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±63000株·hm-2）、第二季度（51000±1500株·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。     

Implications of local policies and institutions on the adoption of improved fallows in eastern Zambia

Planted-tree fallows (syn. improved fallows) have demonstrated great biophysical potential for improving soil fertility on smallholders' farms but efforts to scale up their adoption to more farming households are constrained by lack of permanent ownership rights over land, incidence of bush fires and browsing of tree biomass by livestock. To resolve these institutional bottlenecks, some traditional authorities in Zambia enacted bylaws to prohibit these incursions. Using a combination of village workshops, expert opinion surveys and structured questionnaires, this study assessed the effectiveness of the bylaws across the major cultural communities in eastern Zambia, identified factors that influence the effectiveness of the bylaws and the lessons emanating from the bylaws in the scaling up of improved fallows. The results indicate that fire poses greater risks to the scaling up of agroforestry than does grazing in terms of the risk of occurrence and the extent of damage. Respondents identified mice hunters and `jealous' neighbors as main causes of fire outbreaks. The effectiveness of the bylaws is influenced by many factors such as ambiguous interpretation of the bylaws, relying exclusively on moral persuasion to enforce the bylaws and lack of well defined responsibilities for enforcing the bylaws, conflict of economic interests among different stakeholders within the communities. Formal documentation of the bylaws will be helpful, but that would not be an exclusive panacea to solve all the constraints. The pattern of distribution of benefits (or costs) of an agricultural technology among various sectors of a community may be important factors that affect widespread adoption of a technology. Technological characteristics are important but not exclusive condition for sustained widespread adoption of soil fertility management options. Privatizing seasonal commons is an important issue in the development of institutional regulations within communities. Policy dialogue among community members, increased awareness and diversification of options appear to be the way forward to improve the effectiveness of the bylaws. This revised version was published online in June 2006 with corrections to the Cover Date.     

Adoption of improved fallow technology for soil fertility management in Zambia: Empirical studies and emerging issues

In the subsistence-agricultural region of eastern Zambia, less than 10% of the households have adequate supply of maize (Zea mays L.), the staple food, throughout the year. A major constraint to increasing crop production in the region is poor fertility status of the soil. In order to address this problem, improved fallow has been introduced as a technology for improving soil fertility within a short span of two to three years. Farmers have been testing the technology and a number of empirical studies have been undertaken over the years to identify the factors influencing farmers' decision to adopt the technology. This paper presents a synthesis of the results of adoption studies and highlights generic issues on the adoption of improved fallows in Zambia. The synthesis indicates that farmers' decision on technology adoption does not have a simple directed relationship of some technological characteristics only, but constitutes a matrix of factors including household characteristics, community level factors, socioeconomic constraints and incentives that farmers face, access to information, local institutional arrangements and macro policies on agriculture. The adoption of improved fallows is not strictly speaking a binary choice problem but a continuous process in which farmers occupy a position along a continuum in the adoption path. Further, adoption of improved fallows may not take place in a policy vacuum but needs to be facilitated by appropriate and conducive policy and institutional incentives. Several questions and issues that require further study emerge from the synthesis. These include determination of the relative importance of the factors in the adoption matrix, identification of the conditions under which farmers use a combination of inputs and their profitability under changing price scenarios, exact definition to delineate between `non-adopters', `testers' and `adopters' of agroforestry technologies, and understanding the impact of cash crop farming in farmers' adoption decisions of improved fallows (where off farm opportunities exist). Further, there is a need to determine the inter-relationship between household poverty, labor availability and the adoption of improved fallows and, to evaluate a combination of policy interventions at both national and local level to promote the adoption of agroforestry-based soil fertility management. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mixed planted-fallows using coppicing and non-coppicing tree species for degraded Acrisols in eastern Zambia

