Modelling the Time Course of Self-thinning in Crowded Plant Populations

A logarithmic model for the self-thinning of plants is proposed.This model describes the time course of self-thinning very welland fits data from forest stands and yield tables, which followthe 3/2 power law. An approximated expression of this modelshows that plant density decreases with age along a Gompertzcurve. This appears to be a basic property of the time courseof self-thinning in plants. Pinus strobus L., Pinus densiflora Sieb, et Zucc., stand development, self-thinning, 3/2 power law, logarithmic model, mortality     

The use of water and some inorganic salt solutions to advance sugar beet seed. II. Experiments under controlled and field conditions

The effect of seed advancement on the early growth of sugar beet was determined in two experiments made under controlled conditions and eight field experiments made between 1974 and 1978. The experiments in the growth room suggested that seed advancement increased the rate of emergence and seedling dry weight over a wide range of soil moisture contents but seedling numbers were only greater under sub-optimal moisture conditions. Similarly, in most field experiments, seed advancement resulted in more rapid emergence and larger seedlings. In four field experiments, seed advancement gave up to 16% more plants, and in three experiments a significant increase in final sugar yield of up to 0–7 t h^-1 was detected. The plants were hand-singled to a uniform stand and had the crops been drilled to a stand, some yield responses would have been larger. Seed advancement did not decrease the plant stand or final yield in any experiments. Results in 1977 and 1978 provided some evidence that seed advancement may help reduce the damage caused by field mice and the number of plants which bolt and, by making emergence more rapid, be particularly beneficial on soils prone to slumping or capping. There might, however, be a slightly increased risk of frost injury if advanced seed is used for early sowings.     

Photosynthetic characteristics and productivity of spring rape plants as related to crop density

Optimal density of spring rape (Brassica napus L.) crop stand was determined by plant photosynthetic characteristics at the beginning of flowering. As crop density increased from 100 to 350 plants/m², leaf surface index (LSI) of the crop was found to increase by 18.2–80.2%, and LSI decreased by 38.8–67.3% as compared with the sparsest crop (50–100 plants/m²). LSI depended on the rate of incident PAR reaching 0.5 and 0.25 heights of the crop stand and to the soil surface. When crop density increased from 100 to 350 plants/m², the photosynthetic potential (PP) of the crop increased 1.8 times as compared with the sparsest crop. PP of the densest rape crop stand was 3 times lower than in the sparsest crop. When the crop density increased from 100 to 250 plants/m², the daily increment in biomass calculated per leaf surface unit increased by 27.0% as compared with the sparsest crop and depended on LSI. When leaf area decreased, the daily increment in biomass calculated per leaf surface unit declined; in the densest stand, this characteristic was by 58.3% lower than in the sparsest crop. Rape productivity at the flowering stage depended on the crop density, LSI of plants, rate of PAR reaching 0.5 and 0.25 heights of the crop stand and to the soil surface, PP, and the daily increment in biomass calculated per leaf surface unit. Crop productivity at the flowering stage and the rape seed yield were associated by a significant parabolic relationship. When crop density increased from 100 to 350 plants/m², seed yield per plant considerably decreased (by 33.1–78.5%) as compared with the sparsest crop. The greatest influence on seed yield per plant was exerted by LSI and the daily increment in biomass calculated per leaf surface unit. When crop density increased to 250–300 plants/m², the seed yield considerably rose (by 28.6–58.8%) as compared with the sparsest crop; when this index reached 300–350 plants/m², the seed yield decreased because plant growth was suppressed, with the productivity reduced. The results thus obtained suggest that the photometric characteristics of spring rape were at optimum at crop density of 100–250 plants/m². The agroclimatic conditions of Lithuania ensure potential for rapid accumulation of total biomass and high seed yield.     

