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1.
A study was carried out in an alley cropping system in the Eastern Maize Belt of the United States with the objective of quantifying the competition for water between trees (black walnut (Juglans nigra L.) and red oak (Quercus rubra L.) and maize (Zea mays L.). Spatial and temporal variation in soil moisture, tree and maize rooting patterns, tree and maize water uptake, and maize leaf area expansion were determined in three treatments; barrier (polyethylene root barriers separating maize and tree roots), trench (trenching without a polyethylene barrier), and no barrier (control). Significant temporal variation in soil moisture was observed in both black walnut and red oak alley cropping systems. The barrier and trench treatments resulted in higher soil moisture in the maize alley and lower soil moisture in the tree row than the no barrier treatment. Uptake of water by maize was higher in the barrier than the no barrier treatment. However, tree water uptake was higher in the no barrier treatment than in the barrier treatment because tree roots utilized water from the maize alley. Maize leaf area was negatively impacted by water stress in the no barrier treatment. Quantification of rooting patterns revealed that maize and tree roots were concentrated in the top 30 cm soil layer where water fluctuations were greatest. The barrier and trench treatments successfully eliminated the belowground root competition between trees and maize and resulted in greater leaf area and higher grain yields than the no barrier treatment. Thus, competition for water rather than competition for light seems to be critical in defining the productivity and sustainability of this alley cropping system.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

2.
The susceptibility of some spring wheat varieties to the attack by the two wheat blossom midge species (Dipt., Cecidomyidae) The susceptibility of 20 spring wheat varieties to the attack byContarinia tritici (Kirby) andSitodiplosis mosellana (Géhin) was studied from 1971 to 1976 on small plots. Some varieties proved to be very susceptible, others were partly resistant. Meritus showed a high susceptibility to both wheat blossom midge species, while Arm showed a very low one. The resistance observed could not be explained by incoincidence. Of the varieties still being in trade, Opal should not be grown in areas of damage byS. mosellana. Kleiber and Kolibri are apt for this. Arin can be recommended to be grown in areas, where damage by both wheat blossom midges occurs.  相似文献   

3.
The purpose of this work was to determine the most efficient methods of inoculation to significantly improve nodulation and growth of Acacia senegal and Acacia nilotica, grown under greenhouse conditions. Our results showed that inoculation using dissolved alginate beads containing rhizobia significantly improved the growth of both acacia species better than the growth of plants in other treatments. The experiment with A. nilotica was conducted in two unsterilised soils from different areas. Plants grown in soil from Bel Air were well-nodulated and showed better growth than plants grown in soil from Sangalkam. However, no difference between these soils was shown between the several methods of inoculation and their effect on the nodulation and growth of plants. An interaction between A. senegal and A. nilotica provenances and the effect of inoculation with rhizobia was also demonstrated. Bel Air provenance of A. senegal, Dahra and RIM provenances of A. nilotica grew best of several provenances tested. These results suggest that (1) it may be possible to improve growth and yield of A. senegal and A. nilotica by careful selection of each symbiotic partner; and (2) nursery-grown seedlings of A. senegal and A. nilotica should be inoculated, just after sowing, with dissolved alginate beads containing a mixture of selected rhizobia. Résumé. Lobject de notre travail était de déterminer la méthodologie dinoculation la plus efficace pour améliorer significativement la nodulation et la croissance dAcacia senegal et dAcacia nilotica poussant dans des conditions de serre. Nos résultats ont montré que linoculation utilisant des billes dalginate dissoutes améliorait significativement la croissance des deux espèces dAcacia par rapport à celle des plantes des autres traitements. Lexpérience avec A. nilotica a été faite dans deux types de sol non stériles. Nous avons observé que les plants poussant dans le sol de Bel Air étaient plus nodulés et mieux développés que ceux poussant dans le sol de Sangalkam. Cependant, aucune différence significative entre les deux sols na été mise en évidence en termes deffet des différentes méthodologies dinoculation sur la nodulation et la croissance des plantes. Il a aussi été démontré lexistence dune interaction entre les provenances dA. senegal et dA. nilotica et leffet de linoculation avec rhizobium. Cest ainsi que les provenances Bel Air dA. senegal, Dahra et RIM dA. nilotica ont le mieux poussé parmi les différentes provenances testées. Lensemble de ces résultats suggèrent: (1) il devrait être possible daméliorer la croissance et la production dA. senegal et dA. nilotica par une sélection des deux partenaires symbiotiques; (2) Les jeunes plantules dA. senegal et dA. nilotica produites en pépinière devraient être inoculées juste après le semis avec des billes dalginate dissoutes contenant un cocktail de souches sélectionnées.  相似文献   

