Summary1 Spatially periodic vegetation patterns are well known in arid and semi-arid regions around the world. 2 Mathematical models have been developed that attribute this phenomenon to a symmetry-breaking instability. Such models are based on the interplay between competitive and facilitative influences that the vegetation exerts on its own dynamics when it is constrained by arid conditions, but evidence for these predictions is still lacking. Moreover, not all models can account for the development of regularly spaced spots of bare ground in the absence of a soil prepattern. 3 We applied Fourier analysis to high-resolution, remotely sensed data taken at either end of a 40-year interval in southern Niger. Statistical comparisons based on this textural characterization gave us broad-scale evidence that the decrease in rainfall over recent decades in the sub-Saharan Sahel has been accompanied by a detectable shift from homogeneous vegetation cover to spotted patterns marked by a spatial frequency of about 20 cycles km − 1 . 4 Wood cutting and grazing by domestic animals have led to a much more marked transition in unprotected areas than in a protected reserve. 5 Field measurements demonstrated that the dominant spatial frequency was endogenous rather than reflecting the spatial variation of any pre-existing heterogeneity in soil properties. 6 All these results support the use of models that can account for periodic vegetation patterns without invoking substrate heterogeneity or anisotropy, and provide new elements for further developments, refinements and tests. 7 This study underlines the potential of studying vegetation pattern properties for monitoring climatic and human impacts on the extensive fragile areas bordering hot deserts. Explicit consideration of vegetation self-patterning may also improve our understanding of vegetation and climate interactions in arid areas.

Summary1 Mathematical proofs show that rate estimates, for example of mortality and recruitment, will decrease with increasing census interval when obtained from censuses of non-homogeneous populations. This census interval effect could be confounding or perhaps even driving conclusions from comparative studies involving such rate estimates. 2 We quantify this artefact for tropical forest trees, develop correction methods and re-assess some previously published conclusions about forest dynamics. 3 Mortality rates of > 50 species at each of seven sites in Africa, Latin America, Asia and Australia were used as subpopulations to simulate stand-level mortality rates in a heterogeneous population when census intervals varied: all sites showed decreasing stand mortality rates with increasing census interval length. 4 Stand-level mortality rates from 14 multicensus long-term forest plots from Africa, Latin America, Asia and Australia also showed that, on average, mortality rates decreased with increasing census interval length. 5 Mortality, recruitment or turnover rates with differing census interval lengths can be compared using the mean rate of decline from the 14 long-term plots to standardize estimates to a common census length using λ corr = λ × t 0.08 , where λ is the rate and t is time between censuses in years. This simple general correction should reduce the bias associated with census interval variation, where it is unavoidable. 6 Re-analysis of published results shows that the pan-tropical increase in stem turnover rates over the late 20th century cannot be attributed to combining data with differing census intervals. In addition, after correction, Old World tropical forests do not have significantly lower turnover rates than New World sites, as previously reported. Our pan-tropical best estimate adjusted stem turnover rate is 1.81 ± 0.16% a − 1 (mean ± 95% CI, n = 65). 7 As differing census intervals affect comparisons of mortality, recruitment and turnover rates, and can lead to erroneous conclusions, standardized field methods, the calculation of local correction factors at sites where adequate data are available, or the use of our general standardizing formula to take account of sample intervals, are to be recommended.

La dynamique temporelle des forêts d'une zone de transition forêt-savane, située dans le département de Tanda, à l'est de la Côte d'Ivoire, a été déterminée à partir de l'analyse diachronique de deux images satellitaires de type Landsat TM 1986 et Landsat ETM+ 2000, et de missions de vérification sur le terrain. Les résultats indiquent que les surfaces forestières en 2000 couvraient 29 % de la zone d'étude au lieu de 73 % en 1986. Cette régression forestière a été compensée par une augmentation des zones occupées par les savanes. Les forêts denses sont les plus affectées par cette perte d'habitat, leur superficie ayant diminué de 83 % pendant la période d'étude. La dynamique spatiale du paysage, caractérisée par des indices structuraux, a montré un processus de création de taches de savanes par opposition à la suppression de celles des formations boisées. D'une façon générale, la région d'étude a subi une transformation importante liée essentiellement aux activités agricoles et à la pression démographique. (Résumé d'auteur)

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Questions Soil properties have been shown to partially explain tree species distribution in tropical forests. Locally, species turnover across space can result not only from edaphic heterogeneities but also from limited seed dispersal. To characterize the contribution of each process, contact areas between contrasted soil types offer ideal settings. In the present study, we aimed to test species and species assemblage responses to a sharp edaphic discontinuity in a tropical forest tree community. Location Yoko forest reserve (6975 ha), Democratic Republic of the Congo. Methods We set up four 500–600‐m long parallel transects crossing two contrasted edaphic habitats, one lying on clayey soil and the other on sandy soil. The canopy and subcanopy trees were identified and geo‐referenced along the transects over a width of 50 m and 5 m, respectively, and soil samples were collected every 50 m to characterize each habitat. Results Correspondence analyses indicated a clear differentiation of tree communities between sandy and clayey soils. Using a torus‐translation method combined with Chi‐squared non‐parametric tests, we observed that ca. 40% and 18% of the species represented by at least 12 individuals displayed significant density differences according to habitat in the canopy and subcanopy, respectively, although very few species displayed significant differences in their relative abundance. Nevertheless, whole community tests of differentiation (in species relative abundances) between soil types were significant in both strata, even after removing individual species or families displaying a significant habitat preference. Conclusion While only a minority of species displayed a clear habitat preference, we still observed a community‐wide impact of the edaphic discontinuity on species assemblages at a local scale. Our results provide further evidence for the major contribution of environmental heterogeneity in maintaining biodiversity in tropical forests.

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Située à proximité de Kinshasa, la province du Bas-Congo (R.D. Congo) est l'une des principales pourvoyeuses en produits vivriers de cette ville. Par conséquent, l'augmentation de la pression sur les ressources naturelles s'accentue. L'analyse diachronique du paysage peut servir à montrer l'effet des actions anthropiques sur l'occupation du sol de cette province. La présente étude a pour but la quantification de la dynamique paysagère dans une zone test de cette province entre 1960 et 2005. Elle a permis, grâce à la matrice de transition appuyée par le calcul d'indices de structure spatiale, de montrer que l'occupation du sol a profondément changé. La matrice du paysage, initialement constituée par les forêts secondaires (49,95% du paysage), s'est dégradée au profit des savanes (qui ont conquis 14,23% des forêts secondaires) et des jachères et champs (qui ont conquis 27,23% des forêts secondaires). L'étude a ainsi révélé trois processus de transformation du paysage: la savanisation, la fragmentation de l'écosystème forestier et enfin une la formation de forêt dense (succession). Ces changements-excepté la succession-sont principalement dus à des perturbations anthropiques. La pression démographique et les pratiques agricoles non durables ont orienté la modification de l'occupation du sol. Les écosystèmes forestiers ont été substitués par des écosystèmes anthropisés menaçant alors la biodiversité de cette zone.