Forage production, N uptake, N2 fixation, and N recovery of berseem clover grown in pure stand and in mixture with annual ryegrass under different managements

In Mediterranean countries, forage grasses and legumes are commonly grown in mixture because of their ability to increase herbage yield and quality compared with monocrop systems. However, the benefits of intercropping over a monocrop system are not always realized because the efficiency of a grass–legume mixture is strongly affected by agronomic factors. The present study evaluated productivity, N₂ fixation, N transfer, and N recovery of berseem clover (Trifolium alexandrinum) grown in pure stand and in mixture with annual ryegrass (Lolium multiflorum) under high or low defoliation frequencies and varying plant arrangements (sowing in the same row or in alternating rows). On average, the berseem–ryegrass mixtures resulted in a greater yield and N yield than the monocrops. When mixed together, ryegrass was more efficient than berseem at absorbing soil N, increasing the reliance of berseem on N₂ fixation. Both defoliation management and plant arrangement affected forage yield and the quality of the mixture, modifying the proportion of the two components, the N content of the forage, and the symbiotic N₂ fixation of the legume. Reducing the proximity between plants of the two species may benefit the weaker component of the mixture. No apparent transfer of fixed N from berseem to ryegrass was detected.     

Effect of four plant species on soil 15N-access and herbage yield in temporary agricultural grasslands

Background and aims

We carried out field experiments to investigate if an agricultural grassland mixture comprising shallow- (perennial ryegrass: Lolium perenne L.; white clover: Trifolium repens L.) and deep- (chicory: Cichorium intybus L.; Lucerne: Medicago sativa L.) rooting grassland species has greater herbage yields than a shallow-rooting two-species mixture and pure stands, if deep-rooting grassland species are superior in accessing soil ¹⁵N from 1.2 m soil depth compared with shallow-rooting plant species and vice versa, if a mixture of deep- and shallow-rooting plant species has access to greater amounts of soil ¹⁵N compared with a shallow-rooting binary mixture, and if leguminous plants affect herbage yield and soil ¹⁵N-access.

Methods

¹⁵N-enriched ammonium-sulphate was placed at three different soil depths (0.4, 0.8 and 1.2 m) to determine the depth dependent soil ¹⁵N-access of pure stands, two-species and four-species grassland communities.

Results

Herbage yield and soil ¹⁵N-access of the mixture including deep- and shallow-rooting grassland species were generally greater than the pure stands and the two-species mixture, except for herbage yield in pure stand lucerne. This positive plant diversity effect could not be explained by complementary soil ¹⁵N-access of the different plant species from 0.4, 0.8 and 1.2 m soil depths, even though deep-rooting chicory acquired relatively large amounts of deep soil ¹⁵N and shallow-rooting perennial ryegrass when grown in a mixture relatively large amounts of shallow soil ¹⁵N. Legumes fixed large amounts of N₂, added and spared N for non-leguminous plants, which especially stimulated the growth of perennial ryegrass.

Conclusions

Our study showed that increased plant diversity in agricultural grasslands can have positive effects on the environment (improved N use may lead to reduced N leaching) and agricultural production (increased herbage yield). A complementary effect between legumes and non-leguminous plants and increasing plant diversity had a greater positive impact on herbage yield compared with complementary vertical soil ¹⁵N-access.     

Forage nutritional characteristics and yield dynamics in a grazed semiarid steppe ecosystem of Inner Mongolia,China

In the steppe of Inner Mongolia, forage is the only source of feed for sheep. The forage intake of sheep can be characterized in both quantity and quality terms, which are determined by environmental and anthropogenic factors and grazing has a strong effect on steppe productivity and the grassland ecosystem. Evaluation of forage quality and quantity is therefore of critical importance. The effects of grazing intensity, interannual variability effects, and species composition on aboveground net primary productivity (ANPP) and forage nutritional characteristics were investigated in a controlled grazing experiment along a gradient of 7 grazing intensities (from non-grazed to very heavily grazed) over six years (2005–2010) on typical steppe in Inner Mongolia. Forage nutritional characteristics were defined by the nutritional value (concentrations of crude protein (CP), cellulase digestible organic matter (CDOM), metabolizable energy (ME) and neutral detergent fibre (NDF)) and the nutritional yield. The forage nutritional yield is a function of ANPP and the forage nutritional value. Forage nutritional value increased but ANPP and nutritional yields decreased with increasing grazing intensity. The inter-annual variation of ANPP, forage nutritional value and yield were weakly linked to the inter-annual variability of precipitation. However, ANPP and nutritional value also varied during the growing season, depending on the seasonal distribution of precipitation and temperature, which influence forage digestibility (CDOM) and metabolizable energy (ME), with higher CDOM and ME under high seasonal precipitation and low seasonal mean temperature. Forage nutritional value and yield, as well as ANPP, were predominantly determined by the dominant species rather than by species diversity. The results suggest that forage nutritional yield in the Inner Mongolian steppe is predominantly determined by the ANPP and only to a minor extent by forage nutritional value, and is predominantly determined by the dominant species and only to a minor extent by species diversity. Therefore, herbage productivity seems to be the most limiting factor in managing this steppe ecosystem as a feeding resource for livestock and to be the best ecological and environmental indicator for grassland management practices.     

