Effects of competition and indirect facilitation by shrubs on <Emphasis Type="Italic">Quercus robur</Emphasis> saplings

Indirect facilitation by shrubs has been suggested as a cost-effective way of regenerating oaks in forests of conservation interest. In this study, we tested whether shrubs can enhance growth in pedunculate oak (Quercus robur) by suppressing herbaceous competitors. We studied interactions between young oaks, shrubs, and/or herbaceous vegetation in an open-field experiment, in southern Sweden, over the first 3 years after planting. Oak saplings were grown in four competition treatments: no competing vegetation; with herbaceous vegetation; with shrubs; and with both herbaceous vegetation and shrubs. Competition from shrubs and herbaceous vegetation both reduced stem diameter and biomass accumulation, but they affected biomass partitioning differently. Saplings grown with competition from shrubs partitioned biomass primarily into height growth, while those saplings exposed to competition from herbaceous vegetation invested a relatively higher proportion in root growth. Competition between shrubs and herbaceous vegetation reduced the above-ground biomass of the herbaceous vegetation, resulting in an indirect facilitative effect for the oaks during the first 2 years after planting. However, during the third year, shrubs had a negative effect on biomass accumulation. In summary, results from this study suggest that shrubs indirectly facilitate biomass accumulation of oak saplings by suppressing herbaceous vegetation, possibly by reducing competition for below-ground resources. However, owing to the relatively short duration of positive net outcome for the oak, we recommend that a longer-term assessment of the interaction between oak regeneration and neighboring shrubs be made before the outcome of this study is applied to practical forestry.     

A predictive model to locate ancient forests in the Cross Timbers of Osage County, Oklahoma

The Cross Timbers are a mosaic of upland deciduous forest, savanna, and glade that typifies the broad ecotone between the eastern deciduous forest and the grasslands of the southern Great Plains. The pre-settlement Cross Timbers may have covered some 7,909,700 ha from central Texas, across Oklahama into eastern Kansas, and today may represent the least disturbed forest ecosystem of comparable size still left in the eastern United States. Extensive tree-ring research indicates that ancient forests dominated by 200 to 400 year old post-oaks ( Quercus stellata Wang) survive throughout the Cross Timbers, particularly in Oklahoma. These ancient forests persist largely because the Cross Timbers formation is non-commercial for timber production, and has not experienced large-scale industrial logging. Because ancient forest relics are often found on stressful non-commercial sites in the Cross Timbers and elsewhere, it is possible to design predictive models to locate the specific terrain where undisturbed forests are likely to survive. A predictive model for southern Osage County, Oklahoma, was developed based on the steep, infertile soils of the Niotaze-Darnell complex. We tested the model with field inspection and tree-ring analysis of fifty randomly selected belt transects, and 74% of the sampled terrain is still old-growth Cross Timbers woodland. This translates into 8200 ha of ancient Cross Timbers on this single site type in southern Osage County. The abundance of ancient forest in the Cross Timbers is not widely appreciated. However, large contiguous tracts of ancient Cross Timbers up to 700 ha were identified with this predictive model, strongly supporting inferences concerning the relatively undisturbed nature of this ecosystem.     

Tree seedling survivorship,growth, and allocation in the Cross Timbers ecotone of Oklahoma,USA

In order to better understand tree dynamics and maintenance of the ecotone between eastern deciduous forest and tallgrass prairie, I planted seedlings of five different species into a Cross Timbers area in North West Oklahoma for one-year. The seedlings were planted in four different patch-types under two different herbivore treatments. I found that (1) out of the original 200 seedlings, 58 survived after one-year with lacebark elm (Ulmus parrifolia), osage orange (Maclura pomifera), and eastern redbud (Cercis canadensis) surviving the most and protection from herbivores increasing survivorship by more than an order of magnitude, (2) elm (Ulmus parrifolia) showed the slowest stem growth but protection from herbivory increased stem growth rate by approximately 50%, leaf area ratio was largest for elm (Ulmus parrifolia) but seedlings growing in sumac (Rhus copallina) patches had the smallest values, specific leaf area was the largest for orange (Maclura pomifera) seedlings with tree seedlings growing in grass patches showing greater values than those grown in other patches, and (3) leaf mass ratio was largest for elm (Ulmus parrifolia) with seedlings grown in sumac (Rhus copallina) patches showing the smallest values and rough-leaf dogwood (Cornus drummondii) had the most root biomass relative to shoot biomass compared to all other species. Results suggest tree invasion and establishment across this ecotone is influenced both by species identity and by the variation in resources associated with the distribution and patch dynamics of vegetation from both eastern deciduous forest and tallgrass prairie.     