The widespread planting of Sesbania sesban fallows for replenishing soil fertility in eastern Zambia has the potential of causing pest outbreaks in the future. The pure S. sesban fallows may not produce enough biomass needed for replenishing soil fertility in degraded soils. Therefore, an experiment was conducted at Kagoro in Katete district in the Eastern Province of Zambia from 1997 to 2002 to test whether multi-species fallows, combining non-coppicing with coppicing tree species, are better than mono-species fallows of either species for soil improvement and increasing subsequent maize yields. Mono-species fallows of S. sesban (non-coppicing), Gliricidia sepium, Leucaena leucocephala and Acacia angustissima (all three coppicing), and mixed fallows of G. sepium + S. sesban, L. leucocephala + S. sesban, A. angustissima + S. sesban and natural fallow were compared over a three-year period. Two maize (Zea mays) crops were grown subsequent to the fallows. The results established that S. sesban is poorly adapted and G. sepiumis superior to other species for degraded soils. At the end of three years, sole G. sepium fallow produced the greatest total biomass of 22.1 Mg ha⁻¹ and added 27 kg ha⁻¹ more N to soil than G. sepium + S. sesban mixture. During the first post-fallow year, the mixed fallow at 3.8 Mg ha⁻¹ produced 77% more coppice biomass than sole G. sepium, whereas in the second year both sole G. sepium and the mixture produced similar amounts of biomass (1.6 to 1.8 Mg ha⁻¹). The G. sepium + S. sesban mixture increased water infiltration rate more than sole G. sepium, but both these systems had similar effects in reducing soil resistance to penetration compared with continuous maize without fertilizer. Although sole G. sepium produced high biomass, it was G. sepium + S. sesban mixed fallow which resulted in 33% greater maize yield in the first post-fallow maize. However, both these G. sepium-based fallows had similar effects on the second post-fallow maize. Thus the results are not conclusive on the beneficial effects of G. sepium + S. sesban mixture over sole G. sepium. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon pools in tree biomass and the soil in improved fallows in eastern Zambia

This study quantified tree and soil C stocks and their response to different tree species and clay contents in improved fallows in eastern Zambia. From 2002 to 2003, soil, and destructively harvested two-year old tree, samples were analysed for C. There were significant differences (P < 0.05) in aboveground tree C stocks, and in net organic C (NOC) intake rates across coppicing tree species at Msekera and Kalunga. Aboveground C stocks ranged from 2.9 to 9.8 t ha^-1, equivalent to NOC intakes of 0.8–4.9 t ha^-1 year^-1. SOC stocks in non-coppiced fallows at Kalichero and Msekera significantly differed (P < 0.05) across treatments. SOC stocks to 200 cm depth ranged from 64.7 t C ha^-1 under non-coppicing fallows at Kalunga to 184.0 t ha^-1 in 10-year-old coppicing fallows at Msekera. Therefore, tree and soil C stocks in improved fallows can be increased by planting selected tree species on soils with high clay content.     

Sesbania sesban improved fallows in eastern Zambia: Their inception, development and farmer enthusiasm

In eastern Zambia, nitrogen deficiency is a major limiting factor for increased food production. Soil fertility has been declining because of nearly continuous maize (Zea mays) cultivation with little or no nutrient inputs. The use of short-duration tree fallows was one of several agroforestry options hypothesized to restore soil fertility. Sesbania sesban, an indigenous N₂-fixing tree was the most promising among species tested in screening trials. Several studies since 1987 have demonstrated the dramatic potential of two- or three-year sesbania fallows in restoring soil fertility and increasing maize yields. Analyses showed that these improved fallow systems were feasible, profitable, and acceptable to farmers. Results suggest that high maize yields following fallows are primarily due to improved N input and availability by the fallows. The potential to increase maize production without applying mineral fertilizers has excited thousands of farmers who are enthusiastically participating in the evaluation of this technology. The number of farmers who are testing a range of improved fallow practices has increased from 200 in 1994 to over 3000 in 1997. Presently, a strong network of institutions comprising government, NGOs, development projects, and farmer organizations is facilitating the adaptive research and expansion of improved fallow technology in eastern Zambia. Key elements in the research process that contributed to the achievements are effective diagnosis of farmers' problems, building on farmers' indigenous knowledge, generating several different fallow options for farmers to test, ex-ante economic analysis, farmer participation in on-farm trials, and development of a network for adaptive research and dissemination.This revised version was published online in November 2005 with corrections to the Cover Date.     

