Black polyethylene mulch improves growth of plantation-grown loblolly pine and yellow-poplar

The effects of mulching with perforated black plastic, in combination with fertilization and induced mycorrhizal symbioses, on the growth of loblolly pine (Pinus taeda L.) and yellow-poplar (Liriodendron tulipifera L.) were studied in a plantation under intensive, short rotation management. Mulching effects on soil temperature were also examined in order to assess the potential influence of this treatment on temperature-dependent processes in soils affecting tree growth. Mulching significantly increased height and stem diameter growth of both species, attributable largely to improved water relations resulting from diminished soil surface evaporation and elimination of transpirational losses from competing vegetation. Mulching effects on soil temperature were insufficient to contribute substantially to the growth response exhibited by mulched trees. Multiple applications of urea-N promoted enhanced growth in both loblolly pine and yellow-poplar, an effect accentuated by mulching, but the field performance of trees inoculated in the nursery with selected mycorrhizal fungi was poor relative to that of the other treatments investigated.     

Effects of understorey vegetation on tree establishment and growth in a silvopastoral system in northern Greece

The interaction between understorey plants and trees in a young silvopastoral system was studied in a sub-humid Mediterranean environment with cold winters in northern Greece. The experimental design was a split-split plot with three replications and included: three understorey treatments (grass, legume and control), two tree species (Acer pseudoplatanus and Pinus sylvestris) and two spacings (2.5×2.5 m and 3.5×3.5 m). The competition between herbaceous plants and trees resulted in significant differentiation in tree growth while their spacing did not produce any significant differences. The influence of trees on herbage yield either by plant species or spacing was not significant. During the early stages of establishment, a significant positive correlation was observed between sycamore growth and soil moisture, leaf weight, leaf area and leaf number as well as nitrogen and potassium concentration in leaves. Six years after planting the height increase of the sycamore trees was largest in the control treatment (415%), lowest in the grass treatment (134%) and intermediate (192%) in the legume treatment. Much higher was the increase obtained for the diameter, 161%, 207% and 536% respectively for the grass, legume and control treatments. The Scots pine trees grew faster than sycamore over the course of the experiment. Height increased by 397%, 351% and 400% and diameter by 518%, 443% and 683% respectively for the grass, legume and control treatments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Leaf-water potential and soil-water depletion of walnut mulched with polyethylene and intercropped with alfalfa in Central Italy

In Italy, new agroforestry cultural models could play an important role in the diffusion of plantation forestry timber species. We studied the stem growth rates and leaf water potentials () of common walnut (Juglans regia L.), and gravimetric soil moisture (_DW) depletion during the third and fourth growing seasons in an agroforestry trial in central Italy. Since the establishment of experimental plots in 1992, walnut was intercropped with alfalfa (Medicago sativa L.), with or without polyethylene (PE) mulching along tree rows. By the end of the study period, the unmulched-intercropped walnut was almost 68% smaller in stem diameter and height than the sole-unmulched control, demonstrating walnuts sensitivity to alfalfa competition. The competitive effect of alfalfa on walnut stem growth was effectively controlled/reduced by PE mulching. Stem growth rates of mulched-intercropped walnut were always lower than the control, but much higher (ca. 235% in stem diameter and height) than those of unmulched-intercropped walnut. The higher field performance of mulched-intercropped walnut compared to the un- mulched-intercropped treatment was associated with higher soil moisture and predawn and midday values during the summers driest periods. The presence of PE mulching was also associated with lower levels of soil moisture during the early and late growing season, but this did not negatively affect walnut and growth rates. These results suggest that in the climatic conditions of central Italy, competition for soil moisture between young common walnut trees and alfalfa can be substantially lowered, but not completely eliminated, by PE mulching.This revised version was published online in November 2005 with corrections to the Cover Date.     

Weed control measures in Christmas tree plantations of <Emphasis Type="Italic">Abies nordmanniana</Emphasis> and <Emphasis Type="Italic">Abies lasiocarpa</Emphasis> on agricultural land

Abies nordmanniana and Abies lasiocarpa, established for Christmas tree production in southwestern Norway (58°44′N, 5°38′E), were treated with different weed control methods, including chemicals, use of black plastic mulch, grass or clover as ground cover, living mulch and mechanical hoeing. Ground cover with black plastic mulching resulted in the best growth and quality in A. lasiocarpa; the least favourable treatment was when grasses were allowed to grow close to the trees. Unless measures were taken to remove this competing vegetation, the ground cover grew over the trees, smothering 94% of them. A. nordmanniana plants grew to the same height on plots with no weed control as on plots with thorough weed eradication. Stem diameter decreased on plots with ground vegetation compared to seedlings grown in the weed-free environment. The use of Trifolium repens as ground cover decreased height growth by 30% compared to thorough weed control. Both grasses and clover sown as living mulch, damaged the trees more than did natural weed vegetation.     