Fertilization and plant diversity accelerate primary succession and restoration of dune communities

Plant species richness can increase primary production because plants occupy different niches or facilitate each other (“complementarity effects”) or because diverse mixtures have a greater chance of having more productive species (“selection effects”). To determine how complementarity and selection influence dune restoration, we established four types of plant communities [monocultures of sea oats (Uniola paniculata), bitter panicgrass (Panicum amarum) and saltmeadow cordgrass (Spartina patens) and the three-species mixture] under different soil treatments typical of dune restorations (addition of soil organic material, nutrients, both, or neither). This fully factorial design allowed us to determine if plant identity, diversity and soil treatments influenced the yield of both the planted species and species that recruited naturally (volunteers). Planted species responses in monocultures and mixtures varied among soil treatments. The composition of the plantings and soils also influenced the abundance of volunteers. The mixture of the three species had the lowest cover of volunteers. We also found that the effect of diversity on production increased with fertilizer. We partitioned the biodiversity effect into complementarity and selection effects and found that the increase in the diversity effect occurred because increased nutrients decreased dominance by the largest species and increased complementarity among species. Our findings suggest that different planting schemes can be used to meet specific goals of restoration (e.g., accelerate plant recovery while suppressing colonization of non-planted species).     

Combining field experiments and predictive models to assess potential for increased plant diversity to climate‐proof intensive agriculture

Agricultural production systems face increasing threats from more frequent and extreme weather fluctuations associated with global climate change. While there is mounting evidence that increased plant community diversity can reduce the variability of ecosystem functions (such as primary productivity) in the face of environmental fluctuation, there has been little work testing whether this is true for intensively managed agricultural systems. Using statistical modeling techniques to fit environment–productivity relationships offers an efficient means of leveraging hard‐won experimental data to compare the potential variability of different mixtures across a wide range of environmental contexts. We used data from two multiyear field experiments to fit climate–soil–productivity models for two pasture mixtures under intensive grazing—one composed of two drought‐sensitive species (standard), and an eight‐species mixture including several drought‐resistant species (complex). We then used these models to undertake a scoping study estimating the mean and coefficient of variation (CV) of annual productivity for long‐term climate data covering all New Zealand on soils with low, medium, or high water‐holding capacity. Our results suggest that the complex mixture is likely to have consistently lower CV in productivity, irrespective of soil type or climate regime. Predicted differences in mean annual productivity between mixtures were strongly influenced by soil type and were closely linked to mean annual soil water availability across all soil types. Differences in the CV of productivity were only strongly related to interannual variance in water availability for the lowest water‐holding capacity soil. Our results show that there is considerable scope for mixtures including drought‐tolerant species to enhance certainty in intensive pastoral systems. This provides justification for investing resources in a large‐scale distributed experiment involving many sites under different environmental contexts to confirm these findings.     

