In Kenya, arid and semi-arid lands (ASALs) represent 80% of land area, which Kenyan Government has targeted as highest priority in agricultural development and afforestation . In alley cropping, benefits of fertility and micro-climate improvements are negated by effects of below ground competition for water and nutrients. Adaptations in root architecture, dynamics and functioning have been largely unexplored due to formidable problems in both measurements and interpretations of root behaviour. However, such studies are essential pre-requisites for understanding both survival and competition with crops. Melia volkensii tree is believed by both farmers and academics to be deep rooted and unlikely to seriously interfere with cultivation. Kenyan farmers' interest in M.volkensii has increased due to its compatibility with other crops, insect resistance, fodder and timber use. ICRAF trial results with M.volkensii showed yield reduced to about 20% of that obtained from treeless control plots. These results conflict with farmer perceptions and there are serious concerns in expanding the range of M.volkensii.

To establish whether:

* Differences in intra- and inter-specific tree competition with crops are related to the spatial distribution of the tree root systems.

* Root functioning, especially for water uptake on ASALs, is closely related to the root density and dynamics and the depth of the soil wetting front.

* Root system architectures of root cuttings, natural seedlings and transplants differ and cause variation in competitiveness with crop roots.

* Phenologies, and hence timing of water demand by Melia volkensii provenances differ, and influence competitiveness with crops.

* Partitioning of dry matter to vertical roots is greater in provenances originating from drier sites.

* The socio-economic effects of crop yield reduction by Melia volkensii can be more than compensated for by income generation through timber production in an average tree rotation.

*To clarify the utility of Melia volkensii in agroforestry for the arid and semi-arid lands of Kenya.

• Quantification of the effects of Melia volkensii provenances on crop yield in Kenyan ASALs under a range of rainfall, temperature and altitudinal conditions.
• The results will enable a performance (growth) and competition model to be developed.
• Permit assessment of whether Melia volkensii can be economically integrated into Kenyan agriculture.
• Quantification of comparative competitive effects of a range of tree species on crop yield.
• A simple mathematical model describing competition in terms of root system characteristics which will enable prediction of competition from details of root architecture and function.
• Quantification of the competitiveness of root systems of naturally occurring trees, seedlings, transplanted wildlings and root cuttings.
• The socio-economic implications of utilising Melia volkensii in Kenyan agriculture.