4.
Effect of cutting position on rooting and shoot growth of two poplar clones   总被引：1，自引：0，他引：1  
One-year-old shoots from stooling beds of Populus × deltoides Walker and Populus jackii Northwest were collected in late fall to determine the effect of cutting position on rooting and growth of hardwood poplar cuttings. Cuttings with a minimum diameter of 9.0 mm were obtained from basal, middle and distal locations on the poplar shoot. Rooting and shoot growth were assessed in the greenhouse and under field conditions. Walker poplar cuttings collected from basal portions of the shoot and planted in the field had 87.6% rooting and 103.8 cm growth compared to 78.8% and 103.4 cm and 67.6% and 88.8 cm for middle and distal locations respectively. For Northwest rooting and growth under field conditions was 84.4% and 94.7 cm for basal, 78.4% and 90.5 cm for mid and 64.4% and 84.3 cm for distal locations. Rooting in the greenhouse was superior to the field. Walker cuttings had fewer roots and buds per cutting than Northwest, however, growth of Walker in the field was superior.  相似文献   

5.
This paper describes the salient aspects and analyses the potential of the Paraiso Woodlot System, which is becoming popular as an agroforestry alternative to land use in the degraded acid sandy soils of the humid sub-tropical Guayaybi area of Paraguay. The system consists of a combination of paraiso (Melia azedarach var gigante) with other trees, especially Leucaena leucocephala, and annual crops.The fast growth habit, deep root system, addition of large quantities of organic matter through leaf and litter fall, compatibility with agricultural crops, high value of the sawlogs and production of substantial quantities of poles and firewood make paraiso an excellent species for agroforestry combination. Preliminary results of the trials indicate that the woodlots are successful and with increasing efforts of the extension agencies, they are being accepted as a viable alternative to traditional agricultural systems.Besides discussing the potential of these woodlots based on initial results, the paper identifies the constraints of the system and highlights the priority research areas.  相似文献   