Legume-Cereal Intercropping Improves Forage Yield,Quality and Degradability

Intercropping legume with cereal is an extensively applied planting pattern in crop cultivation. However, forage potential and the degradability of harvested mixtures from intercropping system remain unclear. To investigate the feasibility of applying an intercropping system as a forage supply source to ruminants, two consecutive experiments (experiments 1 and 2) involving a field cultivation trial and a subsequent in vivo degradable experiment were conducted to determine the forage production performance and the ruminally degradable characteristics of a harvested mixture from an alfalfa/corn-rye intercropping system. In experiment 1, the intercropping system was established by alternating alfalfa and corn or rye with a row ratio of 5:2. Dry matter (DM) and nutrient yields were determined. In experiment 2, forages harvested from the different treatments were used as feedstuff to identify nutrient degradation kinetics and distribution of components between the rapidly degradable (a), potentially degradable (b) and the degradation rate constant (c) of ‘b’ fraction by in sacco method in Small-Tail Han wether Sheep. The intercropping system of alfalfa and corn-rye provided higher forage production performance with net increases of 9.52% and 34.81% in DM yield, 42.13% and 16.74% in crude protein (CP) yield, 25.94% and 69.99% in degradable DM yield, and 16.96% and 5.50% in degradable CP yield than rotation and alfalfa sole cropping systems, respectively. In addition, the harvest mixture from intercropping system also had greater ‘a’ fraction, ‘b’ fraction, ‘c’ values, and effective degradability (E value) of DM and CP than corn or rye hay harvested from rotation system. After 48-h exposure to rumen microbes, intercropping harvest materials were degraded to a higher extent than separately degraded crop stems from the sole system as indicated by visual microscopic examination with more tissues disappeared. Thus, the intercropping of alfalfa and corn-rye exhibited a greater forage production potential, and could be applied as forage supply source for ruminants. The improved effective degradability of harvest mixture material could be attributed to greater degradable components involving the rapidly degradable fractions (a), potentially degradable (b) fractions, and degradable rate constant (c), than that of corn and rye hay.     

Sows’ preferences for different forage mixtures offered as fresh or dry forage in relation to botanical and chemical composition

Providing forage to feed-restricted pregnant sows may improve their welfare by reducing their high feeding motivation. The aim of this study was to determine sows’ preferences for four forage mixtures cultivated in Canada. Forage mixtures were compared when offered either fresh or dry. The four forage mixtures were composed of different proportions and species of legumes (alfalfa (Alf) or red clover (Clo)) and grasses (tall fescue (F) and/or timothy (T)): (1) Alf-F, (2) Alf-F-T, (3) Clo-T and (4) Clo-F-T. Voluntary intake was measured, and preference tests were carried out for two experiments: one in spring for fresh forages ( n = 8) and the other in autumn for hays ( n = 8) with different sows housed in individual pens and fed a concentrated diet meeting their nutritional requirements for maintenance and foetal growth. Voluntary intake was measured by offering each forage mixture separately (one forage mixture/day) during 90 min according to a 4 × 4 Latin square design replicated four times. During preference tests, all six combinations of two forage mixtures were offered once (one combination/day) for 45 min to each sow. Individual forage intake was measured, and feeding behaviour was observed. Forages were analysed for botanical and chemical composition. Difference in voluntary intake among the four forage mixtures was determined using a variance analysis followed by Tukey tests for post hoc comparisons. In preference tests, differences between the two forage mixtures offered were determined using a paired Student’s t test, and the most ingested forage mixture was considered the preferred one. Results from both experiments revealed clear preferences for some forage mixtures when offered either fresh or dry. Forage mixtures with a greater proportion of legumes (AlfT and CloT) were preferred over forage mixtures with a higher proportion of grasses (AlfFT and CloFT). The AlfFT and CloFT forage mixtures contained at least 30% of fescue; therefore, the greater preference for the AlfT and CloT forage mixtures could also be due to the absence of fescue. Sows preferred forages with low DM and NDF concentrations and high CP and non-structural carbohydrates concentrations. Based on results from previous studies, the preferences seen in the present study are most likely due to the greater proportion of legumes, although an effect of tall fescue in preference cannot be excluded. Therefore, offering forages with a high proportion of legumes would be a good strategy to maximise both fresh and dry forage intake in pregnant sows.     

Invaded grassland communities have altered stability‐maintenance mechanisms but equal stability compared to native communities

Theory predicts that stability should increase with diversity via several mechanisms. We tested predictions in a 5‐year experiment that compared low‐diversity exotic to high‐diversity native plant mixtures under two irrigation treatments. The study included both wet and dry years. Variation in biomass across years (CV) was 50% lower in mixtures than monocultures of both native and exotic species. Growth among species was more asynchronous and overyielding values were greater during and after a drought in native than exotic mixtures. Mean‐variance slopes indicated strong portfolio effects in both community types, but the intercept was higher for exotics than for natives, suggesting that exotics were inherently more variable than native species. However, this failed to result in higher CV's in exotic communities because species that heavily dominated plots tended to have lower than expected variance. Results indicate that diversity‐stability mechanisms are altered in invaded systems compared to native ones they replaced.     

