Tree windbreaks and shelter benefits to pasture in temperate grazing systems

The effects of windbreaks on pastures are reviewed, with an emphasis on temperate grazing systems. Mechanisms of plant response to shelter are dealt with in brief. Few papers on measured responses of pasture species to shelter were located in a search of the global literature for the period 1972–97. Except in cold climates, where the benefits of snow-trapping on water availability can be demonstrated, there were few reports of increased production of pasture in response to shelter. A significant result was obtained in a summer rainfall environment in Australia, where a 43% increase in wool production was obtained over three years in small plots sheltered with iron sheeting on the fences. The gain was attributed to increased pasture growth. In New Zealand, one study over three years with a narrow, permeable shelterbelt in a windy, dry summer environment showed a 60% increase in pasture growth in the sheltered zone. However, another study on a high rainfall site with a dense, wide shelterbelt found no substantial shelter effect on pasture. In dry, hot and windy climates there appears to be scope for protecting spray-irrigated pasture with windbreaks. The feasibility of evaluating shelter effects on pastures or crops from old windbreaks is questioned. Variability of soil over the site can not be satisfactorily accounted for and there are problems in defining the true ‘unsheltered’ yield. Shelter effects on pastures could best be determined by comparing production in small completely sheltered plots and open plots. Effects in and near the competitive zone should be measured for living windbreaks. Modelling could then be used to evaluate windbreak systems. We are not yet in a position to provide unequivocal advice to farmers on windbreak outcomes for particular purposes or regions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cattle grazing and pine survival and growth in subterranean clover pasture

When combining pines and cattle on tame pasture, grazing is often delayed for several years until trees are large enough to resist injury. As an alternative approach to delayed or deferred grazing during the early years, this study in central Louisiana, USA, examined the effects of cattle grazing in subterranean clover (Trifolium subterraneum L.) pastures on slash (Pinus elliottii Engelm.) and loblolly pine (P. taeda L.) seeding survival and growth during the first 3 years of tree establishment. Pines were planted at about 1200 trees/ha in 3 rows on 0.4-ha subclover units with 1.3 m spacing within rows. Three grazing treatments included: (1) ungrazed pines, (2) limited grazing with a single-wire electric fence above the planted pines, and (3) grazed pines. Thirty Brahman crossbred cows with calves and a bull grazed the tame pasture on a controlled grazing, rotational basis from December through May each year, during the subterranean clover growing season. Pine trampling injury during the year was 8% on the grazed seedlings while essentially none occured under limited grazing. During the first 2 growing seasons, survival and height of the pines were significantly less on the grazed seedlings than on either the limited grazing or ungrazed seedlings. Pine heights from the limited grazing and ungrazed treatments were not different during the 3-year study; loblolly pine heights from the limited grazing treatment continued to be taller than the grazed treatment through the third year while the slash pine heights were similar for all treatments by the third year. Seedling mortality became more acute as severity of grazing injury increased; the greatest mortality occured when the terminal bud and needles were both browsed off.     

Effect of shelter on temperate crops: a review to define research for Australian conditions

The fact that the shelter created by windbreaks can have a significant, positive effect on crop production is supported by eight decades of research from many countries around the world. Although the concept of planting windbreaks to enhance crop production has general currency in Australia, the practice is not as wide as it could be. This review of the last decade of windbreak literature defines the research needed to encourage wider utilisation of windbreak technology. After outlining the principal mechanisms behind the effect of shelter on temperate crops, the review discusses relevant literature of the past decade especially that from Australia. The main mechanisms discussed are: the protection of crops from physical damage; soil conservation; the direct augmentation of soil moisture; and the alteration of the crop energy balance and plant water relations. Also discussed are the elusiveness of the shelter effect, competition from windbreak trees, and the modelling of windbreak systems. Suggestions for future research in Australia include: quantifying the competition of various windbreak species and the effect of root pruning on both crop and tree; a model of crop energy and water relations at the tree-crop interface; an economic model and a farmer-oriented decision support tool. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ecological benefits provided by alley cropping systems for production of woody biomass in the temperate region: a review

