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Author(s): Mahesh Sankaran (corresponding author) [1]; Niall P. Hanan [1]; Robert J. Scholes [2]; Jayashree Ratnam [1]; David J. Augustine [3]; Brian S. Cade [4]; Jacques Gignoux [5]; Steven I. [...]

1 Using a spatially explicit cellular automaton model we explore the effects of tree demography, fire-induced mortality, and seed dispersal on the spatial spread of a single tree species in a humid savanna at Lamto in West Africa. 2 The model system is described by six parameters and consists of a grass-surrounded square grid of connecting cells, each being either inhabited by grass alone or by grass and an individual tree. In the baseline numerical simulations the tree can only recruit seedlings in immediately adjacent cells. These seedlings may perish from annual grass fires in their first year of life if they are not protected from the advancement of the fire by neighbouring reproductively mature trees. 3 Based on preliminary parameter estimates from data collected at field sites at Lamto, we predict that fire slows, but does not stop, the spread of the tree. In the absence of fire the doubling rate of the tree population is about 6 years, whereas we predict that yearly fires prolong this to at least 30 years. 4 The temporal dynamics of the tree population are fairly smooth and predictable as long as there are more than c. 100 cells in the system. As the number of cells is decreased below c. 100 the trajectories become increasingly variable from year to year. 5 Mortalities from fire act in an inverse spatially density-dependent fashion, enhancing tree aggregation. The role of fire in enhancing tree aggregation is supported by additional simulations in which dispersal of seeds to non-adjacent cells can occur. When a small amount of dispersal is possible the rate of tree population growth is greatly accelerated as compared to when no such dispersal occurs. 6 We present several hypotheses to explain why the savanna at Lamto is not tree-dominated as would be predicted by the model, discuss how seed dispersal and fire influence tree dynamics, and make predictions for future testing.

Urban soils are a crucial component of urban ecosystems, especially in public green spaces, because of the ecosystem services they provide (e.g. public recreation, urban cooling or water infiltration). In this study, we describe the chemical, physical and hydrostructural characteristics of 180 forest and lawn surface soil samples, taken along an urban–rural gradient in the Paris region. This was done in order to identify how these soils have been affected by urbanization. Forests and lawns are the main vegetation types found in this region and represent 21% and 22.2% of the territory's surface area, respectively. Many of the properties of urban forest soils differed from those of other sites (e.g. texture, organic carbon content, total nitrogen and carbonate contents), possibly because the urban forests are much older than the lawns and because of the legacy of the historical management of soils in this region (Haussmann period). Urban lawn soils were more compacted than urban forests, probably due to higher foot traffic. The effects of urbanization were, at times, confounded with other factors (e.g. sandier texture of urban forests), which suggests that surface soil characteristics were influenced by past urban planning. Finally, this study constitutes a baseline analysis for the monitoring of soil quality in the region. [ABSTRACT FROM AUTHOR]