Weed seedbank characteristics of arable fields under different fallow management systems in the humid tropical zone of southeastern Nigeria

The composition and pattern of weed flora in arable fields are determined by their seedbank structure; but the influence of fallow management practices on weed seedbank structure is presently unknown. The objective of this study was to investigate weed seedbank characteristics and weed population dynamics in arable fields in natural and planted-fallow systems. The study plots were at Mbaise, a densely populated area of southeastern Nigeria, where farmers regenerate their exhausted soils by maintaining planted fallows of Dactyladenia barteri (Hook. F. ex Oliv.) Prance & F. White, and at Umuahia, a less-densely populated area in the same region, where farmers depend on natural bush fallow for soil regeneration. The effect of three years of fallow on the weed flora of arable fields in the two fallow management systems differed remarkably. The first flush of weeds on fields that were cultivated after three years of planted D. barteri fallow (Mbaise) consisted of 80% broadleaf weeds, 7% grass weeds and 13% sedges. On the other hand, the first flush of weeds on the natural bush fallow fields (Umuahia) of the same fallow duration as the D. barteri fallow system consisted of 17% broadleaf weeds, 70% grasses and 13% sedges. Three years of planted fallow caused 36% decrease in weed seedbank at Mbaise relative to the cropped field while the same duration of natural bush fallow caused a 31% increase in weed seedbank at Umuahia. These results show that the planted D. barteri fallow system has a higher potential to reduce weed pressure in smallholder agriculture than the natural bush fallow system and may explain in part why farmers in this humid forest zone have adopted the practice. This revised version was published online in June 2006 with corrections to the Cover Date.     

Shifting cultivation systems practised in Bhutan

In the highlands of Bhutan shifting cultivation remains an important land use practice, covering an area of approximately 200,000 ha. Two systems, bush fallow and grass fallow, are described and discussed. The traditional methods used in the two systems are well adapted to available resources. The systems differ in fallow vegetation, altitude range, major crops, fallow period, farming tools, clearing method, labour inputs, and erosion risks. Soils used for the grass fallow system are low in P and N. Major limitations for the bush fallow system are weed competition and the steep terrain. Returns from labour wereapproximately 10.6 and 4.2 kg grain per day for the bush fallow and the grass fallow system, respectively. Access to market through a recently built road system provides some cultivators with lucrative alternatives in the form of timber, livestock and potato production. Alternative options are more limited in the bush fallow system.This work was supported by the Department of Agriculture, Royal Government of Bhutan and the Swiss Association for Technical Assistance     

Productivity of intensified crop—fallow rotations in the Trenbath model

Management of crop—fallow rotations should strike a balance between exploitation, during cropping, and restoration of soil fertility during the fallow period. The Trenbath model describes build-up of soil fertility during a fallow period by two parameters (a maximum level and a half-recovery time) and decline during cropping as a simple proportion. The model can be used to predict potential crop production for a large number of management options consisting of length of cropping period and duration of fallow. In solving the equations, the model can be restricted to sustainable systems, where fallow length is sufficient to restore soil fertility to its value at the start of the previous cropping period. The model outcome suggests that the highest yields per unit of land can be obtained by starting a new cropping period after soil fertility has recovered to 50–60% of its maximum value. This prediction is virtually independent of the growth rate of the fallow vegetation. The nature of the fallow vegetation (natural regrowth, planted trees, or cover crops) mainly influences the crop yield by modifying the required duration of fallow periods. Intensification of land use by shortening fallow periods will initially increase returns per unit land at the likely costs of returns per unit labor. When fallows no longer restore soil fertility to 50% of the maximum, overall productivity will decline both per unit land and per unit labor, unless external inputs replace the soil fertility restoring functions of a fallow.This revised version was published online in November 2005 with corrections to the Cover Date.     

