C.R. Black

Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: II. Crop growth and system performance.

Maize and cowpea were grown as sole stands or in agroforestry systems containing grevillea trees (Grevillea robusta A. Cunn.). Crop and system performance were examined over a 4.5-year-period (nine growing seasons) commencing in October 1991; failure of the rains caused the loss of one cropping season. A rotation of cowpea (Vigna unguiculata L. Walp.) and maize (Zea mays L.) was grown during the first five seasons after planting the trees, while maize was grown continuously during the final four seasons. Sole maize was also grown under spectrally neutral shade netting which reduced incident radiation by 25, 50 or 75% to establish the relative importance of shade and below-ground competition for water and nutrients in determining the performance of understorey crops. The above-ground biomass and grain yield of understorey crops were not significantly affected by the presence of grevillea during the first four seasons, but were greatly reduced in subsequent seasons as the trees became increasingly dominant; maize yields reached 50% of the sole crop values only once during the final four seasons, when rainfall was unusually high. The hypothesis that competition for water was the primary limiting factor for understorey crops was supported by the observation that above-ground biomass and grain yield were greater in the shade net treatments than in agroforestry maize, demonstrating that shade was not solely responsible for the substantial yield losses in the latter treatment. Performance ratios (ratio of values for the agroforestry system relative to sole stands) for total above-ground and trunk biomass in grevillea were initially low, reflecting the impact of competition with associated crops during tree establishment, but increased to unity within 2.5 years. Performance ratios for the understorey crops exhibited the reverse trend, initially being close to unity but approaching zero for three of the final four seasons. Performance ratios were never close to unity for both trees and crops during the same season, indicating that there was always competition for available resources irrespective of crop species or tree size. The implications for agroforestry system design and future research are discussed. # 2000 Elsevier Science B.V. All rights reserved.

Using sap flow gauges to quantify water uptake by tree roots from beneath the crop rooting zone in agroforestry systems.

Grevillea (Grevillea robusta A. Cunn.; Proteaceae) is used in agroforestry in many areas of the highlands of East and Central Africa, and is reported to be mainly deep rooted, with few shallow roots and correspondingly low levels of competition with associated crops for water and nutrients. To examine the extent of below-ground complementarily in water use between grevillea and cowpea (Vigna unguiculata L.; Leguminosae), experiments were carried out at the International Centre for Research in Agroforestry (ICRAF) Field Centre at Machakos, Kenya. Sap flux was measured using heat balance gauges attached to the stems of young grevillea trees (10–18 months old), both before and after excavating the crop rooting zone (upper 60 cm of soil) around the stem base, in order to establish the capacity of the grevillea to extract water from below this zone. After excavation, the trees maintained sap fluxes of up to 85% of the unexcavated values, suggesting a high degree of below-ground complementarity.

Allometric estimation of above-ground biomass and leaf area in managed Grevillea robusta agroforestry systems

Non-destructive methods for determining the biomass and leaf area of individual trees throughout their growing cycle are an essential tool in agroforestry research, but must be capable of providing reliable estimates despite the influence that management strategies such as pruning may have on tree form. In the present study, allometric methods involving measurements of the diameter of all branches provided reliable estimates of canopy leaf area and biomass for grevillea trees (Grevillea robusta A. Cunn.; Proteaceae) grown as poles, but proved unsuitable for routine measurements because of their time-consuming nature. An alternative, less laborious method based on measurements of trunk cross-sectional area immediately below the first branch of the canopy provided satisfactory allometric estimates of leaf area and canopy biomass. Trunk biomass was determined from measurements of tree height and diameter at breast height using established methodology based on the assumption that trunk volume may be calculated using a quadratic paraboloid model; biomass was determined as the product of trunk volume and the specific gravity of the wood. The theoretical basis, development and validation of allometric methods for estimating tree growth are discussed and their wider applicability to other agroforestry systems is assessed.

Tree phenology and water availability in semi-arid agroforestry systems

Abstract Four common agroforestry trees, including both exotic and native species, were used to provide a range of leafing phenologies to test the hypothesis that temporal complementarity between trees and crops reduces competition for water in agroforestry systems during the cropping period and improves utilisation of annual rainfall. Species examined included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves once during the long dry season, and the evergreen Croton megalocarpus. Phenological patterns were examined in relation to climatic conditions in the bimodal rainfall regions of Kenya to identify factors which dictate the intensity of competition between trees and crops. The main differences in leaf cover patterns were between indigenous and exotic tree species. The Central American species, S. spectabilis and G. sepium, shed their foliage during the dry season before the short rains, whereas the native species, M. volkensii and C. megalocarpus, exhibited reduced leaf cover during both dry seasons. C. megalocarpus was the only species to maintain leaf cover throughout the 2-year experimental period. M. volkensii and S. spectabilis exhibited similar leafing phenology, losing almost all leaf cover during the long dry season (July–October) and flushing before the onset of the ensuing rains. S. spectabilis lost few leaves during the short dry season, whereas M. volkensii shed a greater proportion of its foliage before flushing prior to the long rains (March–July). M. volkensii lost much of its leaf cover during the 1997/1998 short rains (October–February), when soil water content was unusually high. Although essentially evergreen, leaf cover in C. megalocarpus decreased during the dry season and increased rapidly during periods of high rainfall. G. sepium exhibited a period of low leaf cover during the long dry season and did not regain full leaf cover until mid-way through the short rains. The mechanisms responsible for these phenological changes and the implications of tree phenology for resource utilisation and competition with crops are discussed.

Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth

Abstract The work reported here formed part of a multidisciplinary project to examine the changing tree/crop interactions, expressed in terms of resource capture, tree and crop growth and system performance, that occur as the trees in semi-arid agroforestry systems establish and mature. Grevillea robusta (A. Cunn.; grevillea) trees were grown in a dispersed planting arrangement, either as sole stands or in combination with cowpea (Vigna unguiculata L.) or maize (Zea mays L.). Allometric approaches were used to monitor leaf area and leaf, branch and trunk biomass over a 4.5-year period. Seasonal and annual growth increments and the corresponding mean daily growth rates were calculated; crop growth, development and yield were also determined over nine growing seasons. Tree height, leaf area and trunk and branch biomass were reduced in the agroforestry treatment (CTd) during the first 600 days after planting as a result of competition with the associated crops. Tree height subsequently increased more rapidly in the agroforestry treatment (CTd) and regained parity with the sole grevillea treatment (Td) by ca. 820 days after planting. However, although seasonal and annual biomass increments were comparable in both treatments once the trees became the dominant component of the system, above-ground biomass in CTd grevillea never recovered fully to Td values during the observation period; trunk volume and biomass were consistently lower in CTd than in Td grevillea, reducing their economic value. Tree growth continued throughout the dry season, sometimes at rates similar to or greater than observed during the cropping seasons, indicating that the trees were able to utilise residual soil moisture or deep reserves within the profile. This finding is discussed in relation to recent interpretations of niche separation in savanna vegetation. The factors responsible for the observed reductions in tree growth in the agroforestry system are examined; effects on crop yields and system performance are considered in Part II.

Comparison of methods for determining leaf area in tree rows

Two methods for determining the leaf area of trees growing in rows using an LAI-2000 Plant Canopy Analyser were tested against destructive measurements for Croton megalocarpus and Melia volkensii, species with differing canopy characteristics. The trees ranged between 4.0 and 8.0 m in height and formed part of an agroforestry experiment in semi-arid Kenya where rapid fluctuations in canopy cover rendered allometric approaches inappropriate for determining leaf area. The first method used unmodified theory for determining leaf area in continuous canopies which has proved suitable for isolated bushes. In the second method, path lengths through the canopy were calculated from simple measurements of canopy dimensions and the importance of subsidiary assumptions concerning leaf angle distribution was tested. Leaf angle distribution, which is required for canopy simulation models, was also determined using both direct and indirect approaches and the effect of using assumed leaf angle distributions when calculating leaf area was assessed. The canopy analyser proved unsuitable for measuring leaf angle distributions in isolated canopies, and it was necessary to make direct canopy measurements for this instrument to be used for smaller canopies. It was also shown that, even when path lengths are measured, calibration may be necessary to avoid bias; uncalibrated leaf area density values were, on average, underestimated by 16% for M. volkensii and overestimated by 8% for C. megalocarpus with respect to destructively determined values.

Tree leafing phenology and crop productivity in semi-arid agroforestry systems in Kenya

The hypothesis that temporal separation of resource use between trees and crops minimises competition for wa ter in agroforestry systems during the cropping period and increases utilisation of annual rainfall was tested at Machakos in semi-arid Kenya. Four popular tree species were chosen to provide a range of leafing phenologies. These included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves during the long dry season, and the evergreen Croton megalocarpus. All four species retained their foliage during the long rains, offering little scope for temporal separation of resource use. Maize (Zea mays) yields were reduced by 50–70% in the agroforestry treatments. Reductions in crop yield were strongly correlated with tree growth (r 2 =0.94) and available soil moisture (r 2 =0.88). G. sepium remained leafless for much of the short rains despite the presence of available soil water, and was least competitive with the bean crops (Phaseolus vulgaris) grown at this time. Reductions in crop yield in the agroforestry treatments were closely correlated with tree growth (r 2 =0.99) and available moisture (r 2 =0.79) during the 1996/97 short rains (158 mm), but not during the much wetter 1997/98 season (608 mm). Shading by trees or shade nets reduced crop yield, in contrast to previous studies in the semi-arid tropics. Low off-season rainfall during the study period (9% of annual rainfall compared to the long-term average of 20%) limited the potential for temporal separation of growing periods. Where the prospects for temporal or spatial separation in resource use are limited, shoot and/or root pruning may be necessary to manage competition between trees and crops.

Tree-crop interactions: agroforestry in a changing climate

This new edition provides an update on the considerable amount of evidence on tree-crop interactions which has accumulated during the last two decades, especially on the more complex multi-strata agroforestry systems, which are typical of the humid tropics.

Agroforestry: hydrological impacts

Forests and agroforestry (AF), in which trees are incorporated into cropland, have been widely promoted as viable solutions to overcome the loss of ecosystem functions associated with the conversion of natural landscapes to human use. However, trees in forests and on farms may have both beneficial and detrimental hydrological impacts at farm and watershed levels. In this article, the focus is on the opportunities and challenges of using AF in the semiarid tropics and examines how to: (1) modify water productivity and make better use of unproductive water; (2) enhance the hydrological impacts of simultaneous and sequential AF systems; (3) improve water infiltration and reduce evaporation, runoff, and nutrient leaching; and (4) minimize below-ground competition between trees and crops are explained. The role of trees in improving watershed functions and the importance of hydraulic lift in dryland environments are also considered. It is also shown that there are substantial opportunities to increase water productivity in the semiarid tropics because trees can access unproductive water which crops cannot acquire. Nevertheless, competition for water in AF remains a serious challenge where rainfall is too low to recharge soil below the crop rooting zone unless tree roots have access to deepwater reserves. To avoid competition, it may be necessary to root- or shoot-prune trees, use species with ‘reverse phenology’, or adopt sequential planting of trees and crops to permit spatial complementarity in resource capture.