Effects of organic and mineral fertilizer inputs on maize yield and soil chemical properties in a maize cropping system in Meru South District, Kenya

Soil nutrient depletion as a result of continuous cultivation of soils without adequate addition of external inputs is a major challenge in the highlands of Kenya. An experiment was set up in Meru South District, Kenya in 2000 to investigate the effects of different soil-incorporated organic (manure, Tithonia diversifolia, Calliandra calothyrsus, Leucaena leucocephala) and mineral fertilizer inputs on maize yield, and soil chemical properties over seven seasons. On average, tithonia treatments (with or without half recommended rate of mineral fertilizer) gave the highest grain yield (5.5 and 5.4 Mg ha⁻¹ respectively) while the control treatment gave the lowest yield (1.5 Mg ha⁻¹). After 2 years of trial implementation, total soil carbon and nitrogen contents were improved with the application of organic residues, and manure in particular improved soil calcium content. Results of the economic analysis indicated that on average across the seven seasons, tithonia with half recommended rate of mineral fertilizer treatment recorded the highest net benefit (USD 787 ha⁻¹) while the control recorded the lowest (USD 272 ha⁻¹). However, returns to labor or benefit-cost ratios were in most cases not significantly improved when organic materials were used.     

Effects on site water balance of conversion from native mixed forest to Douglas-fir plantation in N.W. Patagonia

One of the main concerns of afforestation with exotic fast-growing species is their higher soil water consumption compared with the native vegetation they replace. In this regard, sites downstream of the plantations may suffer temporal or permanent droughts. Under this hypotheses, soil water use of Douglas-fir (Pseudotsuga menziesii) plantations and natural woodlands or shrublands (“ñirantales”) was evaluated in two sites during two growing seasons in El Foyel Valley (Río Negro province), Patagonia Argentina. Two methodological approaches were applied: water balances and sapflow measurements. Douglas-fir forests doubled the leaf area index (LAI) of the native forests (12.6 and 5.1 m² m^?2, respectively). Methodological problems were found while constructing water balances because a non-quantified influx of water to the soil was detected in all systems at the beginning and end of the growing season. This flux was not in correlation with precipitation input demonstrating the geological complexity of the valley. For this reason, caution has to be paid to water balance results. However, we have confidence on sapflow measurements, which indicated, as suggested by the water balances, that there are no differences in transpiration between systems (average of 4.8–3.6 mm day^?1 for the exotic and native forests, respectively) in spite of Douglas-fir plantations having twofold LAI than native ñirantales. However, a different response of transpiration to atmospheric demand was found between the exotic and native species, suggesting differences in their ecophysiological characteristics.     

Nutrient use efficiency and biomass production of tree species for rotational woodlot systems in semi-arid Morogoro,Tanzania

Frequent nutrient removals accompanying wood and crop harvests from rotational woodlot systems may contribute to declining site productivity and sustainability because of soil nutrient depletion. However, selecting for nutrient-efficient tree species may well sustain productivity under this system. To test this hypothesis, a randomized complete block experiment was adopted to assess effects of five tree species on soil nutrients status, nutrient use efficiency and wood yield in semi-arid Tanzania. After 5 years rotation, top soils under Gliricidia sepium (Jaqua), Acacia polyacantha Willd. and Acacia mangium Willd. were the most fertile with soil organic carbon and exchangeable cation status raised close to those in natural Miombo systems. Soil inorganic N and extractable P levels reached sufficiency levels for subsequent maize culture. Wood productivity in tree fallows averaged three times higher than that of Miombo woodlands indicating the high potential of the woodlot system to supply fuelwood, and consequently relieve harvesting pressures on the natural forests. Acacia crassicarpa A. Cunn. ex Benth. produced the most wood (51 Mg ha⁻¹) at low nutrient “costs” presumably due to high nutrient use efficiency. Wood yield of this species was 42 and 120% greater than that of A. polyacantha and A. nilotica, respectively, but contained comparatively less nutrients (42–60% less for P, K, and Ca). Gliricidia sepium and A. polyacantha returned the largest amount of nutrients through slash at harvests. Of all test species, A. crassicarpa exhibited the most promise to sustain wood production under rotational woodlot systems due to relatively high productivity and low nutrient export at harvest.     

