Fine root dynamics of shaded cacao plantations in Costa Rica

Root turnover may contribute a significant proportion of recycled nutrients in agroforestry systems and competition between trees and crops for nutrients and water may depend on temporal fine root regrowth patterns. Fine root biomass ( 2 mm) and fine root productivity were measured during one year in plantations of cacao (Theobroma cacao) shaded by Erythrina poeppigiana or Cordia alliodora planted on a deep alluvial soil in Turrialba, Costa Rica. Fine root biomass of approximately 1.0 Mg ha^–1 varied little during the year with maximum values at the beginning of the rainy season of 1.85 Mg ha^–1 in the cacao-C. alliodora system compared to 1.20 Mg ha^–1 for cacao-E. poeppigiana. Fine root productivity of C. alliodora and E. poeppigiana (maximum of 205 and 120 kg ha^–1 4 week^–1, respectively) was greatest at the end of the rainy season, while for cacao it was greatest at the beginning of the rainy season (34–68 kg ha^–1 4 week^–1), which suggests that if nutrient competition occurs between the shade trees and the cacao, it could be minimized by early fertilization during the beginning of the rains immediately after pruning the shade trees. Annual fine root turnover was close to 1.0 in both systems. Assuming that fine root biomass in these mature plantations was constant on an annual basis, nutrient inputs from fine root turnover were estimated as 23–24 (N), 2 (P), 14–16 (K), 7–11 (Ca) and 3–10 (Mg) kg ha^–1 year^–1, representing 6–13% and 3–6% of total nutrient input in organic matter in the C. alliodora and E. poeppigiana systems, respectively.     

Biomass and production of fine and coarse roots of trees under agrisilvicultural practices in north-east India

An understanding of the rooting pattern of tree species used in agroforestry systems is essential for the development and management of systems involving them. Seasonal variation, depth wise and lateral distribution of biomass in roots of different diameter classes and their annual production were studied using sequential core sampling. The investigations were carried out in four tree species under tree only and tree+crop situations at ICAR Research Farm, Barapani (Meghalya), India. The tree species were mandarin (Citrus reticulata), alder (Alnus nepalensis), cherry (Prunus cerasoides) and albizia (Paraserianthes falcataria). The contribution of fine roots to the total root biomass ranged from 87% in albizia to 77% in mandarin. The bulk of the fine roots (38% to 47%) in the four tree species was concentrated in the upper 10 cm soil layer, but the coarse roots were concentrated in 10–20 cm soil depth in alder (46%) and albizia (51%) and at 0–10 cm in cherry (41%) and mandarin (48%). In all the four tree species, biomass of both fine- and coarse-roots followed a unimodal growth curve by showing a gradual increase from spring (pre-rainy) season to autumn (post rainy) season. Biomass to necromass ratio varied between 2 to 3 in the four tree species. The maximum (3.2) ratio was observed during spring and the minimum (2) in the rainy season. In alder and albizia, the fine roots were distributed only up to 1 m distance from the tree trunk but in the other two species they were found at a distance up to 1.5 m from the tree trunk. The annual fine root production varied from 3.6 Mg ha^–1 to 6.2 Mg ha^–1 and total production from 4.2 to 8.4 Mg ha^–1 in albizia to mandarin, respectively. Cherry and mandarin had a large number of woody roots in the surface layers which pose physical hindrance during soil working and intercultural operations under agroforestry. But the high biomass of roots of these two species may be advantageous for sequential or spatially separated agroforestry systems. However, alder and albizia have the most desirable rooting characteristics for agroforestry systems.This revised version was published online in November 2005 with corrections to the Cover Date.     

Biomass production and C-sequestration of Gmelina arborea in plantation and agroforestry system in India

