Stream order controls geomorphic heterogeneity and plant distribution in a savanna landscape

We posed the question: does viewing a savanna as a network of streams linked to a matrix of terrestrial hillslopes provide a useful framework to research and understand plant distribution in these landscapes? Our study area, the Phugwane River network, lies in the semi‐arid savanna of Kruger National Park, South Africa. We examined changes in hillslope geomorphology from first‐, third‐ and fifth‐order hillslopes with regression equations. The distribution of geomorphic boundaries was enumerated by moving window analysis and the relationship between geomorphology and plant distribution was explored through ordination. First‐order hillslopes had a simple geomorphology, fewer geomorphic boundaries and a relatively homogeneous plant assemblage. By contrast, fifth‐order hillslopes were more complex in geomorphology, with more boundaries and a relatively heterogeneous vegetation pattern. Stream order classification of a savanna drainage network resulted in landscape units distinguishable by geomorphology, geomorphic boundaries and vegetation pattern. Therefore, the drainage network is a useful template to expose and organize the complexity in savanna landscapes into easily managed and researched units. This perspective should inform a shift from single‐scale phytosociological views of homogeneous vegetation units towards multi‐scale conceptualizations of savannas as water dependent ecosystems.     

Potential hominin plant foods in northern Tanzania: semi-arid savannas versus savanna chimpanzee sites

Savanna chimpanzees are useful as referential models for early hominins, and here potential differences between chimpanzee and early hominin ecology is the focus. Whereas chimpanzees inhabit only a handful of modern African savannas, there is evidence that early hominins occupied relatively more open and arid savannas than those in which chimpanzees live. In order to help expand potential models of early hominin palaeoecology beyond savanna chimpanzee-like scenarios, and to provide a basis for future modeling and testing of actual hominin diets, this study compares the types of plant foods available in modern semi-arid savannas of northern Tanzania to plant foods at savanna chimpanzee sites. The semi-arid savannas are not occupied by modern chimpanzees, but are potentially similar to environments occupied by some early hominins. Compared to savanna chimpanzee habitats, the northern Tanzania semi-arid savanna has a lower density and fewer species of trees that produce fleshy fruits. Additionally, the most abundant potential hominin plant foods are seasonally available Acacia seeds/pods and flowers, grass seeds, and the underground parts of marsh plants, as evidenced by vegetation surveys and by studies of the diets of baboons that forage in similar areas. The information from this study should be useful for framing hypotheses about hominin diets for sites with palaeoenvironmental contexts similar to those of the northern Tanzania semi-arid savannas and for contextualising tests of actual hominin diets (e.g., those based on dental microwear or isotopes).     

Directional growth of a clonal bromeliad species in response to spatial habitat heterogeneity

Habitat selection by directional growth of plants has previously been investigated but field evidence for this phenomenon is extremely scarce. In this study we demonstrate directional clonal growth in Aechmea nudicaulis, a monocarpic, perennial bromeliad native to spatially heterogeneous sandy coastal plains (restinga) in Brazil. This habitat is characterized by a matrix of bare sand with interspersed vegetation islands. Due to very high soil surface temperatures and other stress factors such as drought, A. nudicauliscan only germinate inside vegetation islands. Nevertheless, this species is very common on bare sand. In this study we tested the hypothesis that clonal fragments occurring at the border and inside vegetation islands show habitat selection by growing preferentially towards the bare sand habitat (i.e. away from the center of vegetation islands).We randomly chose 116 clonal fragments in two distinct micro-environments (inside vegetation islands, and in the border area between bare sand and vegetation islands) in the natural habitat of A.nudicaulisand measured their growth direction in relation to the island center. We measured the growth directions of entire clonal fragments (defined as the line that connects the oldest and the youngest ramets of a clonal fragment) as well as the growth direction of the youngest internode on each fragment (the growth direction of the youngest ramet in relation to its parent ramet). We used Monte Carlo simulations to test for deviations from randomness in the growth direction of clonal fragments and individual internodes. The clonal fragments of A.nudicaulis showed a significant tendency to grow away from the center of vegetation islands. In other words, the main growth direction of clonal fragments growing inside vegetation islands or at the border between bare sand and vegetation islands was preferentially directed towards bare sand environments. Individual internodes at the border of vegetation islands also exhibited this tendency to grow towards the outside of vegetation islands, but internodes growing inside vegetation islands did not show directional growth. These results provide the first field evidence for habitat selection through directional growth of a clonal plant species.Co-ordinationg editor: J. Tuomi     

The scale of landscape fragmentation affects herbivore response to vegetation heterogeneity