山鸡椒水分及氮素利用效率性别特异性动态

【目的】以雌雄异株山鸡椒为研究材料,分析雌雄植株在生殖生长过程中水分及氮素利用策略的性别特异性动态变化规律,以期为雌雄异株植物资源分配动态变化机制提供理论依据。【方法】测定雌雄植株在开花后105～165天共5个时期叶片养分含量(碳含量、氮含量、碳氮比)、稳定碳同位素组成(δ¹³C)、稳定氮同位素组成(δ¹⁵N),并分析其动态变化规律。【结果】1)雌雄株叶片δ¹³C平均值分别为-29. 38‰和-28. 08‰。性别和发育时期对雌雄株叶片δ¹³C值影响均显著,5个时期雌株δ¹³C值均显著低于雄株;且随发育进程雌雄植株δ¹³C值均不断下降。2)雌雄株叶片δ¹⁵N平均值分别为1. 90‰和2. 95‰。性别和发育时期对雌雄株叶片δ¹⁵N值均有极显著影响,开花后105～150天(即雌株果实中精油及柠檬醛含量快速积累期)雌株δ¹⁵N值显著低于雄株;随着发育进程,雌雄株叶片δ¹⁵N值均表现出双峰变化趋势,双峰出现在开花后105天和135天,雌株叶片δ¹⁵N在开花后120天达到最低值,雄株叶片在开花后165天时达到最低值。3)雌雄株叶片碳含量平均值分别为49. 44%和49. 28%。开花后105天和120天,雌株叶片C含量高于雄株叶片C含量,开花后135～165天(果实精油快速积累期到稳定期),雌株叶片C含量低于雄株。雄株叶片C含量随发育进程显著升高,而雌株叶片C含量在不同时期无显著差异。4)雌雄株叶片N含量平均值分别为1. 71%和1. 51%。性别对N含量影响显著,雄株叶片N含量均低于雌株(除开花后135天)。5)雌雄株叶片C/N平均值分别为29. 15和33. 72。性别对叶片碳氮比影响显著,雌株C/N值在不同发育时期均小于雄株,但发育时期对其无显著影响;随着发育时期推进,雄株C/N值下降程度高于雌株。6)雌雄植株叶片δ¹³C与N含量以及δ¹³C与δ¹⁵N之间均无显著相关性。【结论】山鸡椒雌雄植株在养分含量、水分及氮素利用策略存在差异,且在开花后105～165天表现出动态变化规律。雌株水分利用效率低于雄株,从果实精油及柠檬醛含量快速积累期到稳定期,雌株水分利用效率不断下降;雌株氮利用低于雄株,且随着植株发育,雌雄株叶片氮素利用效率均表现出双峰变化趋势;雄株叶片氮含量低于雌株,分配更多的氮素至花芽以保证花粉形成;果实精油快速积累期到稳定期,雌株叶片碳含量低于雄株,为果实及种子的形成提供更多的碳元素。     

水分和热胁迫处理对4种针叶树苗木气体交换和水分利用效率的影响

以杉木、马尾松、花旗松和北美乔柏为研究对象 ,研究了水分和热胁迫对生长期间苗木的气体交换、瞬间水分利用效率和稳定碳同位素组成 (δ1 3C值 )等生理生态指标的影响 ,结果如下 :(1 )水分胁迫使杉木、马尾松、花旗松和北美乔柏 4个树种的净光合速率显著下降。但热胁迫只影响杉木和北美乔柏的净光合速率 ,对马尾松的净光合速率影响不大。 (2 )对针叶气孔导度的研究发现 ,水分和热胁迫均对马尾松苗木针叶气孔导度有显著影响 ,但对杉木和北美乔柏苗木针叶的气孔导度只有轻微影响。 (3)水分和热胁迫处理对马尾松苗木的瞬间水分利用效率有显著影响 ,但对杉木和北美乔柏的瞬间水分利用效率无显著影响。 (4 )水分胁迫处理对杉木、马尾松、花旗松和北美乔柏 4个树种的稳定碳同位素组成 (δ1 3C值 )均有显著影响 ,随着水分胁迫程度的加深 ,4个树种苗木针叶的δ1 3C值增大 (负值绝对值变小 ) ,说明其永久水分利用效率更高。 (5 )热胁迫对杉木和北美乔柏苗木针叶的δ1 3C值有显著影响 ,且在热胁迫下 ,两树种苗木针叶的δ1 3C值减小 (负值绝对值增大 ) ,即永久水分利用效率下降。但热胁迫对马尾松和花旗松苗木的δ1 3C值没有显著影响     