Characteristics of boli formed by dairy cows upon ingestion of fresh ryegrass,lucerne or chicory

This study examined the comminution of fresh herbage, subsequent nutrient release, and the characteristics of swallowed boli from three physically and chemically contrasting forages during ingestive mastication by dairy cows. The extent and pattern of nutrient release will determine their availability to rumen microflora, and potentially influence their efficiency of use. The forages evaluated were perennial ryegrass (ryegrass, Lolium perenne L., cv Alto AR37), lucerne (Medicago sativa L., cv Torlesse) and chicory (Cichorium intybus L., cv Choice). Experimental design was a 3×3 cross-over with three forages and three consecutive 1-day measurement periods, conducted twice. Six non-lactating, pregnant, multiparous Holstein-Friesian×Jersey cows (Bos taurus) were used, with the first cross-over applied to three mature (10.1±0.61 years old; BW 631±64 kg) cows, and the second to three young (4.8±0.02 years; BW 505±19 kg) cows. Fresh cut forage was offered to the cows following partial rumen evacuation. Swallowed boli were collected directly at the cardia at the commencement, middle and end of the first feeding bout of the first meal of the day. Forage species did not affect the fresh weight of ingested boli (mean 169 g, P=0.605) but the proportion of saliva in boli varied between forage. Boli of chicory contained the greatest amount of herbage material and least amount of saliva, whereas ryegrass boli were the opposite. Boli fresh weight tended to increase as time in the meal progressed, but the age of the cow was not shown to affect any boli characteristics or nutrient release. Particle size reduction was affected by forage, with 31%, 38% and 35% of chicory, lucerne and ryegrass herbage reduced to <2 mm. There was little evidence of relationship between comminution and any physical or chemical characteristic of the forage, except in ryegrass where extent of comminution was moderately correlated with herbage strength. Proportional release of herbage soluble carbohydrate exceeded that of N during mastication. Differences in loss of N were moderately correlated with the amount of N in the herbage (R²=0.53) but herbage comminution was not strongly correlated with release of either N or carbohydrate. These findings illustrate the complex animal×forage interactions that occur during mastication, and that it is not possible to infer nutrient loss from herbage based on herbage characteristics as the driver for this differ between species.     

一年生和多年生豆禾混播草地超产与多样性效应的比较

为了测度一年生和多年生豆禾混播草地的超产效应与植物多样性效应的关系, 明确一年生和多年生混播草地的高产优势, 探索豆禾混播草地多样性效应和超产效应对其生态功能的响应机制, 于2013-2015年在新疆伊犁地区昭苏盆地开展了3年的牧草产量观测试验。该试验设置3种牧草混播种类和混播比例, 分别为一年生豆禾混播草地(2种牧草混播, AM2)和多年生豆禾混播草地(2种牧草混播, PM2; 4种牧草混播, PM4; 6种牧草混播, PM6), 豆禾混播比例分别为6:4、5:5和4:6。结果表明: 1) 2013、2014年和3年平均值AM2的超产幅度小于PM2和PM6, 2015年AM2的超产幅度大于PM2、PM4和PM6; 混播群落生产力与群落组分中生产力最高产物种单产以及各组分种平均单产的差值表现出相似的规律。2) 2013、2014年和3年平均值AM2的互补效应大于PM2、PM4和PM6, AM2的选择效应则远小于互补效应, PM2、PM4和PM6的互补效应则比较稳定。3)物种丰富度和物种均匀度与牧草产量(群落生产力)大部分情况下呈单峰的“饱和上升型”模式, 分别在4种牧草混播和豆禾混播比例为5:5时, 具有较高生产力。4)多年生豆禾混播草地的互补效应、选择效应和多样性净效应均随生长年限的延长而呈下降趋势, 也导致了超产幅度、超产效应及其稳定性的下降。由此可见, 在建植初期, 互补效应和选择效应共同主导了多年生豆禾混播草地的超产效应, 而随着生长年限的延长, 选择效应则成为主要影响因素; 一年生豆禾混播草地的超产效应则一直受互补效应的影响。     

Genotypic diversity effects on biomass production in native perennial bioenergy cropping systems