The role of seed predation in the maintenance of the Cross Timbers ecotone of Oklahoma,USA

I investigated how seed predation differed among tree species and among microhabitats across the Cross Timbers and what that variation may tell us about how this ecotone is maintained. The ecotone is located in Oklahoma, USA, between the eastern deciduous forest and tallgrass prairie where seeds of eight common tree species were placed in three microhabitats (oak forest, tallgrass prairie, and sumac shrub/small-tree/grass mix). After nine days in the field, percent seeds remaining were scored for each of the 120 (8 species×3 microhabitats×5 replicates) dishes. I found for both wind-dispersed tree species, (ash, elm) there was significantly more predation in the prairie microsite, with similar small predation levels in the shrub and forest. For two of the three bird-dispersed species (dogwood, hackberry), there was significantly more predation in the prairie and shrub microsites compared to the forest. Red cedar, however, was not taken by predators very much anywhere. Finally, all three mammal-dispersed tree species (two oaks, pecan) showed significantly more predation in the shrub and forest microsites compared to the prairie. Whereas wind- and bird-dispersed species suffered less predation as microsites became more woody and dark, the dominant oaks showed the opposite trend. Consequently, seed predators are not preventing oaks from advancing across this ecotone, but yearly fluctuations in predator population density, especially in the shrub transitional zone, could be helping to maintain it.     

Love of Nurse Plants is Not Enough for Restoring Oak Forests in a Seasonally Dry Tropical Environment

The highest concentration of oak species in the world occurs in Mexico, but human activities have strongly degraded these oak forests. Mexican oaks have high economic, social, and cultural value, and restoring these forests is of paramount importance for the people of Mexico. Here, we propose a method for restoring oak forests using native shrubs that colonize degraded areas as nurse plants for oak seedlings. To test the viability of this proposal, seedling transplant experiments were performed in a degraded area near a protected oak forest relict. Two pioneer shrubs were identified as potential nurse species: Mimosa luisana and Senecio sp. The target oak species was Quercus castanea . Oak seedlings were located beneath the canopies of both shrubs and in the surrounding area without shrub cover. Water is a limiting resource for oak establishment in seasonally dry environments; therefore, we included irrigation systems in our experimental design to determine whether the combination of nurse plants plus watering led to higher rate of survival than the presence of nurse species alone. Seedling survival without watering was less than 20% both beneath nurse species and in the surrounding habitat. When water was supplied, survival rate beneath nurse species increased up to 58% while survival in the surrounding habitat did not differ from that observed in treatments without watering. Our results indicate that survival rate of oak seedlings is increased by the presence of nurse plants only when water is supplied. This suggests that restoration of oak forests in these degraded areas requires both nurse plants and watering.     

Tornado effects on damage,resprouting and spatial heterogeneity in the Cross Timbers ecotone of Oklahoma,USA