In temperate Europe alley cropping systems which integrate strips of short rotation coppices into conventional agricultural fields (ACS) are receiving increasing attention. These systems can be used for crops and woody biomass production at the same time, enabling farmers to diversify the provision of market goods. Adding trees into the agricultural land creates various additional benefits for the farmer and society, also known as ecosystem services. However, tree-crop interactions in the temperate region have not been adequately substantiated which is identified as a drawback to the practical implementation of such systems. In order to bridge this gap, the current paper aims to present a comprehensive overview of selected ecosystem services provided by agroforestry with focus on ACS in the temperate region. The literature indicates that compared with conventional agriculture ACS have the potential to increase carbon sequestration, improve soil fertility and generally optimize the utilization of resources. Furthermore, due to their structural flexibility, ACS may help to regulate water quality, enhance biodiversity, and increase the overall productivity. ACS are shown as suitable land use systems especially for marginal sites. Based on the available data collected, we conclude that ACS are advantageous compared to conventional agriculture in many aspects, and therefore suggest that they should be implemented at a larger scale in temperate regions.     

黑龙江垦区农田防护林生态效益评价

通过定点观测和气象资料分析,得出农田防护林在防风、调温、增湿,改善区域生态环境方面的明显效果。     

Tree/wood quality in slash pine following long-term cattle grazing

Tree height, diameter, and grade were measured on 14 cattle grazing trial plots located on the Palustris Experimental Forest in Louisiana's Kisatchie National Forest. These plots had been established in the early 1960s. Mensurational data was gathered on 28 trees from grazed sites and another 28 from ungrazed plots. Increment cores were also taken from these trees. Statistical analyses showed no effect attributable to grazing on any of the variables measured: tree height, tree diameter at breast height, tree grade, growth rate, amount of latewood, unextracted specific gravity, or tracheid length. This revised version was published online in June 2006 with corrections to the Cover Date.     

Tree phenology and water availability in semi-arid agroforestry systems

Four common agroforestry trees, including both exotic and native species, were used to provide a range of leafing phenologies to test the hypothesis that temporal complementarity between trees and crops reduces competition for water in agroforestry systems during the cropping period and improves utilisation of annual rainfall. Species examined included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves once during the long dry season, and the evergreen Croton megalocarpus. Phenological patterns were examined in relation to climatic conditions in the bimodal rainfall regions of Kenya to identify factors which dictate the intensity of competition between trees and crops.

The main differences in leaf cover patterns were between indigenous and exotic tree species. The Central American species, S. spectabilis and G. sepium, shed their foliage during the dry season before the short rains, whereas the native species, M. volkensii and C. megalocarpus, exhibited reduced leaf cover during both dry seasons. C. megalocarpus was the only species to maintain leaf cover throughout the 2-year experimental period. M. volkensii and S. spectabilis exhibited similar leafing phenology, losing almost all leaf cover during the long dry season (July–October) and flushing before the onset of the ensuing rains. S. spectabilis lost few leaves during the short dry season, whereas M. volkensii shed a greater proportion of its foliage before flushing prior to the long rains (March–July). M. volkensii lost much of its leaf cover during the 1997/1998 short rains (October–February), when soil water content was unusually high. Although essentially evergreen, leaf cover in C. megalocarpus decreased during the dry season and increased rapidly during periods of high rainfall. G. sepium exhibited a period of low leaf cover during the long dry season and did not regain full leaf cover until mid-way through the short rains. The mechanisms responsible for these phenological changes and the implications of tree phenology for resource utilisation and competition with crops are discussed.     

Effects of grazing exclusion and environmental conditions on the soil seed bank of a Mediterranean grazed oak wood pasture

Large seed banks in the soils of Mediterranean wood pastures can allow the composition of the understorey vegetation to adapt to changing conditions such as under-grazing, grazing exclusion and climate change. This three year study investigated the effect of grazing exclusion on the transient and persistent seed banks of 23 areas of a Mediterranean wood pasture of Quercus suber L., Q. ilex L. and Q. pubescens Willd. A canonical correspondence analysis was used to determine the effect of topo-climatic (elevation, aspect, slope, rainfall, temperature, tree coverage), soil (pH, soil texture, and soil nitrogen, phosphorus, lime and organic carbon content) and biodiversity (Shannon index, species richness index, and Pastoral Value) variables on the soil seed bank under grazed and ungrazed conditions. The size of the persistent seed bank increased with rainfall, grazing, and the available phosphorus content of the soil. Specific site by site grazing regimes could increase the abundance of legumes in the soil seed bank and the species richness and diversity of the understorey vegetation. These results can help guide the conservation management of this silvopastoral area.     