6.
Zusammenfassung Unterschiedliche Vektoreignungen können zwischen Rassen, Morphen und Altersstadien einer Blattlausart auftreten. Es wurden verschiedene Morphen vonMacrosiphum euphorbiae (Thomas) undAcyrthosiphon pisum (Harris) hinsichtlich ihrer Übertragungsfähigkeit für das persistente Enationenvirus der Erbse geprüft. Dabei erwiesen sich ovipare Weibchen und adulte Männchen vonM. euphorbiae als ungeeignete Überträger, während die gleichen Morphen von zweiA. pisum-Populationen die Virose übertrugen. Auch die Fundatrix desA. pisum-BastardsTrifolium pratense rot XSarothamnus scoparius grün übertrug zu einem relativ hohen Prozentsatz. Ovipare Weibchen und Männchen der inaktiven RasseLotus uliginosus gelb konnten keine Infektion bewirken; die persistente Virusübertragungsfähigkeit ist im Falle vonAcyrthosiphon pisum (Harris) offenbar ein genetisch fixiertes Merkmal.
Summary Different vector abilities may exist between races, morphs and development instars of a given aphid species. Several morphs ofMacrosiphum euphorbiae (Thomas) and ofAcyrthosiphon pisum (Harris) were tested in their transmission ability for the persistent pea enation mosaic virus. Oviparous females and adult males ofM. euphorbiae proved insuitable for transmission, the corresponding morphs of twoA. pisum-populations, however, were successful in transmitting. The fundatrix of theA. pisum hybridTrifolium pratense red xSarothamnus scoparius green transmitted to a relatively high percentage. Oviparous females and males of the inactiv raceLotus uliginosus yellow failed completely in transmitting; in the present case ofA. pisum the transmission character of the persistent virus evidently is genetically fixed.
Résumé Différentes aptitudes de vecteur peuvent se trouver entre les races, les morphes et les périodes d'âge d'une espèce de pucerons. Différentes morphes deMacrosiphum euphorbiae (Thomas) et deAcyrthosiphon pisum (Harris) ont été examinées en ce qui concerne leurs aptitudes de transmettre le virus persistant du pois(pea enation mosaic). En ce cas les femelles ovipares et les mâles adultes deM. euphorbiae se montraient impropres à la transmission, pendant que les mêmes morphes de deux populations deA. pisum transmettaient la virose. C'était la fondatrice du bâtard deA. pisum Trifolium pratense rouge avecSarothamnus scoparius vert qui transmettait à une pourcentage relativement haute. Les femelles ovipares et les mâles de la race «inactive» deLotus uliginosus jaune ne pouvaient pas effectuer d'infection; en cas deAcyrthosiphon pisum (Harris), l'aptitude persistante à transmettre le virus évidemment est un caractère fixé d'une façon génétique.
, . Macrosiphum euphorbiae (Thomas) Acyrthosiphon pisum (Harris) peaenation mosaic virus). M. euphorbiae , {A. pisum} . A. pisum-Trifolium pratense Sarothamnus scoparius . «» Lotus uliginosus ; Acrythosiphon pisum (Harris) .
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7.
This study used a vibration test method to show that grain angles of face veneer have substantial effects on sound velocities and dynamic Youngs moduli of three types of wood-based composites. The sound velocity at 0° grain angle of face veneer was the highest, and it decreased with increasing grain angle in the range of 0° to 90°. This tendency was similar to that for dynamic Youngs modulus. The relationship between the grain angle of face veneer and the sound velocity of three types of wood-based composites can be expressed in the form of Hankinsons equation or a second-order parabolic equation. This study also showed that the application of orthotropic elasticity theory was valid for the three types of wood-based composites. The relationship between the grain angle of the face veneer and the Youngs modulus of three types of wood-based composites can be expressed in the form of the Jenkin equation, Hankinsons equation, or a second-order parabolic equation. Rule of mixture can also be used to predict the Youngs modulus of wood-based composite from the Youngs moduli of the two elements.Part of this study was presented at the 53rd Annual Meeting of the Japan Wood Research Society, Fukuoka, Japan, March 2003 and the 15th Annual Meeting of Chugoku Shikoku Branch of the Japan Wood Research Society, Higashi-Hiroshima, Japan, September 2003  相似文献   

8.
The extension strategy used by the CARE International in Kenya Agroforestry Extension Project (AEP) is described. This strategy is evaluated in terms of documented changes in farmer's agroforestry practices and the importance of AEP relative to other changes in economic incentives, institutional support, and land use and tenure rules for agroforestry. The project served mainly to increase awareness of the multipurpose potential of agroforestry, expand available technical options, and strengthen local institutions. Its community-based extension approach was more suitable for local conditions than commodity-based, training-and-visit, farming systems, or media-based extension. Projects are encouraged to tailor extension design to local circumstances, use community participation to guide extension priorities, and use extensionists as catalysts and information brokers.Research undertaken while the author was a Principal Scientist at the International Council for Research in Agroforestry, Nairobi, Kenya.  相似文献   

9.
There is growing interest in research to develop potential agroforestry models for temperate climates. In Washington State, recent studies and anecdotal information suggest that agroforestry is already employed by land managers, and if so, this experience should inform future research efforts. Because this population is not well defined, a mail survey was designed to: 1) Assess Washington land manager awareness of agroforestry, 2) assess perceptions of agroforestry as a land management tool, 3) assess the perceived potential opportunities or obstacles for land managers to practice agroforestry, and 4) identify landowner groups believed to be practicing agroforestry in Washington State.Three groups of land managers were surveyed: employees of the Soil Conservation Service (SCS), Washington State University Cooperative Extension Service (WSUCE) and OTHER, consisting of university faculty, private land managers, State and Federal land managers and owners of small natural resource businesses.Agroforestry was not a new concept for most (94%) respondents, further 55% of those familiar with agroforestry were practicing agroforestry or providing advice to landowners who were practicing agroforestry. Use in (government mandated) soil conservation plans on farmland (100% of all respondents) was the most frequently cited potential application for agroforestry in the state followed by range and pasture land and managing non-commercial forest land (both 84%), use on commercial forest plantation (83%) and fruit and nut orchards (61%).Diversifies land use (25%), enhanced productivity (18%), aesthetics (13%) and income diversity (13%) were the four most frequently cited potential advantages to practicing agroforestry. Lack of information (28%), lack of technical assistance (18%), establishment costs (14%) and not an established practice (14%) were the most frequently identified potential obstacles to practicing agroforestry. Respondents suggested there is great potential for application of agroforestry throughout the state, and non-industrial private forest land owners were selected for future study of this potential.  相似文献   

