C. K. Ong

Tree-crop interactions: manipulation of water use and root function

Abstract This paper describes recent research findings on tree–crop interactions in the semiarid tropics focusing on the potential of agroforestry systems to improve the efficiency with which land and water are currently used; the trade-offs between crop productivity and environmental function, and strategies to manipulate tree root function. There is strong evidence that agroforestry has potential for improving water use efficiency by reducing the unproductive components of the water balance, i.e. run-off, soil evaporation and drainage. Examples from India and Kenya show that simultaneous agroforestry systems could double rainfall utilisation compared to annual cropping systems, largely due to temporal complementarity. Where soil loss through erosion is a serious problem, contour hedgerows can provide a viable alternative to conventional soil conservation measures. However, even though soil losses can be dramatically reduced, whether beneficial effects on crops will develop is often unpredictable and usually insufficient to attract widespread adoption of contour hedges. Strategies to reduce the trade-offs between crop and tree interactions or environmental function include the use of high value trees or trees which provide direct benefits to farmers. Recent reviews on root research indicate that there appears to be limited scope for spatial differentiation in rooting between trees and crops (i.e. spatial complementarity) in water-limited environments, unless ground-water is accessible to tree roots. Instead, it is argued that it is more worthwhile to manage below-ground competition by shoot and root-pruning. Pruning of lateral roots could redirect root function and be a powerful tool for improving spatial complementarity, provided that there are adequate resources at depth. However, the downward displacement of functional tree roots following root-pruning must not be allowed to affect their safety net role in the interception of nutrients leaching from the zone of crop rooting and the long-term hydrological implications must not be ignored when attempting to meet demand for trees and their products.

Ecological interactions, management lessons and design tools in tropical agroforestry systems

During the 1980s, land- and labor-intensive simultaneous agroforestry systems (SAFS) were promoted in the tropics, based on the optimism on tree-crop niche differentiation and its potential for designing tree-crop mixtures using high tree-densities. In the 1990s it became clearer that although trees would yield crucial products and facilitate simultaneous growing of crops, they would also exert strong competitive effects on crops. In the meanwhile, a number of instruments for measuring the use of growth resources, exploratory and predictive models, and production assessment tools were developed to aid in understanding the opportunities and biophysical limits of SAFS. Following a review of the basic concepts of interspecific competition and facilitation between plants in general, this chapter synthesizes positive and negative effects of trees on crops, and discusses how these effects interact under different environmental resource conditions and how this imposes tradeoffs, biophysical limitations and management requirements in SAFS. The scope and limits of some of the research methods and tools, such as analytical and simulation models, that are available for assessing and predicting to a certain extent the productive outcome of SAFS are also discussed. The review brings out clearly the need for looking beyond yield performance in order to secure long-term management of farms and landscapes, by considering the environmental impacts and functions of SAFS.

Soil water dynamics in cropping systems containing Gliricidia sepium , pigeonpea and maize in southern Malawi

The water dynamics of cropping systems containing mixtures of Gliricidia sepium (Jacq.) Walp trees with maize (Zea mays L.) and/or pigeonpea (Cajanus cajan L.) were examined during three consecutive cropping seasons. The trees were pruned before and during each cropping season, but were left unpruned after harvesting the maize; prunings were returned to the cropping area in all agroforestry systems to provide green leaf manure. The hypothesis was that regular severe pruning of the trees would minimise competition with crops for soil moisture and enhance their growth by providing additional nutrients. Neutron probe measurements were used to determine spatial and temporal changes in soil moisture content during the 1997/98, 1998/99 and 1999/00 cropping seasons for various cropping systems. These included gliricidia intercropped with maize, with and without pigeonpea, a maizexa0+xa0pigeonpea intercrop, sole maize, sole pigeonpea and sole gliricidia. Soil water content was measured to a depth of 150xa0cm in all treatments at 4–6xa0week intervals during the main cropping season and less frequently at other times. Competition for water was apparently not a critical factor in determining crop performance as rainfall exceeded potential evaporation during the cropping season in all years. The distribution of water in the soil profile was generally comparable in all cropping systems, implying there was no spatial complementarity in water abstraction by tree and crop roots. However, available soil water content at the beginning of the cropping season was generally lower in the tree-based systems, suggesting that the trees continued to deplete available soil water during the dry season. The results show that, under rainfall conditions typical of southern Malawi, the soil profile contains sufficient stored water during the dry season (ca. 75–125xa0mm) to support the growth of gliricidia and pigeonpea, and that gliricidia trees pruned before and during the cropping season did not deleteriously compete for water with associated crops. Water use efficiency also appeared to be higher in the tree-based systems than in the sole maize and maizexa0+xa0pigeonpea treatments, subject to the proviso that the calculations were based on changes in soil water content rather than absolute measurements of water uptake by the trees and crops.