Using alternating bands of weeds and broccoli I experimentally manipulated vegetation composition and the spatial scale at which the landscape was fragmented in a factorial design. This experimental approach allowed me to distinguish the effect of spatial scale from that of simple crop heterogeneity on crop herbivores. The importance of scale depended on which insect species were examined. Cabbage aphids (Brevicoryne brassicae) were influenced by vegetation composition at all tested scales of fragmentation; cabbage butterflies (Pieris rapae) were not affected by scale or by composition and flea beetles (Phyllotreta cruciferae) revealed a striking dependence on scale of fragmentation as well as an interaction between scale and composition. This approach shows the importance of dissecting out the effects of scale from other aspects of landscape manipulation, and emphasizes the challenge of developing a theory that will enable prediction of species–specific responses to scale. Received: 15 February 1998 / Accepted: 10 June 1998     

集合种群动态对生境毁坏空间异质性的响应

首次将分形几何(Fractal geometry)与元胞自动机(Cellular automata)相结合,研究了破碎化生境中集合种群的空间分布格局动态,以及集合种群动态对生境毁坏空间异质性的响应。研究发现:(1)各个物种种群在生境中的分布具有很好的分形特征,物种的计盒维数(Box dimension)不仅可以很好地反映种群的空间分布结构,也能很好地反映种群动态。(2)如果将空间因素考虑进来的话,生境毁坏的灭绝债务(Time debt)将大于空间隐含模式所模拟的结果。(3)物种灭绝同时存在强物种灭绝和弱物种灭绝。并且只有在生境随机毁坏下,才与空间隐含的模拟结果比较接近,即强物种中将是最强物种率先灭绝。而在边缘毁坏这种比较集中成块的开发方式下,将是较强的物种灭绝。(4)边缘毁坏相对随机毁坏有利于物种,尤其是弱物种的长期续存。     

Mound building termites contribute to savanna vegetation heterogeneity

With biomass densities comparable to large ungulates and megaherbivores, termites play a key functional role in many tropical savanna ecosystems. This study focuses on vegetated termite mounds (termitaria) constructed by the Termitidae species Macrotermes herus. We studied how resource rich termitaria affect graminoid herbs (Poaceae and Cyperaceae), forbs and woody species composition and diversity. The density of termitaria explained 89% of the variation in dense thickets in the area. Fire tolerant Acacia species dominated the open savanna while fire sensitive species like Grewia spp. and the succulent Euphorbia candelabrum were restricted to termite mounds. Termitaria plots had four times the mean number of woody species and supported three times as many forb species as the adjacent savanna. For woody species, both the Shannon–Wiener index and the Shannon evenness index were higher on temitaria than on the savanna. There were no differences for graminoid herbs, except for the Shannon evenness index which was higher on termitaria. Our results indicate that graminoid herb richness peaks at lower productivity levels than trees and forbs in savanna ecosystems, as also recently found in temperate areas.     

Tree growth response to drought and temperature in a mountain landscape in northern Arizona, USA

Aim To understand how tree growth response to regional drought and temperature varies between tree species, elevations and forest types in a mountain landscape. Location Twenty‐one sites on an elevation gradient of 1500 m on the San Francisco Peaks, northern Arizona, USA. Methods Tree‐ring data for the years 1950–2000 for eight tree species (Abies lasiocarpa var. arizonica (Merriam) Lemm., Picea engelmannii Parry ex Engelm., Pinus aristata Engelm., Pinus edulis Engelm., Pinus flexilis James, Pinus ponderosa Dougl. ex Laws., Pseudotsuga menziesii var. glauca (Beissn.) Franco and Quercus gambelii Nutt.) were used to compare sensitivity of radial growth to regional drought and temperature among co‐occurring species at the same site, and between sites that differed in elevation and species composition. Results For Picea engelmannii, Pinus flexilis, Pinus ponderosa and Pseudotsuga menziesii, trees in drier, low‐elevation stands generally had greater sensitivity of radial growth to regional drought than trees of the same species in wetter, high‐elevation stands. Species low in their elevational range had greater drought sensitivity than co‐occurring species high in their elevational range at the pinyon‐juniper/ponderosa pine forest ecotone, ponderosa pine/mixed conifer forest ecotone and high‐elevation invaded meadows, but not at the mixed conifer/subalpine forest ecotone. Sensitivity of radial growth to regional drought was greater at drier, low‐elevation compared with wetter, high‐elevation forests. Yearly growth was positively correlated with measures of regional water availability at all sites, except high‐elevation invaded meadows where growth was weakly correlated with all climatic factors. Yearly growth in high‐elevation forests up to 3300 m a.s.l. was more strongly correlated with water availability than temperature. Main conclusions Severe regional drought reduced growth of all dominant tree species over a gradient of precipitation and temperature represented by a 1500‐m change in elevation, but response to drought varied between species and stands. Growth was reduced the most in drier, low‐elevation forests and in species growing low in their elevational range in ecotones, and the least for trees that had recently invaded high‐elevation meadows. Constraints on tree growth from drought and high temperature are important for high‐elevation subalpine forests located near the southern‐most range of the dominant species.     