Tree based land use systems make a valuable contribution to sequester carbon and improve productivity and nutrient cycling within the systems. This study was conducted to determine biomass production, C-sequestration and nitrogen allocation in Gmelina arborea planted as sole and agrisilviculture system on abandoned agricultural land. At 5 years, total stand biomass in agrisilviculture system was 14.1 Mg ha⁻¹. Plantations had 35% higher biomass than agrisilviculture system. At 5 years, leaves, stem, branches and roots contributed 4.1, 65.2, 10.0 and 20.7%, respectively to total standing biomass (17.9 Mg ha⁻¹). Over the 5 years of study, trees had 3.5 Mg ha⁻¹ more C and 36 kg ha⁻¹ more N in plantation than agrisilviculture system. Biomass and C storage followed differential allocation. Relatively more C was allocated in above ground components in plantations compared to agrisilviculture system. C:N ratios for tree components were higher in stem wood (135–142) followed by roots (134–139), branches (123–128) and leaves (20–21). In agrisilviculture system crops recommended are: soybean and cowpea in rainy season; wheat and mustard in winter season. After 5 years, soil organic C increased by 51.2 and 15.1% and N by 38.4 and 9.3% in plantation and agrisilviculture system, respectively. Total C storage in abandoned agricultural land before planting was 26.3 Mg ha⁻¹, which increased to 33.7 and 45.8 Mg ha⁻¹ after 5 years in plantation and agrisilviculture system, respectively. Net C storage (soil + tree) was 7.4 Mg ha⁻¹ in agrisilviculture system compared to 19.5 Mg ha⁻¹ in G. arborea monoculture stands. The studies suggest that competitive interactions played a significant role in agrisilviculture system. Plantations were more efficient in accreting C than agrisilviculture system on abandoned agricultural land.     

Nitrogen and fine root length dynamics in a tropical agroforestry system with periodically pruned Erythrina poeppigiana

The effect of pruning all branches (complete pruning) or retaining one branch (partial pruning) on the dynamics of nitrogen cycling in aboveground biomass, nitrogen supplying power of an amended Eutric Cambisol, and fine root length, was studied in an Erythrina poeppigiana (Walp.) O.F. Cook—tomato (Lycopersicon esculentum Mill.) alley cropping practice in Turrialba, Costa Rica during 1999–2000. Over the 1 year pruning cycle, in which trees were completely or partially pruned four times, respective aboveground biomass production was 4.4 Mg or 7 Mg ha⁻¹ (2-year-old trees) and 5.5 Mg or 9 Mg ha⁻¹ (8-year-old trees); N cycled in aboveground biomass was 123 kg or 187 kg ha⁻¹ (2-year-old trees) and 160 kg or 256 kg N ha⁻¹ (8-year-old trees); mean fine root length was 489 or 821 m (2-year-old-trees), 184 or 364 m per tree (8-year-old-trees). Pruning intensity did not significantly affect net N mineralisation and net nitrification rates during the tomato-cropping season. For the tomato crop, pre-plant mean net N mineralisation rate of 2.5 mg N kg⁻¹ soil day⁻¹ was significantly lower than 16.7 or 11.6 mg N kg⁻¹ soil day⁻¹ at the end of vegetative development and flowering, respectively. Mean net nitrification rates of 3.5, and 4.3 mg N kg⁻¹ soil day⁻¹, at pre-plant and end of vegetative development, respectively, were significantly higher than 0.3 mg N kg⁻¹ soil day⁻¹ at end of flowering. In humid tropical low-input agroforestry practices that depend on organic inputs from trees for crop nutrition, retention of a branch on the pruned tree stump appears to be a good alternative to removal of all branches for reducing N losses through higher N cycling in aboveground biomass, and for conserving fine root length for higher N uptake, although it might enhance competition for associated crops.     

Tree and livestock productivity in relation to tree planting configuration in a silvopastoral system in North Wales, UK

Silvopastoral systems in Europe offer the potential of introducing environmental benefits while at the same time increasing the diversity of farm outputs. The establishment of new silvopastoral systems by planting young trees into existing pasture was investigated at a site in North Wales, UK. Two tree species, sycamore (Acer pseudo-platanus L.) and red alder(Alnus rubra Bong.), were planted into pasture at a range of densities and planting arrangements. Growth of trees planted in farm woodland blocks (2500 stems ha^–1) was compared with the growth of trees planted at 400 stems ha^–1 in clumps and dispersed throughout the plot and at 100 stems ha^–1 (dispersed). Over the first six years after planting, alder trees were significantly taller and larger in diameter than sycamore. Sycamore trees planted at close spacing in farm woodland or clumped arrangements were significantly larger in diameter than widely spaced sycamore at 100 and 400 stems ha^–1. Livestock productivity was unaffected by the presence of trees during the six-year establishment phase of the system. The planting of trees in a clumped pattern appears to combine silvicultural benefits to tree growth with agricultural benefits of maintaining livestock production while trees are established.This revised version was published online in November 2005 with corrections to the Cover Date.     

