Influence of forest clearing methods on nutrient uptake of some tropical food crops

The adverse effects of forest clearing on crop yield have often been directly ascribed to decline in soil fertility. However, the effect of various forest clearing methods on crop nutrient uptake as reflected by the content in the leaf tissues has been the subject of few scientific investigations. A study to determine the effect of forest clearing on nutrient content of some arable crops was, therefore, carried out in the tropical rainforest of south-western Nigeria. Three forest clearing methods—mechanical, semi-mechanical and manual clearing—were used, while maize, cowpea, and cassava were the test crops. Nutrient content in the tissues of these crops was assessed by sampling maize ear leaves at silking, youngest mature leaves (YML) of cowpea at flowering and YML of 3-month-old cassava for analysis. Correlation analysis was carried out to establish the relationship between nutrient content and crop yield. The method of forest clearing had no significant effect on the nutrient content of maize and cowpea during the first cropping cycle. However, nutrient content of maize in the manually cleared plots was higher than those in the semi- and mechanically cleared plots for all elements analyzed except Mg. Nitrogen content of cowpea was much higher than that of maize. Except for P and K, the nutrient content of cassava was not markedly different among the forest clearing methods. Calcium and P content in the leaves of maize and cassava varied inversely with their levels in the soil, implying that crop uptake could have accounted for the differences in their availability within the soil. Nitrogen, P and K had significant positive correlations with maize yield (r = 0.77, 0.89, and 0.71). Potassium content also had significant positive correlation with cassava root yield (r = 0.69). This is not surprising because compared with other crops, cassava requires large amounts of K for starch formation and root development. It is concluded that differences observed in crop yields under different forest clearing methods are a function of nutrient uptake and availability within the soil. However, the uptake of some nutrients does not seem to be determined by the level of soil availability alone. This study was supported with a research grant from the International Foundation for Science (IFS), Sweden.     

Performance and nutrient dynamics of holm oak (<Emphasis Type="Italic">Quercus ilex</Emphasis> L.) seedlings in relation to nursery nutrient loading and post-transplant fertility

Holm oak (Quercus ilex L.) seedlings were exponentially (E) nutrient loaded using incremental increases in fertilizer addition or conventionally (C) fertilized using a constant fertilizer rate during nursery culture. The fertility treatments (mg N plant⁻¹) were control (0), 25E, 100E, and 100C. Subsequently, 1-year-old plants were transplanted under simulated soil fertility gradients in a greenhouse to evaluate effects of nutrient loading and post-transplant fertility on seedling performance. Post-transplant fertility consisted of fertilizing plants at two rates (0 vs. 200 mg N plant⁻¹). A water-soluble fertilizer 20-20-20 was supplied in both nursery and post-transplant experiments. Nutrient loading increased plant N content by 73% in 100E and by 75% in 100C relative to controls, although no significant differences were detected between constant and exponential fertilization regimes at the 100 mg N plant⁻¹ rate. When transplanted, nutrient loading promoted post-transplant root growth relative to shoot, implicating potential to confer competitive advantage to loaded holm oak seedlings after trans-planting. In contrast, post-transplant fertility increased new shoot dry mass by 140% as well as N, P and K content relative to controls. Results suggest that holm oak seedlings can be successfully nutrient loaded in the nursery at higher fertility rates, improving its potential to extend new roots, but alternative fertilization regimes and schedules that better fit nutrient availability to the growth rhythm and conservative strategy of this species must be tested.     

Growth and dry-matter partitioning of young Populus trichocarpa in response to carbon dioxide concentration and mineral nutrient availability

