Trace element concentrations along a gradient of urban pressure in forest and lawn soils of the Paris region (France).

The concentration, degree of contamination and pollution of 7 trace elements (TEs) along an urban pressure gradient were measured in 180 lawn and wood soils of the Paris region (France). Iron (Fe), a major element, was used as reference element. Copper (Cu), cadmium (Cd), lead (Pb) and zinc (Zn) were of anthropogenic origin, while arsenic (As), chromium (Cr) and nickel (Ni) were of natural origin.

Allocation strategies of savanna and forest tree seedlings in response to fire and shading: outcomes of a field experiment.

The forest-savanna ecotone may be very sharp in fire-prone areas. Fire and competition for light play key roles in its maintenance, as forest and savanna tree seedlings are quickly excluded from the other ecosystem. We hypothesized a tradeoff between seedling traits linked to fire resistance and to competition for light to explain these exclusions.

Predator foraging behaviour drives food-web topological structure.

1. The structure and dynamics of prey populations are shaped by the foraging behaviours of their predators. Yet, there is still little documentation on how distinct predator foraging types control biodiversity, food-web architecture and ecosystem functioning.

Determinants of woody cover in African savannas.

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure.

How do sessile dioecious species cope with their males?

In terrestrial plants the segregation of male and female reproductions on different individuals results in the seed-shadow handicap: males do not disperse any seed so that the number of local patches reached by seeds is potentially reduced in dioecious populations in comparison to hermaphrodite populations. An analytical model, incorporating a lottery-based recruitment and dispersal stochasticity, was built. The spatially mediated cost of the seed-shadow handicap has been assessed considering the criterions for the invasion of a resident hermaphrodite species by a dioecious species and the reverse invasion, both species having the same demographic parameters but assuming a likely higher fecundity for dioecious females.

Spatial and temporal variations in leaf area index, specific leaf area and leaf nitrogen of two co-occurring savanna tree species.

Foliage growth, mass- and area-based leaf nitrogen concentrations (Nm and N a) and specific leaf area (SLA) were surveyed during a complete vegetation cycle for two co-occurring savanna tree species: Crossopteryx febrifuga (Afzel. ex G. Don) Benth.

Which functional processes control the short-term effect of grazing on net primary production in grasslands?

Grazing has traditionally been viewed as detrimental to plant growth, but it has been proposed that under certain conditions, grazing may lead to compensatory or overcompensatory growth. However, comprehensive information on the relative role of the main functional processes controlling the response of net primary production (NPP) to grazing is still lacking. In this study, a modelling approach was used to quantify the relative importance of key functional processes in the response of annual canopy NPP to grazing for a West African humid grassland.

Comparing the performances of Diggle's tests of spatial randomness for small samples with and without edge-effect correction: application to ecological data.

Diggle's tests of spatial randomness based on empirical distributions of interpoint distances can be performed with and without edge-effect correction. We present here numerical results illustrating that tests without the edge-effect correction proposed by Diggle (1979, Biometrics 35, 87-101) have a higher power for small sample sizes than those with correction. Ignoring the correction enables detection of departure from spatial randomness with smaller samples (down to 10 points vs.

Does the timing of litter inputs determine natural abundance of (13)C in soil organic matter? Insights from an African tiger bush ecosystem.

We investigated total primary production and natural abundance of (13)C in soil and plants in the landscape of tiger bush, Niger. Tiger bush is viewed as a natural cyclic succession of several types of vegetation (grasses, living trees and senescent vegetation) occurring over very small areas, on soils with similar chemical and physical characteristics. Under the pioneer front, production was 130 g m(-2) year(-1) of which 23% came from C4 plants; under the thicket of mature trees, grass production was 190 g m(-2) year(-1) (all C3 grasses) and under senescent vegetation, 40 g m(-2) year(-1) of which 1.

Alternative fire resistance strategies in savanna trees.

Bark properties (mainly thickness) are usually presented as the main explanation for tree survival in intense fires. Savanna fires are mild, frequent, and supposed to affect tree recruitment rather than adult survival: trunk profile and growth rate of young trees between two successive fires can also affect survival. These factors and fire severity were measured on a sample of 20 trees near the recruitment stage of two savanna species chosen for their contrasted fire resistance strategies (Crossopteryx febrifuga and Piliostigma thonningii).