秋季巴丹吉林沙漠东南缘人工梭梭林水分来源

2009 年秋季( 9 月份) 在巴丹吉林沙漠东南缘的绿洲 - 荒漠过渡带,选择栽植 2,5,10,20 和 30 年的人工梭梭林,测定 20,30,50,100,150 和 200 cm 土深的土壤含水量,应用稳定氧同位素方法研究人工梭梭林的水分来源。结果表明: 梭梭林 20 ～ 30 cm 土层含水量显著高于 150 ～ 200 cm 土层; 土壤浅层的水分来自夏末和秋初的自然降雨; 随林龄增加,梭梭对地下水的利用比例逐渐增加,而对浅层土壤水分的利用比例逐渐降低; 2 年生梭梭依次主要利用 100,20,30,50 和 200 cm 土壤水分,5 年生梭梭依次主要利用地下水以及 200,150,100 和 30 cm 土壤水分,10 年生梭梭依次主要利用地下水以及 200,150,30 和 20 cm 土壤水分,20 年生梭梭依次主要利用地下水以及200 和 150 cm 土壤水分,而 30 年生梭梭依次主要利用 50,100,150,200,30 和 20 cm 土壤水分; 在巴丹吉林沙漠东南缘,随着林龄增加,梭梭对降雨补充的土壤浅层水分的利用能力降低,而对土壤深层水分与地下水的利用又不足以维持其正常生长需要,这可能是导致人工梭梭林大面积退化的一个原因。     

Estimating water use of sclerophyllous species under East-Mediterranean climate: III. Tabor oak forest sap flow distribution and transpiration

The annual course of daily transpiration and the hydrological balance of a Tabor oak forest were determined. The study was done in a representative forest within the natural geographical range of the species in the lower Galilee region of Israel. The climate is sub-humid with a rainless dry season from May to October. A partial water balance of a 0.1 ha area supporting an average of 14 trees was calculated from: (a) soil water content (SWC) measured by a Neutron Probe at depths of from 0.2 to 8 m, and (b) daylight transpiration rate measured with sap flow sensors by the heat pulse technique.Soil–bedrock complex water content (%) in the first 2 m of the profile fluctuated strongly between 5 and 20% depending on the season. The water content increased with depth from about 10% at 2.0 m depth to more than 20% at 5.0 m depth. For depths exceeding 5.5 m seasonal fluctuations in water content were negligible and water content ranged from 30 to 35%. After a dry winter, water content generally decreased within the main root zone down to about 2.0 m depth. Monthly changes in water content (mm) were greatest at depths of 0.35–1.0 m. Only minor changes in the soil–bedrock complex water content were recorded at greater depths. After a very rainy winter (2002/2003), decreases in soil–bedrock complex water content in the upper 2 m were much larger than after a dry winter. Fluctuations of soil–bedrock complex water content in deeper regions were larger in the wetter year, probably the result of drainage.Sap velocity was measured at six depths in the sapwood, from 4 to 44 mm, at 8 mm intervals. Sap velocity declined with depth, hence, sap flux density too.Based on sap velocity measurements performed during 4 years, the annual average daily transpiration (T) was 0.796 mm/day. This sums up to 239 mm during ∼300 days of leaf carriage, i.e. 41.3% of the 578 mm average annual rainfall for the area in the last 50 years. In a relatively dry year (rainfall of 432.7 mm) total water withdrawal from the 8 m soil–bedrock profile was 81% of the annual rainfall; of this amount 69% were transpired by the oak trees (239.0 mm), or 55% of the annual rainfall. In a relatively wet year (annual rainfall 801.4 mm) total water withdrawal was 67%; of this amount 45% would be transpired by the oak trees, or 30% of the annual rainfall.     

Evaluating a simple radiation/dry matter conversion model using data from Eucalyptus globulus plantations in Western Australia