Young individuals of a single black cottonwood (Populus trichocarpa Torr. & Gray) clone were raised for three growing seasons in whole-tree chambers and exposed to either ambient or elevated atmospheric carbon dioxide concentration ([CO2]), with either a high or a low mineral nutrient supply, in a factorial experimental design. Nutrient availability had a larger effect on growth and dry matter partitioning than did [CO2]. Total biomass did not differ significantly with CO2 treatment when nutrient availability was low. However, elevated [CO2] increased whole-plant biomass by 47% in the high nutrient availability treatment. Carbon dioxide enrichment reduced leaf area ratio and specific leaf area significantly, but had no significant effect on mean leaf size or leaf mass ratio. Root mass ratio was significantly increased by elevated [CO2] at low, but not at high nutrient availability. A modified "demographic harvesting approach" made possible the retrospective estimation of stem and branch dry masses for different years. The relative growth rates of stem and branch were significantly enhanced by elevated [CO2] with high, but not with low nutrient availability. Canopy productivity index (CPI), i.e., the amount of stem and branch wood produced annually per unit leaf area, was raised 12% by elevated [CO2] when nutrient availability was high, but was reduced when nutrient availability was low, because of increased below ground allocation.     

Effects of nutrient availability on performance and mortality of <Emphasis Type="Italic">Rumex obtusifolius</Emphasis> and <Emphasis Type="Italic">R. crispus</Emphasis> in unmanaged grassland

Several authors record high mortality of the weedy species Rumex obtusifolius in unmanaged grasslands, but there have never been any studies on how the performance and mortality of R. obtusifolius and R. crispus are affected by different nutrient availability in unmanaged temperate grasslands. To investigate this question, we established a seedlings transplantation and fertiliser experiment on unmanaged Dactylis glomerata grassland in Czech Republic. We monitored plant height, number of leaves per plant, fertility and mortality of transplanted R. obtusifolius and R. crispus plants, from 2008 to 2011, in an unfertilised (U) and a fertilised (F) treatment (manure applied in 2008 at the rate of 200 kg of nitrogen, 42 kg of phosphorus and 230 kg of potassium per ha). In 2010, taller plants of both species were recorded in treatment F than in treatment U, but there was no effect of treatment on the number of leaves. In 2010, fertility for both species was 50% in treatment F, but only 20% for R. obtusifolius and 10% for R. crispus in treatment U. Over 4 years, no mortality of R. obtusifolius in treatment F contrasted with 30% mortality in treatment U. In the case of R. crispus, mortality was 50% in treatment F and 28% in treatment U. An increase in nutrient availability can decrease mortality of R. obtusifolius plants in grasslands, but increase mortality of R. crispus. It seems that leaving grassland unmanaged for several years is only an effective strategy for control of R. obtusifolius under conditions of low nutrient availability.     

Effect of dothistroma needle blight on needle and shoot lengths

The needles of pine trees are indicative of the overall health of the tree, and their length is affected by many factors. This study describes the effect of high levels of pathogen infection on lengths of both needles and shoots. Dothistroma septosporum is an important foliar pathogen of pines causing necrosis and premature defoliation with successive years of high infection leading to growth reduction and in extreme cases tree death. Corsican pine trees with all foliage infected by D. septosporum had needles of primary, secondary and tertiary shoots reduced by 31.8%, 44.0% and 64.2%, respectively, compared to non‐infected trees. Needle lengths were reduced both in the upper and lower portions of the canopy, with a greater reduction lower in the canopy. Trees with high levels of infection had shorter shoots in the upper canopy with fewer, shorter needles on these shoots compared to trees with low levels of infection. The results demonstrate the substantial negative effect on needle and shoot lengths of trees with high levels of D. septosporum infection, comparable to factors such as water and nutrient availability known to have a strong influence on these parameters. The reductions in length reduce the photosynthetic capacity of the tree and compound the immediate reductions caused by necrosis and premature defoliation. These effects on needle and shoot lengths contribute to the reductions in volume growth of affected trees and, moreover, are longer lasting than the immediate effects of necrosis and premature defoliation.     