Aims In order to better understand how tornados structure the ecotone between eastern deciduous forest and tallgrass prairie, we sampled trees in a recent tornado blowdown area and in the adjacent undisturbed forest. Both are part of the Cross Timbers ecotone of Osage County, Northeastern Oklahoma, USA.Methods We set up a 1-ha square plot in each area and sampled all tree stems at least 50 cm tall where stems were identified to species and measured for size and spatial location in both plots. For the stems in the blowdown plot, we also scored damage and resprouting. We then used the spatial location data to compute the spatial heterogeneity and degree of clumping in both areas.Important findings Significant clumping was observed in the control forest at the small spatial scale of 1–8 m and at the medium spatial scale of 30–36 m. The blowdown area: showed domination by post oak (Quercus stellata) with small stems, branch damage and stem resprouting above 1 m most common; had no trees that either lost their leaves without branch or stem damage or lived without some sort of resprouting, had a significant negative correlation between damage and resprouting and clumped only at small spatial scales for stems of medium size, for stems with branch damage and for stems that resprouted at or below 1 m height. We conclude that in addition to largely eliminating a forest's spatial heterogeneity, tornados may also structure this ecotone by increasing tree persistence through resprouting.     

Seasonal physiology and growth of bottomland oaks of differing planting stocks in afforestation sites on the U.S. Gulf Coastal Plain

Hurricane Katrina caused large losses of bottomland hardwood forests on the Gulf Coastal Plain. Heavy‐seeded species such as oaks (Quercus) generally require direct planting for restoration after such losses. However, evaluating the performance of various oak planting stocks using biometric data alone can be challenging due to their slow juvenile growth and belowground carbon allocation. Our study goals were to evaluate physiological parameters including photosynthesis, stomatal conductance, and water‐use efficiency (WUE) and their correlation with annual height growth to determine differences in functional performance and drought tolerance between seedling types and whether physiology can predict height growth. Monthly growing season gas exchange measurements were made on two oak species (Quercus texana and Quercus shumardii) and three planting stocks (bare root, conventional containerized, large containerized [LC]) planted on two sites in coastal Mississippi. We found that photosynthesis decreased throughout the growing season while stomatal conductances increased leading to decreasing WUEs in all seedling types. Physiological parameters differed across planting stocks but not species. Particularly, LC seedlings exhibited greater WUEs and sensitivity to vapor pressure deficit (VPD) suggesting increased moisture stress compared with other planting stocks. Across seedling types, photosynthesis, stomatal sensitivity to VPD, and seasonal changes in intrinsic WUE measured in year one of the study were significantly correlated with year two, but not year one height growth, suggesting belowground allocation of carbon during the first growing season. In total, these results highlight the use of physiological measurements in selecting successful planting stocks for various site conditions.     

地上竞争与地下竞争对科尔沁沙地榆树幼苗生长的影响

榆树疏林草原对科尔沁沙地植被恢复和景观保护有着重要意义.本文采用双因素两水平控制试验,从幼苗生物量、地下/地上生物量、茎高、根茎比、叶片数等方面,研究了草-树地上、地下竞争对科尔沁沙地榆树幼苗生长的影响.结果表明:对于1年生榆树幼苗,单株平均生物量表现为无竞争＞地上竞争＞全竞争＞地下竞争;地下/地上生物量表现为地下竞争＞全竞争＞无竞争＞地上竞争;幼苗高度表现为地上竞争＞无竞争＞全竞争＞地下竞争;根茎比表现为地下竞争＞全竞争＞无竞争＞地上竞争;叶片数表现为地上竞争＞无竞争＞地下竞争＞全竞争.地下竞争对1年生榆树幼苗生长影响显著,而地上竞争对榆树幼苗生长无显著影响.地上竞争与地下竞争对2年生榆树幼苗生长的影响均不显著.科尔沁沙地草本植物对榆树幼苗生长的影响主要通过地下竞争的方式实现,但地下竞争并没有改变榆树幼苗的资源分配方式.随榆树幼苗龄级的增长,草本植物竞争作用的影响逐渐减弱.     