Weed seedbank response to planted fallow and tillage in southwest Nigeria

Planted fallows are an alternative to the unsustainable bush fallow for improved soil and weed management in the tropics. However, the interactive effects of planted fallows and tillage on the weed seedbank are not well documented in the tropical environment. The effect of fallow type and tillage on the weed seedbank in the soil was assessed in 1995 and 1996 at Ibadan, southwest Nigeria. The planted fallow species consisted of a herbaceous legume (Pueraria phaseoloides) and three woody legumes (Acacia auriculiformis, Leucaena leucocephala, and Senna siamea). Natural bush fallow and continuous cassava/maize plots were controls. Tillage treatments were minimum tillage and mounding. Continuous maize/cassava plots had the largest weed seedbank in both years. After six years of continuous fallow, the weed seedbank was 86% lower in A. auriculiformis, 79% in P. phaseoloides, 68% in S. siamea, 53% in L. leucocephala, and 35% in natural bush fallow plots than in continuously cultivated plots. Compared to minimum tillage, mounding reduced the seedbank by 47% in 1995 and 66% in 1996. Redundancy analysis showed that tillage contributed significantly to the variance in species composition. Euphorbia hyssopifolia, E. heterophylla, and Cynodon dactylon showed no preference in terms of tillage. Perennial and annual grasses (Digitaria horizontalis, Eleusine indica, Paspalum orbiculare, Cynodon dactylon) with Cyathula prostrata and Desmodium scorpiurus, an annual and perennial broadleaf, respectively, were most abundant in the seedbank of continuously cultivated plots. There were more annual broadleaf weeds in the seedbank of planted fallow plots than in the control plots. Species diversity of the seedbank was greatest in plots under minimum tillage. Mounding as a seedbed preparation method, especially within the improved fallow system, could reduce the high weed pressure experienced by smallholder farmers in southwest Nigeria.     

Ecosystem fertility and fallow function in the humid and subhumid tropics

The regeneration of natural vegetation (fallowing) is a traditional practice for restoring fertility of agricultural land in many parts in the tropics. As a result of increasing human population and insufficient fertilizer inputs, the ecosystem fertility functions of traditional fallows must now be improved upon via the use of managed fallows. Interactions between vegetation and soil determine nutrient losses and gains in crop—fallow systems and are influenced by fallow species, patterns and rates of biomass allocation, and crop and fallow management. Nutrient losses occur through offtake in crop harvests during the cropping phase and through leaching, runoff, and erosion in the cropping phase and the initial stage of fallows $#x2014; when nutrient availability exceeds nutrient demand by vegetation. Gains in nutrient stocks in later stages of fallow are generally more rapid on soils with high than low base status due to greater quantities of weatherable minerals and lack of constraints to N₂ fixation, deep rooting, and retrieval of subsoil nutrients by fallow vegetation. On low base status soils (exchangeable Ca < 1 cmol_c kg^–1), N₂ fixation and atmospheric inputs are likely to be the main sources of nutrient additions. On high base status soils limited by N, gains in N stocks by inputs from N₂ fixation and retrieval of subsoil nitrate can occur relatively rapidly; hence short-term fallows can often improve crop performance. Large losses of Ca associated with soil organic matter (SOM) mineralization and soil acidification during cropping and fallow establishment, combined with chemical barriers to root penetration, suggest that long-duration fallows (> 5 yr) are needed for recovery of cation stocks and crop performance on low base status soils. On both soils, however, residual benefits of fallows on crop yields usually last less than three crops.This revised version was published online in November 2005 with corrections to the Cover Date.     

Fuelwood Resources and Forest Regeneration on Fallow Land in Uganda

Abstract

East African forests have been largely converted to agriculture. The remaining forests hold many endangered species but are threatened by the heavy local demand for fuelwood. Here we evaluate fallow land in western Uganda as an alternate fuel source to diverse forests. We quantify the regeneration process on fallows, calculate tree bio-mass increases, and measure grass and woody herb biomass over 44 months. The biomass values we measured were typical or slightly below the average from 11 studies elsewhere in the tropics. Variation in biomass between our neighboring study sites exceeded that between sites on different continents, indicating the sensitivity of vegetation regeneration to local land use. Tree regeneration was extremely slow (0.46 g/m²/year); however, the woody herbs and grasses on a 4 year old fallow of ～0.5 ha can provide much of a family's domestic fuel. Fallow land is generally abundant in western Uganda and can partially alleviate pressure on forests for domestic fuels. Fallows cannot however provide the trees demanded by charcoal, brick, and gin manufacturers. In the future, conserving forests while meeting fuelwood demands will require improving local land tenure security, enhancing the productivity of cultivated and abandoned land, promoting more efficient stoves, stills and kilns, and curtailing illicit, inefficient charcoal manufacture.     