The potential of alley cropping in improvement of cultivation systems in the high rainfall areas of Zambia. III. Effects on soil chemical and physical properties

A detailed study of the soil chemical and physical properties in seven-year-old alley cropping trial containingLeucaena leucocephala andFlemingia congesta in Northern Zambia is described. There was a strong correlation between the maize yield and the total amount of nitrogen applied, both from prunings and fertiliser, suggesting that a major reason for the observed benefit from alley cropping, particularly withLeucaena, was due to an improvement in nitrogen supply.Leucaena produced significantly more biomass, and its leaves had higher concentrations of nitrogen, phosphorous and potassium and lower C/N and C/P ratios than did those ofFlemingia. There was also evidence that the trees had a beneficial effect on other soil chemical properties; under the hedgerows, particularly those ofLeucaena, there were higher levels of organic carbon, Mg, K and ECEC, and pH values were also highest.It is suggested that higher levels of organic carbon in the alley crop treatments were responsible for the improvements observed in soil physical properties. Lower bulk density, lower penetration resistance, and a higher infiltration rate and pore volume fraction were measured in the alley crops, although there was no significant change in the soil water release parameters.A deteriorating effect of constant applications of nitrogen fertiliser on soil fertility was observed; as the level of urea application increased, there were significant decreases in Mg, K and pH, increases in Al and soil acidity, and higher penetrometer resistance. These results highlight the urgent need for further research on biological methods of maintaining soil fertility.     

Dry matter production and nutrient cycling in agroforestry systems of mandarin grown in association with Albizia and mixed tree species

A study on dry matter production and nutrient cycling in agroforestry systems of mandarin grown in association with N₂-fixing Albizia and mixed tree species (non-N₂-fixing) was carried out in the Sikkim Himalaya. A site with Albizia was referred asAlbizia-mandarin stand and the other site with mixed tree species as mandarin stand. The stand total biomass, net primary productivity and mandarin fruit production was higher under the influence of Albizia. Agronomic yield of crops remained nearly the same in both the stands. Nitrogen and phosphorus concentrations of different components of Albizia were higher than those of mixed tree species, whereas their back translocation from leaf to branch before abscission was lower inAlbizia. The mandarin-based agroforestry is a highly nutrient-exhaustive system evaluated in terms of nutrient exit through the removal of agronomic yield. This system, under the influence of Albizia, was more productive with faster rates of nutrient cycling. Nutrient use efficiency increased under the influence of Albizia, in contrary to the hypothesis that efficiency should decrease with increasing rate of uptake. The poor nutrient conservation of Albizia, and malleability of nutrient cycling under its influence make it an excellent association which promotes higher availability and faster cycling of nutrients.Albizia should be utilised more extensively in the management of mandarin-based agroforestry systems.     

Use of soil solarization to improve growth of eucalyptus forest nursery seedlings in Argentina

Damping-off and root rot are major diseases affectingseedlings of Eucalyptus species in forestnurseries in temperate regions in Argentina. The mostcommon fungi associated with these diseases andaffecting the vigor of the root system are Fusarium and Pythium species. Two forestnursery experiments were conducted in the province ofBuenos Aires, Argentina, to determine the effect ofsoil solarization on growth of Eucalyptusviminalis seedlings and relate this effect to thepresence of pathogenic and native ectomycorrhizaepopulations in roots and nutrient availability insoil. Changes in populations of soilborne pathogenswere determined by a bioassay that relates theirpotential to induce disease. Changes in nativeectomycorrhizae were assesed by measuring colonizationlevels in roots. Nutrient availability was determinedby the amount of nitrates released by solarization.Solar heating decreased pathogenic and ectomycorrhizalinoculum potential and increased soil nitrates.Seedling growth in solarized seedbeds may be relatedto a low initial pathogenic population and/or toincreases in nitrate availability. Solarization mayinduce soil suppressiveness against re-establishmentof major seedling pathogens in treated soils.     

Soil C and N dynamics, nutrient leaching and fertility in a pine plantation amended with wood ash under Mediterranean climate

Wood ash addition to forest soils can balance exported nutrients by tree harvesting and decrease soil acidity, but its effectiveness in Mediterranean areas has been scarcely evaluated. The aim of the present study was to assess the effects of wood ash application on soil C and N dynamics, nutrient leaching and fertility in a pine stand. Treatments were loose and pelleted ash application (11 Mg ha^?1), alone or combined with N fertilizer, and a control treatment. Nutrient leaching and soil chemical and biological properties were periodically evaluated for a 30-month period. Wood ash increased leaching of base cations (Ca, Mg, Na and K) and P, mainly at the beginning of the study. The effect was more pronounced for the loose formulation. As a consequence, a positive effect on soil nutrient availability (exchangeable base cations and extractable P) and soil acidity reduction was observed for the loose formulation in the 0–10-cm soil layer. Carbon and N dynamics were only affected when ash was applied with N fertilizer, which enhanced CO₂ flux during the study period.     

