122 C.K. Ong et al. Agriculture, Ecosystems and Environment 80 2000 121–141
1. Introduction
Agroforestry has attracted considerable attention because of its potential to maintain or increase pro-
ductivity in areas where high energy input, large-scale agriculture is impractical Kidd and Pimentel, 1992. It
is often assumed that appropriate agroforestry systems can provide the essential ecological functions needed
to ensure sustainability and maintain microclimatic and other favourable influences, and that such benefits may
outweigh their greater use of water in areas of limited water availability. Young 1989 suggested that trees
and shrubs play an essential role in minimising erosion by reducing runoff, so improving water conservation.
However, supporting evidence is either limited Lal, 1989; Kiepe, 1995 or extrapolated from plantations or
natural forests Sanchez, 1987.
The hydrological and biological factors determining the success or failure of agroforestry systems are still
poorly understood, although the existence of traditional agroforestry systems under arid and semi-arid condi-
tions provides evidence that they have a useful role even in the water-limited environments of north-west India
Shankarnarayan, 1984 and the Sahel Vandenbelt and Williams, 1992. Agroforestry research in dryland ar-
eas falls into two broad categories: firstly, studies of microclimatic and soil amelioration around scattered
trees in traditional parkland systems Kater et al., 1992; Kessler, 1992; Vandenbelt and Williams, 1992; Belsky
et al., 1993; Jonsson, 1995; and secondly, alley crop- ping systems, in which closely planted trees are pruned
regularly to produce hedges Singh et al., 1989; Rao et al., 1991; Ong et al., 1992. Several previous agro-
forestry trials have been criticised on the grounds that interference resulting from the extension of tree roots
into adjacent mono-crop control plots may have been underestimated; such effects would introduce a bias in
favour of the agroforestry treatment Rao et al., 1993.
The present paper describes a trial involving Gre- villea robusta
A. Cunn., maize Zea mays and cowpea Vigna unguiculata in which interference by tree roots
was controlled by periodic trenching around the plots. Recent alley cropping studies at Machakos, Kenya in-
volving Senna spectabilis grown in rotation with maize and cowpea McIntyre et al., 1996, 1997 have shown
that there was little difference in light interception or evapotranspiration between mono-crop and alley crop-
ping treatments, and hence no improvement in pro- ductivity during the cropping season. This observa-
tion is consistent with other alley cropping trials at Machakos involving root trenching and species such
as Leucaena leucocephala Govindarajan et al., 1996 which demonstrated that alley cropping is detrimen-
tal to crop productivity in semi-arid areas because soil fertility benefits resulting from the application of tree
mulch or increased root turnover are outweighed by increased competition for water. Other studies indi-
cate that regular pruning encourages the proliferation of fine tree roots near the soil surface, decreasing spa-
tial niche separation between tree and crop roots and hence the potential for complementarity in the use of
below-ground resources Van Noordwijk and Purno- mosidhi, 1995. Grevillea was chosen for the present
study owing to its popularity with farmers in Machakos District and because experimental studies of root distri-
bution Laycock and Wood, 1963; Jonsson et al., 1995 suggested it may extract substantial quantities of wa-
ter from beneath the crop rooting zone. Grevillea is a fast-growing member of the Proteacae with potential as
a multi-purpose cash crop as its timber is suitable for flooring and the manufacture of plywood and light fur-
niture, and its flowers are suitable for honey production Harwood and Getahun, 1990.
1.1. Rationale Cannell et al. 1996 argued that agroforestry may in-
crease productivity provided the trees capture resources which are under-utilised by crops. In annual systems
where the land lies bare for extended periods, residual water remaining in the soil after harvest and off-season
rainfall are often unused, particularly in areas of uni- modal rainfall. For instance, at Hyderabad, India an-
nual rainfall 800 mm, substantial available water re- mained in the 45–90 cm horizons after harvesting
sorghum and pigeonpea Ong et al., 1992 and ca. 20, or 152 mm, of the annual rainfall occurs outside the nor-
mal cropping season when it could be used by perennial species. The scope for improving water use is therefore
considerable, as a maximum of 40 of the annual rain- fall was utilised by the most effective cropping systems
and the remainder was lost as runoff 26 or deep drainage 33; Ong et al., 1992. In drier areas such
as Syria and Niger, soil evaporation may account for 30–60 of the annual rainfall Cooper et al., 1983;
Wallace, 1991. Thus any decrease in soil evaporation,
C.K. Ong et al. Agriculture, Ecosystems and Environment 80 2000 121–141 123
runoff or deep drainage resulting from increased shad- ing, reduced soil temperature, the windbreak effect of
the trees, or increased abstraction of water at depth or during the dry season would increase the proportion of
rainfall used for transpiration. A possible disadvantage is that interception losses resulting from the evapora-
tion of rainfall from the tree canopy may range from 10 to 30 in agroforestry systems Ong and Black,
1994, although interception losses are lower when the tree canopy is sparse 5–10; Wallace et al., 1995.
