A model for the explanation of vegetation stripes (tiger bush)

Abstract. Regular vegetation patterns appear on aerial views of plateaux in SW Niger where densely and sparsely populated zones alternate with each other. This spatial organization of the vegetation is an endogenous phenomenon which is not limited to specific plants or soils; it is a characteristic landscape of many arid regions throughout the world. The phenomenon is interpreted as the result of a spatial range difference between two biologically distinct interactions operating at the plant population level. The proposed mechanism is independent of external heterogeneities deriving from soil geomorphology or meteorology. We present a model to simulate the genesis of vegetation stripes. In addition, the model predicts the occurrence of vegetation hexagons corresponding to higher or lower density spots arranged in a hexagonal lattice. The distinction between the two spatial symmetries is discussed in terms of their Fourier transforms.     

Differences between banded thickets (tiger bush) at two sites in West Africa

Abstract. This paper deals with the influence of edaphic conditions on the spatial structure of banded thickets or tiger bush (brousse tigrée). It is based on two sites in West Africa, with similar climatic conditions but located on contrasting substrates. The spatial structure was described with standardized characteristics including thicket spacing, thicket/inter‐thicket contrast, upslope/downslope asymmetry and species zonation throughout the vegetation band. Recruitment and senescence features of woody stands were emphasized in order to understand current dynamics. Data were collected on transects oriented perpendicular to the contours and so to the thickets as well. A standardized analytical procedure was applied to data from both sites to ensure consistent and thorough delineation of thickets. The overall periodicity of thickets, the woody flora and the dominant species Combretum micranthum were similar at the two sites. However, thicket spacing, thicket/inter‐thicket contrast and upslope/downslope floristic asymmetry of the thickets were higher in the less favourable site. Also seedlings were less abundant, with a greater dependence on pre‐existing thickets. Not all banded vegetation systems show sharp contrasts and are strongly asymmetric, since such characteristics are likely to be reinforced by adverse environmental conditions. As a consequence, current dynamics may be more diverse than expected. Quantified inter‐site comparisons can greatly help to classify African banded vegetation systems and to discuss potential dynamic outcomes.     

Soil organic matter dynamics in tiger bush (Niamey,Niger). Preliminary results

Some typical features of soil organic matter dynamics and soil texture were studied to discuss the particular spatial pattern of tiger bush in Niger and its dynamics. The soil texture through silt and clay contents showed a high variability in the vegetation arc as well as in the bare area. These variations were clearly linked to water/wind erosion and termite activity. Tiger bush soils showed a high capacity to store soil organic matter despite a moderate primary production, even in the bare area where the input of plant debris has been nil for many decades. The carbon content was higher within the vegetation arc (0.93 %) than within the bare area (0.45 %). Additionally, potential carbon mineralisation significantly varied in relation to the total carbon content and thus to primary production. Then, the vegetation arcs can be viewed as `fertility islands' as in many arid ecosystems. The measurements of δ¹³C showed a dominant contribution of C₃ plants to the soil organic matter pool. Nevertheless, the contribution of C₄ plants was not negligible. Two hypotheses could be proposed: a different mineralisation rate between C₃ and C₄ plants; or (ii) a better physical protection of C₄ compounds against biodegradation. The soil variables depending totally or partly on biological factors, such as carbon and nitrogen contents, carbon isotopic composition, carbon potential mineralisation, did not show any symmetry in their variations along the studied transects. It was expected in the vegetation arc because the vegetation cover does not show symmetry in its specific composition and spatial structure. In the bare area, a clear asymmetry was observed on some of the variables: carbon content, fine material content and natural abundance of ¹³C. This supports the hypothesis that the vegetation arcs move upslope, and weakens the hypothesis of the alternance of contraction and spreading periods of the vegetation cover.     