Growth and nitrogen status of young walnuts as affected by intercropped legumes in a Mediterranean climate

Walnut trees (Juglans nigra L. and Juglans nigra × regia NG23) were intercropped with alfalfa (Medicago sativa L.) or sainfoin (Onobrychis sativa L.) forage legumes compared to a grass (Festuca arundinacea Schr., fescue), or to spontaneous weeds as a control in two Mediterranean sites (Castries and Notre-Dame de Londres) near Montpellier (France). Tree growth, soil water depletion and nitrogen content of the tree leaves were monitored to assess the impact of both water competition and possible facilitation resulting from fixed nitrogen transfer from the leguminous crops to the trees. At Castries, where alfalfa and fescue were compared, they were found to have the same impact on tree growth. At Notre-Dame de Londres where sainfoin was compared with a spontaneous grassing treatment, the sainfoin crop was more competitive to the walnuts. The nitrogen content of walnut leaves was enhanced when intercropped with nitrogen fixing species at both sites. Perennial leguminous intercrops were more competitive for soil water resources than fescue or weeds, but in the long term this may be compensated by the improved nitrogen status of the trees. This compensating effect was observed during a rather rainy year. The overall impact of leguminous intercrops on tree growth may depend on the frequency of dry (competitive) and wet (non-competitive) years. This revised version was published online in June 2006 with corrections to the Cover Date.     

Status of thousand cankers disease on eastern black walnut in the eastern United States at two locations over 3 years

Thousand cankers disease (TCD), a lethal fungal dieback of eastern black walnut (Juglans nigra), caused by Geosmithia morbida, and spread by the walnut twig beetle, Pityophthorus juglandis, was documented in 2009 to be very destructive in the western United States and was identified in the native range of J. nigra at Knoxville, Tennessee, in 2010, and in 2011 at Richmond, Virginia. Beginning late 2010, we studied branch dieback levels (per cent live crown) and new TCD symptom development at these two quarantined locations monthly for 3 years. Of the 106 trees studied (53 at each location), 31 trees had low live crown ratings of 70 to 0% with little change over the 3 years of the study. One per cent of the trees developed new symptoms on a per‐year basis. Thus, a moderate level of TCD (mean = 76% live crown) was present in these two locations, and most trees were in a quiescent or dormant TCD condition for 3 years, an important finding not previously reported. We found new TCD symptoms developed in Richmond in 2011 and 2012 when precipitation from January 1 to the end of August was low (60–64 cm), and not when the precipitation in Richmond was higher (99 cm). In late 2012, in Richmond, soil water potential assays indicated that some black walnut trees were under severe physiological stress (?15 bars). In contrast, in 2013, high precipitation levels (99 and 130 cm) and high soil water potentials (?0.1 to ?3 bars) at both locations were associated with extensive new foliage and stem growth and recovery from TCD. Further research is needed on water relationships in regard to TCD and black walnut health.     

Beneficial Effects of Intercropping on the Growth and Nitrogen Status of Young Wild Cherry and Hybrid Walnut Trees

Growing concern for economic and environmental issues emphasizes the potential value of intercropping systems in temperate regions. However, the selection of relevant tree species to be associated with crops has been little documented. The growth and the nitrogen nutrition of two economically valuable species, wild cherry (Prunus avium L.) and hybrid walnut (Juglans nigra L.×Juglans regia L.), were compared over six years after plantation. These two species were associated with non-irrigated cereal crops in the agroforestry treatment or grown separately (weeded control and fallow). Intercropping increased diameter growth as soon as year 2 in the two species. Leaf biomass assessment using allometric models showed an earlier and greater leaf biomass increase in hybrid walnut than in wild cherry tree. After six years, the relative growth increase of the agroforestry trees with respect to the control trees varied with the parameter considered (diameter at breast height from +26 to +65%, leaf biomass from +54 to +142%) and with the tree species (higher relative growth for hybrid walnut trees). The beneficial effect on tree growth can be accounted for in terms of enhanced nitrogen nutrition. The tree–crop association in intercropping systems, which improves tree growth, might thus allow the planting of more demanding trees on soils of lower fertility.     