Dynamics of Self-Thinning Plant Stands

The growth of plants in even-aged, pure stands and the declinein the number of these plants are represented with a set ofS-system differential equations. The S-system representationcaptures the observed interdependence between average size andnumber of plants, demonstrates how a plant stand approachesthe well-established 3/2 power relationship between number andsize, and also accounts for the observation that in stands ofperennial species, old plants usually remain smaller than predictedby the relationship. The model includes, as special cases, earliermodels that only partially represent the mutual dependence betweennumber and average size. The S-system model incorporates well-knowngrowth laws, and thus circumvents the problem of identifyingthe most appropriate growth and decline functions in the analysisof actual data. For particular parameter settings, the S-systemcan be solved analytically to yield explicit closed-form relationshipsbetween the numbers and sizes of plants, not only in the rangewhere the stand dynamics moves along the size-limiting relationship,but also for large numbers of small plants and small numbersof large plants. Growth, plant stands, self-thinning, 3/2 power rule, S-system model     

An Individual Stand Growth Model for Mean Plant Size Based on the Rule of Self-thinning

A growth model for pure, even-aged stands of plants is asymptoticallybounded above by the self-thinning rule that relates maximumplant size to stand density. The model characterizes accretionin mean size as a deviation from the limiting size. It consistsof a function relating mean size to time and density and a companionsurvival model. The growth model is obtained by substitutingthe survival model for density in the mean size relationship.Model flexibility is demonstrated by fitting it to annual remeasurementsof mean size and number of plants per unit area in a stand ofPinus taeda L. 3/2-power rule, mortality, survival, stand dynamics, plant growth model, loblolly pine     

THE EFFECTS OF DEFOLIATION AND LOSS OF STAND UPON YIELD OF SUGAR BEET

Seedling pests of sugar beet cause defoliation and loss of stand. Attempts were made to assess the effects of these types of injury by means of field trials in which the defoliation and loss of stand were produced artificially shortly after singling. The main effects of these treatments were to modify the size of the plants without greatly influencing their percentage sugar content. Little loss of crop resulted until 50% of the leaf area or 50% of the plant population had been destroyed. The yield from re-sowing after removal of all the plants was inferior to that from stands depleted of half or two-thirds of their initial plant population. The relation between the results of artificial injury and the injury produced by seedling pests is briefly discussed.     

Unraveling the role of mitochondria during oxidative stress in plants

The sedentary habit of plants means that they must stand and fight environmental stresses that their mobile animal cousins can avoid. A range of these abiotic stresses initiate the production in plant cells of reactive oxygen and nitrogen species that ultimately lead to oxidative damage affecting the yield and quality of plant products. A complex network of enzyme systems, producing and quenching these reactive species operate in different organelles. It is the integration of these compartmented defense systems that coordinates an effective response to the various stresses. Future attempts to improve plant growth or yield must consider the complexity of inter-organelle signaling and protein targeting if they are to be successful in producing plants with resistance to a broad range of stresses. Here we highlight the role of pre-oxidant, antioxidant, and post-oxidant defense systems in plant mitochondria and the potential role of proteins targeted to both mitochondria and chloroplasts, in an integrated defense against oxidative damage in plants.     

Evolutionarily stable leaf area production in plant populations

Using an analytical model, it was shown that for a given amount of nitrogen in the canopy of a stand (N(T)), there exists an evolutionarily stable leaf area index (ES-LAI), and therefore an evolutionarily stable average leaf nitrogen content (n(ES)(av);n(ES)(av) =N(T)/ES-LAI), at which no individual plant in the stand can increase its photosynthesis by changing its leaf area. It was also shown that this ES-LAI is always greater than the optimal LAI that maximizes photosynthesis per unit N(T) of the stand. This illustrates that the canopy structure that maximizes photosynthesis of a population is not the same as the canopy structure that maximizes photosynthesis of individuals within a population. It was further derived that the ES-LAI at given N(T) increases with the ratio between the light-saturated photosynthesis and the N content per unit leaf area (leaf-PPNUE) and that it decreases with the canopy extinction coefficient for light (K(L)), the light availability and the apparent quantum yield (phi). These hypotheses were tested by comparing calculated ES-LAI and n(ES)(av) values to actual LAIs and leaf N contents measured for stands of a large variety of herbaceous plants. There was a close correspondence between the calculated and measured values. As predicted by the model, plants with high leaf-PPNUEs produced more leaf area per unit nitrogen than those with low leaf-PPNUEs while plants with horizontal leaves, forming stands with higher K(L) values, produced less leaf area than those with more vertically inclined leaves. These results suggest that maximization of individual plant photosynthesis per unit of nitrogen plays an important role in determining leaf area production of plants and the resulting canopy structure of stands of vegetation. They further suggest this optimization to be a mechanism by which leaf traits such as leaf-PPNUE and leaf inclination angle are causally related to structural characteristics of the population, i.e. the leaf area index of the stand.     