Seedling survivorship,growth, and response to disturbance in Belizean mangal

Species zonation patterns across tidal gradients in mangrove forests are formed by successful seedling establishment and maintained by replacement of adults by conspecific seedlings. These two processes rarely have been examined experimentally in neotropical mangal. We studied survivorship and growth of seedlings of two species of mangrove, Rhizophora mangle L. and Avicennia germinans (L.) Steam, across a tidal gradient in Belize, Central America. Propagules of each species were planted in common gardens at tidal elevations corresponding to lowest low water (LLW), mean water (MW), and highest high water (HHW). Sixty-nine percent of Rhizophora seedlings planted at MW and 56% of those planted at LLW survived 1 year. Forty-seven percent of MW Avicennia seedlings also survived 1 year. No individuals of either species survived at HHW, and neither did any LLW Avicennia seedlings. Among the surviving Rhizophora seedlings, LLW seedlings grew more rapidly in terms of height, diameter, leaf production, and biomass than did MW seedlings. Insect herbivory was twice as high on MW seedlings as on LLW Rhizophora seedlings. We also examined the response of established Rhizophora seedlings to experimental removal of the adult Rhizophora canopy. Seedlings in canopy removal areas had higher survivorship, grew twice as fast, produced more leaves, and had less than half the herbivory of seedlings growing beneath an intact canopy. These results provide insights into underlying causes and maintenance of zonation in Caribbean mangrove forests.     

Competition by herbs as a limiting factor in shrub invasion in grassland: a test with different growth forms

Abstract. We tested the hypothesis that seedling establishment, the critical stage in the invasion of grassland by shrubs, is limited by competition with perennial grasses in seasonally wet/dry savannas. We placed seeds of two invasive exotic shrubs – Cryptostegia grandiflora, a woody vine, and Acacia nilotica, an arborescent legume – into pots with a wide range of existing above- and below-ground herbaceous biomass provided by either a tussock or a stoloniferous perennial grass. We also imposed different levels of watering frequency (5, 10 and 21 d), nutrient addition (+ and -) and grass clipping intensity (no clipping, clipped to 5 cm and clipped to 25 cm). There was no effect of any treatment on shrub seedling emergence or survival and all of the seedlings that emerged survived the 90-d growing period. Herbaceous competition also failed to have an effect on biomass accumulation in shrub seedlings. More frequent watering significantly increased above- and below-ground biomass accumulation for both shrub species and nutrient addition significantly increased Cryptostegia biomass accumulation. Based on these results, we question the proposition that reduction in competition by herbs via livestock grazing has been a significant factor in determining the rate or pattern of exotic shrub increase in the seasonally wet/dry tropics. We also question the suitability of the two-layer soil moisture hypothesis as a basis for management practices to control the ingress of woody species into grasslands and open savannas.     

植物群落类型和围栏封育对高山林线岷江冷杉幼苗成活的影响

林线树种幼苗的生长紧密关系着高山林线的形成。通过室内发芽实验测定了岷江冷杉种子萌发能力。2015年春,在川西巴郎山岷江冷杉林线之上的高山林线交错带,通过播种处理(播种和不播种),研究了不同植物群落类型(灌木和草地群落)和围栏处理(围栏和不围栏)对岷江冷杉幼苗成活率的影响,以期为进一步探讨高山林线形成机制提供科学参考。结果表明:(1)岷江冷杉室内发芽率31.4%,成活率随时间下降。(2)灌木群落的全年日平均空气温湿度显著高于草本群落,而两群落的全年日平均土壤温度无显著差异。(3)未播种样地没有发现岷江冷杉幼苗,说明林线交错带缺乏岷江冷杉种子。(4)与不围栏样地相比,围栏封育分别显著提高了岷江冷杉幼苗成活率2.0%(2015年)和2.2%(2016年);与灌木群落相比,草本群落中岷江冷杉幼苗成活率显著提高0.8%(2015年)和1.2%(2016年),说明灌木对林线交错带幼苗更新具有更强的竞争作用。围栏处理下,草本群落中岷江冷杉幼苗成活率显著高于灌木群落;不围栏处理下,草本群落中岷江冷杉幼苗成活率显著低于灌木群落,2017年在不围栏样地没有幼苗存活,说明动物干扰对林线交错带森林更新有阻碍作用。研究表明,高山林线交错带森林更新,受种子、植物竞争和动物干扰(例如,放牧)等多方面因素的共同影响,可以通过播种、植物剔除(去除竞争)和围栏等促进幼苗更新和成活。     