Establishing trees in an Appalachian silvopasture: response to shelters, grass control, mulch, and fertilization

In order to successfully introduce trees into existing pastures, it is important to determine and recommend a whole range of tree establishment practices. In the spring of 1995, approximately 350 bare-root seedlings each of black walnut (Juglans nigra L.) and honeylocust (Gleditsia triacanthos L.) were planted in six randomized paddocks within a silvopastoral study area at the Agroforestry Research and Demonstration Site in Blacksburg, Virginia. Three seedling establishment studies were tested, including (1) a tree protection study, (2) a water retention study, and (3) a fertilization study. Seedlings were planted using two different tree shelters (60 cm-tall poultry wire cage and 1.2 m-tall plastic Tubex), two water retention treatments (mulch and herbicide spray), and one fertilizer treatment. All treatments were compared to untreated controls. Tree survival, damage, and stem volume were compared for each species. Tree survival was comparable among all studies over three growing seasons. Tree establishment using poultry wire and Tubex shelters resulted in significant reduction of deer damage and significant increase in stem volume from 1996 to 1998. Tubex shelters had a pronounced positive impact on tree height and also on stem form; height of both black walnut and honeylocust was twice the height of control seedlings. Mulch and herbicide treatments for moisture control resulted in significant stem volume increases over thecontrol treatment from 1997 to 1998. However, mulching was less effective than the herbicide treatment. There was no significant tree growth response resulting from fertilization during this same period. This revised version was published online in June 2006 with corrections to the Cover Date.     

Vegetation succession process induced by reforestation in erosion areas

Reforestation is one of the most important and efficient measures of water and soil conservation. Based on field investigations in the Shangyang Soil Conservation and Reforestation Station in Huizhou, Guangdong Province, China, we studied the variation in vegetation development, vegetation succession processes and soil erosion. The regional vegetation consists mainly of monsoon evergreen broad-leaved forests (MEBF). The area was deforested and became a denuded hill area with extremely high soil erosion in the 1960s and 1970s. Then, the area was closed in order to allow recovery of the vegetation. Under natural conditions the vegetation development and succession processes were slow during which soil erosion and strong sunshine and evaporation slowed down the development of the vegetation. About 25 years later, the vegetation cover was still merely 35% or so. The dominant vegetation types were heliophilous herbage and shrubs which formed a poorly developed shrub-herbage community and erosion remained high. In contrast, reforestation with selected tree species dramatically speeded up the vegetation succession process. About 12 years after reforestation, vegetation cover of the Acacia auriculiformis plantations in the Shangyang Station was 90% and erosion was under control. After 23 years, understory vegetation, consisting of indigenous species, had developed in the plantations. The planted trees and naturally developing herbage, shrubs, bamboo, local trees and liana formed a complex vegetation community in three layers. It will take 60 years for the vegetation to succeed from bare land to a secondary growth forest under natural conditions. Reforestation may speed up the vegetation succession process. The time may be reduced to 20 years. Reforestation is the most effective measure of vegetation restoration and erosion control in this area. __________ Translated from Acta Ecologica Sinica, 2006, 26(8): 2558–2565 [译自: 生态学报]     

Forage legumes as living mulches for trees in agroforestry practices -- preliminary results

Successful establishment and growth of tree seedlings in an agroforestry practice is dependent on the control of competing herbaceous vegetation. Conventional weed control methods (i.e., chemical, mechanical, and physical suppression) are effective but can be costly in terms of time, damage to non-target vegetation, or increased soil erosion. Alternatively, some living mulches can exclude undesirable vegetation, protect the soil, compete minimally with associated trees, and supplement soil nitrogen. In this study, small and large white clover (Trifolium repens L.), red clover (T. pratense L.), kura clover (T. ambiguum Bieb.), strawberry clover (T. fragiferum L.), birdsfoot trefoil (Lotus corniculatus L.), hairy vetch and ’AU Early‘ hairy vetch (Vicia villosa Roth) were selected for evaluation as living mulches. These species were established in 9 m ×9 m replicated plots along with two controls [tall fescue (Festuca arundinacea Schreb.) and bare soil kept vegetation free using glyphosate]. The controls represent ’high competition‘ and ’no competition‘, respectively. Eastern black walnut (Juglans nigra L.), honeylocust (Gleditsia triacanthos L.), and hybrid pitch × loblolly pine (Pinus rigida L. × P. taeda L.) seedlings were planted in association with each living mulch. Tree seedling heights and diameters were measured during the first two years and forage performance evaluated. All seedlings grew better in the absence of competition. Tall fescue greatly reduced the growth of hardwood seedlings. Red clover and ladino clover showed the greatest promise as living mulches for use with the hardwood and pine seedlings, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of herbicides and felling,fertilization, and prescribed fire on longleaf pine establishment and growth through six growing seasons