Seed production of cut-leaved teasel (<Emphasis Type="Italic">Dipsacus laciniatus</Emphasis>) in central Missouri

Cut-leaved teasel is an invasive weed in Missouri that reduces the diversification of native species along roadsides and impairs traffic visibility. Teasel is a biennial and grows as a rosette in the first year and flowers the second year. Reproduction is only by seed. Field studies were conducted in 2004 and 2005 at two locations to assess the seed production of cut-leaved teasel. From a natural stand, fifteen plants were tagged at the onset of flowering. Selected plants included those considered growing in a group and those growing alone; a plant was considered alone when no other plant was adjacent for at least 60 cm. Whenever a seedhead completed flowering, it was covered with a cellophane bag and harvested one month later. Linear regression was used to correlate the weight of seeds from a single seedhead and number of seeds to estimate the total seed production per seedhead. The number of seedheads per plant varied from 3 to 56. On average, plants growing alone had 64% more seedheads per plant than plants occurring in a group. Seed numbers in the primary seedhead ranged from 511 to 1,487. Total seed production per plant ranged from 1,309 to 33,527. Seed production was 61% greater for plants growing alone versus those growing in a group and was more prolific in 2005 than in 2004. In addition, seed production per plant varied between locations for plants growing alone, but seed yield per plant was similar for plants growing in groups. Colonization of teasel in new areas is facilitated by higher seedhead numbers per plant and total seed production compared to reproduction of plants in areas of intraspecific competition.     

Architecture and growth of an annual plant <Emphasis Type="Italic">Chenopodium album</Emphasis> in different light climates

Light climates strongly influence plant architecture and mass allocation. Using the metamer concept, we quantitatively described branching architecture and growth of Chenopodium album plants grown solitarily or in a dense stand. Metamer is a unit of plant construction that is composed of an internode and the upper node with a leaf and a subtended axillary bud. The number of metamers on the main-axis stem increased with plant growth, but did not differ between solitary and dense-stand plants. Solitary plants had shorter thicker internodes with branches larger in size and number than the plant in the dense stand. Leaf area on the main stem was not different. Larger leaf area in solitary plants was due to a larger number of leaves on branches. Leaf mass per area (LMA) was higher in solitary plants. It did not significantly differ between the main axis and branches in solitary plants, whereas in the dense stand it was smaller on branches. Dry mass was allocated most to leaves in solitary plants and to stems in the dense stand in vegetative growth. Reproductive allocation was not significantly different. Branch/main stem mass ratio was higher in solitary than dense-stand plants, and leaf/stem mass ratio higher in branches than in the main axis. Nitrogen use efficiency (NUE) (dry mass growth per unit N uptake) was higher and light use efficiency (LUE) (dry mass growth per unit light interception) was lower in the plant grown solitarily than in the dense stand.     

Influence of Meloidogyne hapla on Alfalfa Yield and Host Population Dynamics

Self-thinning in alfalfa, a dynamic process involving the progressive elimination of the weakest plants, was enhanced by Meloidogyne hapla. Alfalfa stand densities decreased exponentially with time and were reduced 62% (P = 0.05) in the presence of M. hapla. As stand densities decreased over time, mean plant weights increased at a rate 2.59 times faster in the absence of M. hapla. In a stepwise multiple regression analysis, 65% of the total variation in yield could be explained by changes in stand density and 85% by average weight of individual stems. Alfalfa yields were suppressed (P = 0.05) by M. hapla, with suppression generally increasing with time and as the nematode population density increased. Yield suppression was attributable primarily to the decline in plant numbers and to suppression in individual plant weights.     