Spectral irradiance and stomatal conductance of enriched fallows with fast-growing trees in eastern Amazonia,Brazil

The biomass production of both indigenous and introduced plant species in tropical fallow systems depends on the degree to which these species can acclimate to the light and water environments. Results for light spectral composition monitoring within the canopy of enriched fallows and for the leaf stomatal conductance of fast-growing leguminous trees and indigenous fallow species are presented. All measurements were made in a smallholder farm in Igarapé-Açu, northeastern Pará State, Brazil. Light spectral composition (330 to 1100 nm) was monitored at two heights (ground level and 1 m) in a six-year-old natural fallow, 1.5-year-old natural fallow, and 1.5-year-old fallows enriched with Acacia angustissima, Acacia mangium, Clitoria racemosa, Inga edulis, Sclerolobium paniculatum, and a mixture of these trees. Light-quality parameters including photosynthetically active radiation, phytochrome active radiation, and blue active radiation changed most drastically in the stands enriched with A. mangium. Stomatal conductance was higher for A. mangium than the other trees and four common indigenous fallow vegetation species (Phenakospermum guyannense, Davilla rugosa, Lacistema pubescens, and Myrcia bracteata). Results suggest that the enrichment of fallows with A. mangium may promote changes in light and water vapor exchange regimes, with potential effects on species diversity in fallows.This revised version was published online in November 2005 with corrections to the Cover Date.     

Optimal tree fallow rotations: some principles revealed by modelling

A model (ROTATE) of the nitrogen (N) cycle during the tree and crop phases of fallow systems [Robertson, 1994] was used to determine the primary factors influencing longterm crop yields. The model simulated the expected patterns of increase in old (recalcitrant) soil organic N during tree rotations and their decrease under continuous cropping. After 3–4 fallow cycles an equilibrium soil organic N content is reached, where N losses by crop removal are balanced by N gains by the trees (either by fixation or pumping from depth) plus small inputs in rain. The rotation period has two variable components; the cycle length (tree plus crop period) and the fraction of years in each cycle occupied by trees (1 = sole trees, 0 = sole crops). Both components have optima determined by the time taken for the trees to increase the old soil organic N pool to an optimal (but not maximal) size. This optimum exists because the rate of increase in old organic N slows as the tree fallow progresses and a time is reached (often soon after the trees reach full size) when the benefits of further improvements in soil fertility are outweighed by crop yield foregone. In the example chosen of Acacia/sorghum in the Sahel, the optimum cycle seemed to be about 50 years with half of the time in trees. The optimum fallow period is shortened by growing fast-growing trees, and the benefit of fallow periods are greatest when (i) a large proportion of the N in litter (above and belowground) is transferred to the recalcitrant soil pool, and (ii) the trees attain a large size with correspondingly large annual additions of N to the soil.     

Ethnicity, land use and woody vegetation: a case study from south-western Burkina Faso

Woody vegetation in cultivated landscapes in Burkina Faso is influenced by agricultural activities that are in turn influenced by institutional arrangements. Research was undertaken in a village in south-west Burkina Faso to investigate the relationship between species composition, diversity, density, species accumulation and land use category. Additionally the relationship between number of trees, size of farmed land and farmer ethnicity was investigated. Indigenous Tiéfo farmers had on average more than double the number of large (>4 m) trees of Vitellaria paradoxa Gaertn. f., Parkia biglobosa (Jacq.) R. Br. Ex G. Don and Anacardium occidentale L. than farmers belonging to other ethnic groups, but this was partly explained by a larger area being available. Differences between ethnic groups were not significant when expressed per ha. Botanical inventories of fallows more than 4 years old, cultivated parklands and plantations of Mangifera indica L. and A. occidentale showed that tree density and Simpson’s index of diversity for trees were highest in the fallow. Simpson’s index was not significantly different for regeneration, but sample-based species accumulation curves indicated that species accumulation in parklands was faster than in plantations when expressed per individual. The average regeneration density was 12,605, 1,995 and 6,772 seedlings ha^-1 for fallow, parklands and plantations, respectively. This low density for parklands means that species accumulation is slow compared to the other land uses when expressed per unit area. Fallow seems the most efficient way of keeping tree diversity in the agricultural landscape.     