A simple model that describes growth in terms of physical and physiological processes is needed to predict growth rates and hence the productivity of trees at particular sites. The linear relationship expected between absorbed photosynthetically active radiation (phi(pa), MJ m(-2)) and dry mass production (G(t)); i.e., G(t) = epsilonphi(pa), where epsilon is the radiation utilization coefficient, was fitted to three years' data from five Western Australian Eucalyptus globulus Labill. plantations for which monthly growth measurements, leaf area indices, weather data and soil water measurements were available. Reductions in growth efficiency relative to absorbed photosynthetically active radiation were associated with high vapor pressure deficits (D, kPa) so the relationship between monthly aboveground biomass increments and D was used to calculate utilizable phi(pa). Plotting cumulative aboveground growth against utilizable phi(pa) gave strong linear relationships with slope epsilon. Values of epsilon ranged from 0.93 to 2.23 g dry mass MJ(-1) phi(pa). The variation could not be explained either in terms of soil water content in the root zones, because all plantations appeared to have access to groundwater, or in terms of soil chemistry. A value of epsilon approximately 2.2 is considered near the maximum likely to be applicable to Eucalyptus plantations. An interesting peripheral finding was a strong relationship between allometric ratios and soil phosphorus; this, if confirmed elsewhere, will be of considerable value in converting biomass increments to wood production. There was also a strong negative relationship between the average ratio of leaf/total aboveground biomass and soil nitrogen content.     

Nitrogen fertilization of poplar plantations on agricultural land: effects on diameter increments and leaching

Fast-growing poplar plantations on agricultural land require intensive management activities, often involving fertilization. The present paper aims at investigating the effect of fertilization on growth and on groundwater quality, by examining four trials established in commercial poplar plantations in central and south Sweden. The treatments consisted of nitrogen applications (Urea N46, Tot. N: 46%) in two different dosages, 75 and 150?kg?ha^?1 for two years, and a control, in three replicates (plot size: 20?×?20?m) following a randomized block design. Diameters were measured at each plot at the end of each growing season for the period 2012–2015. At the same time, groundwater pipes were installed in the center of each plot, at ca. 1.5?m depth. Samples were regularly collected and analyzed for NO₃-N and PO₄-P. The results show a large variation in the diameter growth response to nitrogen fertilization and the leaching of poplar plantations after canopy closure. In young plantations, the effect on growth was clear with moderate fertilization rates although it was not observed on sandy soils with already good growth, leading to high nitrogen leaching.     

内蒙河套平原盐碱地水盐运动研究

土地盐渍化是干旱和半干旱地区普遍存在的问题。防止土地盐渍化和盐渍土的改造利用,是一项具有重要生态意义和经济效益的工作。在“三北”防护林体系工程建设中,盐碱一直影响造林成活、保存和生长,成为进一步治理这一地区的主要限制因子之一。为探讨“三北”地区河套平...     

Soil water content and water relations in planted and naturally regenerated Pinus halepensis Mill. seedlings during the first year in semiarid conditions

The capacity of Aleppo pine (Pinus halpensis Mill.) seedlings to overcome the planting shock in dry conditions was assessed by firstly studying the survival and water status during the first year after planting in relation to soil water content. In spite of receiving only 177 mm rainfall during the year studied, survival of planted Aleppo pine was very high (88.5%). Soil water during summer months (after receiving 67 mm rainfall in winter and spring) was only available at 30–60 cm depth, with tension values of –1.1 and –1.3 MPa in July and August respectively; in these conditions, July predawn xylem water potential measured was –2.5 MPa, and midday potential was –3.6 MPa. According to different authors, these values don't jeopardize the survival of Aleppo pine. In addition, acclimation of outplanted seedlings to environmental conditions was followed by comparing their water relations with those of naturally regenerated seedlings on the site. Predawn and midday xylem water potential showed differences in favour of outplanted seedlings since June, indicating an adjustment to this dry site. Compared to naturally regenerated trees, nursery grown stock of the same age before field planting had much more biomass and higher N and P concentrations and contents; although shoot:root balance and Dickson quality index were not significantly different. Finally, planted seedlings acclimation level during first year was also evaluated by Transplant Stress Index, which value (–0.1278) indicated a slight planting impact.     

海拉尔西山30a生不同密度沙地樟子松人工林调查研究

对海拉尔西山30a生不同密度沙地樟子松人工林生长状况和土壤水分进行对比分析,结果证实不同密度沙地樟子松人工林的土壤含水量,林分结构、组成、更新能力均存在显著差异。初步认为海拉尔西山密度为7株/100m2或8株/100m2(即47株/亩或53株/亩)的30a生沙地樟子松人工林生长状况和综合效益较好。随着单株个体的生长,不同密度沙地樟子松人工林易引起光、水分及养分的竞争,不利于林分健康而稳定生长,因此,进入中龄期后,必须对初植密度进行人为调控,才能保证沙地樟子松人工林可持续、健康生长与林分的稳定性。     

Performance of Grevillea robusta in plantations and on farms under varying environmental conditions in Rwanda

The growth of Grevillea robusta A. Cunn. ex R.Br. in plantations and on farms was assessed on 67 sites selected at random in the major ecological zones of Rwanda. The following parameters were assessed: diameter at breast height, tree height, altitude above sea level, annual rainfall and temperature, soil pH, soil type, soil fertility, soil depth, spacing and forest type (plantation or farm).