Effects of above- and below-ground competition from shrubs on photosynthesis, transpiration and growth in Quercus robur L. seedlings

For a tree seedling to successfully establish in dense shrubbery, it must maintain function under heterogeneous resource availability. We evaluated leaf-level acclimation in photosynthetic capacity, seedling-level transpiration, and seedling morphology and growth to gain an understanding of the effects of above- and below-ground competition on Quercus robur seedlings. Experimental seedlings were established in a typical southern Swedish shrub community where they received 1 of 4 competition levels (above-ground, below-ground, above- and below-ground, or no competition), and leaf-level responses were examined between two growth flushes. Two years after establishment, first-flush leaves from seedlings receiving above-ground competition showed a maximum rate of photosynthesis (A_max) 40% lower than those of control seedlings. With the development of a second flush above the shrub canopy, A_max of these seedlings increased to levels equivalent to those of seedlings free of light competition. Shrubby competition reduced oak seedling transpiration such that seedlings exposed to above- and below-ground competition showed rates 43% lower than seedlings that were not exposed to competition. The impaired physiological function of oak seedlings growing amid competition ultimately led to a 60-74% reduction in leaf area, 29-36% reduction in basal diameter, and a 38-78% reduction in total biomass accumulation, but root to shoot ratio was not affected. Our findings also indicate that above-ground competition reduced A_max, transpiration and biomass accumulation more so than below-ground competition. Nevertheless, oak seedlings exhibited the ability to develop subsequent growth flushes with leaves that had an A_max acclimated to utilize increased light availability. Our findings highlight the importance of flush-level acclimation under conditions of heterogeneous resource availability, and the capacity of oak seedlings to initiate a positive response to moderate competition in a shrub community.     

Composted sewage sludge can improve the physiological properties of<Emphasis Type="Italic">Betula schmidtii</Emphasis> grown in tailings

We investigated how the application of composted sewage sludge to tailings affects the physiological response of woody plants growing on abandoned coal-mining sites. Twenty seedlings ofBetula schmidtii were transplanted to pots containing various combinations of artificial soil plus nursery soil, tailings, composted soil, or tailings amended with composted soil. Dry weights, shoot to root ratios, relative growth rates (RGR), chlorophyll content and fluorescence, and carbohydrate concentrations were assessed at the end of the experiment. Growth responses differed significantly among soil types. For example, dry weights were greatest for seedlings grown in composted soil and smallest for plants raised in pure tailings. Shoot to root ratios were higher for seedlings in composted soil compared with those in either tailings or nursery soil. Leaf chlorophyll content was twice as high for seedlings from composted soil than for those in the nursery soil or tailings; chlorophyll fluorescence (Fv/Fm) was lower for seedlings in either nursery soil or tailings than for those in composted soil. In contrast, plants grown in either nursery soil or tailings had higher starch concentrations in their stems, whereas the carbohydrate allocation of seedlings in composted soil was highest in the leaves, followed by stems and roots. Overall, the carbohydrate content was highest in the leaves, except for seedlings treated with tailings. Therefore, we believe that composted soil can improve the physiological and biochemical properties of trees growing in tailings when appropriate nutrients are supplemented.     

Solute accumulation in leaves and roots of woody plants subjected to water stress

Summary Young seedlings of English Oak, Quercus robur L., and Silver Birch, Betula verrucosa Ehrl., were subjected to a number of consecutive periods during which water was withheld. During one 14-day period leaf-and soil-water potentials and leaf- and root-solute potentials of two groups of plants were sampled at noon of each day. One group of plants was watered every day while water was withheld from the other group. Solute accumulation in roots and leaves of oak seedlings subjected to water stress resulted in maintenance of turgor and high leaf conductance as the soil dried. In birch seedlings turgor was only maintained by stomatal closure at high soil water potential.Fourteen consecutive water stress cycles greatly reduced the growth of birch seedlings but had little effect on oak seedlings other than to alter root morphology. Water stress treatment resulted in the production of long thin roots in this plant. Stomatal behaviour in oak and birch seedlings during the 14-week stress period was consistent with observed changes in leaf water and solute potentials. Daily solute accumulation in oak leaves was presumably responsible for the maintenance of plant growth as water potentials fell.     