The persistent decline of patterned woody vegetation: The tiger bush in the context of the regional Sahel greening trend

Following 25 years of below average annual rainfall in the Sahel between 1970 and 1995, the return to more humid conditions has led to rapid postdrought recovery of the woody cover. However, the increase in the woody cover is not spatially homogeneous raising questions about the resilience of some woody vegetation types. Based on the analysis of field and remote sensing data collected on the tiger bush systems in the northern Sahel in Mali, this study noted the current and persistent degradation of these systems in the Sahel since the 1970s despite the recent improvement in rainfall since the mid‐1990s and the general Sahel re‐greening. Profound changes in the woody population pattern, tree density and cover, and floristic composition took place regardless of the site location along the south–north rainfall gradient. Associated with definite structural changes of the woody population, surface hydrology shifted from a sheet to concentrated run‐off accelerating the collapse of the patterned woody population. Currently, there is no evidence in favour of reversing the current degradation process, at least at a decadal scale, although very sparse recolonization by pioneer woody vegetation has been observed in the driest sites along recently formed gullies. These observations support the hypothesis of an ecosystem shift, with long‐term implications for the structure and functioning of the patterned vegetation, as well as the whole watershed landscape through increased run‐off leading to stronger water flows in enlarged wadis, increased soil erosion upstream and sediment deposition downstream, enhanced water storage in ponds, and greater recharge of aquifers, which is an illustration of the “Sahelian paradox”.     

Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes

It is a well-known phenomenon that islands can support populations of gigantic or dwarf forms of mainland conspecifics, but the variety of explanatory hypotheses for this phenomenon have been difficult to disentangle. The highly venomous Australian tiger snakes (genus Notechis) represent a well-known and extreme example of insular body size variation. They are of special interest because there are multiple populations of dwarfs and giants and the age of the islands and thus the age of the tiger snake populations are known from detailed sea level studies. Most are 5000-7000 years old and all are less than 10,000 years old. Here we discriminate between two competing hypotheses with a molecular phylogeography dataset comprising approximately 4800 bp of mtDNA and demonstrate that populations of island dwarfs and giants have evolved five times independently. In each case the closest relatives of the giant or dwarf populations are mainland tiger snakes, and in four of the five cases, the closest relatives are also the most geographically proximate mainland tiger snakes. Moreover, these body size shifts have evolved extremely rapidly and this is reflected in the genetic divergence between island body size variants and mainland snakes. Within south eastern Australia, where populations of island giants, populations of island dwarfs, and mainland tiger snakes all occur, the maximum genetic divergence is only 0.38%. Dwarf tiger snakes are restricted to prey items that are much smaller than the prey items of mainland tiger snakes and giant tiger snakes are restricted to seasonally available prey items that are up three times larger than the prey items of mainland tiger snakes. We support the hypotheses that these body size shifts are due to strong selection imposed by the size of available prey items, rather than shared evolutionary history, and our results are consistent with the notion that adaptive plasticity also has played an important role in body size shifts. We suggest that plasticity displayed early on in the occupation of these new islands provided the flexibility necessary as the island's available prey items became more depauperate, but once the size range of available prey items was reduced, strong natural selection followed by genetic assimilation worked to optimize snake body size. The rate of body size divergence in haldanes is similar for dwarfs (h(g) = 0.0010) and giants (h(g) = 0.0020-0.0025) and is in line with other studies of rapid evolution. Our data provide strong evidence for rapid and repeated morphological divergence in the wild due to similar selective pressures acting in different directions.     

On the origin of plastids

The buoyant density in CsCl of ribosomes from chloroplasts of the green algaChlorella pyrenoidosa and two species of higher plants,Pisum sativum andChenopodium album, has been studied. From the relative protein content it was calculated that 70S ribosomes from chloroplasts are much smaller than 80S cytoplasmic ribosomes (3.0–3.1×10⁶ and 4.0×10⁶ daltons) and slightly larger than 70S ribosomes from abcteriaE. coli 2.5×10⁶ daltons). Chloroplast ribosomes from pea seedlings were analyzed by two-dimensional polyacrylamide gel electrophoresis. They appear to contain 71 proteins. This indicates that chloroplast ribosomes contain a larger number of proteins than do the ribosomes fromE. coli and other species of Enterobacteriaceae. Further study will permit a probable evaluation of the validity of Mereschkowsky's hypothesis that the photosynthetic plastids of eukaryotic plant cells are the evolutionary descendants of endosymbiotic blue-green algae.