Water is vital for most plant processes as it is transporting carbohydrates and nutrients required for growth, and at the same time is an important part of the structure of the plant. However, water is continuously lost from plants by transpiration. In fact, water availability restricts the vegetation distribution on global scale. In the semi-arid regions, water limits productivity of plants including trees, which are essential in rural people’s lives and traditional agricultural systems at social, environmental and economic levels. However, research on plant water content and its relation to leaf phenology, and on the root distribution of Sahelian species is scarce. Eco-physiological aspects must be better understood in order to achieve the conservation of trees in agroforestry systems. This PhD study consists of three manuscripts and aims to improve knowledge on the water storage and rooting distribution of different agroforestry tree species in the Sahel.
The study presented in Manuscript 1 was conducted in three sites along a climatic gradient in Senegal and aimed at characterizing root systems of three important agroforestry tree species. An assessment of root distribution showed a great variation of root density across sites, soil depth and distance to the trunk. Comparison between root biomass across the climatic gradient revealed that tree species produced more superficial roots under humid conditions with loamy sandy soils than under semi-arid conditions in sandy soils. It was also found that roots grow deeply in the site characterized by low rainfall. Variation in root distribution was also observed among species. Borassus akeassii and Faidherbia albida tended to produce more roots between 30-60 cm depth while Adansonia digitata roots were concentrated in the surface layers at 0-30 cm depth.
Manuscript 2 concerns root growth and biomass partitioning of seedlings of nine tree species under three water regimes. Seedlings were established in 2 m long cylinders buried vertically in the soil. Three different treatments including top irrigation, deep irrigation at 50 cm depth and drought treatment were applied at the nursery of the Centre National de Research Forestiere (Senegal) during one year. This study revealed that most species enhanced deep root growth under drought stress and reached the maximum depth of 2 m after 12 month in the cylinder. This study also showed that A. digitata, Sterculia setigera and Anacardium occidentale produced high amounts of root biomass in the upper soil layers under top irrigation, while Acacia senegal, Acacia nilotica and F. albida produced deeper root systems. Under drought, A. digitata and S. setigera showed steady leaf water potential, while the Acacia species had a low leaf water potential.
The study in the Manuscript 3 presents the first results on variation in tree water content and its relation to leaf phenology in Sahelian drylands. Both ground based measurements and satellite observations were used to determine the tree water content, leaf phenology and water potential over one year.
This study found a synchronous development of precipitation and water storage for most of the eight tree species studied. Water contents as assessed from ground measurements and satellite-based on the L-band Vegetation Optical Depth (L-VOD) signals were higher in the rainy season than in the dry season. Both results from satellite observations and gravimetric methods indicate that leaf shedding coincided with the dry season and low leaf water potentials for most species. The exception was F. albida, which opposite to the other species has foliage in the dry season, but not in the wet season. However, the single tree assessments of Normalized Vegetation Difference Index (NDVI) by satellite (Planetscope and MODIS) were not able to accurately capture the defoliation of tree species during the wet (F. albida) and the dry (A. digitata, C. pinnata and B. costatum) season. For most species, leaf flush occurred at the end of the dry season suggesting that intrinsic factors such as water storage play crucial roles in leaf display of dry tropical species.
Deep rooting and water storage appear to be two strategies that help plants overcome water stress and could advantageous under long dry periods where the soil water availability becomes very low. A better understanding of how species differ with respect to acquisition and storage of water can have major implications for management of agroforestry systems and afforestation programs and could be used in the modeling of global ecosystem processes.