Fine root and nodule dynamics of Erythrina poeppigiana in an alley cropping system in Costa Rica

Fine root and nodule production and turnover in pruned 2- and 8-yr-old Erythrina poeppigiana (Walp.) O.F. Cook trees were estimated under humid tropical conditions by applying the compartment flow model (CFM) to fine root and nodule biomass and necromass measured in sequentially taken core samples. Shoot pruning intensities compared were complete pruning (i.e., complete removal of shoots) and partial pruning (i.e., retention of one branch on the pruned stump). The CFM provided reasonable estimates of nodule dynamics but did not apply to fine root data. Over a five-month observation period, nodule production in completely and partially pruned 2-yr-old trees was 58.2 and 115 g tree^–1, respectively, and the corresponding values in 8-yr-old trees were 26.8 and 26.4 g tree^–1. Senescent nodules and fine roots pass to soil organic matter via decomposition. Partially and completely pruned 2-yr-old trees added 95.4 and 50.4 g tree^–1 decomposed nodules to soil, respectively. The respective value for 8-yr-old trees were 26.7 and 36.5g tree^–1. Nodule and fine root turnover was compensated for by new production at 10–14 weeks after pruning. The retention of a branch on the pruned E. poeppigiana tree stump allows better fine root and nodule survival, and enhances tree biomass production.This revised version was published online in November 2005 with corrections to the Cover Date.     

Above- and below-ground biomass dynamics in a sole cropping and an alley cropping system withGliricidia sepium in the semi-deciduous rainforest zone of West Africa

A considerable amount of data is available about above-ground biomass production and turnover in tropical agroforestry systems, but quantitative information concerning root turnover is lacking. Above- and below-ground biomass dynamics were studied during one year in an alley cropping system withGliricidia sepium and a sole cropping system, on aPlinthic Lixisol in the semi-deciduous rainforest zone of the Côte d'Ivoire. Field crops were maize and groundnut. Live root mass was higher in agroforestry than in sole cropping during most of the study period. This was partly due to increased crop and weed root development and partly to the presence of the hedgerow roots. Fine root production was higher in the alleys and lower under the hedgerows compared to the sole cropping plots. Considering the whole plot area, root production in agroforestry and sole cropping systems was approximatly similar with 1000–1100 kg ha^–1 (dry matter with 45% C) in 0–50 cm depth; about 55% of this root production occured in the top 10 cm. Potential sources of error of the calculation method are discussed on the basis of the compartment flow model. Above-ground biomass production was 11.1 Mg ha^–1 in sole cropping and 13.6 Mg ha^–1 in alley cropping, of which 4.3 Mg ha^–1 were hedgerow prunings. The input of hedgerow root biomass into the soil was limited by the low root mass ofGliricidia as compared to other tree species, and by the decrease of live root mass of hedgerows and associated perennial weeds during the cropping season, presumably as a result of frequent shoot pruning.     

The agronomic and economic performance of banana,bean and tree intercropping in the highlands of Burundi: an interim assessment

To meet their wood, fodder and fruit needs, resource-poor farmers with only small land holdings are forced to mix trees in their food crop plots. An experiment was conducted to study the effect of nine tree species planted at 312.5 trees ha^–1 (4×8 m) on the yield of bananas planted at 625 stools ha^–1 (4×4 m) and beans (80000 plants ha^–1) as well as the wood production of the trees when intercropped. In addition, an economic analysis was done to compare the different tree/banana/bean associations.After three and one-half years, wood volume (in m³ ha^–1) ofGrevillea robusta (18.1), was highest and that ofErythrina poeppigiana (2.7),Cedrela odorata (2.4) orMarkhamia lutea (0.8) was the lowest. Volume ofCedrela serrata (13.7) was not significantly different from that ofAlbizia chinensis (12.8) but was significantly higher than that ofLeucaena diversifolia (6.8),Acrocarpus fraxinifolius (6.7) orCalliandra calothyrsus (6.0).None of the tree species had a significant influence on the yields of the bananas and none affected the yield of the bean crops until the seventh cropping season, three years after the trees were planted. In that year, Grevillea reduced bean yield by 29%, Albizia by 34% and Leucaena by 36%. From the economic analyses, all the treatments except Leucaena and Markhamia had positive net benefits relative to the control (banana/bean) but the results were highly variable.C. serrata was found to be the best tree to be intercropped in a banana/bean system.     