林分因子对神木垒不同森林群落林下植物多样性的影响

为探究神木垒不同森林群落林下植物多样性的差异，本研究采用典型样地法，以夹金山神木垒的5种主要森林群落：云杉林、丽江云杉林、红杉林、针阔混交林、阔叶林为研究对象，对不同森林群落林下植物物种组成和物种多样性进行比较，并对林分因子和林下植物多样性进行冗余分析，确定影响林下植物多样性的主要林分因子，为当地森林经营管理提供理论依据。结果表明：（1）在研究区内共记录林下植物147种，隶属于61科，108属；云杉林群落林下植物的科属种组成最丰富。（2）各类型群落的H值、H"值、D值、JSW值均为：草本层＞灌木层；灌木层多样性最高的群落为云杉林群落，草本层多样性最高的群落为丽江云杉林群落，针阔混交林群落、阔叶林群落林下植物多样性较差。（3）平均枝下高与林分密度是影响灌木层物种多样性的主要林分因子（P<0.01），平均枝下高与灌木层的D值、H值、H"值呈负相关关系，林分密度与灌木层4个多样性指数均呈正相关关系；平均枝下高是影响草本层物种多样性的主要林分因子（P<0.01），平均枝下高与草本层H值、H"值、JSW值呈正相关关系。本研究认为，云杉林群落与丽江云杉林群落的林下植物多样性水平较高，平均枝下高与林分密度是影响神木垒不同森林群落林下植物多样性的主要林分因子。     

Growth and nitrogen use in Xanthium canadense grown in an open or in a dense stand

Plants develop branches profusely when grown solitarily, while less so when grown in a dense stand. Such changes in architecture are associated with changes in dry mass allocation and nitrogen use. Here, we studied what traits in plant growth and nitrogen use were influenced by different light climates in the stand. Annual plants (Xanthium canadense) were grown solitarily or in a dense stand. Dry mass growth was analyzed as the product of the net assimilation rate (NAR) and leaf area (LA). Nitrogen use efficiency (NUE) was analyzed as the product of nitrogen productivity (NP) and the mean residence time (MRT) of nitrogen. These growth variables were further factorized into their components. Solitary plants maintained a high NAR, whereas plants in the dense stand decreased the NAR due to mutual shading. Plants in the dense stand developed a larger LA with a higher specific leaf area than solitary plants. Solitary plants had higher NUE due to higher NP. A temporal increase in NUE was attributed to the increase in MRT of nitrogen. Light climate was different between solitary and dense-stand plants, but they took up a comparable amount of nitrogen and used it differently in response to the given light climate. NUE was thus demonstrated to be a useful tool for analyzing the mechanism leading to different N use in plant growth.     

Density Effects on the Size Structure of Annual Plant Populations: An Indication of Neighbourhood Competition

Size variability among plants has been observed to increasewith higher stand density, leading to the speculation that resourcedistribution among competing plants is primarily asymmetricrather than symmetric. The relationships between size variability,stand density, and type of resource distribution among competingplants were investigated using a spatially explicit, individual-plantmodel of annual plant population dynamics. When plants variedin neighbourhood competition, size variability increased withhigher stand densities whether shared resources were symmetricallyor asymmetrically distributed among competing plants. Size variabilitydid not increase with higher stand densities when neighbourhoodcompetition was constant for all plants. These simulations indicatethat increased size variability among competing plants doesnot distinguish between symmetric and asymmetric resource distribution,but rather is direct evidence for neighbourhood competition. Size hierarchy, neighbourhood competition, density effects, asymmetric competition, symmetric competition     

Insect pest damage increases faba bean (Vicia faba) yield components but only in the absence of insect pollination

Identifying and quantifying crop stressors interactions in agroecosystems is necessary to guide sustainable crop management strategies. Over the last 50 years, faba bean cropping area has been declining, partly due to yield instabilities associated with uneven insect pollination and herbivory. Yet, the effect of interactions between pollinators and a key pest, the broad bean beetle Bruchus rufimanus (florivorous and seed predating herbivore) on faba bean yield has not been investigated. Using a factorial cage experiment in the field, we investigated how interactions between two hypothesized stressors, lack of insect pollination by bumblebees and herbivory by the broad bean beetle, affect faba bean yield. Lack of bumblebee pollination reduced bean weight per plant by 15%. Effects of the broad bean beetle differed between the individual plant and the plant‐stand level (i.e., when averaging individual plant level responses at the cage level), likely due to high variation in the level of herbivory among individual plants. At the individual plant level, herbivory increased several yield components but only in the absence of pollinators, possibly due to plant overcompensation and/or pollination by the broad bean beetle. At the plant‐stand level, we found no effect of the broad bean beetle on yield. However, there was a tendency for heavier individual bean weight with bumblebee pollination, but only in the absence of broad bean beetle herbivory, possibly due to a negative effect of the broad bean beetle on the proportion of legitimate flower visits by bumblebees. This is the first experimental evidence of interactive effects between bumblebees and the broad bean beetle on faba bean yield. Our preliminary findings of negative and indirect associations between the broad bean beetle and individual bean weight call for a better acknowledgment of these interactions in the field in order to understand drivers of crop yield variability in faba bean.     