Analyses of variance demonstrated that fertile soil, intercropping and cultivation between trees considerably improved growth performance. However, growth was negatively correlated with altitude, with stands above 2300 m above sea level showing poor growth. In Rwanda with proper management, the species can be grown in plantations and on farms for firewood, poles and sawlogs.     

Soil moisture and water use by pastures and silvopastures in a sub-humid temperate climate in New Zealand

Soil moisture content from 0 to 2 m depth was monitored under 2–6 year old radiata pine (Pinus radiata) with three understoreys of bare ground, lucerne (Medicago sativa) and ryegrass/clover (Lolium perenne/Trifolium spp.) and under adjacent open-grown lucerne and ryegrass/clover pastures. By the fifth year soil moisture depletion/recharge pattern under the trees alone was similar to that under open pasture and under trees with pasture understoreys. Maximum plant available moisture storage was 207–223 mm in the top meter of this Templeton silt loam soil but only 69–104 mm at 1–2 m depth where coarse textures often predominated. Lucerne reduced soil moisture content (SMC) to lower levels during drier summers and extracted more water from 1 to 2 m depth than ryegrass/clover. Evapotranspiration (ET) during early summer when soil moisture was high was close to the Penman potential evapotranspiration (E _p ), but the difference increased when SMC in the top meter dropped below 200 mm. The silvopasture treatments had higher ET in winter than pasture alone but this was still less than E _p . Soil moisture deficits (SMD) at the end of each summer were sufficiently large to require slightly higher than normal winter rainfall and ET < E _p to recharge the soil to field capacity before the next summer. The soil moisture results, taken together with root and growth data, suggest that trees and understorey pastures are complementary in the first three or four growing seasons but this balance subsequently declines in favor of the pine trees. Management options, to extend the period that understorey pastures are productive, include reducing tree stockings, more vigorous pruning, using competitive understoreys and changing from pines to deciduous trees. Research on new silvopastoral combinations is suggested.     

First-year survival and height-growth of red ceiba following various site preparation techniques on vertic soils in northern Colombia

If growth of red ceiba, Bombacopsis Quinata (Jacq.) Dugand, is to be increased in plantations established on Vertisols, site preparation techniques must be identified to overcome the soil management difficulties of these soils. This research was conducted to identify specific site preparation techniques appropriate for Vertisols that increase survival and height growth of red ceiba. The effects of subsoiling within the row (prior to planting) to depths of 40 and 60 cm, subsoiling between the rows, and additions of organic mill waste on red ceiba survival and height growth, soil water content, bulk density, and penetrometer resistance were investigated. The soil was a very fine, montmorillonitic, isohyper-thermic Entic Chromustert. No difference in survival due to subsoiling was found, but the additions of mill waste significantly decreased survival after 1 year (94.7%) compared to the control subplot (98.7%). Height was increased by subsoiling to the 40-cm depth (92.3 cm) and to the 60-cm depth (97.4 cm) over no subsoiling (80.7 cm). Trees on the subplots receiving mill waste were significantly lower in height (83 cm) compared to the control subplots (93.1 cm) and the subsoiling between the planting rows subplots (94.2 cm). Survival was most highly correlated with bulk density at the 50-cm depth within the planting row (r=–0.44) and height with soil water content at the 30-cm depth within the planting row (r=–0.56). Subsoiling affected height more than survival. Subsoiling to 40 cm is the most cost-effective treatment. Mill waste addition did not improve height or survival, possibly due to harmful decomposition by-products.     

Water-use strategies in two co-occurring Mediterranean evergreen oaks: surviving the summer drought