Exploring Below Ground Complementarity In Agroforestry Using Sap Flow And Root Fractal Techniques

Indices of shallow rootedness and fractal methods of root system study were combined with sapflow monitoring to determine whether these ‘short-cut’ methods could be used to predict tree competition with crops and complementarity of below ground resource use in an agroforestry trial in semi-arid Kenya. These methods were applied to Grevillea robusta Cunn., Gliricidia sepium (Jacq.) Walp., Melia volkensii Gürke and Senna spectabilis syn. Cassia spectabilis aged two and four years which were grown in simultaneous linear agroforestry plots with maize as the crop species. Indices of competition (shallow rootedness) differed substantially according to tree age and did not accurately predict tree:crop competition in plots containing trees aged four years. Predicted competition by trees on crops was improved by multiplying the sum of proximal diameters squared for shallow roots by diameter at breast height2, thus taking tree size into account. Fractal methods for the quantification of total length of tree root systems worked well with the permanent structural root system of trees but seriously underestimated the length of fine roots (less than 2 mm diameter). Sap flow measurements of individual roots showed that as expected, deep tap roots provided most of the water used by the trees during the dry season. Following rainfall, substantial water uptake by shallow lateral roots occurred more or less immediately, suggesting that existing roots were functioning in the recently wetted soil and that there was no need for new fine roots to be produced to enable water uptake following rainfall.

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.

Tree and crop productivity in gliricidia/maize/pigeonpea cropping systems in southern Malawi.

This study examined the hypothesis that incorporation of Gliricidia sepium (Jacq.) Walp.) (gliricidia), a fast-growing, nitrogen-fixing tree, into agroforestry systems in southern Malawi may be used to increase the input of organic fertilizer and reduce the need for expensive inorganic fertilizers. The productivity of maize (Zea mays L.), pigeonpea (Cajanus cajan L.) and gliricidia grown as sole stands or in mixed cropping systems was examined at Makoka Research Station (latitude 15° 30′xa0S, longitude 35° 15′xa0E) and a nearby farm site at Nazombe between 1996 and 2000. Treatments included gliricidia intercropped with maize, with or without pigeonpea, and sole stands of gliricidia, maize and pigeonpea. Trees in the agroforestry systems were pruned before and during the cropping season to provide green leaf manure. Maize yields and biomass production by each component were determined and fractional light interception was measured during the reproductive stage of maize. Substantial quantities of green leaf manure (2.4 to 9.0xa0Mgxa0ha−1xa0year−1) were produced from the second or third year after tree establishment. Green leaf manure and fuelwood production were greatest when gliricidia was grown as unpruned sole woodlots (c. 8.0 and 22xa0Mgxa0ha−1xa0year−1 respectively). Improvements in maize yield in the tree-based systems also became significant in the third year, when c. 3.0xa0Mgxa0ha−1 of grain was obtained. Tree-based cropping systems were most productive and exhibited greater fractional light interception (c. 0.6 to 0.7) than cropping systems without trees (0.1 to 0.4). No beneficial influence of pigeonpea on maize performance was apparent either in the presence or absence of gliricidia at either site in most seasons. However, as unpruned gliricidia provided the greatest interception of incident solar radiation (>0.9), coppicing may be required to reduce shading when gliricidia is grown together with maize. As pigeonpea production was unaffected by the presence of gliricidia, agroforestry systems containing gliricidia might be used to replace traditional maize + pigeonpea systems in southern Malawi.

Roots, soil water and crop yield : tree crop interactions in a semi-arid agroforestry system in Kenya

Variations in soil water, crop yield and fine roots of 3–4 year-old Grevillea robusta Cunn. and Gliricidia sepium (Jacq.) Walp. growing in association with maize (Zea mays L.) were examined in semiarid Kenya during the long rains of 1996 and 1997. Even although tree roots penetrated more deeply than maize roots, maximum root length densities for both tree species and maize occurred in the top 200 mm of the soil profile where soil moisture was frequently recharged by rains. Populations of roots in plots containing trees were dominated by tree roots at the beginning of the growing season but because tree roots died and maize root length increased during the cropping season, amounts of tree and maize roots were similar at the end of the season. Thus, there was evidence of temporal separation of root activity between species, but there was no spatial separation of the rooting zones of the trees and crops within that part of the soil profile occupied by crop roots. Tree root length density declined with increasing distances from rows of trees and with depth in the soil profile. Although Grevillea trees were largest, plots containing G. sepium trees always contained more tree roots than plots containing G. robusta trees and Gliricidia was more competitive with maize than Grevillea. Overall, Gliricidia reduced crop yield by about 50% and Grevillea by about 40% relative to crop yield in control plots lacking trees and reductions of crop yield were greatest close to trees. There was less soil moisture in plots containing trees than in control plots. Such difference between control plots and plots containing trees were maximal at the end of the dry season and there was always less soil moisture close to trees than elsewhere in the plots. Plots containing Gliricidia trees contained less soil water than plots containing Grevillea trees.

