Root Gravitropism and Below-ground Competition among Neighbouring Plants: A Modelling Approach

Competition for nutrients among neighbouring roots occurs whentheir individual depletion volumes overlap, causing a reductionin nutrient uptake. By exploring different spatial niches, plantswith contrasting root architecture may reduce the extent ofcompetition among neighbouring root systems. The main objectivesof this study were: (1) to evaluate the impact of root architectureon competition for phosphorus among neighbouring plants; and(2) to compare the magnitude of competition among roots of thesame plant vs. roots of neighbouring plants. SimRoot, a dynamicgeometric model, was used to simulate common bean root growthand to compare the overlap of depletion volumes. By varyingthe gravitropism of basal roots, we simulated three distinctroot architectures: shallow, intermediate and deep, correspondingto observed genetic variation for root architecture in thisspecies. Combinations of roots having the same architectureresulted in more intense inter-plant competition. Among them,the deep-deep combination had the most intense competition.Competition between deep root systems and shallow root systemswas only half that of deep root systems competing with otherdeep root systems. Inter-plant root competition increased assoil diffusivity increased and the distance among plants decreased.In heterogeneous soils, co-localization of soil resources androots was more important in determining resource uptake thaninter-plant root competition. Competition among roots of thesame plant was three- to five-times greater than competitionamong roots of neighbouring plants. Genetic variation for rootarchitecture in common bean may be related to adaptation todiverse competitive environments. Copyright 2001 Annals of BotanyCompany Root architecture, phosphorus, competition, common bean, Phaseolus vulgaris L. nutrient uptake, gravitropism     

Effects of Ultraviolet-B Irradiance on Intraspecific Competition and Facilitation of Plants: Self-Thinning,Size Inequality,and Phenotypic Plasticity

(1) The effects of facilitation on the structure and dynamics of plant populations have not been studied so widely as competition. The UV-B radiation, as a typical environmental factor causing stress, may result in direct stress and facilitation. (2) The effects of UV-B radiation on intraspecific competition and facilitation were investigated based on the following three predictions on self-thinning, size inequality, and phenotypic plasticity: i) Self-thinning is the reduction in density that results from the increase in the mean biomass of individuals in crowded populations, and is driven by competition. In this study, the mortality rate of the population is predicted to decrease from UV-B irradiance. ii) The size inequality of a population increases with competition intensity because larger individuals receive a disproportionate share of resources, thereby leaving limited resources for smaller individuals. The second hypothesis assumes that direct stress decreases the size inequality of the population. iii) Phenotypic plasticity is the ability to alter one’s morphology in response to environmental changes. The third hypothesis assumes that certain morphological indices can change among the trade-offs between competition, facilitation, and stress. These predictions were tested by conducting a field pot experiment using mung beans, and were supported by the following results: (3) UV-B radiation increased the survival rate of the population at the end of self-thinning. However, this result was mainly due to direct stress rather than facilitation. (4) Just as competitor, facilitation was also asymmetric. It increased the size inequality of populations during self-thinning, whereas stress decreased the size inequality. (5) Direct stress and facilitation influence plants differently on various scales. Stress inhibited plant growth, whereas facilitation showed the opposite on an individual scale. Stress increased survival rate, whereas facilitation increased individual variability on the population scale. (6) Trade-offs between competitions, facilitation, and direct stress varied in different growing stages.     

Comparative Study of Wastewater Lagoon with and without Water Hyacinth

A 3-year study was conducted on an existing, one-cell, facultative sewage lagoon having a total surface area of 3.6 ha and receiving a BOD₅ loading rate of 44 kglhald (40 Iblald). The comparative experimental periods ran from July through November for 3 consecutive years. During the first period, water hyacinths completely covered the lagoon. The water hyacinth coverage was reduced to 33% of the total surface area the second year. The lagoon, free of all vascular aquatic plants the third year, was monitored for comparative purposes. The most significant improvement overall in the effluent quality occurred when water hyacinths covered the entire lagoon. During this period the effluent BOD₅ and TSS were 23 and 6 mgll, respectively. Without water hyacinths, the effluent BOD₅ and TSS were 52 and 77 mgll, respectively. The effluent total organic carbon concentration with water hyacinths averaged 40 mgll, and without water hyacinths, 72 mgll. A discussion of the results from this 3-year study is presented in this paper along with associated problems that were observed when water hyacinths were introduced into the lagoon and altered its behavior from that of a normal facultative lagoon.     