Crop yields of rice and wheat grown in rotation as intercrops with three tree species in the outer hills of Western Himalaya

Studies on tree crop interaction under rainfed condition in Dehradun valley were conducted for 13 years during 1977 to 1990. Grewia optiva (Bhimal), Morus alba (Mulbery) and Eucalyptus hybrid were tried along with rice (CV: Akashi) — wheat (CV: RR-21) rotation. One-year-old tree seedlings of the above tree species were planted in line, 5 m apart in N-S direction, in July 1977, in the middle of the plot (size 20 m × 20 m). Eucalyptus was first harvested in 1987. Grewia optiva, Morus alba and coppice of Eucalyptus were harvested in 1990. All tree species had depressing effect on crop yields. Eucalyptus had maximum effect in depressing crop yield till the first harvest and had least effect thereafter. From 1987 onwards, Morus alba affected rice most, while wheat was mostly affected by Grewia optiva. The depressing effect on an average varied from 28 to 34% depending upon the species.Distance of tree line from the crop significantly affected the crop yield upto a distance of 5 m and there was 39% decrease in crop yield upto 1 m, 33% from 1–2 m, 25% from 2–3 m and 12% from 3–5 m distance. Annual removal of lops and tops from trees partly compensated the deficit. Grewia optiva could produce 1.08 t ha^–1 yr^–1 of branches and 0.26 t ha^–1 yr^–1 of leaves (air dry) and 1.28 t ha^–1 yr^–1 of branches and 0.28 t ha^–1 yr^–1 of leaves were obtained from Morus alba. Wood (ADT) produced by the trees was 33.6 t ha^–1 from Eucalyptus, 9.5 t ha^–1 from Grewia optiva and 11.6 t ha^–1 from Morus alba.     

Biomass, carbon and nitrogen dynamics of multi-species riparian buffers within an agricultural watershed in Iowa, USA

This study was conducted to determine biomass dynamics, carbon sequestration and plant nitrogen immobilization in multispecies riparian buffers, cool-season grass buffers and adjacent crop fields in central Iowa. The seven-year-old multispecies buffers were composed of poplar (Populus×euroamericana Eugenei) and switchgrass (Panicum virgatum L.). The cool-season grass buffers were dominated by non-native forage grasses (Bromus inermis Leysser., Phleum pratense L. and Poa pratensis L). Crop fields were under an annual corn-soybean rotation. Aboveground non-woody live and dead biomass were determined by direct harvests throughout two growing seasons. The dynamics of fine (0–2 mm) and small roots (2–5 mm) were assessed by sequentially collecting 35 cm deep, 5.4 cm diameter cores (125 cm deep cores in the second year) from April through November. Biomass of poplar trees was estimated using allometric equations developed by destructive sampling of trees. Poplar had the greatest aboveground live biomass, N and C pools, while switchgrass had the highest mean aboveground dead biomass, C and N pools. Over the two-year sampling period, live fine root biomass and root C and N in the riparian buffers were significantly greater than in crop fields. Growing-season mean biomass, C and N pools were greater in the multispecies buffer than in either of the crop fields or cool-season grass buffers. Rates of C accumulation in plant and litter biomass in the planted poplar and switchgrass stands averaged 2960 and 820 kg C ha^–1 y^–1, respectively. Nitrogen immobilization rates in the poplar stands and switchgrass sites averaged 37 and 16 kg N ha^–1 y^–1, respectively. Planted riparian buffers containing native perennial species therefore have the potential to sequester C from the atmosphere, and to immobilize N in biomass, therefore slowing or preventing N losses to the atmosphere and to ground and surface waters.This revised version was published online in November 2005 with corrections to the Cover Date.     

Fine roots refilling process in an artificial gap in a Picea mongolica forest

Picea mongolica is an endemic but endangered species in China. The spruce forest is only found in sandy forest-steppe ecotones. In this study, we examined the initial response of the quantity and refilling process of fine roots in an artificial canopy gap with a diameter of 36 m in a P. mongolica forest. Under the canopy, the fine root length densities of trees, shrubs and herbs were 2,622, 864 and 3,086 m·m–2, respectively. The fine root biomass of trees, shrubs and herbs were 148, 62 and 65 g·m–2, respect...     