Effects of landscape restoration on soil water storage and water use in the Loess Plateau Region,China

Large-scale vegetation restoration in China's Loess Plateau has been initiated by the central government for controlling soil and water losses in the past three decades. However, there is virtually no guidance for plant species selection for the restoration purpose. We investigated the effects of planting trees and shrubs, land-use conversions from croplands to pastures and native grasslands on soil water dynamics and water stresses by using long-term field measurements from 1986 to 1999 in the semi-arid loess hilly area. Our research has led to the following three major findings. Firstly, the amount of soil water storage within a 100 cm depth decreased as measured at both the beginning and the ending point of growing seasons regardless land cover types. The soil water replenishment by rainfall during rainy seasons was not sufficient to fully recharge the soil water storage. Landscape restoration through shrub planting may help retain more soil water than other land cover types. Secondly, the ratio of actual evapotranspiration (ET) and pan evaporation (PET) generally declined for all land cover types during the study period. Shrub lands had the largest ET/PET ratio, followed by native grassland, cropland/alfalfa, and pine woodland. The ET/PET ratio of native grasslands declined fastest, followed by pine woodlands, shrub lands, alfalfa, and croplands. Pine woodland's low ET/PET ratios were mainly caused by its higher runoff due to soil compaction resulted from soil desiccation. Lastly, we found that the soil water storage at the beginning of growing season was important in determining the ET/PET ratios. This study suggested that pine plantations may not be appropriate for landscape restoration in such a semi-arid loess hilly area while shrubs may be highly recommended.     

Effects of Water and Nutrient Availability in Pinus pinaster Ait. Open Pollinated Families at an Early Age: Growth, Gas Exchange and Water Relations

Growth and physiological responses of Pinus pinaster Ait. to water and nutrient availability were compared in four open pollinated families of Arenas de San Pedro provenance (Central Iberian Peninsula), looking for more useful parameters and growing conditions to be included in early selection programmes of forest trees. Young seedlings were grown in a greenhouse, subjected to high and low water and nutrient regimes, during 14 weeks. Interfamily differences were significant with regard to growth, nutrient content and water-use efficiency (WUE): differences in growth were higher under more favourable growing conditions; gas exchange differences were higher in the short term (9 weeks), indicating a faster reaction to water stress of the better adapted families to these conditions; the highest growth rates were found in the families with the largest plasticity, highest WUE and highest nutrient content. Water regime and nutrients had significant effects on growth, δ¹³C, gas exchange and water relation parameters. For these parameters the ranking of the families was kept regardless the water × nutrient supply combination and there was not a significant family × treatment interaction. In the long term (14 weeks) higher water supply and nutrient content significantly increased instantaneous WUE. The positive relationships between growth, intrinsic WUE (A/g _wv) and δ¹³C demonstrate that it should be possible to use physiological parameters (e.g. g _wv, δ¹³C) as a surrogate for the efficiency in the use of water, at least if short periods of water stress (up to 2 months long) were frequent, as it occurs in the Mediterranean basin. There was also some indication that the decrease in nitrogen or potassium supply led to increasing stomatal conductance and hence lower WUE.     

Gas exchange responses of <Emphasis Type="Italic">Eucalyptus</Emphasis>, <Emphasis Type="Italic">C. africana</Emphasis> and <Emphasis Type="Italic">G. robusta</Emphasis> to varying soil moisture content in semi-arid (Thika) Kenya