Optical porosity and windspeed reduction by coniferous windbreaks in Southern Ontario

Relative windspeed reduction was measured behind nine relatively narrow, homogeneous windbreaks in southern Ontario, Canada to assess whether any characteristics of the windspeed reduction curve could be predicted from optical porosity. The latter was determined for each windbreak using high contrast black and white photographic silhouettes on a computer digitizing system. Minimum windspeeds behind the windbreaks ranged from 29 to 71% of open windspeed; these minima were located 2 to 6 multiples of windbreak height away from the windbreak. Optical porosities of the bottom half of the windbreak ranged from 0 to 31%. Multiple regression of the shelter parameters (location and value of minimum relative windspeed) on the independent variables (optical porosity, open windspeed, surface roughness, approaching wind direction relative to the windbreak, average tree diameter and average tree spacing) showed that the minimum relative windspeed could be predicted from the optical porosity of the bottom half of the windbreak. The results suggest that optical porosity can be used to predict minimum relative windspeeds and may therefore be useful as a guide in the field evaluation of windbreaks.     

CT-measured macropores as affected by agroforestry and grass buffers for grazed pasture systems

Agroforestry and grass buffers have been proposed for improving water quality in watersheds. Soil porosity can be significantly influenced by buffer vegetation which affects water transport and water quality. The objective of the study was to compare differences in computed tomography (CT)-measured macroporosity (>1,000-μm diam.) and coarse mesoporosity (200- to 1,000-μm diam.) parameters for agroforestry and grass buffer systems associated with rotationally grazed and continuously grazed pasture systems. Soils at the site were Menfro silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalf). Six replicate intact soil cores, 76.2 mm diam. by 76.2 mm long, were collected using a core sampler from the four treatments at five soil depths (0–50 cm at 10-cm intervals). Images were acquired using a hospital CT scanner and subsequently soil bulk density and saturated hydraulic conductivity (K _sat) were measured after scanning the cores. Image-J software was used to analyze five equally spaced images from each core. Bulk density was 5.9% higher and saturated hydraulic conductivity (K _sat) values were five times lower for pasture treatments relative to buffer treatments. For the 0–10 cm soil depth, CT-measured soil macroporosity (>1,000 μm diam.) was 13 times higher for the buffer treatments compared to the pasture treatments. Buffer treatments had greater macroporosity (0.020 m³ m⁻³) compared to pasture (0.0045 m³ m⁻³) treatments. CT-measured pore parameters were positively correlated with K _sat. The project illustrates benefits of agroforestry and grass buffers for maintaining soil porosity critical for soil water and nutrient transport.     

Tree diversity and composition in Mexican traditional smallholder cocoa agroforestry systems

Agroforestry Systems - The cultivation of cocoa is a commodity of great importance worldwide. In Mexico, cocoa is grown in Tabasco and Chiapas states in agroforestry systems since pre-Hispanic...     

Tree leafing phenology and crop productivity in semi-arid agroforestry systems in Kenya

The hypothesis that temporal separation of resource use between trees and crops minimises competition for wa ter in agroforestry systems during the cropping period and increases utilisation of annual rainfall was tested at Machakos in semi-arid Kenya. Four popular tree species were chosen to provide a range of leafing phenologies. These included Melia volkensii, which sheds its leaves twice a year, Senna spectabilis and Gliricidia sepium, which shed their leaves during the long dry season, and the evergreen Croton megalocarpus. All four species retained their foliage during the long rains, offering little scope for temporal separation of resource use. Maize (Zea mays) yields were reduced by 50–70% in the agroforestry treatments. Reductions in crop yield were strongly correlated with tree growth (r ² =0.94) and available soil moisture (r ² =0.88). G. sepium remained leafless for much of the short rains despite the presence of available soil water, and was least competitive with the bean crops (Phaseolus vulgaris) grown at this time. Reductions in crop yield in the agroforestry treatments were closely correlated with tree growth (r ² =0.99) and available moisture (r ² =0.79) during the 1996/97 short rains (158 mm), but not during the much wetter 1997/98 season (608 mm). Shading by trees or shade nets reduced crop yield, in contrast to previous studies in the semi-arid tropics. Low off-season rainfall during the study period (9% of annual rainfall compared to the long-term average of 20%) limited the potential for temporal separation of growing periods. Where the prospects for temporal or spatial separation in resource use are limited, shoot and/or root pruning may be necessary to manage competition between trees and crops. This revised version was published online in June 2006 with corrections to the Cover Date.     