Faidherbia albida and its effects on Ethiopian highland Vertisols

On a Vertisol under 850 mm rainfall, at an altitude of 1850 m, Faidherbia albida trees that occurred at 6.52 trees ha^–1, with a canopy cover that range from 97 m² ha^–1 to 229 m² ha^–1 were recorded. Studies on the lateral and vertical influence of the tree canopy on some physical and chemical properties of the soil, show apparent higher organic matter on the West side of the tree than the East due to accumulated wind blown litter by prevalent wind direction, and organic matter, N, P and K levels were higher under the tree canopy than outside for all directions and depths studied. Soil pH, exchangeable Na, Ca and Mg under the canopy and outside were similar. Available water capacity was 1.5 to 2 times more under than outside the tree. Noticeable increases in the silt fraction under the tree was recorded and discussed in relation to soil condition improvement and plant growth.     

Biomass production and allometric above- and below-ground relations for young birch stands planted at four spacings on abandoned farmland

The objective of the study was to quantify above- and below-stumpbiomass of silver (Betula pendula Roth) and downy (Betula pubescensEhrh.) birches planted at four spacing intervals and growingon two soil types on an area of farmland. The 12-year-old bircheshad been grown at four spacings (1.3, 1.5, 1.8 and 2.6 m) ontwo sites: one on medium clay soil and the other on fine sandsoil. The dry weight of the stem, branches, leaves, stumps androots was estimated by drying and weighing sub-samples. Theprojected leaf area (PLA) m^–2 of trees, leaf area indexof stands and basic density (kg m^–3)of stems were alsoestimated. A significant greater dry weight of stem, branches,stump and roots and species and spacing for pendula birch werefound. The root length of silver birch was significantly greaterthan that for downy birch and for both species the root lengthwas greatest at the widest spacing (2.6 m). There was also asignificant difference between leaf weights of birch of thesame species growing on the two soil types. Significant differenceswere also found between PLA and species, and for both species,between PLA spacing. Basic density of stems was significantlydifferent between soil types. Equations for estimating the above-groundbiomass and root biomass from diameter at breast height weredeveloped for birches growing on fine sand and on medium claysoils. The total biomass production per hectare on fine sandwas higher for silver birch (19.9–65.9 tonnes ha^–1),than for downy birch (13.0–48.3 tonnes ha^–1). Onmedium clay soil, total biomass production for silver and downybirches was 30.8–52.8 and 16.8–42.8 tonnes ha^–1,respectively.     

Mixed-species plantations of Acacia mangium and Eucalyptus grandis in Brazil: 2: Nitrogen accumulation in the stands and biological N2 fixation

The sustainability of plantation forests is closely dependent on soil nitrogen availability in short-rotation forests established on low-fertility soils. Planting an understorey of nitrogen-fixing trees might be an attractive option for maintaining the N fertility of soils. The development of mono-specific stands of Acacia mangium (100A:0E) and Eucalyptus grandis (0A:100E) was compared with mixed-species plantations, where A. mangium was planted in a mixture at a density of 50% of that of E. grandis (50A:100E). N₂ fixation by A. mangium was quantified in 100A:0E and 50A:100E at age 18 and 30 months by the ¹⁵N natural abundance method and in 50A:100E at age 30 months by the ¹⁵N dilution method. The consistency of results obtained by isotopic methods was checked against observations of nodulation, Specific Acetylene Reduction Activity (SARA), as well as the dynamics of N accumulation within both species. The different tree components (leaves, branches, stems, stumps, coarse roots, medium-sized roots and fine roots) were sampled on 5–10 trees per species for each age. Litter fall was assessed up to 30 months after planting and used to estimate fine root mortality. Higher N concentrations in A. mangium tree components than in E. grandis might be a result of N₂ fixation. However, no evidence of N transfer from A. mangium to E. grandis was found. SARA values were not significantly different in 100A:0E and 50A:100E but the biomass of nodules was 20–30 times higher in 100A:0E than in 50A:100E. At age 18 months, higher δ¹⁵N values found in A. mangium tree components than in E. grandis components prevented reliable estimations of the percentage of N derived from atmospheric fixation (%Ndfa). At age 30 months, %Ndfa estimated by natural abundance and by ¹⁵N dilution amounted to 10–20 and 60%, respectively. The amount of N derived from N₂ fixation in the standing biomass was estimated at 62 kg N ha⁻¹ in 100A:0E and 3 kg N ha⁻¹ in 50A:100E by the ¹⁵N natural abundance method, and 16 kg N ha⁻¹ in 50A:100E by the ¹⁵N dilution method. The total amount of atmospheric N₂ fixed since planting (including fine root mortality and litter fall) was estimated at 66 kg N ha⁻¹ in 100A:0E and 7 kg N ha⁻¹ in 50A:100E by the ¹⁵N natural abundance method, and 31 kg N ha⁻¹ in 50A:100E by the ¹⁵N dilution method. The most reliable estimation of N₂ fixation was likely to be achieved using the ¹⁵N dilution method and sampling the whole plant.     