Root proliferation and seed yield in response to spatial heterogeneity of below-ground competition

Here, we tested the predictions of a 'tragedy of the commons' model of below-ground plant competition in annual plants that experience spatial heterogeneity in their competitive environment. Under interplant competition, the model predicts that a plant should over-proliferate roots relative to what would maximize the collective yield of the plants. We predict that a plant will tailor its root proliferation to local patch conditions, restraining root production when alone and over-proliferating in the presence of other plants. A series of experiments were conducted using pairs of pea (Pisum sativum) plants occupying two or three pots in which the presence or absence of interplant root competition was varied while nutrient availability per plant was held constant. In two-pot experiments, competing plants produced more root mass and less pod mass per individual than plants grown in isolation. In three-pot experiments, peas modulated this response to conditions at the scale of individual pots. Root proliferation in the shared pot was higher compared with the exclusively occupied pot. Plants appear to display sophisticated nutrient foraging with outcomes that permit insights into interplant competition.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

Predictors of elephant poaching in a wildlife crime hotspot: The Ruvuma landscape of southern Tanzania and northern Mozambique

Understanding the spatial distribution of elephant carcasses in relation to ecological characteristics and human activities is critical to developing targeted management strategies for reducing poaching. We employ a spatial modelling approach to quantify the relative contribution of multiple climatic, ecological, human and protected area management predictors of the number of elephant carcasses in a recognized poaching hotspot: the Ruvuma landscape of northern Mozambique and southern Tanzania. This includes the Niassa Reserve in the south and the Selous Game Reserve in the north. In Mozambique, the number of elephant carcasses is positively associated with State-managed protected areas such as Niassa Reserve, but particularly with environmental variables including low rainfall and high temperatures. In Tanzania, elephant carcasses are positively associated with community-managed sites. A strong focus on effective management of protected areas in the Ruvuma landscape is crucial to reducing the killing of elephants.     

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.     

Response of ant and terrestrial spider assemblages to pastoral and military land use,and to landscape position,in a tropical savanna woodland in northern Australia

This study aims primarily to assess the response of two invertebrate groups to the effects of pastoralism and military training, at one site in the tropical savanna of north‐eastern Queensland. The richness and species composition of ants and terrestrial spiders were examined at two contrasting times of year across three land use treatments (pastoralism, military training and undisturbed) and four landscape positions (upper slope to riparian). Ant species richness was least in the grazed sites, and a high proportion of the ant species recorded varied significantly in frequency between the grazed and the two ungrazed land uses. This variation was generally greater than that associated with landscape position. Although variation in the richness of spiders was significantly related to land‐use type, this effect was less pronounced than for ants, was less marked than variation associated with landscape position and was confounded by a strong interaction between land use and landscape position. Quadrat‐scale variation in the composition of spider assemblages was influenced most by season of sampling. For both spiders and ants, there were few differences in richness or species composition between undisturbed land and that managed for military use.     

Plant species richness and environmental heterogeneity in a mountain landscape: effects of variability and spatial configuration

The loss of biodiversity has become a matter of urgent concern and a better understanding of local drivers is crucial for conservation. Although environmental heterogeneity is recognized as an important determinant of biodiversity, this has rarely been tested using field data at management scale. We propose and provide evidence for the simple hypothesis that local species diversity is related to spatial environmental heterogeneity. Species partition the environment into habitats. Biodiversity is therefore expected to be influenced by two aspects of spatial heterogeneity: 1) the variability of environmental conditions, which will affect the number of types of habitat, and 2) the spatial configuration of habitats, which will affect the rates of ecological processes, such as dispersal or competition. Earlier, simulation experiments predicted that both aspects of heterogeneity will influence plant species richness at a particular site. For the first time, these predictions were tested for plant communities using field data, which we collected in a wooded pasture in the Swiss Jura mountains using a four-level hierarchical sampling design. Richness generally increased with increasing environmental variability and "roughness" (i.e. decreasing spatial aggregation). Effects occurred at all scales, but the nature of the effect changed with scale, suggesting a change in the underlying mechanisms, which will need to be taken into account if scaling up to larger landscapes. Although we found significant effects of environmental heterogeneity, other factors such as history could also be important determinants. If a relationship between environmental heterogeneity and species richness can be shown to be general, recently available high-resolution environmental data can be used to complement the assessment of patterns of local richness and improve the prediction of the effects of land use change based on mean site conditions or land use history.     