The perennial grass species that are being developed as biomass feedstock crops harbor extensive genotypic diversity, but the effects of this diversity on biomass production are not well understood. We investigated the effects of genotypic diversity in switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) on perennial biomass cropping systems in two experiments conducted over 2008–2014 at a 5.4‐ha fertile field site in northeastern Illinois, USA. We varied levels of switchgrass and big bluestem genotypic diversity using various local and nonlocal cultivars – under low or high species diversity, with or without nitrogen inputs – and quantified establishment, biomass yield, and biomass composition. In one experiment (‘agronomic trial’), we compared three switchgrass cultivars in monoculture to a switchgrass cultivar mixture and three different species mixtures, with or without N fertilization. In another experiment (‘diversity gradient’), we varied diversity levels in switchgrass and big bluestem (1, 2, 4, or 6 cultivars per plot), with one or two species per plot. In both experiments, cultivar mixtures produced yields equivalent to or greater than the best cultivars. In the agronomic trial, the three switchgrass mixture showed the highest production overall, though not significantly different than best cultivar monoculture. In the diversity gradient, genotypic mixtures had one‐third higher biomass production than the average monoculture, and none of the monocultures were significantly higher yielding than the average mixture. Year‐to‐year variation in yields was lowest in the three‐cultivar switchgrass mixtures and Cave‐In‐Rock (the southern Illinois cultivar) and also reduced in the mixture of switchgrass and big bluestem relative to the species monocultures. The effects of genotypic diversity on biomass composition were modest relative to the differences among species and genotypes. Our findings suggest that local genotypes can be included in biomass cropping systems without compromising yields and that genotypic mixtures could help provide high, stable yields of high‐quality biomass feedstocks.     

寒温带牧林交错区生境复杂度对啮齿类物种多样性的影响

采用铗日法对嫩江流域牧林交错区5个生境梯度中啮齿动物的多样性水平进行了研究,以探讨栖息地复杂程度对地面啮齿类物种多样性的影响.结果表明,黑线姬鼠、黑线仓鼠和大林姬鼠是嫩江流域牧林交错区中的优势种和常见种,不同物种在不同生境类型中呈明显的不均匀分布,反映了它们对不同生境的选择倾向性.同时,地面草本植物的覆盖度和丰富度对地面活动的小型哺乳动物的物种组成和群落结构具有较大的影响,啮齿动物的捕获率和生物量与草本植物的多样性水平呈现出显著的相关性关系,其物种丰富度随草本植物多样性指数的增加呈递增的趋势.此外,各生境中啮齿动物的物种多样性指数与草本植物多样性的相关关系并不显著,可能是因为灌丛区和林-灌交错区内大量的低矮灌丛与萌生丛改善了生境的隐蔽条件与食物资源,为更多物种的共存创造了良好的微环境,使得两生境中啮齿动物的物种多样性水平尤为突出.研究表明,小型啮齿动物的物种组成与栖息地的复杂程度有关,但地面植被对小型兽类生物量和物种多样性的影响力度并不是等同的,对地面小型兽类物种多样性影响的研究,统计生境复杂度或异质性时应考虑不同植被型/生境类型距地面的高度,不同植被型对小型兽类物种多样性的贡献程度随其距地面高度的增加而降低.     

Biodiversity effects on yield and unsown species invasion in a temperate forage ecosystem

Background and Aims

Current agricultural practices are based on growing monocultures or binary mixtures over large areas, with a resultant impoverishing effect on biodiversity at several trophic levels. The effects of increasing the biodiversity of a sward mixture on dry matter yield and unsown species invasion were studied.

Methods

A field experiment involving four grassland species [two grasses – perennial ryegrass (Lolium perenne) and cocksfoot (Dactylis glomerata) – and two legumes – red clover (Trifolium pratense) and white clover (Trifolium repens)], grown in monocultures and mixtures in accordance with a simplex design, was carried out. The legumes were included either as single varieties or as one of two broad genetic-base composites. The experiment was harvested three times a year over three years; dry matter yield and yield of unsown species were determined at each harvest. Yields of individual species and interactions between all species present were estimated through a statistical modelling approach.

Key Results

Species diversity produced a strong positive yield effect that resulted in transgressive over-yielding in the second and third years. Using broad genetic-base composites of the legumes had a small impact on yield and species interactions. Invasion by unsown species was strongly reduced by species diversity, but species identity was also important. Cocksfoot and white clover (with the exception of one broad genetic-base composite) reduced invasion, while red clover was the most invaded species.