Root distribution of Fagus sylvatica in a chronosequence in western France

The distribution of fine (<2 mm diameter) and small roots (2–20 mm diameter) was investigated in a chronosequence consisting of 9-year-old, 26-year-old, 82-year-old and 146-year-old European beech (Fagus sylvatica) stands. A combination of trench wall observations and destructive root sampling was used to establish whether root distribution and total biomass of fine and small roots varied with stand age. Root density decreased with soil depth in all stands, and variability appeared to be highest in subsoil horizons, especially where compacted soil layers occurred. Roots clustered in patches in the top 0–50 cm of the soil or were present as root channels at greater depths. Cluster number, cluster size and number of root channels were comparable in all stands, and high values of soil exploitation occurred throughout the entire chronosequence. Overall fine root biomass at depths of 0–120 cm ranged from 7.4 Mg ha⁻¹ to 9.8 Mg ha⁻¹, being highest in the two youngest stands. Small root biomass ranged from 3.6 Mg ha⁻¹ to 13.3 Mg ha⁻¹. Use of trench wall observations combined with destructive root samples reduced the variability of these estimates. These records showed that variability in fine root distribution depended more on soil depth and edaphic conditions than on stand age, and suggest that trench wall studies provide a useful tool to improve estimates of fine root biomass.     

Estimates of above-ground biomass and nutrient accumulation in Mimosa scabrella fallows in southern Brazil

Naturally regenerated stands of bracatinga (Mimosa scabrella Bentham) are harvested for firewood after six to eight years of unregulated growth, debris burnt and the area planted to one cycle of intercropped maize (Zea mays L.) and beans (Phaseoulus vulgaris L.). Burning breaks dormancy of bracatinga seed (> 80% germination) marking the onset of a new fallow-crop cycle. This production system has been practiced for nearly 100 years in Southern Brazil, covering some 60,000 ha in 3,000 small farms. An estimation of above-ground biomass and nutrient accumulation was made using literature data on stand age, population numbers, tree sizes, tree biomass partitioning and concentration of major nutrients in tree tissues. A simple simulation model, used to quantify above-ground nutrient pathways and their temporal dynamics, confirmed that six to eight years is the optimal rotation length. Biomass and nutrients deposited onto the soil, peak at stand age six years, which may result in significant soil fertility improvement prior to crop planting. At year six, estimated total above-ground biomass amounts to 83 Mg ha^–1; 44 Mg ha^–1 available as firewood and 39 Mg ha^–1 to be returned to the soil. Roughly half the amount of nutrients fixed in the above-ground bracatinga biomass would be exported in firewood and subsequent grain crops.     

Root development in a Sesbania sesban fallow-maize system in Eastern Zambia

Roots of trees (Sesbania sesban) and crops (Zea mays) were quantified during two tree/crop cycles in a sequential tree — crop system at Chipata, Eastern Zambia. The experiment included one- and two-year fallows as well as fertilized and unfertilized controls. The roots of S. sesban represent a standing biomass in the soil of 3 Mg ha^t-1 in the top 1.5 m after two years, with 45–60% and 70–75% being in the top 25 and 50 cm respectively. S. sesban fallow improved early rooting and growth of the following maize crop. Increased soil infiltration was also observed in the two-year fallow treatment, as well as decreased bulk density and resistance to penetration in the soil. No differences between maize root parameters could be detected at tasselling, nor differences between nutrient status of the different treatments. Study results indicate that under the drought-prone conditions of Eastern Zambia, where improved soil physical conditions are important for early deep rooting of crops and access to water and nutrients, tree roots could play an important role in the fallow effect. Further studies are required to assess the relative importance of the improvement of soil chemical and physical properties.Submitted as ICRAF Journal Article # 95/28.     