Forestry insularity effect of four <Emphasis Type="Italic">Mimosa</Emphasis> L. species (Leguminosae-Mimosoideae) on soil nutrients of a Mexican semiarid ecosystem

Information about forestry insularity of plants on soil nutrients will be critical for selecting plant species for agrosilvopastoral or fertility reclamation programs in dry ecosystems. We explored the effects of four Mimosa species (M. lacerata, M. luisana, M. polyantha and M. texana var. filipes) and of rainfall seasonal variation on soil nutrients in a semiarid ecosystem located at the Tehuacán-Cuicatlán Valley, Mexico. Soil samples were taken from outside and under the canopy at three positions (trunk, middle, edge) in all four Mimosa species; ten plants per species. The soil pH, organic matter (SOM), organic carbon (SOC), total nitrogen (Nt), available phosphorus (Pi), and major cations (Ca, Mg, K and Na) were determined. Our results showed that Mimosa species improve the soil under their canopies creating fertile islands with higher SOM, SOC, total N and Pi cycling than the soil in open areas (OA). The insularity effect was significantly species-dependent, where SOM, SOC, Nt and Pi decreased consistently from trunk to OA in all four Mimosa species; however, magnitude varied among species. Likewise, differences in the quantity of soil cations were observed among Mimosa species; though, an insularity gradient trunk-open area was not observed. All these effects were consistent across the species studied and showed little seasonal variability, suggesting a strong forestry insularity of Mimosa species on soil fertility. Of all the four Mimosa species studied, M. lacerata was the most effective in accumulating SOM and nutrients in the soil, for which it would be a good option to implement in agrosilvopastoral or fertility reclamation programs in this semiarid ecosystem.     

光皮桦生长与环境因子的关系研究

以雅长林场九龙分场13 a光皮桦纯林作为研究对象,探索土壤酶活性、土壤化学元素以及林分温度对光皮桦生长的影响,采用相关性分析探究影响光皮桦生长的主要环境因子。结果表明,土壤酶活性与光皮桦树高、胸径、冠幅呈正相关,且相关性均较高,光皮桦生长受土壤酶活性的影响作用较大;除速效磷外,土壤中其他化学元素含量与光皮桦生长量相关性较高,其中总氮含量相关性最高,对光皮桦的生长起主导作用;光皮桦林分内地表温度、空气温度与其生长量的相关系数均较低,其与光皮桦的生长没有直接影响;土壤中的蔗糖酶活性、酸性磷酸酶活性、土壤总氮与光皮桦的相关性相对较高,因此,土壤蔗糖酶活性、酸性磷酸酶活性、总氮含量可能是影响光皮桦生长的主导环境因子。     

Effects of thinning on nutrient content pools in two Pinus sylvestris forests in the western Pyrenees

Abstract

Nutrient content [nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg)] in needles, branches and stems before (P0, not thinned) and after thinning (P20 and P30, 20% and 30% of basal area removed, respectively) was studied in 37- and 32-year-old Pinus sylvestris L. forests in the western Pyrenees: Garde and Aspurz. Thinning significantly decreased all nutrient pools in P20 and P30 relative to P0 on both sites, but no significant differences were found between P20 and P30 owing to low statistical power. Thinning increased the differences between the two forests for total above-ground content of P and Mg, and for the content of N, P and K in the 1999 needles cohort. The former result was due in part to the higher concentration of P (needles and branches) and Mg (needles) in Garde. Therefore, the importance of needles relative to the other nutrient pools increased for N and P after thinning. This may have resulted from the fact that the removal of dead trees with low needle biomass was greater in Garde than in Aspurz. Based on the percentage of basal area removed, nutrient removal in Garde for P30 was higher than expected, apparently owing to an increase in branch removal relative to P20. These results indicate that effects of thinning on nutrient pools were influenced by differences in stand structure and nutrient tissue concentrations between sites.     