Conclusions

The results show that it is possible to increase, and stabilize, the yield of a grassland crop and reduce invasion by unsown species by increasing its species diversity.Key words: Cocksfoot, Dactylis glomerata, diversity effect, invasion, legumes, perennial ryegrass, Lolium perenne, red clover, Trifolium pratense, simplex design, statistical modelling, transgressive over-yielding, white clover, T. repens     

Effects of plant diversity on invasion of weed species in experimental pasture communities

Studies have shown that weed invasion into grasslands may be suppressed if the resident plant community is sufficiently diverse. The objective of this study was to determine whether increased forage plant diversity in grazed pasture communities might be associated with reduced weed abundance both in the aboveground vegetation and soil seed bank. Data were collected from a pasture experiment established in 1994 in Missouri, USA. The experiment consisted of 15 m×15 m plots sown with Festuca arundinacea Schreb. or Bromus inermis Leysser as a base species in mixtures of 1, 2, 3, 6, or 8 forage species. The plots were grazed by cattle during each growing season from 1998 to 2002. Aboveground plant species composition in each plot was measured using a point step method. Soil cores were collected in 1999 and 2002, and the species composition of germinable weed seeds in plots were evaluated by identifying seedlings as they germinated over an 8-week period. Species diversity was measured using several indices: species richness (S), Shannon–Wiener diversity index (H^′), and forage species evenness (J). Aboveground weed abundance in plots was unrelated to forage species richness (S), but weed abundance declined as the evenness (J) of resident forage species increased in mixtures. The species composition of mixtures may have affected weed abundance. Weeds both in the soil seed bank and aboveground vegetation were less abundant in mixtures that contained F. arundinacea compared with mixtures that contained B. inermis. Although variables like forage plant productivity may also suppress weed abundance in pastures, our results suggest that maintaining an evenly distributed mixture of forage species may help suppress weeds as well.

Zusammenfassung

Untersuchungen haben gezeigt, dass die Unkrautinvasion in Grünländer unterdrückt sein kann, wenn die ansässige Pflanzengemeinschaft ausreichend divers ist. Die Zielsetzung dieser Untersuchung war es zu bestimmen, ob eine erhöhte Futterpflanzendiversität in beweideten Grünlandgemeinschaften mit einer verringerten Unkrautabundanz sowohl bei der oberirdischen Vegetation als auch in der Bodensamenbank verbunden sein kann. Die Daten wurden in einem Weidelandexperiment gesammelt, das 1994 in Missouri, USA, etabliert wurde. Das Experiment bestand aus 15 m×15 m Probeflächen, die mit Festuca arundinacea Schreb. oder Bromus inermis Leysser als Basisarten in Mischungen von 1, 2, 3, 6 oder 8 Futterarten eingesät waren. Die Probeflächen wurden während jeder Wachstumssaison von 1998 bis 2002 stark mit Vieh beweidet. Die oberirdische Pflanzenartenzusammensetzung wurde in jeder Fläche mit einer Punktstopmethode gemessen. Bodenproben wurden 1999 und 2002 gesammelt und die Artenzusammensetzung der keimfähigen Unkrautsamen wurde in den Probeflächen bewertet, indem die Keimlinge identifiziert wurden, die in einer 8-wöchigen Periode keimten. Die Artendiversität wurde unter Verwendung verschiedener Indizes gemessen: Artenreichtum (S), Shannon–Wiener-Diversitätsindex (H^′) und Futterarten-Äquitabilität (J). Die oberirdische Unkrautartenabundanz in den Probeflächen stand in keiner Beziehung zum Futterartenreichtum (S), aber die Unkrautabundanz nahm ab, wenn die Äquitabilität (J) der ansässigen Futterarten in den Mischungen zunahm. Die Artenzusammensetzung der Mischungen könnte die Unkrautabundanz beeinflusst haben. Sowohl die Unkräuter in der Bodensamenbank, als auch in der oberirdischen Vegetation waren weniger abundant in Mischungen, die F. arundinacea enthielten, im Vergleich zu denen, die B. inermis enthielten. Obgleich Variablen wie die Futterpflanzenproduktivität möglicherweise ebenfalls die Unkrautabundanz im Weideland unterdrücken, lassen unsere Ergebnisse vermuten, dass die Aufrechterhaltung einer gleichmäßigen Mischung von Futterarten ebenfalls helfen kann, die Unkräuter zu unterdrücken.     