A dramatic decline in forest cover in eastern Africa along with a growing population means that timber and poles for building and fuelwood are in short supply. To overcome this shortage, the region is increasingly turning to eucalyptus. But eucalyptus raises environmental concerns of its own. Fears that it will deplete water supply, affect wildlife and reduce associated crop yields have caused many countries in the region to discourage farmers from planting this exotic. This paper is part of a series of investigations on the growth and water use efficiency of faster growing eucalyptus hybrids, which was introduced from South Africa to Kenya. The hypothesis is that the new hybrids are more efficient in using water and more suitable for the semi-arid tropics than existing eucalyptus and two popular agroforestry species. Gas exchange characteristics of juvenile Eucalyptus grandis (W. Hill ex Maiden), two eucalyptus hybrids (E. grandis × Eucalyptus camaldulensis Dehnh.), Grevillea robusta (A. Cunn) and Cordia africana (Lam) was studied under field and pot conditions using an infrared gas analyzer was used to measure photosynthetic active radiation (PAR), net photosynthetic rate (A), stomatal conductance (g _s) and transpiration rate (E) at CO₂ concentrations of 360 μmol mol⁻¹ and ambient humidity and temperature. A, E and g _s varied between species, being highest in eucalyptus hybrid GC 15 (24.6 μmol m⁻² s⁻¹) compared to eucalyptus hybrid GC 584 (21.0 μmol m⁻² s⁻¹), E. grandis (19.2 μmol m⁻² s⁻¹), C. africana (17.7 μmol m⁻² s⁻¹) and G. robusta (11.1 μmol m⁻² s⁻¹). C. africana exhibited high E values (7.0 mmol m⁻² s⁻¹) at optimal soil moisture contents than G. robusta (3.9 mmol m⁻² s⁻¹) and eucalyptus (5.3 mmol m⁻² s⁻¹) in field experiment and G. robusta (3.2 mmol m⁻² s⁻¹) and eucalyptus (4.2 mmol m⁻² s⁻¹) in pot-grown trees. At very low soil moisture content, extremely small g _s values were recorded in GC 15 and E. grandis (8 mmol m⁻² s⁻¹) and G. robusta (14 mmol m⁻² s⁻¹) compared to GC 584 (46.9 mmol m⁻² s⁻¹) and C. africana (90.0 mmol m⁻² s⁻¹) indicating strong stomatal control by the species. Instantaneous water use efficiency ranged between 3 and 5 μmol mmol⁻¹ and generally decreased with decline in soil moisture in pot-grown trees but increased with declining soil moisture in field-grown trees.     

Improving the Traditional Acacia Senegal-Crop System in Sudan: The Effect of Tree Density on Water Use, Gum Production and Crop Yields

The traditional Acacia senegal bush-fallow in North Kordofan, Sudan, was disrupted and the traditional rotational fallow cultivation cycle has been shortened or completely abandoned, causing decline in soil fertility and crop and gum yields. An agroforestry system may give reasonable crop and gum yields, and be more appealing to farmers. We studied the effect of tree density (266 or 433 trees ha⁻¹) on two traditional crops; sorghum (Sorghum bicolor) early maturing variety and karkadeh (Hibiscus sabdariffa), with regard to physiological interactions, yields and soil water depletion. There was little evidence of complementarity of resource sharing between trees and crops, since both trees and field crops competed for soil water from the same depth. Intercropping significantly affected the soil water status, photosynthesis and stomatal conductance in trees and crops. Gum production per unit area increased when sorghum was intercropped with trees in low or high density. However, karkadeh reduced the gum yield significantly at high tree density. Yields of sorghum and karkadeh planted within trees of high density diminished by 44 and 55% compared to sole crops, respectively. Intercropping increased the rain use efficiency significantly compared to trees and field crops grown solely. Karkadeh appears to be more appropriate for intercropping with A. senegal than sorghum and particularly recommendable in combination with low tree density. Modification of tree density can be used as a management tool to mitigate competitive interaction in the intercropping system.     