In the Mediterranean evergreen oak woodlands of southern Portugal, the main tree species are Quercus ilex ssp. rotundifolia Lam. (holm oak) and Quercus suber L. (cork oak). We studied a savannah-type woodland where these species coexist, with the aim of better understanding the mechanisms of tree adaptation to seasonal drought. In both species, seasonal variations in transpiration and predawn leaf water potential showed a maximum in spring followed by a decline through the rainless summer and a recovery with autumn rainfall. Although the observed decrease in predawn leaf water potential in summer indicates soil water depletion, trees maintained transpiration rates above 0.7 mm day(-1) during the summer drought. By that time, more than 70% of the transpired water was being taken from groundwater sources. The daily fluctuations in soil water content suggest that some root uptake of groundwater was mediated through the upper soil layers by hydraulic lift. During the dry season, Q. ilex maintained higher predawn leaf water potentials, canopy conductances and transpiration rates than Q. suber. The higher water status of Q. ilex was likely associated with their deeper root systems compared with Q. suber. Whole-tree hydraulic conductance and minimum midday leaf water potential were lower in Q. ilex, indicating that Q. ilex was more tolerant to drought than Q. suber. Overall, Q. ilex seemed to have more effective drought avoidance and drought tolerance mechanisms than Q. suber.     

Observations of evapotranspiration in a break of slope plantation susceptible to periodic drought stress

Break of slope (BOS) plantations are advocated as a means of water table control in areas where groundwater flows through colluvial deposits overlying low permeability bedrock. It is also believed that BOS plantations can supplement their water use requirements by exploiting shallow groundwater at the breaks in topographic slope. We compared measurements of BOS plantation and pasture evapotranspiration during spring, when the weather was warm and soils moist, and late summer when drought conditions prevailed. Microlysimeters and ventilated chambers were used to determine pasture and plantation floor evaporation, and heat pulse sensors were used to determine transpiration of the plantation. In spring, pasture evapotranspiration was 65% of that of the plantation, whereas, in summer, pasture evapotranspiration was equivalent to only 35% of that of the plantation. Rainfall interception by the canopy of the plantation was twice that of the pasture, reinforcing the notion that trees can help reduce groundwater recharge and alleviate dryland salinity and waterlogging. During the summer drought period, daily plantation transpiration was only 20% of that measured during spring, suggesting that the plantation was not utilizing groundwater supplies but was instead drawing from soil water supplies. This hypothesis was supported by the comparison of relative abundances of the isotopes of water ((2)H and (18)O) in soil and wood samples. We conclude that the BOS plantation is not behaving in the manner predicted, and our findings raise doubts about the predicted advantages of establishing plantations in break of slope positions.     

The annual pattern of sap flow in two Eucalyptus species established in the vicinity of gold-mine tailings dams in central South Africa

Several hundred mine tailings dams occur in the Witwatersrand Basin Goldfields in central South Africa. Seepage of acid mine drainage (AMD) from these unlined structures is widespread, and a variety of contaminants is released into soil and groundwater. The ‘Mine Woodlands Project’ is aimed at evaluating the use of high-density tree stands over surrounding contaminant plumes to limit the spread of contaminants through hydraulic control of groundwater and enhanced uptake or immobilisation of contaminants. The annual pattern of hourly sap flow in four contiguous Eucalyptus dunnii trees (aged three years) was followed over a full year in a species trial situated near Carltonville. The annual pattern of hourly sap flow was also recorded in four contiguous sample trees (aged four years) of the clonal hybrid E. grandis × E. camaldulensis (E. G×C) at another trial near Orkney. Both species showed high sap flow rates close to reference evaporation rates in response to summer rains. Both showed greatly reduced sap flow rates during the latter half of the dry winter season. Sap flow rates only recovered after the arrival of the first spring rains. Annual sap flow (E. dunnii, 673 mm; E. G×C, 767 mm) was similar to the recorded annual rainfall at each site (E. dunnii, 629 mm; E. G×C, 795 mm), and was substantially lower than total annual reference evaporation (E. dunnii, 1 273 mm; E. G×C, 1 330 mm). We conclude that the roots of both species are not yet deep enough to access the AMD-influenced groundwater, which lies at depths of 14 and 10 m below the ground at the Carltonville and Orkney sites, respectively. Despite prolonged water deficits, both species survived well and maintained sufficient vigour to permit the quick recovery of high transpiration rates in the following summer. This resilience is essential to hasten root growth and improve the chance of contact with groundwater plumes.     