A Field Experiment to Assess the Transplant Success of Salt Marsh Plants into Tidal Wetlands

Field plots were used to assess the restoration potential of three salt marsh species, Juncus maritimus, Leptocarpus similis and Schoenoplectus pungens, within an established salt marsh near Christchurch, New Zealand. A split-block design was used to asses the effects of soil type, (soil from a proposed marsh construction site or estuarine mud from a natural salt marsh), species and plant source (commercial nursery stocks or natural marsh stock). In December, after 9 months, there was no mortality in L. similis and a significant increase in the biomass compared with J. maritimus. S. pungens failed to regenerate following seasonal die-back. Plant biomass was unaffected by soil type, despite occasional higher salinities recorded in the reclamation soil. Natural stocks had significantly higher biomass than nursery stocks (p < 0.01). These results suggest that both L. similis and J. maritimus are appropriate species for transplanting in the Canterbury region and that the soil from the proposed area is suitable for the restoration and construction of tidal wetlands. The success of the transplants could be enhanced by sourcing hardy stock and management regimes may be necessary to reduce salinity extremes and herbivory by rabbits.     

Reproductive Systems and Sibling Competition in Plants

Abstract Competition among relatives can modify the genetic structure of plant populations; in turn, competitive outcomes can depend on the genetic relatedness of the individuals competing. The offspring from individual parents exhibit a continuum of genetic relatedness, depending on parental reproductive systems. Competition among relatives may have evolutionary significance as a selection pressure; sibling competition, for example, has been invoked to explain the evolution of sexual systems, seed packaging within fruits, seed dispersal dimorphisms, and germination behavior.
Density-dependent fitness consequences of sibling competition have been documented in a population of the annual grass Sporobolus vaginiflorus . This species produces seeds matured in cleistogamous spikelets within leaf sheaths along tillers in autumn: when seeds along the tillers of a maternal parent (i.e. a sibship) germinate in situ the following spring in close proximity to one another, sibling competition results in a high-density zone centered around the original senescent parent. Both intra- and intersibship interactions can occur within a population. Although fitness is much reduced for siblings inside the zone of competition, potential seed rain and net primary productivity per unit area are significantly higher relative to outside the zone. This annual is functionally analogous to a perennial ramet producer with a phalanx growth strategy and the unit of selection may be the sibling group. It is not yet known whether sibling competition is a significant selection pressure in other species, but indirect evidence suggests it may be relatively widespread.     

Photosynthesis and Growth of Water Hyacinth under CO(2) Enrichment

Water hyacinth (Eichhornia crassipes [Mart.] Solms) plants were grown in environmental chambers at ambient and enriched CO₂ levels (330 and 600 microliters CO₂ per liter). Daughter plants (ramets) produced in the enriched CO₂ gained 39% greater dry weight than those at ambient CO₂, but the original mother plants did not. The CO₂ enrichment increased the number of leaves per ramet and leaf area index, but did not significantly increase leaf size or the number of ramets formed. Flower production was increased 147%. The elevated CO₂ increased the net photosynthetic rate of the mother plants by 40%, but this was not maintained as the plants acclimated to the higher CO₂ level. After 14 days at the elevated CO₂, leaf resistance increased and transpiration decreased, especially from the adaxial leaf surface. After 4 weeks in elevated as compared to ambient CO₂, ribulose bisphosphate carboxylase activity was 40% less, soluble protein content 49% less, and chlorophyll content 26% less; whereas starch content was 40% greater. Although at a given CO₂ level the enriched CO₂ plants had only half the net photosynthetic rate of their counterparts grown at ambient CO₂, they showed similar internal CO₂ concentrations. This suggested that the decreased supply of CO₂ to the mesophyll, as a result of the increased stomatal resistance, was counterbalanced by a decreased utilization of CO₂. Photorespiration and dark respiration were lower, such that the CO₂ compensation point was not altered. The photosynthetic light and CO₂ saturation points were not greatly changed, nor was the O₂ inhibition of photosynthesis (measured at 330 microliters CO₂ per liter). It appears that with CO₂ enrichment the temporary increase in net photosynthesis produced larger ramets. After acclimation, the greater total ramet leaf area more than compensated for the lower net photosynthetic rate on a unit leaf area basis, and resulted in a sustained improvement in dry weight gain.     

