatial pattern analysis yielded no evidence supporting a hypothesis of stand density regulation through competition between individuals. Other processes, as surface sealing of bare soils or insufficient recruitment, may play a more important role in preventing a savanna-like vegetation from turning into denser woodlands or thickets.     

Size-dependent mortality in a Neotropical savanna tree: the role of height-related adjustments in hydraulic architecture and carbon allocation

Size-related changes in hydraulic architecture, carbon allocation and gas exchange of Sclerolobium paniculatum (Leguminosae), a dominant tree species in Neotropical savannas of central Brazil (Cerrado), were investigated to assess their potential role in the dieback of tall individuals. Trees greater than ∼6-m-tall exhibited more branch damage, larger numbers of dead individuals, higher wood density, greater leaf mass per area, lower leaf area to sapwood area ratio (LA/SA), lower stomatal conductance and lower net CO₂ assimilation than small trees. Stem-specific hydraulic conductivity decreased, while leaf-specific hydraulic conductivity remained nearly constant, with increasing tree size because of lower LA/SA in larger trees. Leaves were substantially more vulnerable to embolism than stems. Large trees had lower maximum leaf hydraulic conductance ( K _leaf) than small trees and all tree sizes exhibited lower K _leaf at midday than at dawn. These size-related adjustments in hydraulic architecture and carbon allocation apparently incurred a large physiological cost: large trees received a lower return in carbon gain from their investment in stem and leaf biomass compared with small trees. Additionally, large trees may experience more severe water deficits in dry years due to lower capacity for buffering the effects of hydraulic path-length and soil water deficits.     

Spatial heterogeneity in the herbaceous layer of a semi-arid savanna ecosystem

Despite increasing recognition of the role spatial pattern can play in ecosystem function, few studies have quantified spatial heterogeneity in savanna ecosystems. The spatial distribution of herbaceous biomass and species composition was measured across three scales in a semi-arid savanna in central Kenya, and patterns were related to environmental variables at different scales. Herbaceous biomass declined across a rainfall gradient and from upper to lower topographic positions, but variation within a site (across 5–50 m) was similar in magnitude to among-site variation associated with rainfall and topography. Geostatistical analyses showed that patchiness at scales of 5–25 m explained 20% of total variation in herbaceous biomass. This pattern arose from the presence of both 5–10-m diameter patches containing high herbaceous biomass (> 170 g m^–2) and 5–10-m diameter patches characterized by nearly bare soil surfaces (< 40 g m^–2). Patch structure was contingent on topography, with larger bare patches at ridgeline and upper hillslope positions. Grass species distributions showed the greatest degree of patch structure and species turnover across distances of 5–45 m. Additional community variation was associated with topography, with minimal variation in species composition across the rainfall gradient. Pattern diversity significantly exceeded levels reported for four other grassland ecosystems, suggesting fundamental differences in local processes generating spatial pattern. It is hypothesized that heterogeneously distributed grazing pressure, interacting with the distribution of shrub canopies, is an important factor generating such high levels of small-scale patch structure in this savanna.