Separating rhizosphere respiration from total soil respiration in two larch plantations in northeastern China

The potential capacity of soil to sequester carbon in response to global warming is strongly regulated by the ratio of rhizosphere respiration to respiration by soil microbial decomposers, because of their different temperature sensitivities. To quantify relative contributions of rhizosphere respiration to total soil respiration as influenced by forest stand development, we conducted a trenching study in two larch (Larix gmelini (Rupr.) Rupr.) plantations, aged 17 and 31 years, in northeastern China. Four plots in each plantation were randomly selected and trenched in early May 2001. Soil surface CO2 effluxes both inside and outside the plots were measured from May 2001 to August 2002. Soil respiration (i.e., the CO2 effluxes outside the trenched plots) varied similarly in the two plantations from 0.8 micromol m(-2) s(-1) in winter to 6.0 micromol m(-2) s(-1) in summer. Rhizosphere respiration (i.e., CO2 efflux outside the trenched plots minus that inside the plots) varied from 0.2 to 2.0 micromol m(-2) s(-1) in the old forest and from 0.3 to 4.0 micromol m(-2) s(-1) in the young forest over the seasons. Rhizosphere respiration, on average, accounted for 25% of soil respiration in the old forest and 65% in the young forest. Rhizosphere and soil respiration were significantly correlated with soil temperature but not with soil water content. We conclude that the role forests play in regulating climate change may depend on their age.     

Can agricultural management emulate natural ecosystems in recharge control in south eastern Australia?

Water balance information at locations across southern Australia is analysed to identify mechanistic causes for higher water tables under agricultural systems compared to natural vegetation. Contrasting patterns of water use indicate pronounced physiological activity in summer by natural ecosystems ensuring the persistence of the dominant perennial species. A strategy of tempered water use during periods of rainfall excess in winter/spring enables seasonal carryover of soil water to be withdrawn by deep roots for summer functioning. Characteristic patterns of water use by agricultural systems, comprised mainly of determinant annual species, are truncated in time but feature elevated peak rates coinciding with periods of maximum soil water availability during the cooler months. Low seasonal vapour pressure deficit explains the observed benefits of enhanced biomass production due to high water use efficiency. The associated limited scope for soil water uptake carries the penalty of increased frequency of drainage and the subsequent resource degradation currently associated with farmland. Inclusion of a perennial lucerne phase in rotation with crops confers environmental benefit through summer uptake from a rooting depth double that of crops. An effective buffer is thus provided to restrict drainage under an ensuing cropping phase in a manner that more closely emulates natural systems. Simulations of phase farming point to a halving in long term drainage. They suggest adoption over a range of arable land units could cause local retreat of water tables by 0.3 m y⁻¹ initially, leading to a new equilibrium level which would still remain elevated with respect to that under natural vegetation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Seasonality of vertically partitioned soil CO<Subscript>2</Subscript> production in temperate and tropical forest

Soil CO₂ production seasonality at a number of depths was investigated in a temperate forest in Japan and in a tropical montane forest in Thailand. The CO₂ production rates were evaluated by examining differences in the estimated soil CO₂ flux at adjacent depths. The temperate forest had clear temperature seasonality and only slight rainfall seasonality, whereas the tropical montane forest showed clear rainfall seasonality and only slight temperature seasonality. In the temperate forest, the pattern of seasonal variation in soil respiration was similar at all depths, except the deepest (0.65 m–), and respiration was greater in summer and less in winter. The contribution of the shallowest depth (around 0.1 m) was more than 50% of total soil-surface CO₂ flux all year round, and the annual mean contribution was about 75%. CO₂ production mostly appeared to increase with temperature in shallower layers. In contrast, in the tropical forest, soil CO₂ production seasonality appeared to differ with depth. The CO₂ production rate in the shallowest layer was high during the rainy season and low during the dry season. Soil CO₂ production at greater depths (0.4 and 0.5 m–) showed the opposite seasonality to that in the shallower layer (around 0.1 m). As a result, the contribution from the shallow depth was greatest in the tropical forest during the rainy season (more than 90%), whereas it decreased during the dry season (about 50%). CO₂ production appeared to be controlled by soil water at all depths, and the different ranges of water saturation seemed to cause the difference in seasonality at each depth. Our results suggest the importance of considering the vertical distribution of soil processes, particularly in areas where soil water is a dominant controller of soil respiration.     