Competition between Invasive and Indigenous Species: Impact of Spatial Pattern and Developmental Stage

Elodea canadensis (indigenous) and Elodea nuttallii (invasive) were grown in experimental tanks in monocultures and mixtures in two spatial patterns (aggregated or mixed) and two developmental stages (small or large plants of E. canadensis, and small plants of E. nuttallii). Competitive interactions between the two species were assessed by monitoring the area colonised by each species, the number of rootings and biomass after 10 weeks. In monocultures the growth of E. canadensis was significantly lower than that of E. nuttallii. In mixtures the number of rootings and biomass of E. canadensis were always significantly less than those of E. nuttallii. The tank surface area colonised by E. canadensis was always significantly less than that occupied by E. nuttallii, but it was higher in the aggregated treatment, where the colonisation of E. nuttallii was lower. Therefore both spatial pattern and developmental stage of an indigenous species (E. canadensis) may influence the outcome of competition with potential invaders (E. nuttallii).     

Escape Tactics and Alarm Responses in Badgers Meles meles: A Field Experiment

A typical badger (Meles meles) territory contains a primary burrow or 'main sett' plus several secondary burrows or 'outlier setts'. The main aim of our study was to test the hypothesis that outlier setts are used as emergency refuges, by subjecting foraging radio-collared badgers to three levels of experimental disturbance (low, moderate and high). In addition, we recorded the occurrence of potential alarm signals. With low-level disturbance, badgers usually returned to the main sett even when this was further away than the nearest outlier sett; with high-level disturbance they always took refuge in the nearest outlier; and with moderate-level disturbance they showed no clear preference. We conclude that outlier setts do act as emergency refuges, but only when an animal is badly frightened. Possible alarm signals (pilo-erection, head-flagging, snorting and growling) occurred mainly during moderate or high-level disturbance but even then they were relatively infrequent. Signals were no more likely to be emitted when conspecifics were near by than when the signaller was alone, and when conspecifics were present they rarely reacted either to the flight of the disturbed animal or to any signals that it emitted. We conclude that such alarm signals as do occur constitute threats directed towards the predator rather than warnings for the benefit of conspecifics.     

How Ant Nests Increase Soil Biota Richness and Abundance: A Field Experiment

Although many studies have shown that ant nests tend to increase soil nutrient concentrations, only a few have examined ant impact on soil biota. To date, no one has examined the mechanism behind this complex ‘ant effect.’ In this study, we employed a 2 × 2 complete factorial design (water × food) in the field to mimic the effects of harvester ant nests (Messor andrei) on soil. We hypothesized that, in the absence of ants, addition of moisture and food (seeds and insects) would interact to produce conditions found in ant nests. Our results indicated that the addition of food to the soil (regardless of water addition) best mimicked the conditions found inside M. andrei nests. Both food-treated and ant-nest soils supported higher numbers of bacteria, nematodes, miscellaneous eukaryotes, and microarthropods compared to the other soil treatments. Microbial richness was also highest in ant and food-treated samples. Moreover, the ant effect in our experiment occurred in just two months. Because ants are a widespread, abundant group with many long-lived species, they could substantially influence soil properties and belowground food webs and may have important restoration/conservation implications for terrestrial communities.     

A Comparison of Plastic Responses to Competition by Invasive and Non-invasive Congeners in the Commelinaceae

Evidence supporting an association between phenotypic plasticity and invasiveness across a range of plant taxa is based primarily on comparisons between invasive species and native species whose potential invasiveness is typically unknown. Comparison of invasive and non-invasive exotic species would provide a better test of whether plasticity promotes invasion. Such comparisons should distinguish between adaptive and non-adaptive plasticity because they have different consequences for invasiveness. Adaptive plasticity is expected to promote the invasion of multiple habitats, but non-adaptive plasticity may reflect specialization for invading more favorable habitats only. We grew four invasive and four non-invasive species of the Commelinaceae with and without competitors and compared their putatively adaptive plasticity of three traits related to competitive ability and non-adaptive plasticity in performance. The invasive species grew significantly more than the non-invasive species only in the non-competitive environment. The invasive species had greater plasticity of performance (total biomass) in response to competition than non-invasives, but there was no consistent difference in the plasticities of the traits related to competitive ability. These results are consistent with specialization of these invasive taxa for invading the more productive non-competitive environment rather than a superior ability to invade both competitive and non-competitive environments. A comprehensive understanding of the relationship between plasticity and invasiveness will require many more comparisons of the plasticity of invasive and non-invasive taxa in a range of traits in response to a variety of environments.     