Shrub–tree interactions and environmental changes drive treeline dynamics in the Subarctic

Treelines have drawn persistent research interest as they can respond markedly to climate. However, the mechanisms that determine tree seedling recruitment and the response of the forest‐tundra ecotone to environmental changes remain poorly understood. We hypothesise that treeline tree seedling performance depends on the interplay between climatic and soil nutritional changes and facilitative and competitive interactions between trees and shrubs. We conducted a seedling transplantation experiment with Betula pubescens at a subarctic treeline, in northern Sweden, which followed a full factorial design with four treatment factors relating to environmental regimes of stress and resource availability: site (forest vs treeline); temperature (+/? passive warming); shrub presence (+/?Vaccinium myrtillus removal); and nutrient availability (+/? NPK addition). During three growing seasons we assessed the establishment and performance of Betula. The experimental manipulations caused highly significant effects on seedling performance. Although Vaccinium enhanced seedling survival and reduced the effects of excessive solar radiation and insect herbivory, the seedlings growing with the shrub had a poorer performance by the end of the experimental period. Also, seedlings in the forest had a poorer performance than those at the treeline. Betula seedlings showed a very pronounced and positive response to passive warming and to nutrient addition, but such effects were more evident at the treeline site and often interacted with the presence of Vaccinium. This experiment shows that shrub–tree interactions are important drivers of subarctic treeline dynamics and that they vary with time and space. Facilitation, competition, herbivory and environmental changes at the tree seedling stage act as important filters in structuring the forest–tundra ecotone. We demonstrate that changes in this ecotone cannot be simply predicted from changing temperature patterns alone, and that complex interactions need to be considered, not only between shrubs and trees, but also with herbivores and between warming and soil nutrient availability.     

模拟淹水对枫杨和栓皮栎气体交换、叶绿素荧光和水势的影响

研究了人工模拟淹水胁迫对两年生栓皮栎(Quercus variabilis)和枫杨(Pterocarya stenoptera)树苗的影响。经过70 d的淹水处理两种植物的存活率均为100%。淹水对两种植物生理生态过程的早期影响是快速降低了二者的最大净光合速率(P_max)、气孔导度(G_s)、最大光化学量子效率(F_v/F_m)。经过7 d的淹水处理后,受淹栓皮栎的最大净光合速率是对照的39%,枫杨是对照的42%;受淹栓皮栎的气孔导度是对照的38.8%,枫杨是对照的71.9%;水淹5 d后,枫杨和栓皮栎的最大光化学量子效率分别为0.694和0.757。但经过最初的下降后,枫杨的最大净光合速率、气孔导度和最大光化学量子效率逐渐恢复,而栓皮栎的则持续下降。到淹水70 d时栓皮栎的最大净光合速率下降了94.1%,最大光化学量子效率的平均值为0.537。在试验过程中,枫杨产生了有利于吸收氧气的不定根和肥大的皮孔,而栓皮栎没有产生不定根。随淹水时间的增加枫杨的叶绿素含量与对照没有显著差异;而栓皮栎的叶绿素含量在第33 d后大幅降低,Chla/Chlb的比值下降。淹水后第10 d和第70 d测定的清晨水势,受淹栓皮栎比对照高,而受淹枫杨比对照低,因此淹水导致的叶片水势的变化可能与树种相关。以上的试验结果表明受淹的栓皮栎的光合机构运转受到了严重影响,因此栓皮栎应属于对淹水较敏感的树种。从两种植物受淹水胁迫的形态和生理变化看,枫杨比栓皮栎更耐淹,更适合库塘消落区生境。     

Post-Planting Treatments Increase Growth of Oregon White Oak (Quercus garryana Dougl. ex Hook.) Seedlings