Integration and role of planted trees in a bush-fallow cultivation system in central Zambia

The purpose of natural fallow in bush-fallow cultivaton systems is to improve soil fertility following a phase of cultivation and to provide useful forest products, including livestock feed. When natural fallow fails to serve these purposes, it can be supplemented or replaced by planted trees. This paper describes the development and function ofAcacia fallow in the Soli tribal land of central Zambia and examines the supplementary role of planted trees.The 31 woody species at five fallow sample sites were dominated byAcacia polyacantha. Thirty-nine percent of the species are leguminous and N-fixing while 42% are fodder plants. These species make thisAcacia fallow suitable for regeneration of soil fertility and production of fodder. The dominant species,A. polyacantha, has an extensive lateral root system and may attain a mean maximum girth at breast height (BH) of 125 cm within 20 years. In the study areaAcacia fallow regenerates from seed and root-stocks and with a maximum annual stem basal area increment of 0.87 m²ha^–1 at BH can acumulate a total of 17.4 m²ha^–1 in 20 years.However,Acacia fallow is poor in edible wild fruits and durable construction wood. The scarcity of fruits in the study area has been compensated by widespread planting of exotic fruit trees. Ninety percent of households have fruit trees (mean 2.3 fruit species per household). However, in spite of the scarcity of good construction wood in the Soli tribal land, no local initiative has developed to plant timber trees. A clear opportunity therefore exists to promote timber tree planting. Such a tree planting programme was started in the study area during the 1985/86 planting season (December-February) when 25,100 seedlings ofEcualyptus grandis, Gmelina arborea andLeuceana leucocephala were distributed, free of charge and the programme will continue until 1988. A survey carried out five to seven months after planting revealed that the survival ofE. grandis, L. leucocephala andG. arborea seedlings was 65%, 90% and 92%, respectively. The high mortality ofE. grandis seedlings was due to termite damage.     

Structural root growth of young Veronese poplars on erodible slopes in the southern North Island,New Zealand

In New Zealand poplars are commonly planted on moist, unstable pastoral hill country to prevent or reduce soil erosion, thereby maintaining hillslope integrity and pasture production. Mechanical reinforcement by poplar root systems aids slope stabilisation. Root mass and distribution were determined for three Populus deltoides × nigra ‘Veronese’ trees aged 5, 7 and 9.5 year planted as 3 m poles at 8 m × 8 m spacing on a hillslope near Palmerston North in the southern North Island. Most of the structural roots (≥2 mm diameter) were distributed in the top 40 cm of soil. Vertical roots penetrated to about 1.0 m, being the depth of the soil above a fragipan. Total structural root dry masses (excluding root crown) were 0.57, 7.8 and 17.90 kg for the trees aged 5, 7 and 9.5 year, respectively. Total structural root length was 79.4 m for the 5 year tree and 663.5 m for the 9.5 year tree. Surrounding trees were estimated to increase root mass density to 3 times and root length density to 4–5 times the contribution of the single tree at 9.5 year. The study indicated that root development of wide-spaced poplar trees on hillslopes was minimal in the first 5 years but then increased rapidly. These results suggest that poplar trees established from poles may take at least 5 years to develop a structural root network that will effectively bind soil.     

Evaluation of Populus deltoides clones under nursery,field and agrisilviculture system in subhumid tropics of Central India

Populus deltoides Bartr., a native of North America, is generally grown in India above latitude 28 °N. One hundred and six clones were evaluated for four years at Raipur situated at 21°12N latitude and 81°36E longitude. These were grown on vertisol soil. Based on growth and survival performance in the nursery for two successive years, nineteen clones were selected for field evaluation. The best five clones (G3, G48, 65/27, D121 and S7C1) were planted in an agrisilviculture system at a spacing of 4 × 4 m with soybean grown as an intercrop. After 4 years these clones had an increment of DBH by 66.5 to 77.5% and of height by 42.2 to 78.6% within one year when compared to that observed at 3 years of age. In rank order of growth the best five clones were 65/27 > G3 > D121 > G48 > S7C1. Total biomass varied between 20.9 to 35.8 Mg ha^–1 in different clones. Among the tree components, stemwood accounted for 52–61% of the total biomass, followed by branches (20–25%), bark (9–13%) and leaves (7–10%). No significant variation between net primary productivity and photosynthetic efficiency was found in different clones. Soybean productivity decreased as the trees aged, reaching 40.5 to 58.1% in 4-year-old trees.