Light Masking in the Field: An Experiment with Nocturnal and Diurnal Spiny Mice Under Semi-natural Field Conditions

Light masking has been studied almost exclusively in the laboratory. The authors populated four field enclosures with locally coexisting nocturnal Acomys cahirinus and diurnal A. russatus, and monitored their body temperatures (T_b) using implanted temperature-sensitive radio transmitters. A 3-h light pulse was initiated at the beginning of two consecutive nights; preceding nights were controls. A. cahirinus T_b and calculated activity levels decreased significantly during the light pulse, demonstrating a negative light masking response (light effect on T_b: ?0.32°C?±?0.15°C; average calculated activity records during the light pulse: 7?±?1.53, control: 9.8?±?1.62). Diurnal A. russatus did not respond to the light pulse. We conclude that light masking is not an artifact of laboratory conditions but represents a natural adaptive response in free-living populations. (Author correspondence: Shayroti@post.tau.ac.il)     

Causes and Consequences of Invasive Plants in Wetlands: Opportunities,Opportunists, and Outcomes

Wetlands seem to be especially vulnerable to invasions. Even though ≤6% of the earth's land mass is wetland, 24% (8 of 33) of the world's most invasive plants are wetland species. Furthermore, many wetland invaders form monotypes, which alter habitat structure, lower biodiversity (both number and “quality” of species), change nutrient cycling and productivity (often increasing it), and modify food webs. Wetlands are landscape sinks, which accumulate debris, sediments, water, and nutrients, all of which facilitate invasions by creating canopy gaps or accelerating the growth of opportunistic plant species. These and other disturbances to wetlands, such as propagule influx, salt influx, and hydroperiod alteration, create opportunities that are well matched by wetland opportunists. No single hypothesis or plant attribute explains all wetland invasions, but the propensity for wetlands to become dominated by invasive monotypes is arguably an effect of the cumulative impacts associated with landscape sinks, including import of hydrophytes that exhibit efficient growth (high plant volume per unit biomass).     

Experiences of Stigma and Discrimination among Caregivers of Persons with Schizophrenia in China: A Field Survey

In China, caregivers for family members with schizophrenia play an important role in treatment and recovery but may experience stigma and discrimination simply because of their family relationship. The object of this study was to measure the degrees and correlates of stigma and discrimination experiences among this group. Four hundred twenty-seven caregivers participated in this hospital-based and cross-sectional study in Ningbo and Guangzhou, China. Data were collected by trained interviewers using fixed questionnaires. Stigma and discrimination experiences were measured by the Modified Consumer Experiences of Stigma Questionnaire (MCESQ). Caregivers’ social support was measured by the Social Support Rating Scale. Parametric analysis, nonparametric analysis and multivariate linear regression were used. The mean (SD) score of MCESQ was 2.44(0.45), 2.91(0.71) for stigma experiences and 1.97(0.37) for discrimination experiences on a five-point score (“1 = never” and “5 = very often”). Approximately 65% of caregivers reported that they tried to conceal their family members’ illness, and 71% lacked the support of friends. The experience of stigma was significantly negatively associated with the perceived social support of caregivers (standard β = −0.2,p<0.001). Caregivers who were children of the patients experienced fewer stigmas than other (standard β = −0.18, p<0.001). Urban residence (standard β = −0.12, p<0.01) and patients did not complete primary school education (standard β = −0.13, p<0.01) were negatively related with stigmas. In addition, stigma and discrimination was more experienced in Zhejiang than in Guangdong (p<0.05). In conclusion, this study performed that caregivers of people with schizophrenia in China experienced general stigmas and rare discrimination and found the relations with social support, kinship, patient’s educational level and regional differences. More interventions and supports should been given to caregivers who are lack of social support, who live in rural area and who are the patients’ parents, spouses or siblings.