Effects of <Emphasis Type="Italic">Eucalypt</Emphasis> and <Emphasis Type="Italic">Acacia</Emphasis> plantations on soil water in Sumatra

Indonesia’s pulp and paper industry needs a large area of sustainably grown plantations to support its continued development. Acacia mangium has been the key species underpinning the pulp and paper industries in Sumatra, however increased disease pressure on A. mangium is expected to require large-scale conversion of Acacia plantations to Eucalyptus in the near future. The effect of such a large scale change in plantation species on soil moisture, for both tree production, and catchment hydrology is unknown. In this study we sought to characterize the impacts of plantation species (Acacia or Eucalyptus) and nitrogen management, on soil moisture, soil water depletion and depth to groundwater under stands of Acacia mangium and Eucalyptus pellita over the first 2–3 years after establishment. The study was conducted in experiments at four sites in Sumatra, Indonesia. Soil moisture and soil water depletion were not influenced by plantation species or fertilizer treatment. Soil moisture content and soil water depletion were strongly influenced by shallow groundwater at two of the four sites, however depth to groundwater did not influence stem growth. Results from the field trials cautiously suggest that large scale conversion of Acacia mangium to Eucalypt species in these regions is unlikely to result in increased moisture stress, nor is conversion of plantation species likely to lead to substantial differences in catchment hydrology. This study demonstrated the importance of conducting multi-site studies when investigating biophysical relationships in forest/plantation systems.     

A six-year study of sapling and large-tree growth and mortality responses to natural and induced variability in precipitation and throughfall

Global climatic change may cause changes in regional precipitation that have important implications for forest growth in the southern United States. In 1993, a stand-level experiment was initiated on Walker Branch Watershed, Tennessee, to study the sensitivity of forest saplings and large trees to changes in soil water content. Soil water content was manipulated by gravity-driven transfer of precipitation throughfall from a dry treatment plot (-33%) to a wet treatment plot (+33%). A control plot was included. Each plot was 6400 m2. Measurements of stem diameter and observations of mortality were made on large trees and saplings of Acer rubrum L., Cornus florida L., Liriodendron tulipifera L., Nyssa sylvatica Marsh, Quercus alba L. and Quercus prinus L. every 2 weeks during six growing seasons. Saplings of C. florida and A. rubrum grew faster and mortality was less on the wet plot compared with the dry and control plots, through 6 years of soil water manipulation. Conversely, diameter growth of large trees was unaffected by the treatments. However, tree diameter growth averaged across treatments was affected by year-to- year changes in soil water status. Growth in wet years was as much as 2-3 times greater than in dry years. Relationships between tree growth, phenology and soil water potential were consistent among species and quantitative expressions were developed for application in models. These field growth data indicate that differences in seasonal patterns of rainfall within and between years have greater impacts on growth than percentage changes in rainfall applied to all rainfall events.     

Concentrations of nitrate in soil water following herbicide treatment of tree planting positions in an upland agroforestry system

Concentrations of nitrate have been measured in soil water samples collected during the first year of an agroforestry trial at an upland site in Wales. Nitrate concentrations were higher in water samples collected from herbicide-treated areas around trees than from undisturbed pasture between the trees. These differences were statistically significant fo samples collected during the summer but not for those from the main winter leaching period. The enhanced nitrate concentrations that were measured beneath the tree planting positions may have implications for the nutrition of the trees but, at the planting densities included in the trial (100 and 400 trees/ha), are estimated to represent only a small increase in the nitrate content of water draining from the agroforestry area as a whole, relative to that from unplanted pasture. There was some evidence that preferential deposition of urine, but not of dung, contributed to the increased nitrate contents in the soil around the planting positions.     

Competition for light, water and nitrogen in an association of hazel (Corylus avellana L.) and cocksfoot (Dactylis glomerata L.)

Competition for light, water and nitrogen between hazel trees and cocksfoot grown under the trees was studied in two hazel plantations managed in different ways. The first plantation with a light transmission of at least 70% at grass level was compared with two control hazel and cocksfoot monocrops. The soil was a calcareous heavy clay, 1.4 m deep. Water stress during summer was severe in the intercropped stand as well as the grass- only plots, while it was much less in the sole stand of trees (without grass). Hazel tree water potential in the intercropped stand improved in the fourth year as the trees extended their roots similarly to the grass-free ones, despite their much lower canopy growth rate. The fourth year, local N fertilisation began to be effective as evidenced by the N content per leaf area unit and growth of the intercropped trees: grass root competition was high and root barriers were needed to exclude it. The second plantation was managed on a deep soil with a water table at approximately 2.5 m depth. The trees did not suffer from the presence of grass, and allowed a 15 to 75% light transmission rate. A control monocropped grass plot was established for comparison. On the basis of the multiple limitation hypothesis, a grass dry matter production model was built which accounts for radiation transmitted to the understorey, water supply, temperature and air moisture deficit in interaction with the radiation transmission rate (r2 = 0.716; 590 observations). The limits of such models are discussed with regard to insufficient knowledge on root dynamics. This revised version was published online in June 2006 with corrections to the Cover Date.