The influence of vegetation pattern on the productivity,diversity and stability of vegetation: The case of `brousse tigrée' in the Sahel

Sample sites of `brousse tigrée' and related vegetation types are described for Mali and Niger. Species composition and physical structure of the herbaceous layer as well as woody plant population were recorded at all sites together with data on soils and natural resource management. Herbage yield was measured whereas foliage yield and wood mass were calculated using allometry equation calibrated for each species. `Brousse tigrée' is characterized by the regularly alternating bare-soil stripes with dense linear thickets arranged perpendicularly to the slope. There was no clear superiority in total plant production of `brousse tigrée' when compared to neighbouring site with diffuse vegetation. However, the pattern of `brousse tigrée' tended to favour woody plant yield to the detriment of herbage yield. The number of herbaceous species recorded per site (22–26) was slightly above Sahelian vegetation average despite low number of species per 1-m² quadrat (6–9), bare soil excluded. This species richness reflects the diversity in edaphic niches resulting from the redistribution and local concentration of water resources and shade. The high spatial heterogeneity and species richness of the herbaceous layer in `brousse tigrée' did not attenuate the interannual variation in herbage yield despite low yields. Except for the herb layer, little evidence was found of grazing influence on the vegetation structure and yield a few hundred metres away from livestock concentration points. On the other hand, the clearing of thickets for cropping led to severe soil erosion which threaten the resilience of `brousse tigrée'. These observations and the well-defined climatic, soiland topographic situations under which the `brousse tigrée' occurs invalidate the hypothesis of an anthropic origin of that vegetationpattern.     

Spatial aggregation facilitates coexistence and diversity of wild plant species in field margins

European agri-environment schemes encourage farmers to establish sown field margin strips to protect and enhance wild plant diversity. However, plant diversity in such wild plant sowings based on seed mixtures is often low due to the high competitiveness of few, common species. Here we analysed whether intraspecific aggregation could enhance the performance of less competitive species, and how plant performance is influenced by the number of species in a mixture. We focused on inter- and intraspecific competition between six agricultural wild plant species (Centaurea cyanus, Calendula arvensis, Melilotus officinalis, Poa annua, Bromus mollis, Medicago lupulina), and tested (i) two different seeding patterns (intraspecifically aggregated vs. randomly dispersed) and (ii) three different species mixtures (monocultures, three-species, and six-species mixtures). Plant performance was measured in terms of number of individuals, biomass per individual, and biomass per m². Intraspecific aggregation resulted in higher numbers of individuals of all species, while mixtures generated lower numbers of individuals. The performance of plant species differed depending on their position in the competitive hierarchy. Competitively weak species suffered much less from intraspecific than interspecific competition in terms of biomass, and the competitively weakest species became even excluded in the most species rich and randomly dispersed sowings with high interspecific competition. In conclusion, the performance of wild plant species was influenced by both seeding pattern and number of species in a mixture. Intraspecific aggregation enabled the coexistence of competitively weak species by reducing interspecific competitive exclusion processes. Consequently, agri-environmental schemes designed to preserve and enhance biodiversity should consider small-scale processes influencing the distribution and abundance of plants, and develop new agricultural sowing technologies to cultivate competitively weak and endangered wild plant species.     

Biomass production, relative competitive ability and water use efficiency of two dominant species in semiarid Loess Plateau under different water supply and fertilization treatments

Water stress and nutrient deficiency are considered to be the main environmental factors limiting plant growth and species interaction in semiarid regions. However, less is known about the interactive effects of soil water, nitrogen and phosphorus on native species growth and relative competitive ability. A replacement series design method was used with 12 mixed plants of Bothriochloa ischaemum and Lespedeza davurica grown in a pot experiment under three water regimes and four fertility treatments. Intercropping systems were assessed on the basis of indices such as biomass production and allocation, relative competitive ability, aggressivity, relative yield total and water use efficiency (WUE). Water stress decreased significantly the total biomass production for each species, either in monoculture or in mixtures. N, P, or NP application can significantly improve biomass production of the two species in their mixtures. There was no obvious change trend in root/shoot ratio of B. ischaemum or L. davurica in different mixture proportions. Relative yield total (RYT) values ranged from 0.98 to 1.39. Aggressivity values of B. ischaemum to L. davurica were positive in all water regimes and fertilizations, implying that B. ischaemum was the dominant species. Relative competition intensity values of B. ischaemum (i.e., RCI_B) were less than zero, while greater than zero for L. davurica (i.e., RCI_L), indicating that the effects of intraspecific competition with L. davurica were stronger for B. ischaemum, and the opposite for L. davurica. WUE increased gradually as the proportion of B. ischaemum increased in mixtures, and a 10:2 B. ischaemum:L. davurica mixture proportion had significantly higher WUE. Results suggest that it is advantageous to grow the two species together to maximize biomass production and the recommended mixture ratio was 10:2 of B. ischaemum to L. davurica because it gave higher RYT and significantly higher WUE under conditions of water deficit.     