The extent of Oregon white oak woodland and savanna ecosystems in the U.S. Pacific Northwest has diminished significantly during the past century due to land use changes and fire suppression. Planting Oregon white oak seedlings is often necessary when restoring these plant communities. Our objective was to determine the efficacy of post‐planting treatments for establishing Oregon white oak seedlings on sites characterized by low growing season precipitation and coarse‐textured soils. We evaluated the effects of control of competing vegetation, tree shelters, fertilization, irrigation, and planting date on growth of planted seedlings. Survival was generally high (90%), but growth rate varied substantially among treatments. Plastic mulch increased soil water content and increased annual seedling height growth by an average of 56% relative to one‐time manual removal of competing vegetation. Solid‐walled tree shelters reduced browse damage and increased mean annual height growth compared to mesh shelters and no shelter by averages of 7.5 and 10.9 cm, respectively. Controlled‐release fertilizer applied at planting did not consistently increase seedling growth. Weekly irrigation (3.8 L/seedling) increased first‐year seedling growth only where mulch also was applied. Seedlings planted by late February had greater root growth by summer than those planted in early April. Soil water management was necessary for best seedling growth, and the improved height growth in solid‐walled tree shelters allowed the terminal shoot to grow more quickly above the height of animal browse. Our results indicate effective methods for establishing Oregon white oak seedlings, but these results may also be applicable to establishment of other tree species on similarly droughty sites.     

Root System Development of Oak Seedlings Analysed using an Architectural Model. Effects of Competition with Grass

A dynamic 3D model of root system development was adapted to young sessile oak seedlings, in order to evaluate the effects of grass competition on seedling root system development. The model is based on a root typology and the implementation of a series of developmental processes (axial and radial growth, branching, reiteration, decay and abscission). Parameters describing the different processes are estimated for each root type. Young oak seedlings were grown for 4 years in bare soil or with grass competition and were periodically excavated for root system observation and measurements (topology of the root system, length and diameter of all roots with a diameter greater than 0.3 mm). In the fourth year, 40 cm×20 cm×20 cm soil monoliths were excavated for fine root measurement (root density and root length). Root spatial development was analysed on a sub-sample of roots selected on four seedlings. The model was a guideline that provided a complete and consistent set of parameters to represent root system development. It gave a comprehensive view of the root systems and made it possible to quantify the effects of competition on the different root growth processes. The same root typology was used to describe the seedlings in bare soil and in grass. Five root types were defined, from large tap roots to fine roots. Root system size was considerably reduced by grass competition. Branching density was not affected but the branch roots were always smaller for the seedlings grown in competition. Reiteration capacity was also reduced by competition. Cross sectional areas before and after branching were linearly related with a scaling coefficient close to 1, as predicted by the pipe model theory. This relationship was not affected by grass competition.     

Gibberellic acid improves water deficit tolerance in maize plants

The combination effects of water stress and gibberellic acid (GA₃) on physiological attributes and nutritional status of maize (Zea mays L. cv., DK 647 F1) were studied in a pot experiment. Maize plants were grown in the control (well watered WW) and water stress subjected to treated both water stress and two concentrations of gibberellic acid (GA₃ 25 mg L⁻¹, 50 mg L⁻¹). WS was imposed by maintaining the moisture level equivalent to 50 % pot capacity whereas the WW pots were maintained at full pot capacity. Water stress reduced the total dry weight, chlorophyll concentration, and leaf relative water content (RWC), but it increased proline accumulation and electrolyte leakage in maize plants and appears to affect shoots more than roots. Both concentrations of GA₃ (25 and 50 mg L⁻¹) largely enhanced the above physiological parameters to levels similar to control. WS reduced leaf Ca²⁺ and K⁺ concentrations, but exogenous application of GA₃ increased those nutrient levels similar or close to control. Exogenous application of GA₃ improved the water stress tolerance in maize plants by maintaining membrane permeability, enhancing chlorophyll concentration, leaf relative water content (LRWC) and some macro-nutrient concentrations in leaves.