From alley cropping to rotational fallow: Farmers' involvement in the development of fallow management techniques in the humid forest zone of Cameroon

Alley cropping was introduced in the humid forest zone of Cameroon to increase soil fertility in 1987, but until 1992 the adoption rate had remained low. To better understand the reasons behind this, three types of on farm trials were established from fully researcher controlled to fully farmer controlled. During the evaluation of the technology with farmers a number of modifications were registered (1) Pruning height and frequency: Because of the difficulties to consistently cut back at 30 cm, farmers decided to slash at ground level as they normally do while slashing the natural fallow vegetation. This equally allowed for more flexibility in time of pruning. (2) Cropping intensity and pattern: Initially, alleys were cropped each year. This however had several shortcomings. Having observed the positive impact of incidental fallow period in a farmer's field, it was decided to introduce a fallow phase of at least one year. During the fallow period the plot can be used for fodder production, bee farming and production of stakes. (3) Residue management: Fire went incidentally in a farmer's tree plot after slashing, and the trees were not affected. This gave farmers an alternative way to manage the residue, by controlling the fire, before bringing in other crops such as groundnut and cassava. (4) Agroforestry species: Because Leucaena leucocephala, in spite of it soil fertility restoration potential, rapidly became a noxious weed, farmers have asked for a less invasive species. Calliandra calothyrsus was introduced for this purpose and became a good bee forage. With these modifications, the original alley cropping system has evolved into a rotational tree fallow with higher adoption potential. From about 15 farmers who were testing the technology in 1992, the number increased to 52 in 1996, 120 in 1997 and 236 in 1998. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil microarthropod populations under natural and planted fallows in southwestern Nigeria

Microarthropods, such as soil mites (Acari) and springtails (Collembola), with body width between 0.08 mm and 0.5 mm play important roles in soil fertility maintenance through their regulatory activities in decomposition and nutrient turnover. Observations were made at IITA, Ibadan, southwestern Nigeria to evaluate the effects of natural regrowth of vegetation — mainly the shrub Chromolaena odorata — and three planted woody fallow species (Acacia leptocarpa, Senna siamea, and Leucaena leucocephala) on soil microarthropods in a degraded Alfisol. Populations of soil microarthropods were higher in the rainy season than the dry season, and populations were greater under natural fallow than for continuous cropping with maize (Zea mays) and cassava (Manihot esculenta). Populations of soil microarthropods were comparable under leucaena and natural fallow, but populations in the rainy season were 38% higher under senna than natural fallow and 36% higher under acacia than natural fallow. Regression analysis indicated that soil microarthropod population under fallow species was positively correlated with the lignin contents of leaf litter.This revised version was published online in November 2005 with corrections to the Cover Date.     

Chromolaena odorata in slash-and-burn rice systems of Northern Laos

Chromolaena odorata, introduced to Laos in the 1930s, has become the most abundant weed and fallow species in slash-and-burn fields over a wide range of land use systems, elevation, and pH ranges. Regeneration from roots, high seed production and easy dispersal allow for the rapid colonization of fields in the initial fallow period. At rice harvest, after a 1-year and a 2-year fallow, the total aboverground biomass in monitoring plots was 1.4, 10, and 15.4 t ha^–1, with 16, 48, and 29% contribution byC. odorata, respectively. With progressing fallow periodC. odorata is gradually replaced by tree and bamboo species. Slash-and-burn farmers preferredC. odorata over other fallow species common in their fields.Chromolaena odorata is an excellent fallow species considering its fast expansion after crop harvest, high biomass production, weed suppression, and fast decomposition rate. Some of these properties may, however, become a serious disadvantage when farmers gradually change to land use systems that integrate grazed fallow, crop rotation, and/or fruit and timber plantations.     