The hypothesis that agroforestry may increase pro- ductivity by capturing a larger proportion of the annual
rainfall Ong et al., 1992 was supported by the Hyder- abad studies, which demonstrated that improvements in
annual rainfall utilisation from 40 to 80 were possi- ble in perennial pigeonpea Cajanus cajangroundnut
agroforestry systems, primarily because the use of off- season rainfall was increased Marshall, 1995. These
observations demonstrate the potential of agroforestry for temporal complementarity in areas where signifi-
cant rainfall occurs outside the normal cropping season. However, the short-term nature of these experiments,
often involving only one or 2 years of measurements, made it impossible to assess the long term implications.
The presence of trees may also modify microclimatic conditions in ways that improve the water use effi-
ciency of understorey crops, although regular pruning limits the extent of such effects in alley cropping sys-
tems Wallace, 1996. Several factors may be involved. Firstly, shading by the trees may increase the fraction
of available water used for transpiration by decreasing soil evaporation, particularly when the crop canopy is
sparse and rain is received as frequent, low intensity events. Under these circumstances, any reduction in
the quantity of radiation reaching the soil decreases evaporation as this process is primarily energy-limited.
Decreased windspeed at ground level may also limit evaporation. Secondly, agroforestry may confer mi-
croclimatic benefits by decreasing the air temperature, windspeed and saturation deficit experienced by un-
derstorey crops, thereby reducing evaporative demand Monteith et al., 1991. In C3 crops, in which pho-
tosynthesis becomes light saturated at relatively low irradiances, the reduced flux of photosynthetically ac-
tive radiation PAR resulting from partial shading may have little effect on assimilation Stirling et al., 1990,
although this is less likely to apply to C4 species with their much higher light-saturated photosynthetic rates.
The potential benefits of shade are therefore likely to depend on tree spacing and age, canopy structure, in-
cident radiation, shading intensity and the photosyn- thetic pathway of the understorey crop. Thirdly, shad-
ing may alter the surface temperature of understorey crops in ways that benefit their phenology and produc-
tivity Monteith et al., 1991; Vandenbelt and Williams, 1992. In areas of high incident radiation and ambi-
ent temperature, tissue temperatures frequently exceed optimal levels in unshaded crops, particularly during
drought periods; under such conditions, partial shade may exert an ameliorative influence by bringing tem-
peratures within the optimum range.
There are, therefore, several mechanisms whereby agroforestry may use available water more effectively
than mono-crops and improve microclimatic condi- tions for understorey crops. The key question is whether
the potential benefits outweigh the detrimental influ- ence of competition for water and nutrients between
trees and crops. The present study, extending over a pe- riod of almost 6 years, is one of the most comprehensive
and highly instrumented agroforestry experiments ever attempted. Its objectives were to quantify the changing
influence of trees on the microclimatic and hydrolog- ical conditions experienced by understorey crops un-
der semi-arid conditions as the system matured, and to establish the consequences for water use, light inter-
ception and tree and crop performance. Fig. 1 shows the main categories of measurements, the periods when
they were conducted, and seasonal rainfall. This paper describes the experimental design, instrumentation and
measurement protocols, evaluates effects on the atmo- spheric and soil environment, and examines their influ-
ence on tree and crop growth. More detailed consider- ation of the soil water balance, partitioning of light and
water between the trees and crops, system productiv- ity and the modelling aspects of the work is presented
elsewhere.
2. Materials and methods
