Agriculture Reference
In-Depth Information
Trees also increase soil nitrogen availability through nitrogen fixation. Vitex
payos and Cordeauxia edulis have been reported to nodulate naturally
(Bekele-Tesemma, 1993; Mbabu and Wekesa, 2004). However, there are no
studies that report the amount of nitrogen fixed by these tree species.
Despite the high soil fertility under trees, reports in the literature on the
effects of some of the priority indigenous trees in agroforestry parkland systems
on productivity of associated crops are contradictory. For example, Kater et al .
(1992), Kessler (1992) and Boffa (1999) reported that Vitellaria paradoxa
reduced sorghum and millet yields but the yield of cotton was unaffected.
Belsky et al . (1989) and Grouzis and Akpo (1997), on the other hand, found
higher herbaceous biomass under the crowns of Balanites aegyptiaca and
Adansonia digitata than in the open. The negative effects of trees on sorghum
as reported by Kater et al . (1992), Kessler (1992) and Boffa (1999) are
generally associated with the higher tree density in agroforestry parkland
systems in dry subhumid zones, where irradiance seems to be the main factor
limiting growth. The positive effects of trees on herbaceous layer production
reported by Belsky et al . (1989) and Grouzis and Akpo (1997) are, however,
generally associated with low tree density in agroforestry parklands in arid and
semi-arid zones, where water is the main limiting factor in primary production.
By reducing air temperature, solar radiation and wind velocity, trees decrease
potential evapotranspiration (PET) under their crowns, resulting in higher levels
of soil moisture under trees than in the open. Lower air temperature under tree
crowns because of shading has been reported by Grouzis and Akpo (1997),
who observed a difference of 6°C between direct sunlight and the shade of
Balanites aegyptiaca . Soil temperature was also substantially reduced under
crowns of Adansonia digitata and Vitellaria paradoxa by at least 5°C, as
reported by Belsky et al . (1989) and Jonsson et al . (1999), respectively.
Agroforestry parklands have also been reported to reduce wind velocity by
Jonsson et al . (1999), who measured a significant difference in wind speed
under Vitellaria paradoxa trees compared with the open. As a result of reduced
temperature and wind speed and consequently reduced PET, significantly
greater soil moisture under tree crowns than in the open has been reported by
Belsky et al. (1989) and Boffa (1999) under Balanites aegyptiaca and Vitellaria
paradoxa , respectively.
The influence of trees in agroforestry parklands on the microclimate and soil
fertility is therefore extremely favourable to the growth and development of
associated crops in Eastern Africa's dryland agriculture, which is normally
characterized by aridity and the depletion of nitrogen and phosphorus from the
soil. This improvement of soil fertility and the microclimate by trees is exploited
by farmers to sustain crop production in dryland agricultural systems with
spaced indigenous trees; these are referred to as agroforestry parkland systems
(Boffa, 1999; Teklehaimanot, 2003). However, agroforestry parkland systems in
the drylands of Eastern Africa are undergoing losses of indigenous trees as a
result of habitat alteration through monocrop agriculture and overcutting of trees
for construction, fuelwood and charcoal as a result of increased population
density and pressure (IPED, 1994; FAO, 2003). This loss of indigenous trees has
affected the ecosystem functions of dryland agriculture in many areas, posing a
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