Biodiversity and agro-ecology in field margins

This multidisciplinary study investigates agro-ecological functions (nature conservation, agriculture, environment) and implications of newly created, mown sown and unsown field margin strips installed on ex-arable land to increase biodiversity. From conservational concern, the development of species rich field margin strips was not strongly affected by the installed type of margin strip since species diversity converged over time, whether strips were sown or not. Convergence between unsown and sown margin strips occurred also in terms of species composition: unsown and sown strips became similar over time. Mowing without removal of cuttings significantly reduced species richness, yielded more grassy margin strips and delayed similarity in species composition between sown and unsown margin strips. Species richness on the longer term was not significantly affected by light regime nor by disturbance despite significant temporary effects shortly after the disturbance event. On the contrary vegetation composition in terms of importance of functional groups changed after disturbance: the share of spontaneous species within functional groups increased resulting in higher similarity between the sown and unsown vegetation. Furthermore risk of invasion was highest in the disturbed unsown community on the unshaded side of a tree lane. A positive effect of botanical diversity on insect number and diversity was found. However the effects of botanical diversity on insect number was mediated by light regime. At high light availability differences between plant communities were more pronounced compared to low light availablilty. The abundance of some insect families was dependent on the vegetation composition. Furthermore light availability significantly influenced insect diversity as well as the spatial distribution of families. From agricultural concern, installing margin strips by sowing a species mixture and a mowing regime with removal of cuttings are good practices to diminish the risk of species ingrowth into adjacent crops by creeping roots and rhizomes. Seed dispersal was only problematic one year after the installation of the field margin strips particularly nearby the unsown margin strip and wind-borne seeds were dispersed over limited distances, mainly within 4 m of field margins. Annual herbage yield was not affected by mowing management. DM yield of sown/unsown communities converged over time. Compared to herbage from an intensively managed fertilized grassland, field margin herbage revealed a low feeding value, owing to a low crude protein content, a low digestibility and a high crude fibre content. The unsown community had a higher forage quality than communities sown to bred commercially available grass varieties. Both digestibility and crude protein content decreased over time irrespective of plant community or location. Mid June cuts were more productive than mid September cuts but digestibility and crude protein content was lower. The use of herbage from field margins as hay for horses or as a component in farmland compost are good alternatives. A strong relation was found between the distribution of pest insects and their antagonist families along field margin strips indicating a status of biological equilibrium. From environmental concern, field margin strips buffered boundary vegetation and watercourses against cropped areas loaded with high levels of mineral nitrogen. Margin strips reduced the mineral nitrogen content of the soil in the margin and mineral nitrogen loss during wintermonths. Mineral nitrogen loss was not affected by field margin type but by distance from the field crop. A minimal width of 5 m is necessary to reach an optimal reduction in mineral soil N and N losses.     

泌桐高速公路生态护坡不同草灌混播10a后的植被群落特征

稳定的边坡群落是公路边坡生态防护的目标,为探究高速公路生态护坡工程10 a演替后群落特征,该研究以泌桐高速公路为依托,布设自然恢复、单一狗牙根播种和5个不同播种密度的草灌混播生态防护措施,分析不同恢复方式对边坡群落组成、生活型和物种多样性的影响。结果表明:(1)调查样方内共有52种植物,分属于17科49属。禾本科、菊科和豆科植物共26属29种,占种总数的55.77%,表明这三大科植物在边坡群落演替过程中起着重要作用。(2) 10 a演替后不同恢复方式下群落中多年生植物比例高于一年生草本植物。(3)在草灌混播样地中,群落物种多样性指数随着播种密度的增加呈先增加后降低的单峰变化趋势,在播种密度为每平方米500株时达到最大。(4)草灌混播的生态恢复效果优于纯草本种植和自然恢复方式。(5)播种密度对草灌混播群落类型,地上生物量和物种多样性指数没有显著影响。从植物的生长效果及成本方面考虑,初播密度每平方米为500~600株的草灌混播可构建较为稳定的边坡植物群落,实现最佳的边坡恢复效果,可应用于类似区域边坡生态恢复工程。