Influence of remnant trees on nutrients and fallow biomass in slash and burn agroecosystems in Guinea

Three tree species are traditionally conserved in the traditional slash and burn agricultural system practiced for the production of upland rice (Oryza sativa Linn.) in the Fouta Djallon region of Guinea, i.e. (Parinari excelsa [Sabine], Parkia biglobosa [Jacq.) Benth.], Erythrophleum guinensis [G. Don.]). Sampling a chronosequence of fallow sites indicated that extractable P; exchangeable K, Ca and Mg; cation exchange capacity (CEC) and pH all decreased over the fallow period with patterns that differed with soil depth. Soils under the remnant Parinari excelsa and Parkia biglobosa trees had higher concentrations of organic C; total N; extractable P; exchangeable K, Ca and Mg; total P and Ca; and CEC than did the open microsites. Extractable P; exchangeable K, Ca and Mg; total P and Ca; and CEC were greater under Parinari excelsa than under Erythrophleum guinensis. Intensive measurements of a single mature fallow site showed that the foliar nutrient concentration of the large trees was not an accurate index of which microsite had the greatest accumulation of biomass and nutrients in the aboveground fallow vegetation. The biomass and the total amounts of N, P, K, Ca and Mg in the fallow vegetation were significantly greater for the Parinari excelsa and Parkia biglobosa microsites than for the open microsites. We conclude that although the nutrient status of the soils decreased over the 8-year fallow period, the microsites under the large trees were generally more fertile than the open microsites. However, there were important differences in the effects of the different tree species. This revised version was published online in June 2006 with corrections to the Cover Date.     

Fallow enrichment of a traditional slash/mulch system in southern Costa Rica: comparisons of biomass production and crop yield

The slash and mulch system of frijol tapado or covered bean is a pre-hispanic system of bean production found throughout much of Central America. However, land use pressures have forced farmers to shorten the traditional fallow period, thus resulting in decreasing productivity. A potential solution is to enrich the fallow by using leguminous nitrogen-fixing trees. The enriched fallow systems evaluated in this study include both single and mixed species treatments: 1) Erythrina poeppigiana; 2) Calliandra colothrysus; 3) Gliricidia sepium; 4) Inga edulis; 5) Inga edulis and Erythrina poeppigiana; 6) Inga edulis and Calliandra calothrysus; and 7) Inga edulis and Gliricidia sepium. Biomass production of the fallow vegetation is shown to be of greater quality and quantity in all fallow enrichment treatments. However, bean yields did not show a significant response to the fallow enrichment treatments.     

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.     

The T'OLCHE', a Maya system of communally managed forest belts: the causes and consequences of its disappearance

The T'OLCHE' is a protected forest belt about 10–20 m wide that runs through the Mayan landscape on the limits of the maize fields of the farmers. This study explores its traditional use and management and traces how it has changed in the last 30 years in the study village in Yucatán, Mexico. A major change is in its extent; little remains of the 10–20% of the area it occupied in the past. Two of the causes of this are identified as being a credit programme of the local bank and 1973 state legislation concerning cattle. The causes and consequences of this decline are discussed in relation with other farmer-perceived changes in overall landuse, including a fall in maize yields. The ecological roles of the T'OLCHE' are examined. It is argued that the T'OLCHE' plays an essential role in the regeneration of the fallow vegetation and in the maintenance of biological diversity. The case of the T'OLCHE' suggests the need for a diversified and politicized look on Common Property Resource Management.     

城市居民休闲生活方式的阶层差异与空间环境的再创造

城市是指适宜人居住的场所,人们在城市中生活应该感到自由、舒适和快乐,其本质在于休闲功能的实现.本文通过对老年人、中青年人以及儿童这几种不同阶层人群的特点分析,了解不同人群对住区休闲环境的需要,住区休闲娱乐活动空间在很大程度上决定了居民的休闲模式,直接影响到人们休闲生活的质量,对于满足居民的日常休闲娱乐活动需求、更好的进...     