Effect of wide spacing on increment and branch properties of young Norway spruce

The effects of stand density on increment and branch properties were studied in three spacing experiments of Norway spruce [Picea abies (L.) Karst.]. The stand densities ranged from 350 stems ha⁻¹, regarded as open-grown trees, up to 1,600 stems ha⁻¹, corresponding to the density recommended for forestry practice. Properties of all the branches were measured from the stem apex downwards. The study material included a total of 5,661 branches from 45 trees. Increasing stand density resulted in a decrease in radial increment as well as shorter and narrower crowns, but it had no effect on height increment. The average number of spike knots per tree was 0.87, 0.27, and 0.33 in densities of 350, 700 and 1,600 ha⁻¹, respectively. Additionally, in the widely spaced stands of 350 stems ha⁻¹, the fraction of trees having spike knots was high (over 50%). At a density of 1,600 ha⁻¹, the sample trees had somewhat less branches in a whorl compared with the more widely spaced plots. The most pronounced effect of stand density was the increase in branch diameter with decreasing stand density. At a density of 350 ha⁻¹, the maximum branch diameter of all the sample trees exceeded the diameter limit of quality class B in the European quality requirements for round wood. The results give some indication that trees subjected to severe competition would produce smaller branches per unit of crown projection area. However, the possibilities for reducing branch dimensions relative to stem and crown size through competition appear quite restricted.     

Nutrient availability alters belowground respiration of ozone-exposed ponderosa pine

Exposure to ozone (O(3)) and changes in soil fertility influence both the metabolism of plant roots and their interaction with rhizosphere organisms. Because one indication of altered root metabolism is a change in belowground respiratory activity, we used specially designed measurement chambers to assess the effects of O(3) and nutrient availability on belowground respiratory activity of potted three-year-old ponderosa pine (Pinus ponderosa Dougl. ex Laws.). Seedlings were exposed to a factorial combination of three O(3) treatments and three fertilization treatments in open-top O(3) exposure chambers. Ozone exposure decreased and high nutrient supply increased total plant dry weight, but root/shoot ratios were not affected. In general, exposure to O(3) increased rates of belowground O(2) uptake and CO(2) release and the respiratory quotient (RQ, CO(2)/O(2)), although seasonal differences were detected. In October, following the second season of O(3) exposure, rates of belowground O(2) uptake and CO(2) release and RQ were increased in trees in the high-O(3) exposure treatment by 22, 73 and 32%, respectively, over values in control trees in charcoal-filtered air. Increasing nutrient supply resulted in decreasing rates of belowground O(2) uptake and CO(2) release but it had little effect on RQ. In the high-nutrient supply treatment, rates of belowground O(2) uptake and CO(2) release were decreased by 38 and 39%, respectively, compared with rates in the low-nutrient supply treatment. At the end of the second growing season, the high-nutrient supply treatment had decreased lateral root total nonstructural carbohydrates by 22% compared with the low-nutrient supply treatment. Nutrient availability altered the belowground respiratory response to O(3), such that the response to O(3) was greatest in the low-nutrient supply treatment. Significant O(3) effects on belowground respiratory activity were apparent before any reduction in total plant growth was found, suggesting that roots and rhizosphere organisms may be early indicators of physiological dysfunction in stressed seedlings.     