景观空间异质性对生态系统服务形成与供给的影响

景观空间异质性与生态系统服务的关系极为密切,适当调整景观空间异质性有助于生态系统服务的持续形成与稳定供给。研究景观空间异质性和生态系统服务形成与供给之间的相互影响作用及响应机制具有重要的理论与现实意义,是保护生物多样性、管理生态系统服务与优化景观空间配置的基础。现有研究大多在不同尺度上探讨了景观格局与生态过程或生态系统服务间的相互影响关系,而缺乏景观格局-生态过程-生态系统服务三者间有效联结等方面的研究。景观空间异质性是怎样直接或间接地作用于生态系统服务形成与供给的,目前还没有一个较为明确的解释。因此,通过分析国内外文献,回顾了景观格局或景观空间异质性与生态系统服务之间关系的研究进展、研究内容和研究方法;从景观组成、景观构型的变化入手,讨论了景观空间异质性对生态系统服务形成与供给的影响及其强度,并认为景观组成异质性变化能够直接影响生态系统服务,而景观构型异质性变化会通过改变生态过程而间接影响生态系统服务;阐述了景观空间异质性在影响生态系统服务形成与供给的同时,也使生态系统服务在空间上产生了异质性分布,并从自然因素和人为因素两个方面对其进行解释;强调了尺度问题在景观空间异质性与生态系统服务研究中的重要性;最后,明确了对生态系统服务形成与供给的景观空间异质性影响研究不仅有助于生态系统服务的维持与调节,也能更深层次地揭示其中的生态学意义。     

Spatial scaling of ecosystem C and N in a subtropical savanna landscape

Widely occurred woody encroachment in grass‐dominated ecosystems has the potential to influence soil organic carbon (SOC) and total nitrogen (TN) pools at local, regional, and global scales. Evaluation of this potential requires assessment of both pool sizes and their spatial patterns. We quantified SOC and TN, their relationships with soil and vegetation attributes, and their spatial scaling along a catena (hill‐slope) gradient in the southern Great Plains, USA where woody cover has increased substantially over the past 100 years. Quadrat variance analysis revealed spatial variation in SOC and TN at two scales. The larger scale variation (40–45 m) was approximately the distance between centers of woody plant communities and their adjoining herbaceous patches. The smaller scale variation (10 m) appeared to reflect the local influence of shrubs on SOC and TN. Litter, root biomass, shrub, and tree basal area (a proxy for plant age) exhibited not only similar spatial scales, but also strong correlations with SOC and TN, suggesting invasive woody plants alter both the storage and spatial scaling of SOC and TN through ecological processes related primarily to root turnover and, to a lesser extent litter production, as mediated by time of occupancy. Forb and grass biomass were not significantly correlated with SOC and TN suggesting that changes in herbaceous vegetation have not been the driving force for the observed changes in SOC and TN. Because SOC and TN varied at two scales, it would be inappropriate to estimate SOC and TN pools at broad scales by extrapolating from point sampling at fine scales. Sampling designs that capture variation at multiple scales are required to estimate SOC and TN pools at broader scales. Knowledge of spatial scaling and correlations will be necessary to design field sampling protocols to quantify the biogeochemical consequences of woody plant encroachment at broad scales.     

Spatial heterogeneity in ecosystem structure and productivity in a moist Kenyan savanna

Topographic variations and disturbances are key sources of spatial heterogeneity in the ecosystem and may influence its functioning, productivity, and carbon (C) storage. In water controlled ecosystems, structural and functional heterogeneity become distinct during drought when the ecosystem processes are operating at their limits. We examined spatial heterogeneity arising from grazing, abandoned cropland, presence of Acacia trees, and termite mounds (termitaria). Soil water content (SWC) was significantly (P < 0.05) higher in termitaria and fenced (un-grazed) plots. Higher soil nitrogen (N) content occurred in the Acacia, termitaria, and fenced plots while total biomass was highest in the fenced plot. The termitaria plots showed the highest net ecosystem CO₂ exchange (NEE), ecosystem respiration (R _eco), and gross primary production (GPP) and were the only plots that were net CO₂ sinks. Except in fenced plots, maximum GPP was positively correlated with SWC and green biomass in all the other plots. Green biomass and R _eco were positively correlated with SWC. Shifting cultivation (abandoned farmlands) negatively affected soil quality, ecosystem CO₂ assimilation, and productivity. Removal of grazing (cattle) from the ecosystem negatively influenced GPP, while the presence of termitaria and Acacia trees facilitated soil water and N availability and ecosystem productivity. We concluded that soil water availability was responsible for most of the localized differences in the savanna and has a strong influence on ecosystem C capture and storage. We recommend that future studies on savanna productivity and ecosystem CO₂ fluxes should consider heterogeneity in the ecosystem in order to avoid bias and increase the accuracy of any estimates made.