10.
Summary It is stated that in the spreading of epizooties among small wild animals, a factor of biocoenosis played a main role, first of all some phenomena among small rodents, the field mice. In the spreading of rabies infection, a role is ascribed to an insect, theNecrophorus vespillo.The author suggests a research work to be done in that respect, and perhaps in the regions where rabies was never registered and are still free of rabies, as to establish what in a such biocoenosis is lacking that could be a factor of rabies epizooty.
Résumé A l'extension de grandes épizooties chez les animoux féroces, on considère que le grand rôle joue le facteur de biocenose, tout d'abord certains phénomènes entre les menus rongeurs, souris de champs. On attribute surtout le rôle de l'extension de l'épizootie de rage à l'insectNecrophorus vespillo. On recommende de faire des recherches dans ce but avec les autres branches de sciences naturelles et peut-être dans ces regions du monde où il n'y en avait pas et il n'y a pas aujourd'hui de rage, pour établir ce qu'il manque dans une telle biocenose et que peut être pourrait on prendre en considération comme le facteur des épisooties de rage.
, — . Necrophorus vespillo L. . , .
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11.
Zusammenfassung Samenuntersuchungen an 12 Eichenarten und -formen, 20 Birkenarten und 14 Ahornarten zeigten, daß die Samen der Eichen und Ahorne, nordamerikanischer Birken und Ahorne nordasiatischer Abstammung in der Ukraine praktisch von Schädinsekten nicht beschädigt werden, während der Samenertrag der heimischen durch Schadinsekten stark herabgemindert wird.Die Widerstandsstärke der Samen gegen Schädlinge ändert sich. nicht nur bei den verschiedenen Arten, sondern auch innerhalb der Bäume einer Art je nach ihrem physiologischen Zustand. Die Widerstandsfaktoren der Samen gegen Schädinsekten sind entweder anatomisch-morphologische Besonderheiten (Schalenstärke), oder das Vorhandensein von Schutzstoffen in den Samen oder beide zusammen. Besonders deutlich sind diese Faktoren in den Eicheln von Bäumen der nordamerikanischen Abstammung gegen Eichelrüssler ausgeprägt. Chromatographische Analyse der Auszüge aus den widerstandsfähigen Samen der Schmidt'schen Birke und des Eschenahorns zeigten, daß für die Rolle der Schutzstoffe die Phenolstoffe verantwortlich sind von denen z. B. die Chlorogensäure ermittelt wurde.
12 , 20 14 - , - , - - . . - / /, . - . , , , , .
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12.
In the assessment of the structural adequacy of oriented strand board (OSB) members by design calculation, the need for the boards Poissons ratios often becomes evident. However, mention of Poissons ratios as a relevant mechanical property is seldom made in wood-based panel test and design standards. The short-term principal Poissons ratios of a typical OSB has been determined from tension tests. Mean values of 0.23 and 0.16 have been obtained for the major and minor Poissons ratios, respectively. The experimentally determined Poissons ratios appear consistent with the values estimated from the panel shear and tension moduli of elasticity. Complementary values of the Poissons ratios in bending have been estimated from the bending moduli of elasticity and tension test results for the panel. Efficiency in the assessment of OSB by design calculation follows from knowledge of Poissons ratios. However, for this benefit to be realised in routine design, the Poissons ratios of structural grade OSB need to be available.  相似文献   