城市绿地节水机制与技术研究

文中对城市植物节水机制和城市绿地节水技术进行了探讨。植物节水机制的研究主要包括植物抗旱以及节水的生理、遗传学基础等方面,可用于城市绿地的节水技术主要有中水回用和雨水利用、合理灌溉、选择乡土植物与节水耐旱植物进行合理配置、实施有机覆盖物的土壤养护和政策管理等。       

Photosynthesis,growth and yield of soybean and maize in a tree-based agroforestry intercropping system on the Loess Plateau

Agroforestry is the most effective way to restore the disturbed lands on the Loess Plateau and to develop the poor local economy. In order to maximize the potential benefits of tree-based intercropping systems, photosynthesis, growth and yield of soybean and corn were studied by measuring photosynthetic active radiation (PAR), plant water deficit and soil moisture in a 4-year-old plantation of walnut (Juglans regia L.) and plum (Prunus salicina) grown at a spacing of 5 m × 3 m on the Loess Plateau. The effects of tree competition significantly reduced PAR, net assimilation (NA), growth and yield of individual soybean or corn plants growing nearer (1 m near tree row) to tree rows. NA was highly correlated with growth and yield of the both crops. These correlations were higher for corn than soybeans, with corn, rather than soybeans being more adversely impacted by tree shading. Plum, rather than walnut had the greatest competitive effect on PAR and NA. Daily plant water deficits were non-significantly and poorly correlated with NA and growth and yield of the both crops. However, soil moisture (20 cm depth) was significantly correlated with biomass and yield of both crops. Possible remediation strategies are discussed to reduce tree competitive interactions on agricultural crops.     

大青山阳坡油松和山杏光合生理参数与土壤含水量的关系研究

在大青山南麓浅山地带的阳坡造林地,使用Li-6400便携式光合测定仪对不同土壤水分处理下的油松和山杏苗木的光合生理参数进行了测定,研究了叶片净光合速率（Pn）、羧化效率（Ce）、蒸腾速率（Tr）、水分利用效率（WUE）对土壤含水量（SWC）的响应过程,并探讨了在不同土壤水状况下2个树种叶片光合作用的变化规律,最后确定了...     

Influence of woody and herbaceous vegetation control on leaf gas exchange,water status,and nutrient relations of eastern white pine (Pinus strobus L.) seedlings planted in a central Ontario clearcut

The influence of herbaceous and woody vegetation control, either singly or in combination, on leaf gas exchange, water status, and nutrient relations of planted eastern white pine (Pinus strobus L.) seedlings was examined in a central Ontario clearcut over four consecutive growing seasons (GSs). Net carbon assimilation (A_n), leaf conductance to water vapour (G_wv), water use efficiency (WUE), and midday leaf water potential (ψ_m) were measured periodically during the second to fourth GSs of vegetation control treatments, while leaf nutrient relations were examined in GS five. Leaf A_n and G_wv were reduced (p ≤ 0.05) in the presence of herbaceous vegetation in GS two, by both herbaceous and woody vegetation in GS three, and only by woody vegetation (largely trembling aspen (Populus tremuloides Michx.)) in GS four. Leaf WUE was increased (p ≤ 0.05) in all three GSs in which herbaceous vegetation control was applied and where woody vegetation provided partial shading of planted white pine. Leaf water status was comparatively less responsive to vegetation control treatments, but leaf ψ_m was increased (p ≤ 0.05) in the presence of woody vegetation in GSs two and four, likely due to shading and reduced atmospheric evaporative demand of the white pine seedling environment. Within a given GS, the effects of vegetation control on A_n, G_wv, and ψ_m were strongly linked to treatment-induced changes in total vegetative cover, and light and soil moisture availability. Seedling height, diameter, and volume growth rates were positively correlated with A_n and WUE in GSs two and three, but less so in GS four. Vector analysis suggested that herbaceous competition induced foliar N, P, and K deficiencies in five-year-old white pine seedlings while competition from aspen resulted in foliar Ca deficiency.     

Review on Leaf Water Use Efficiency of Major Trees Species in Arid and Semi-Arid Area

The tree specie selection is the critical part in the process of vegetation restoration and reconstruction in arid and semi-arid areas.As a reliable indicator of drought resistance and water use characteristics of tree species,water use efficiency（WUE） has become a hotspot in the research on tree specie selection.This paper introduced the concept and correspondingly research method of leaf WUE and summarized the research progress of leaf WUE in terms of the variation in leaf WUE between species,positions in the canopy,and characteristics of temporal and spatial variation,its influence on environmental factors like illumination,moisture,temperature, CO₂ and O₃ concentrations.Furthermore,it put forward the problems currently existing in the research and prospected the trend of future research on leaf WUE.