Recovery of longleaf pine (Pinus palustris P. Mill.) is necessary to arrest the decline of many associated plants and animals, and the establishment of longleaf pine on much of its original range requires artificial regeneration and diligence. In central Louisiana, USA, two fertilization levels (No [NF] or Yes [F-36 kg/ha N and 40 kg/ha P]) in combination with three vegetation treatments (check, two prescribed fires [PF], or multi-year vegetation control by herbicidal and mechanical means [IVM]) were applied to container-grown longleaf pine plantings in two studies. In Study 1 (grass dominated), 6-year-old longleaf pine survival was 52% on the F–checks, 78% on the F–PF plots, and averaged 93% on the other four treatment combinations. Longleaf pine trees on the IVM plots (3.4 m) were significantly taller than on the other two vegetation treatments, and trees on the PF plots (1.8 m) were taller than trees on the check plots (1.2 m). In Study 2 (brush dominated), survival averaged 65% across the six-treatment combinations after 6 years. The longleaf pine trees were 4.7 m tall on the IVM plots and averaged 3.9 m tall on the check and PF plots. Fertilization increased P concentrations in the soil and longleaf pine foliage, while fertilization did not significantly affect longleaf pine height growth. Native fertility was not apparently limiting longleaf pine development contrary to prior research recommendations for these soils. In both studies, the IVM treatment reduced early herbaceous competition and the number and height of arborescent plants. The PF treatment reduced arborescent plant height on the grassy site where fires were more intense than on the brushy site.     

Growth and water potential of j-rooted loblolly and eastern white pine seedlings over three growing seasons

The effects of j-rooting on water stress and growth of loblolly (Pious taeda L.) and eastern white pine (Pious strobus L.) were examined over three growing seasons in the field. Seedlings were planted in an area with severe herbaceous competition with either their roots planted straight or bent into a j shape. All seedlings were planted with their root collars placed at the soil surface. During the first year j-rooted seedlings consistently had lower water potentials but never statistically significant. Since both treatments were planted with the root collar at the soil surface, this trend was likely due to an initial shallower root system in j-rooted seedlings. In year three no differences in water potential were significant and no trends were evident. Growth did not differ significantly by treatment at any time but, by year three, j-rooted plants were consistently larger for both species.     

陇东山地核桃抗旱保墒试验初报

为了解决陇东山地核桃挂果晚、产量低、品质差的问题,2013—2014年连续2a于4—10月份在环县刘家塬项目区选择长势基本一致的3a生核桃幼树进行覆草、覆膜试验,定期、定点测定并记载土壤含水量和核桃树新梢生长量。结果表明:覆草较对照可提高土壤含水量12.70%,覆膜较对照可提高土壤含水量4.68%;通过对核桃树体生长量调查,覆草较对照树体年新梢生长量平均提高18.42%,覆膜较对照核桃年新梢生长量平均提高9.01%,其中覆草对土壤含水量和新梢生长量的影响最大。     