13.
Zusammenfassung Es wurden in Kiefernkulturen Untersuchungen über die Tagesdynamik des Harzdruckes und der Intensität der Harzabsonderung geführt und die Ergebnisse in Beziehung zum Befallsgrad des KiefernknospentriebwicklersRhyacionia buoliana Schiff. gesetzt.Im Verlauf eines Tages variierte der Harzdruck wenig, die Intensität der Harzabsonderung dagegen stark. Die höchste Harzfluß-Intensität fällt in den Zeitraum zwischen 13 und 16 Uhr. Die Flugzeit des Wicklers, nach 18 Uhr, liegt somit außerhalb der Zeit des stärksten Harzflusses.Weder der Index der Harzfluß-Intensität noch jener des Harzdruckes zeigten eine Beziehung zur Stärke des Wicklerbefalls. Sie sind daher als Kriterien der Widerstandsfähigkeit einer Kulturkiefer gegenRh. buoliana, zumindest unter den Verhältnissen der Süd-Ukraine, nicht verwendbar.
Summary Studies on the diurnal rhythm of resin pressure and resin secretion in connection with the degree of pine-infestation byRhyacionia buoliana Schiff.In the course of one day pines in the age of 15 years showed only a small variation in resin pressure but a great one in secretion of resin. The maximal intensity of secretion was observed early in the afternoon. The moths ofRh. buoliana are flying after this time when the secretion of resin is low.There was no connection between resin pressure and secretion of resin on the one hand and the degree of pine-infestation byRh. buoliana on the other. The two factors are not suitable for indicating the degree of pine-infestation by this tortricid.
- . / 0 26 / /., , . , 13 15 . , , , , .
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14.
In response to the request of its member countries from the Latin America and the Caribbean to help promote agroforestry, FAO has initiated a number of activities in the region. These include a regional project Demonstration and Training in Agroforestry in Latin America, an International Workshop on the Formulation of a Project on Demonstration and Training in Agroforestry for the Amazon, held in January 1993, and the project Participative Forestry Development in the Andes. Currently, FAO assistance to the countries of the region in their efforts to promote agroforestry include The Latin American Technical Cooperation Network on Agroforestry Systems, FAO's country-specific technical cooperation projects, and assistance in obtaining development-bank loans through missions of the FAO Investment Center.  相似文献   

15.
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.  相似文献   

16.
Abstract Although the Economic Injury Level (EIL) concept belongs to the keystones of IPM theory, its applicability to all pest problems is believed not to be universal. Current IPM theory claims that the EIL concept is of limited use (i) in situations where an injury-damage function cannot be established, (ii) if pest monitoring is impossible or EIL is very low, and (iii) with preventive measures of pests and pathogens. In this work, I argue that the two latter points may not be true. First, within IPM all types of chemical treatment, including preventive ones, should be economically justified via calculation of EIL, based on the comparison of the cost of preventive and responsive control measures and the cost of the forecasted/expected damage. The expected damage should be based on long-term (historical) damage records, manipulative experiments, risk assessment and biomathematical modelling of the evaluated pathosystem. Second, the absence of EIL in preventively controlled pests hampers completion of the consistent Stern–Pedigos classification of pest organisms according to mutual position of EIL and General Equilibrium Position (GEP) or Stationary Distribution of Population Densities (SDPD).  相似文献   

17.
Bergmann  Ben A. 《New Forests》2003,25(3):185-199
Performance of Paulownia elongata, Paulownia fortunei, and Paulownia×Henan 1 was monitored at three sites in North Carolina between 1996 and 2000, and P. elongata clones were tested at an additional two sites. The primary objective was to define the potential for growing Paulownia in the state. Species and clones within species differed in survival and height and diameter growth whenever genotype was included as a treatment. P. elongata was superior at two and P. fortunei was superior at one of the three sites that included P. elongata, P. fortunei, and P.×Henan 1. Trees produced through vegetative means were more likely to survive and were taller and of greater DBH than seed produced trees. Mortality in trees produced from seed was greater for P. fortunei and P. ×Henan 1 than for P. elongata. Application of animal waste or different rates of nitrogen as inorganic fertilizer did not have a pronounced effect on tree survival and growth.  相似文献   