Willows rapidly affect microclimatic conditions and forage yield in two temperate short-rotation agroforestry systems

Temperate short-rotation agroforestry systems are promising solutions for environmentally and economically sound production of crops and woody biomass. Accurate estimates of microclimatic conditions and tree-crop interactions in these systems are clearly needed to improve their adoption and management, especially in northeastern North America. The objective of this study was to determine the effects of a windbreak and an alley cropping system that were planted with strips of short-rotation willow on microclimate and forage crop yields. Microclimatic conditions (wind speed, air temperature, air relative humidity, light availability, soil moisture, snow depth) and forage yields were measured at different distances from the willow strips in these two agroforestry systems and in agricultural control plots between the third (2020) and fourth years (2021) after experiment establishment. Willow strips significantly reduced wind speed in both experiments, despite their relatively short stature (260 cm, June 2021). Wind speed reduction was greatest (50 and 58% in alley cropping and windbreak systems, respectively) close to the willow strips (5 m), intermediate at 20 m (22% in both systems), and negligible at 50 m (windbreak system). A significant increase in daytime air temperature that reached almost 1 °C was measured close to the willow strips in the agroforestry systems, compared to control plots. Higher soil moisture was measured close to the windbreak. In both agroforestry systems, snow depth was significantly greater near the willow strips. In both systems, forage yield close to the willow strips was not different from that at any other measured distances and in the control plots, which suggests that no competition for resources occurred between the willows and the forage crop during the study. In July 2021, forage yield within the windbreak (0–50 m from the willow strip) was 44% greater than that in the control plots. The alley cropping system had significant negative (-12%, June 2020) and positive (+?13%, September 2021) effects on forage yield, possibly as a result of temporal variation in climatic conditions. Results of this study stress the importance of better understanding of intra- and inter-annual forage crop variability within agroforestry systems.

     

Tree-root systems and herbaceous species-characteristics under tree species introduced into grazing lands in subhumid Cameroon

The effect of tree species on the characteristics of the herbaceous stratum, during the first five years of a fallow, was evaluated in the North of Cameroon (average annual temperature 28.2 °C, total annual rainfall 1050 mm). Treatments included a natural grazed herbaceous fallow, a natural ungrazed herbaceous fallow and three planted tree fallows (Acacia polyacantha Willd. ssp. campylacantha (Hochst. ex A. Rich.), Senna siamea Lam. and Eucalyptus camaldulensis Dehnh.), which were protected against grazing. Because tree species influenced light interception in different ways, as well as having different root patterns, they had different effects on the herbaceous stratum in terms of species composition and biomass. The grazed herbaceous fallow maintained the greatest species richness. Protection against grazing or the introduction of tree species associated with the absence of grazing induced both a progressive evolution to a particular species composition. The ungrazed herbaceous fallow consisted mainly of Andropogon gayanus Kunth, which provided the greatest biomass (8 t dry matter ha^–1 at the end of the fallow period). E. camaldulensis provided little shade and the lowest fine root mass in the top layer allowing the growth of A. gayanus and thus a greater herbaceous biomass (3.5 t DM ha^–1) than that found under the other tree species. Under the heavy shade of A. polyacantha, the herbaceous stratum consisted mainly of annual Pennisetum spp. (2.2 t DM ha^–1) and showed the greatest N concentration (1.3%), probably due to N₂ fixation by the tree species. After the fourth year, despite the relatively open tree canopy, S. siamea, which showed the highest fine root mass, had a strong depressive effect on the herbaceous stratum. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root length density and carbon content of agroforestry and grass buffers under grazed pasture systems in a Hapludalf

Enhancement of root development helps to improve soil physical properties, carbon sequestration, and water quality of streams. The objective of this study was to evaluate differences in root length density (RLD) and root and soil carbon content within grass buffer (GB), agroforestry buffer (AgB), rotationally grazed pasture (RG) and continuously grazed pasture (CG) treatments. Pasture and GB areas included red clover (Trifolium pretense L.) and lespedeza (Kummerowia stipulacea Maxim.) planted into fescue (Festuca arundinacea Schreb.) while AgB included Eastern cottonwood trees (Populus deltoids Bortr. ex Marsh.) planted into fescue. One-meter deep soil cores were collected from each treatment in August 2007 and 2008 with a soil probe. Three soil cores were sampled at six replicate sampling positions. Soil cores were collected in plastic tubes inserted inside the metal soil probe. Soils were segregated by horizons, and roots were separated into three diameter classes (0–1, 1–2, >2 mm) by soil horizon. Root length was determined using a flatbed scanner assisted with computer software. Buffer treatments (167 cm/100 cm³) had 4.5 times higher RLD as compared to pasture treatments (37.3 cm/100 cm³). The AgB treatment had the highest (173.5 cm/100 cm³) RLD and CG pasture had the lowest (10.8 cm/100 cm³) value. Root carbon was about 3% higher for the buffers compared to RG treatment. Soil carbon was about 115% higher for the buffers compared to pasture treatments. Results from this study imply that establishment of agroforestry and GB on grazed pasture watersheds improve soil carbon accumulation and root parameters which enhance soil physical and chemical properties thus improving the environmental quality of the landscape.     