Role of weeds in the management of nitrogen in a young Pinus radiata plantation

Pinus radiata trees were grown on a podzolized sandy soil at a second rotation site under the following treatments: total weed control, total weed control plus ammonium nitrate, strip weed control and no weed control. During the first two summers after planting the differences in needle water potential between trees under no, strip or total weed control were very small. Despite similar rates of net N-mineralization in strip and total weed control treatments, which averaged 64 kg ha^–1 yr^–1 in the 0–15 cm soil depth, weeds in the strip weed control treatment reduced soil mineral-N concentrations by 50–80%, leaching of N by the end of the first growing season by 45%, foliar-N concentrations by 4–14% and stem biomass at 20 months after planting by 46%. Although N-uptake by above-ground vegetation (trees plus weeds) was 49% higher in the strip weed control treatment, the amount of N apportioned to trees during the first 20 months after planting was reduced from 15.5 to 9.0 kg ha^–1. These effects of weeds were even more pronounced in the no weed control treatment. Since weeds had little effect on the needle water potential of trees and the annual rates of N-mineralization, but adversely affected N-uptake by trees, results indicate that weeds directly competed with trees for N, and thereby aggravated N-deficiency in trees. Application of ammonium nitrate after complete weed control increased foliar-N concentrations, and N-uptake and growth of trees, but also induced severe stem deformation.     

Effect of shading,mulching and vegetation control on Douglas-Fir seedling growth and soil water supply

Harsh environmental conditions on many harvested sites in southwest Oregon necessitate site modifications for successful regeneration of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco]. We conducted a 2-year study with 350 seedlings to assess the effects of twelve soil-shading, mulching, and vegetation control techniques on seedling growth and soil temperature and moisture environments. Treatments modified a variety of environmental conditions. Major effects of treatments were to lower soil surface temperature, reduce soil surface evaporation, and reduce vegetative competition for soil water. These modified conditions affected seedlings by adjusting the timing of seedling growth and reducing soil water loss to increase available water for seedling use. Final seedling shoot volume and stem diameter both differed among treatments. Seedlings in treatments where competing vegetation was controlled showed significantly greater growth than seedlings in other treatments. Soil water loss in treatments where either soil surface evaporation was controlled by mulching, or where competing vegetation was controlled, was significantly less than water loss from the shaded and control treatments. Soil water loss in treatments with vegetation controlled by herbicide was significantly less than in treatments with vegetation controlled by scalping. Seedlings showed greatest growth with treatments that elicited the most efficient use of available microsite water either by reducing soil surface evaporation or vegetation competition.     

Competition for nitrogen between adult European beech and its offspring is reduced by avoidance strategy

Plant growth, reproduction, and biomass allocation may be affected differently by nitrogen availability depending on tree size and age. In this context, competition for limited N may be avoided by different strategies of N acquisition between different vegetation components (i.e., seedlings, mature trees, other woody and herbaceous understorey). This study investigated in a field experiment whether the competition for N between different vegetation components in beech forests was prevented via seasonal timing of N uptake and affected by microbial N use. For this purpose, a removal approach was used to study the seasonal effects on N uptake and N metabolites in adult beech trees and beech natural regeneration, as well as soil microbial processes of inorganic N production and utilisation. We found that the competition for N between beech natural regeneration and mature beech trees was reduced by seasonal avoidance strategies (“good parenting”) of N uptake regardless of the N sources used. In spring, organic and inorganic N uptake capacity was significantly higher in beech seedlings compared to adult beech trees, whereas in autumn mature beech trees showed the highest N uptake rates. Removal of vegetation components did not result in changes in soil microbial N processes in the course of the growing season. Thus, N resources released by the removal of vegetation components were marginal. This consistency in soil microbial N processes indicates that competition between plants and soil microorganisms for N was not avoided by timing of acquisition during the vegetation period, but existed during the entire growing season. In conclusion, N nutrition in the studied forest ecosystem seems to be optimally attuned to European beech.     

Weeds in <Emphasis Type="Italic">Eucalyptus globulus</Emphasis> subsp. <Emphasis Type="Italic">maidenii</Emphasis> (F. Muell) establishment: effects of competition on sapling growth and survivorship

The effect of herbaceous vegetation on growth and survival was assessed in planted eucalyptus saplings grown under four levels of weed cover. Seedlings of Eucalyptus globulus subsp. maidenii were planted with 0 (W0), 25 (W25), 50 (W50) and 100% (W100) weed cover. Weed species composition and aboveground biomass was determined. Soil water content was evaluated by the gravimetric method. Seedling leaf area, diameter and height were evaluated at planting and during the 3 months following establishment. Tree height, diameter and stem volume was estimated at 12, 24 and 36 months. First year tree survival was recorded. The ratio of cumulative stem growth under W100: cumulative stem growth under W0 was used to measure competitive performance. Regression analysis was used to determine competition thresholds. Weeds seriously threaten the growth of E. globulus subsp. maidenii. Early negative effects of competition on growth were evident as from the second month after establishment, and both seedlings and 1-year-old saplings were more affected than 2- and 3-year-old ones. A “minimum-response threshold” was determined near 500 kg/ha (corresponding to W25 cover). No clear effect was found between sapling survival and weed biomass.     

