Key message Acacia senegal and Acacia seyal present different drought stress coping mechanisms that are independent of substrate fertility. Higher substrate fertility increased aboveground plant growth, even with low watering. Abstract The potential of native African tree species for agriculture and forestry have not yet been thoroughly investigated. In this experiment, we studied the early growth of Acacia senegal and Acacia seyal plants in an experiment with two substrates of contrasting fertility (low/ high) and two watering frequency regimes (low = 24.40 l/m 2 per month, high = 48.80 l/m 2 per month). Our objectives were: (1) to study whether the mechanisms by which nutrients affect plant growth at the seedling stage operate differently when water availability varies, and (2) to look for differences in the growth strategies of the two species in early stages. Higher substrate fertility increased aboveground plant growth at the expense of roots in both water regimes. Though water stress significantly limited growth under both soil conditions, substrate fertility effects were relatively higher in plants with low water supply than in those with high water supply. However, even with low resources the root-to-shoot ratio was between 0.7 and 0.9, plants presented adequate nutrition and no mortality was observed. This indicates opportunistic mechanisms for water and nutrient use. A. seyal showed the lowest negative pre-dawn stem water potential value (-0.15 MPa) and shed nearly all leaves in the hottest month of the assay, which suggests a different drought avoidance strategy and adaptation to water stress than A. senegal. Both species can be produced successfully in local nursery conditions and can survive and thrive with low watering. The study also demonstrated that fast growing genotypes can be effectively isolated in nursery conditions.

African dry forests and Acacia savanna woodlands have tended to disappear due to severe environmental conditions and anthropogenic impacts such as human population increase and increased land demand for crops, which have resulted in deforested areas and poor and unfertile soils. On the other hand, relative aridity, precipitation and water availability in the soil are among the most important environmental factors influencing vegetation in arid and semiarid lands of Africa and fostering degradation processes. So, knowledge on the germination process, and on the initial growth and development of forest species from arid and semiarid lands of Africa, is a key point to avoid their over-exploitation and degradation, as well as for promoting vegetation cover restoration and for water and soil resources conservation. The genus Acacia has great potential for the improvement of tree cover in arid and semi-arid lands due to their drought stress resistance abilities and generation of different goods and services such as fodder for animals, sources of wood and non-wood products (e.g. gums, resins and pharmaceuticals) for the local human communities, provision of shade and live fencing, and in maintaining soil fertility through nitrogen fixation. Another very important environmentally, economically and socially genus is Boswellia. For example Boswellia papyrifera (Del) is the principal producer species of incense of higher quality. In order to achieve knowledge adequate to foster the use of these species in restoration projects, the main objectives of this thesis were: 1) to study germination of three important species of arid zones of Ethiopia: Acacia senegal (Linne), Acacia seyal (Del) and Boswellia papyrifera, simulating water stress through polyethylene glycol (?); 2) to study the effect of different thermal shocks, defined by 4 temperatures and 3 exposition times, on the germination of Acacia senegal and Acacia seyal seeds; 3) to evaluate the effect of two watering frequencies (12 mm/m 2 watered weekly or fortnightly through the whole essay) and two types of substrate (poor-nutrient or local, and rich-nutrient or commercial substrate) on the initial growth and development of Acacia senegal and Acacia seyal seedlings. Different statistical methods were used, but basically they consisted on analyses of variance and logistic regression models. On one hand, results showed that water stress is a key factor which influences negatively on Acacia seeds germination. However, it was not a limiting factor. In the case of Boswellia papyrifera, just very few seeds (four from X) germinated during our essay. Thus, this suggests the necessity of studying other secondary factors that may influence more on the seeds germination, such as dormancy. On the other hand, the higher proportion of germinated seeds in both Acacia species was observed with the thermal shock of 90ºC and 5 minutes of 4 exposure. It was also observed that germination probability decreased along with the temperature increment. However, germination percentages were high...