A general model is presented describing ecosystem degradation to help decide when restoration, rehabilitation, or reallocation should be the preferred response. The latter two pathways are suggested when one or more “thresholds of irreversibility” have been crossed in the course of ecosystem degradation, and when “passive” restoration to a presumed predisturbance condition is deemed impossible. The young but burgeoning field of ecological restoration, and the older field of rehabilitation and sustainable range management of arid and semiarid lands (ASAL), are found to have much in common, especially compared with the reallocation of lands, which is often carried out without reference to pre‐existing ecosystems. After clarifying some basic terminology, we present 18 vital ecosystem attributes for evaluating stages of degradation and planning experiments in the restoration or rehabilitation of degraded ecosystems. Finally, we offer 10 hypotheses concerning ecological restoration and rehabilitation as they apply to ASAL and perhaps to all terrestrial ecosystems.

Phenological patterns of herbaceous and woody plants well represented in the Sudano-Sahelian savannas of northern Cameroon were studied in relation to water availability in three contrasted soil types. Mean annual precipitation in this region is 800 mm, entirely restricted to the period May-June to September-October, but redistribution of rainwater by varying soil substrates leads to considerable heterogeneity in available water resources for plants. Results are discussed in terms of probable competition for water resources among co-occurring plants and the adaptations shown by observed patterns to prevailing environmental conditions. Water stress is seen to be a limiting factor to plant growth but does not constitute the sole trigger for the phenological phases observed, nor is it the principal factor responsible for the preponderance of annual species in the herbaceous stratum.

Abstract. Relationships between plant form, plant function, and environmental factors are analyzed in order to test the efficiency of growth form attributes and phenomorphology for studying vegetation on a regional scale. Examples of sclerophyllous evergreen coppices with Quercus ilex in the mediterranean climate of southern France are examined. The results for three growth form attributes (renewal bud location, leaf consistency, leaf size) and some environmental factors show that these criteria are efficient along a gradient of increasing stress. Concerning phenomorphology, some trends along the main environmental gradient become evident, but these results should be confirmed with descriptions of more species. It is suggested that man's ancient, very strong influence upon these ecosystems could explain some results showing that growth form distribution does not fit the pattern of variation in environmental factors.

A model of ecosystem degradation and three possible responses to it—restoration, rehabilitation, and real‐location—is applied to ongoing projects in the arid mediterranean region of southern Tunisia, the subhumid mediterranean region of central Chile, and the semiarid tropical savannas of northern Cameroon. We compare both nonhuman and human determinants of ecosystem degradation processes in these contrasted regions, as well as interventions being tested in each. A number of quantifiable “vital ecosystem attributes” are used to evaluate the effects of ecosystem degradation and the experimental responses of rehabilitation on vegetation, soils and plant‐soil‐water relations. We argue that attempts to rehabilitate former ecosystem structure and functioning, both above‐ and below ground, are the best way to conserve biodiversity and insure sustainable long‐term productivity in ecosystems subjected to continuous use by people in arid and semi‐arid lands of “the South.” The success of such efforts, however, depends not only on elucidating the predisturbance (or slightly disturbed) structure and function of the consciously selected “ecosystem of reference,” but also on understanding and working with the socioeconomic, technical, cultural, and historical factors that caused the degradation in the first place.