肯尼亚西部印度田菁改良休耕地对玉米生产力和黄独脚金寄生杂草(Sesbania sesban)侵扰的作用

在撒哈拉以南的非洲地区,黄独脚金寄生杂草（Strigahermonthica）侵扰是限制小农产自给性农业生产的主要因素之一。土壤肥力低加之总体环境退化是寄生杂草侵扰产生的重要原因。引入改良的耕作制度来解决寄生杂草侵扰和土壤肥力下降的问题势在必行。本文对肯尼亚西部双峰高原地区内,用豆科植物--印度田菁改良的休耕地对玉米产量和农田寄生杂草侵扰的作用进行了研究。实验处理分阶段进行,处理包括田菁改良6和18个月的休耕地、未经耕作自然植物再生6和18个月的休耕地、连续种植玉米未施肥的耕地和连续种植玉米同时施加氮和磷肥的耕地。结果表明,与未施肥玉米地相比,田菁改良休耕地明显（p〈0.5）增加玉米产量.除草管理降低了第一季度（428000&#177;63000株&#183;hm-2）、第二季度（51000&#177;1500株&#183;hm-2）玉米地寄生杂草植株种群。实验周期内,除草管理降低玉米地土壤中寄生杂草种子种群数。短期田菁改良休耕地对玉米产量的促进作用明显好于未施肥的玉米地,但是短期杂草休耕地对玉米产量无显著影响。种植玉米和除草控制寄生杂草效果要好于休耕。     

Characterisation of Afzelia africana Sm. habitat in the Lama forest reserve of Benin

A study was carried out in the Lama forest reserve of Benin to characterize the habitat of Afzelia africana Sm., an endangered multipurpose tree species (found in African humid, dry forests and woodlands), in order to define a sustainable management strategy for its conservation. An estimation of species density was done on 100 square plots of 1 ha each, while tree height and dbh of all the species were measured on subplots of 50 m × 30 m within the 1 ha plots. The regenerations of A. africana (dbh < 10 cm) were counted in the diagonal quadrats of the principal plots. Presence–absence data of the species was subjected to multidimensional scaling and results showed four vegetation communities including: young fallow, old fallow, typical dense forest and degraded dense forest. Significant differences were noted between the four communities with respect to dendrometric parameters of the species. High values of these parameters were noted for the species in typical dense forest (5.2 stems/ha, 66.7 cm, 17.9 m, 7.9 m²/ha and 38.8% for the tree-density, the mean diameter, the mean height, basal area and basal area contribution of the species, respectively) whereas the lowest values were obtained for the old preforest fallow as far as the mean diameter (59.7 cm), the mean height (15.7 m) and the basal area contribution (27.7%) of the species were concerned. In general, the basal area of A. africana in the over vegetation types was less than 3 m²/ha. No A. africana tree was found in the young preforest fallow while more than 80% of A. africana trees were found in the typical dense forest community. Stem diameter and height structures of the species in all the four communities showed a left dissymmetric Gaussian shape and were well adjusted to Weibull distribution.     

Prospects for easing land tenure conflicts with agroforestry in Mediterranean France: a research approach for intercropped timber orchards

Agroforesty plots offer the opportunity to combine annual agricultural production and long-term wood capitalization. The fallow scheme now in operation in Europe intends to reduce the surplus of agricultural products. Agroforestry management could be a valuable alternative to fallow or to forestry plantations on agricultural land.Agroforestry is attractive to agricultural landowners in Mediterranean France, as agroforestry stands are less costly than forestry plantations, are less prone to fire hazards, provide agricultural returns from intercropping during the first twenty years and have many environmental benefits.An experimental network of on-farm agroforestry plots has been set up since 1988 in the Mediterranean part of France. These plots include grazed paddocks (silvopastoral systems) and ungrazed fields (silvoarable systems). They gained support both from landowners and tenants: the former expect a high value treestock, while the latter have a free use of the space between plantation lines. Plantations have been designed to allow easy mechanical intercropping. First results from two experimental sites are discussed. The growth of the trees in the agroforestry plots is much higher than in the control forestry plots.