A review on the potential of improved fallows and green manure in Rwanda

Agricultural production in the densely populated highlands of Rwanda is subject to serious soil fertility constraints. As the use of imported mineral fertilizers is beyond the economic means of resource-poor farmers, research and extension efforts of several projects, institutes and organizations concentrated during the last 15 years on the development and promotion of improved fallows with woody and herbaceous legumes, like Tephrosia sp., Cajanus sp., Crotalaria sp., Sesbania sp., Mucuna sp., and Mimosa sp., planted over one or more seasons as pure green manure, in hedgerows (alley cropping), or on fields as seasonal inter- or relay-crop. Green manuring proved to be a risky enterprise, due to highly variable biomass production and residual effects. Yield increments on-farm of up to 74% in the first season and 46% in the second season did not compensate loss of yields and labour investments during green manuring. Even where biomass production was sufficient, residual effects were in most cases unsatisfactory, due to rapid nutrient leaching (N, K) or inappropriate foliage incorporation on-farm. In researcher-managed trials, residual effects were in general somewhat higher, but more than a mere compensation of lost yields was not possible and farmers' adoption of these labour-intensive technologies was rather low. Due to acute land shortage, farmers were reluctant in allocating land to fallows or hedgerows also, with the exception of fields already out of production. Consequently, the concept of improving soil fertility and crop yields with the help of planted fallows or green manure in rotation failed. Woody legumes might have a future on abandoned fields and in wide spaced contour hedges, mainly for the production of firewood and bean stakes. For soil fertility management, the production and availability of farmyard manure and country-own mineral fertilizers, such as travertin and volcanic ashes should be supported. The question is raised as to whether sustainable agricultural development is possible without a credit system for small farmers, reallocating land and creating off-farm employment.     

Induced resistance of Norway spruce,variation of phenolic compounds and their effects on fungal pathogens

Three clones of Norway spruce (Picea abies) were studied for their response to mass‐inoculation with the blue‐stain fungus Ceratocystis polonica. The effect of different pretreatments (fungal inoculation and wounding) before mass‐inoculation was investigated for their possible role in an acquired resistance reaction. Pretreated trees showed enhanced resistance to the subsequent mass‐inoculation relative to control trees that received no pretreatment. Furthermore, the fungal colonization of inoculated trees was less than that of wounded trees. The phenolic content of the bark, analysed by RP‐HPLC, was compared in trees receiving different treatments. Trees inoculated with C. polonica had higher average concentration of (+)‐catechin, taxifolin and trans‐resveratrol than wounded trees. Both inoculated and wounded trees had higher average concentrations of these compounds than control trees. The effect of the phenolic extract of Norway spruce bark on the growth of the root rot fungus Heterobasidion annosum and the blue‐stain fungi C. polonica and Ophiostoma penicillatum were investigated in vitro. Heterobasidion annosum was not negatively affected, and the extracts had fungistatic effects on the blue‐stain fungi. The growth of O. penicillatum was more inhibited than the growth of the more aggressive C. polonica.     

Variation in herbivory-induced responses within successively flushing Quercus serrata seedlings under different nutrient conditions

Herbivore damage can induce the host plant to alter the chemical and physical qualities of its leaves, which is thought to be a plant strategy—termed “induced response”—for avoiding further herbivory. In woody plants, many studies have considered variation in induced response with resource availability, but few studies have examined this variation in relation to growth patterns of woody plants. We studied the phenotypic variability of induced response within successively flushing Quercus serrata seedlings. Q. serrata seedlings were grown under controlled conditions. The controlled factors were herbivore damage (herbivore-damaged and -undamaged) and soil fertility (low and high). At each flush stage, the concentrations of condensed tannin (CT), total phenolics (TP), and nitrogen (N) in leaves were analyzed, and the leaf mass per area (LMA) was measured. CT and TP concentrations in leaves and LMA were higher in herbivore-damaged seedlings. Leaves of the first flushes showed greater sensitivity to herbivore damage and had a higher CT concentration than leaves of the later flushes. Furthermore, seedlings growing in low-fertility soil showed a greater induced response. The results suggest that the induced response of Q. serrata seedlings was related to the contributions of the tissue to current productivity. Leaves of the first flush showed a greater induced response, possibly because they play an important role in subsequent growth. The potential of Q. serrata seedlings to adjust the properties of leaves depending on herbivory and soil fertility in relation to growth patterns may be advantageous on the forest floor, where seedlings grow in soil of heterogeneous fertility and are constantly exposed to herbivory.     