Tree root characteristics as criteria for species selection and systems design in agroforestry

This literature review presents information about the role of tree root systems for the functioning of agroforestry associations and rotations and attempts to identify root-related criteria for the selection of agroforestry tree species and the design of agroforestry systems. Tree roots are expected to enrich soil with organic matter, feed soil biomass, reduce nutrient leaching, recycle nutrients from the subsoil below the crop rooting zone and improve soil physical properties, among other functions. On the other hand, they can depress crop yields in tree-crop associations through root competition. After a brief review of favourable tree root effects in agroforestry, four strategies are discussed as potential solutions to the dilemma of the simultaneous occurrence of desirable and undesirable tree root functions: 1) the selection of tree species with low root competitiveness, eventually supplemented by shoot pruning; 2) the identification of trees with a root distribution complementary to that of the crops; 3) the reduction of tree root length density by trenching or tillage; and 4) the use of agroforestry rotations instead of tree-crop associations. The potential and limitations of these strategies are discussed, and deficits in current understanding of tree root ecology in agroforestry are identified. In addition to the selection of tree species and provenances according to root-related criteria, the development of management techniques that allow the manipulation of tree root systems to maximize benefit and minimize competition are proposed as important tasks for future agroforestry research.     

Tree and crop productivity in gliricidia/maize/pigeonpea cropping systems in southern Malawi

This study examined the hypothesis that incorporation of Gliricidia sepium (Jacq.) Walp.) (gliricidia), a fast-growing, nitrogen-fixing tree, into agroforestry systems in southern Malawi may be used to increase the input of organic fertilizer and reduce the need for expensive inorganic fertilizers. The productivity of maize (Zea mays L.), pigeonpea (Cajanus cajan L.) and gliricidia grown as sole stands or in mixed cropping systems was examined at Makoka Research Station (latitude 15° 30′ S, longitude 35° 15′ E) and a nearby farm site at Nazombe between 1996 and 2000. Treatments included gliricidia intercropped with maize, with or without pigeonpea, and sole stands of gliricidia, maize and pigeonpea. Trees in the agroforestry systems were pruned before and during the cropping season to provide green leaf manure. Maize yields and biomass production by each component were determined and fractional light interception was measured during the reproductive stage of maize. Substantial quantities of green leaf manure (2.4 to 9.0 Mg ha⁻¹ year⁻¹) were produced from the second or third year after tree establishment. Green leaf manure and fuelwood production were greatest when gliricidia was grown as unpruned sole woodlots (c. 8.0 and 22 Mg ha⁻¹ year⁻¹ respectively). Improvements in maize yield in the tree-based systems also became significant in the third year, when c. 3.0 Mg ha⁻¹ of grain was obtained. Tree-based cropping systems were most productive and exhibited greater fractional light interception (c. 0.6 to 0.7) than cropping systems without trees (0.1 to 0.4). No beneficial influence of pigeonpea on maize performance was apparent either in the presence or absence of gliricidia at either site in most seasons. However, as unpruned gliricidia provided the greatest interception of incident solar radiation (>0.9), coppicing may be required to reduce shading when gliricidia is grown together with maize. As pigeonpea production was unaffected by the presence of gliricidia, agroforestry systems containing gliricidia might be used to replace traditional maize + pigeonpea systems in southern Malawi. This revised version was published online in June 2006 with corrections to the Cover Date.     

歌乐山绿色屏障工程建设重要意义的分析与评价

“重庆市歌乐山绿色屏障工程”是重庆市“十五”重点生态环境建设项目。近年来，由于森林植被覆盖率低、树种单一、森林生态系统稳定性差等原因，已导致项目区水土流失现象加剧，环境质量下降，对重庆市中心城区及沙坪坝区生态环境改善和生态安全构成较大威胁。实施本项工程将充分发挥重庆市西郊歌乐山绿色屏障的重要防护功能，维护重庆市中心城区及三峡库区生态环境的安全与稳定。