Adaptation of five co-occurring tree and shrub species to water stress and its implication in restoration of degraded lands

The open savanna woodlands in Ethiopia, distributed over very large areas of the Rift valley in southern and eastern parts of the country, have suffered from deforestation due to excessive tree cutting and overgrazing. Previous reforestation programs attempting to counteract the effects of deforestation have failed mainly due to lack of knowledge on site–species matching. This research was set out to asses the effect of disturbance on water status of both plants and their respective sites, explain differences among species in survival and distribution and determine their relative suitability for restoration of degraded lands. Plant water potentials of five common co-occurring species were measured in the field at midday and predawn in the dry season. The water potential values of the species at different sites of biophysical settings and diurnal differences revealed significant difference among the species. The differences in degradation status in terms of tree density, ground cover and overgrazing did not show a clear pattern in water potential changes among the species and sites. Acacia tortilis, Balanites aegyptiaca and Dichrostachys cinerea, which exhibited low midday (−3.05 to −4.85 MPa), predawn (−1.98 to −3 MPa) and wide diurnal (1.16–2.25 MPa) plant water potential ranges had wider capacities to withstand water changes and, hence, can be considered as suitable candidates for reforestation in drought prone areas. Though Acacia senegal and Acacia seyal with highest midday (−1.55 to −2.68 MPa), predawn (−0.83 to −2.09 MPa) and narrow diurnal (0.32–1.1 MPa) plant water potential ranges could be categorized as drought avoider (sensitive) species, they have a combination of mechanisms to avoid drought and grow in dry areas. Particularly, their better growth performances during moist years could make them suitable candidates in areas of non-limiting water conditions. Both exotic and indigenous species, with similar water potentials to those found in the respective sites, may have a better chance of survival in those sites and in areas that have similar ecological and climatological conditions to the ones the current research covered.