Functional relationships between leaf structure and photosynthetic traits as modulated by irradiance and nutrient availability in a sclerophyllous and a non-sclerophyllous mediterranean oak species

Plasticity of structural and physiological leaf traits elicited by irradiance and soil nutrients was investigated in two sympatric mediterranean oaks: a sclerophyllous (Quercus suber L.) and a non-sclerophyllous species (Q. canariensis Willd.). Seedlings were grown for 2 years in pots in a 2-way crossed factors design. Leaf mass-to-area ratio (LMA) and nitrogen were recorded, and photosynthetic capacity (i.e. the apparent maximal carboxylation rate by rubisco, V _cmax) was derived from response curves of net CO₂ assimilation (A) versus intercellular CO₂ mol fraction (C _i). Structural equation modelling was applied to the data for disentangling the complex correlation structure between variables. The two species differed significantly in photosynthetic nitrogen use efficiency (PNUE). They displayed the expected responses to irradiance, with large increases in LMA, V _cmax and nitrogen per unit leaf area and decreases in mass-based nitrogen content. Nutrient availability modulated severely leaf N content (mass- and area-based) and mass-based maximal carboxylation rate, but not the plastic response of all these parameters to irradiance. Irradiance primarily modulated leaf structure (LMA), and secondarily nitrogen content, while nutrient availability modulated directly nitrogen content. Nitrogen content in turn had a severe impact on mass-based photosynthetic capacity. It is concluded that in young trees solely leaf structure displayed irradiance-elicited plasticity. This plasticity was not modulated by nutrient availability and was similar in a sclerophyllous and a non-sclerophyllous species.     

Untersuchungen über Lebensdauer und Eivorrat der Wacholdersamenwespe Megastigmus bipunctatus Swederus (Hym., Chalc., Torymidae)

Investigations on longevity and fertility of Megastigmus bipunctatus Swederus (Hym., Chalc., Torymidae)Investigations on the influence of different climatic and nutritional factors on longevity and fertility of adult femaleMegastigmus bipunctatus showed that differences in relative humidity had little effect. In higher temperatures longevity and fertility were reduced; the number of undeveloped eggs was reduced more than the number of mature eggs. Food formulas containing protein in the form of chicken egg albumen had a negative effect on longevity and fertility when compared with data fromM. bipunctatus femals kept without food and water. An increase of both, life span and number of eggs, was observed after giving distilled water, aqueous honey solution or aqueous honey solution enriched with vitamin mixture. A maximum increase was obtained with femals living only on honey solution.     

Leucaena + maize alley cropping in Malawi. Part 2: Residual P and leaf management effects on maize nutrition and soil properties

Agroforestry systems involving leaf removal for animal fodder may result in rapid depletion of soil fertility. The purpose of this research was to determine if the effects of leaf removal on soil fertility parameters and maize yield in a Leucaena leucocephala alley cropping system could be reversed. Three leaf management strategies in a Leucaena alley cropping trial that had been in effect from 1987 to 1991 were investigated: 1) leaves returned, 2) leaves removed, and 3) leaves removed, with 100 kg inorganic N ha⁻¹ added. In the 1990/91 season, a 3⁴ confounded factorial design was utilized to investigate the effects of leaf management strategy, N rate (0, 30 and 60 kg N ha⁻¹); maize plant population (14,800, 29,600, and 44,400 plants ha⁻¹); and P rate (0, 18, and 35 kg P ha⁻¹). In the 1991/92 and 1992/93 seasons, leaves were applied equally to all plots, and no P was applied. The N rate and plant population treatments were continued, and the same confounded factorial design was implemented to investigate residual leaf management strategy, residual P rate, n rate, and plant population. The yield gap between the plots where leaves had been returned vs. removed narrowed each season due to uniform leaf application. Application of N improved yields during both seasons. Residual effects of the initial P application decreased to only 10% of the total yield in 1992/93. Plant population affected yields only during the season of very good rainfall. Leaf additions resulted in a relative increase in soil pH, total N,and exchangeable Ca, Mg, and K and a decrease C/N ratio in plots that had not previously received leaves. Leaves supplied more K and Zn to the upper 15 cm of soil than were being extracted by the maize crop, but uniform leaf additions eliminated differences in K and Zn uptake. Plant Zn uptake decreased with increasing P rate and plant population, and increased with increasing N rate and a history of leaf return. The results show that applying leaves equilibrated yields within two seasons, and resulted in a relative improvement of several soil properties. The residual effect from P applications was not adequate to maximize yields.