Arthropod diversity in alley cropped black walnut (Juglans nigra L.) stands in eastern Missouri, USA

Knowledge of the complex interactions among trees, crops and their associated fauna is necessary to determine the viability of a particular agroforestry practice. Information is lacking concerning these interactions, particularly in temperate agroforestry practices. We examined the effects of two forages on the growth, nut production, and arthropod communities of alley cropped eastern black walnut, Juglans nigra L. Experimental plots of eastern black walnut, intercropped with alfalfa, Medicago sativa L., smooth bromegrass, Bromis inermis Leyss., or no vegetation were sampled with sweep nets prior to each cutting date for the forages. Comparisons were made between treatments and sampling dates. Tree growth measurements, nut yield and other nut quality measurements were taken at the end of each growing season. There were no differences in tree growth among alleyway treatments. The first season's nut yield was greater from trees with vegetation-free alleyways; otherwise nut production did not differ among the treatments. Arthropods were more numerous and diverse in alley cropped alfalfa than in alley cropped bromegrass or in the vegetation-free controls. Alley cropped bromegrass supported a more diverse population of arthropods than did the vegetation-free control. Arthropod diversity in the tree canopies did not differ among treatments. Alley cropped forages supported a more diverse and even arthropod fauna than did adjacent monocropped forages. We conclude that alley cropped forages had a relatively minor impact on the growth and nut yield of walnut trees.This revised version was published online in November 2005 with corrections to the Cover Date.     

Rooting depth explains [CO2] x drought interaction in Eucalyptus saligna

Elevated atmospheric [CO(2)] (eC(a)) often decreases stomatal conductance, which may delay the start of drought, as well as alleviate the effect of dry soil on plant water use and carbon uptake. We studied the interaction between drought and eC(a) in a whole-tree chamber experiment with Eucalyptus saligna. Trees were grown for 18 months in their C(a) treatments before a 4-month dry-down. Trees grown in eC(a) were smaller than those grown in ambient C(a) (aC(a)) due to an early growth setback that was maintained throughout the duration of the experiment. Pre-dawn leaf water potentials were not different between C(a) treatments, but were lower in the drought treatment than the irrigated control. Counter to expectations, the drought treatment caused a larger reduction in canopy-average transpiration rates for trees in the eC(a) treatment compared with aC(a). Total tree transpiration over the dry-down was positively correlated with the decrease in soil water storage, measured in the top 1.5 m, over the drying cycle; however, we could not close the water budget especially for the larger trees, suggesting soil water uptake below 1.5 m depth. Using neutron probe soil water measurements, we estimated fractional water uptake to a depth of 4.5 m and found that larger trees were able to extract more water from deep soil layers. These results highlight the interaction between rooting depth and response of tree water use to drought. The responses of tree water use to eC(a) involve interactions between tree size, root distribution and soil moisture availability that may override the expected direct effects of eC(a). It is essential that these interactions be considered when interpreting experimental results.     

Influence of alfalfa (Medicago sativa L.) intercropping and polyethylene mulching on early growth of walnut (Juglans spp.) in central Italy

Combined production of timber and forage species is considered to be a desirable land-use system to offset some of the current land management problems of Italy. This paper reports the early results of a study on intercropping young plants of walnut with alfalfa (Medicago sativa L.), with or without polyethylene mulching, in an experimental plantation in central Italy. Two genotypes of walnut were used:Juglans regia L. and the French hybrid NG23×RA. The seasonal changes of absolute and relative growth rates of walnut stem diameter and apical shoot length were investigated.During both growing seasons, alfalfa intercropping strongly decreased walnut growth, whereas polyethylene mulching promoted stem height and diameter growth. Stem growth of intercropped-and-mulched walnut was the same as that of sole walnut under clean cultivation management. The use of polyethylene mulching reduced or avoided the competitive effect of associated alfalfa on walnut growth. During both growing seasons, the intercropped-and-mulched walnut had higher relative growth rates than intercropped-unmulched walnut. The results show the effectiveness of polyethylene mulching in promoting early growth of walnut, even when intercropped with alfalfa during early stages of plantation establishment.