Tree species and pruning regime affect crop yield on bench terraces in SW Uganda

Integration of trees on farms may exert complementary or competitive effects on crop yield. This 4xa0year study examined novel systems in which Alnus acuminata (alnus), Calliandracalothyrsus (calliandra), Sesbania sesban (sesbania) or a mixture of all three were grown on the degraded upper part of bench terraces in Uganda; beans or maize were grown on the more fertile lower terrace during the short and long rains. Three pruning treatments (shoot, root or shootxa0+xa0root pruning) were applied to the tree rows adjacent to the crops; shoot prunings were applied as green manure to the woodlot from which they came. Pruning increased survival in calliandra and reduced survival in sesbania; alnus was unaffected. Pruning reduced tree height and stem diameter in alnus, but did not affect calliandra or sesbania. Maize yield adjacent to unpruned calliandra, alnus and sesbania or a mixture of all three was reduced by 48, 17, 6 and 24% relative to sole maize. Shoot pruning initially sustained crop performance but shootxa0+xa0root pruning became necessary when tree age exceeded 2xa0years; shootxa0+xa0root pruning increased maize yield by 88, 40, 11 and 31% in the calliandra, alnus, sesbania and tree mixture systems relative to unpruned trees. Bean yield adjacent to unpruned calliandra, alnus, sesbania and the tree mixture was 44, 31, 33 and 22% lower than in sole crops and pruning had no significant effect on crop yield. The results suggest that sesbania fallows may be used on the upper terrace without reducing crop yield on the lower terrace, whereas pruning of alnus is needed to sustain yield. Calliandra woodlots appear to be unsuitable as crop yield was reduced even after pruning.

Manipulating phenology and water relations in Senna spectabilis in a water limited environment in Kenya

The effect of shoot pruning on leaf phenology, stem wood anatomy and sap flow was investigated on Senna spectabilis (DC.) Irwin and Barneby in Machakos, Kenya. Unpruned trees (single stem) were compared to hedges (two to four stems), pruned 4xa0times a year during two rainy seasons (April–June, 1997 and November, 1997–January, 1998) separated by a dry season (July–October 1997). Trees attained peak leaf area of 55xa0m2xa0plant−1 during the rainy seasons, and shed all their leaves naturally during the dry season. Maximum hedge leaf area was 4xa0m2xa0plant−1 between pruning events and 5.2xa0m2xa0plant−1 during the dry season. Pruning induced multiple stems and narrow xylem vessels with low hydraulic conductivity. Average cross sectional area of conducting wood per plant was at least 1.8xa0times greater in trees than in hedges. Xylem lumen diameter at 5xa0mm depth below the cambium was significantly (Pxa0<xa00.001) larger in trees (53.6xa0±xa06.21xa0μm) than that in hedges (36.2xa0±xa08.21xa0μm). Maximum sap flow occurred in the wet season for trees (4800xa0gxa0d−1xa0plant−1) and in the dry season for hedges (1400xa0gxa0d−1xa0plant−1). Wet season pruning suppressed crown expansion and modified the natural phenology of senna, reducing transpiration rate and therefore soil water depletion, causing crowns to grow. This enhanced the ecological combining ability of senna managed as hedges with annual crops.

Early growth performance and water use of planted West African provenances of Vitellaria paradoxa C.F. Gaertn (karité) in Gonsé, Burkina Faso.

A participatory selection trial of five provenances from Burkina Faso (3), Mali (1) and Senegal (1) was established in Burkina Faso in 1997 with the aim of addressing issues of the long juvenile phase and the large variability in annual fruit yields of Vitellaria paradoxa C.F. Gaertn in West Africa. The objectives of the present study were to evaluate survival rate and the growth performance of the five provenances, characterize the wetting profile under which the trees of these provenances are growing and quantify the variation in their seasonal transpiration. The design was a randomized complete block design (RCBD) with single tree as the experimental unit which was replicated 70 times. The results showed a mean survival rate of 50% for all provenances. Passoré (Burkina Faso), Djonon-Karaba (Mali) and Botou-Fada (Burkina Faso) provenances showed the highest height and collar diameter whereas the provenance of Gonsé (Burkina Faso) performed poorly. Djonon-Karaba provenance displayed the highest water use (2.70xa0lxa0day−1 tree−1 in 2004 and 2.85xa0lxa0day−1 tree−1 in 2005). Soil water content under Passoré provenance was the lowest (9.38%) whereas its content under Gonsé provenance (11%) was the highest with no clear pattern according to the distance from tree trunk. Samecouta and Djonon-Karaba provenances showed the highest transpiration values per sapwood unit area in 2004 (0.079xa0lxa0cm−2xa0day−1) and in 2005 (0.069xa0lxa0m−2xa0day−1), respectively. Based on growth performance and water use, it can be recommended at this early stage the selection of Djonon-Karaba and Passoré provenances as the most suitable for semi-arid regions of West Africa. However, there is a need for further data to model the long term effects of these provenances on soil water balance and their fruit production before reliable recommendations can be made to farmers.