18.
Zusammenfassung DurchRickettsiella melolonthae (Krieg) PHILIP infizierte Maikäfer-Engerlinge wandern während des Spätherbstes und Winters im Freiland absterbend auf die Erdoberfläche. Als Begleitarten inMelolontha Larvenpopulationen kamen im Gebiet vor:Amphimallon solstitiale (L.),Anomala dubia aenea (DeG.) undMaladera brunnea (L.). MitR. melolonthae künstlich infiziert, zeigten sie im Orientierungsversuch die gleiche Aufwanderungstendenz wie kranke Maikäferengerlinge, am deutlichsten die vonM. brunnea. Auf dem Waldboden fanden sich kranke Larven dieser Art in einigen Fällen.
Summary Larvae ofMelolontha spp. infested withRickettsiella melolonthae (Krieg) PHILIP move during October– January to the top of the soil where they eventually die. Larvae ofAmphimallon solstitiale (L.),Anomala dubia aenea (DeG.) andMaladera brunnea (L.) are susceptible forR. melolonthae, too. In experiments, artificially infested individuals showed a similar tendency to move upward. This was most pronounced inM. brunnea, of which some infested larvae were found on the forest floor.
Résumé Larves du hanneton, infectées parRickettsiella melolonthae (Krieg) PHILIP, se rendent à la surface de la terre au fin de l'automme et dans l'hivers. Comme espèces accompagnant les populations larvaires deMelolontha nous avons trouvés:Amphimallon solstitiale (L.),Anomala dubia aenea (DeG.) etMaladera brunnea (L.). Ils out démonstré, après une infection artificielle avecR. melolonthae, la mème tendence dans les essais d'orientation comme les vers blancs malades deMelolontha. Le phé nomène etait le plus prononcé parmi les larves deMaladera. De cette espèce nous avons trouvés plusieurs larves malades à la surface de la terre en plein champs.
Rickettsiella melolonthae (Krieg) Philip Melolontha- . . Melolontha- :Amphimallon solstitiale (L.),Anomala dubia aenea (De G.) Maladera brunnea (L.). R. melolonthae , Melolontha-, M. brunnea. .
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19.
Zusammenfassung In Freiland- und Laborversuchen wurde die Wirkung des Pflanzenschutzmittels Sevin auf Regenwürmer untersucht. Dieser Stoff ist in Pulverform und als Suspension für diese Organismen außerordentlich gefährlich. Schon der Kontakt mit sehr geringen Mengen bewirkt schwere Lähmungserscheinungen und irreversible histopathologische Schäden.
Summary Experiments in the field as well as in the laboratory showed up the effect of the pesticide Sevin on rainworms. In the form of powder as well as a suspension this material is extremely toxic for them. Contact with smallest quantities even causes severe paralysis and irreversible histopathologic damages.
Résumé Aussi bien dans la nature que au laboratoire nous avons étudié l'activité de l'insecticide Sevin sur les vers de terre. Ce produit est en poudre ou en suspension extrémement dangereux pour ces organismes. Le seul contact avec des quantités minimes de ce produit provoquent des phénomènes de paralysie et des troubles histopathologiques irreversibles.
SEVIN . . .
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20.
Many aspects of scaling up must be considered in the spectrum between promising results of new technologies in experimental plots and wide adoption by farmers. These aspects include extrapolating in time to optimize management decisions, extrapolating in space from small plots to large fields and to other farms and regions, and enlarging the range of (presumed) beneficiaries. Models can help in all these aspects to lay a biophysical foundation on which socioeconomic decisions can be built. We focus here on improved fallow systems where trees are planted to restore soil fertility for subsequent food crops. The restoration of soil fertility — based on biomass production, litterfall, and build-up of dynamic soil organic matter pools — depends on total resource capture by the fallow vegetation. Where lateral resource capture and lateral resource flow play a substantive role in the performance of the fallow, the size (scale) of fallow and cropped plots may influence both the build-up and the decline of soil fertility during a cycle. On small farms, fallow systems should not be seen as pure sequential systems, but as mosaics of spatially interacting fallow and cropped plots. Border effects depend on the lateral spread of the root system of the fallow vegetation, as well as on rainfall and N supply. Scale effects in technology adoption include both positive and negative feedback effects, because the spread of a technology may both accelerate innovations as well as increase threats from pest and disease attack.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   

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