Aims Classification of vegetation is an essential tool to describe, understand, predict and manage biodiversity. Given the multiplicity of approaches to classify vegetation, it is important to develop international consensus around a set of general guidelines and purpose‐specific standard protocols. Before these goals can be achieved, however, it is necessary to identify and understand the different choices that are made during the process of classifying vegetation. This paper presents a framework to facilitate comparisons between broad‐scale plot‐based classification approaches. Results Our framework is based on the distinction of four structural elements (plot record, vegetation type, consistent classification section and classification system) and two procedural elements (classification protocol and classification approach). For each element we describe essential properties that can be used for comparisons. We also review alternative choices regarding critical decisions of classification approaches; with a special focus on the procedures used to define vegetation types from plot records. We illustrate our comparative framework by applying it to different broad‐scale classification approaches. Conclusions Our framework will be useful for understanding and comparing plot‐based vegetation classification approaches, as well as for integrating classification systems and their sections.

A new approach to characterise geographical areas with a drought risk index (DRI) is suggested, by applying an artificial neural network (ANN) classifier to bioclimatic time series for which operational temporal units (OtUs) are defined. A climatic database, corresponding to a grid of 8 km x 8 km cells covering the Italian peninsula, was considered. Each cell is described by the time series of seven variables recorded from 1989 to 2000. Sixteen cells were selected according to land cover homogeneity and completeness of the time series data. The periodic components of the time series were calculated by means of the fast Fourier transform (FFT) method. Temporal units corresponding to the period of the sinusoidal functions most related to the data were used as OtUs. The ANN for each OtU calculates a DRI value ranging between -1 and 1. The value is interpretable as the proximity of the OtUs to one of two situations corresponding to minimum and maximum drought risk, respectively. The former set (DRI = -1) is represented by an ideal OtU with minimum values of temperatures and evapo-transpiration, and maximum values of rainfall, normalised difference vegetation index (NDVI) and soil water content. The second set (DRI = 1) is represented by the reciprocal OtU to the former one. The classification of the cells based on DRI time profiles showed that, at the scale used in this work, DRI has no dependence on land cover class, but is related to the location of the cells. The methodology was integrated with GIS (geographic information system) software, and used to show the geographic pattern of DRI in a given area at different periods.

Vegetation and environment have been analyzed along an altitudinal gradient in Harena Forest, Bale Mountains National Park, southeastern Ethiopia. Vegetation data include numbers of each tree and shrub species and cover-abundance values of each herbaceous species. Environmental data comprise edaphic factors, altitude and topography. The two vegetation layers data were analysed separately.Probabilistic similarity coefficients were computed between the releves, and these values were used in subsequent computations for classification and ordination. Two sets of stratocoena, comprising 6 types each, derived on the basis of separate analyses of tree-shrub and herb layers of the forest were recognised.

-This study reports measurements of stomatal conductance, relative water content and water potential (Ψ L ) from three dominant evergreens (Ceratonia siliqua L., Quercus coccifera L. and Olea oleaster Hoffmgg. et Link) growing in four coastal sites of Turkey. In particular, a fully vegetated site (H) was selected and compared for the above parameters to three degraded sites (D1, D2 and D3) with decreasing vegetation covers. From the integral of the diurnal time course of Ψ L , the water stress impact on each species (WSIS) was calculated. C. siliqua and Q. coccifera showed similar WSIS's, increasing significantly from H to D3. O. oleaster was sensitive both to summer drought and to increasing site degradation. The impact of water stress was scaled up from the species to the vegetation level (WSIV) as WSIV = Σ WSIS S (1-f s ) where f s was the relative frequency of the species studied. WSIV was rather sensitive to the impoverishment of vegetation and was correlated to vegetation density as estimated both by field observations and remotely sensed Normalized Difference Vegetation Index. desertification / leaf water potential / Mediterranean evergreens / Normalized Difference Vegetation Index / water stress impact Résumé -Diagnostiquer l'état de l'eau dans la plante : un outil pour quantifier le stress hydrique au niveau régional dans les régions sèches méditerranéennes. Cette étude rapporte les mesures de conductance stomatique, de la teneur relative en eau et du potentiel hydrique (Ψ L ) d'arbres à feuilles persistantes (Ceratonia siliqua L., Quercus coccifera L. et Olea oleaster Hoffmgg. Et Link) croissant sur 4 sites côtiers de Turquie. En particulier, un site totalement recouvert de végétation (H) a été sélectionné et comparé, pour les paramètres ci-dessus, à 3 sites dégradés (D1, D2 et D3) ayant une couverture végétale de plus en plus faible. A partir de l'intégrale de Ψ L , pour le cycle diurne, l'impact du stress hydrique de chaque espèce (WSIS) a été calculé. C. siliqua et Q. coccifera montrent des WSIS similaires, augmentant significativement de H à D3. O. oleoaster a été sensible à la fois à la sécheresse estivale et à l'accroissement de la dégradation du site. Un changement d'échelle, du niveau de l'espèce à celui de la végétation, a été réalisé pour l'impact du stress hydrique (WSIV) par la transformation WSIV = Σ WSISs (1-f s ) ou f s est la fréquence relative de l'espèce étudiée. WSIV est particulièrement sensible à l'appauvrissement de la végétation et est corrélé à la densité de la végétation estimée à la fois par des observations aux champs et par l'indice normalisé de différentiation de la végétation par observation satellitaire. désertification / potentiel hydrique des feuilles / arbre à feuilles persistantes méditerranéen / index normalisé de différentia-tion des espèces / impact du stress hydrique Ann. For. Sci. 58 (2001) 113-125 113

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