Effects of Elasmopalpus lignosellus (Lepidoptera: Pyralidae) damage on sugarcane yield

Feeding by lesser cornstalk borer, Elasmopalpus lignosellus (Zeller) (Lepidoptera: Pyralidae), larvae on sugarcane (a complex hybrid of Saccharum spp.) causes leaf damage, dead hearts, and dead plants that can result in stand and yield loss. A 2-yr greenhouse experiment was conducted to examine sugarcane variety and plant age-specific feeding responses to E. lignosellus. Plants growing from single-eye setts of three varieties were exposed to a single generation of E. lignosellus larvae beginning at the three-, five-, and seven-leaf stages. Results indicated that the physical damage and resulting yield loss of plants attacked by E. lignosellus larvae were dependent on the variety and leaf stage at which they were infested. Significantly more plant damage was observed in all three varieties when infested at the three- than at the seven-leaf stage. Larvae caused significantly more plant damage and reduced yield in CP 89-2143 than in CP 78-1628. Tiller production increased in CP78-1628 and CP 88-1762 when infested at the three-leaf stage, whereas tiller production, biomass and sugar yield decreased in CP 89-2143 when infested at all leaf stages, compared with the untreated control. There was no reduction in yield when CP 78-1628 was infested at the three- or five-leaf stages. Biomass was reduced in CP 88-1762 when plants were infested at any of the leaf stages, but sugar yield was reduced only when infested at the seven-leaf stage. These results indicate that compensation in response to E. lignosellus damage was variety dependent and declined with the delay in infestation time.     

高寒区青海云杉人工林密度与林下植物多样性的关系

为了研究青海云杉人工林密度对林下植物多样性的影响及二者的关系,探明维持植物多样性、人工林生态系统稳定性及其健康状况的合理林分密度,以青海大通宝库林区6种不同密度（3 400株/hm²、2 900株/hm²、2 600株/hm²、2 300株/hm²、2 150株/hm²、1 750株/hm²）25年生青海云杉人工林林下植物的重要值、丰富度指数、多样性指数、均匀度指数为指标,对林下植被多样性随林分密度的变化规律进行分析。结果表明:(1) 青海云杉人工林下共有维管束植物60种,非维管束植物1种,隶属于29科45属。（2）灌木层植被优势种受密度影响较大,草本层植被优势种受密度影响较小。（3）青海云杉密度分别与灌木层和草本层的S指数、H′指数、Ea指数均呈显著相关关系,且相关系数分别为0.688、0.937、0.762和 0.679、0.870、0.505。（4）随着青海云杉密度的增加,林下植被多样性呈先减少后增大再减少的变化规律,且密度为2 600株/hm²时,林下植物多样性综合得分最高。研究认为,青海云杉林下植被多样性对其密度响应具有非同步性;密度为2 600株/hm²时比较适合林下植被生长,并保持林下植物多样性水平。     

Intergenotypic competition and shoot production in wheat plants

In field experiments, the competition among plants of two distinct wheat genotypes in binary mixtures, and among three genotypes being grown in adjacent rows was investigated. The competition greatly affected the over-ground dry weight, weight of stem and grain per plant. This effect was undercompensatory, the depression in poor competitor was higher than the enhancement of strong competitor for all these characteristics. From final components of grain yield per plant, the number of fertile stems and number of grains per ear were more affected than weight per grain. More expressively than weight of grain on the main-stem ear, the weight of grain per average secondary ear-bearing stem was affected by competition in binary mixture. The enhancement in standard height—a strongly competitive genotype— reached in binary mixture about 10% for this last characteristic in comparison with the value from the pure stand. Some morphological characteristics of plants being in relation with competition for grain yield per plant are discussed in these experiments.     

Analysis of competition effects in mono- and mixed cultures of juvenile beech and spruce by means of the plant growth simulation model PLATHO

Inter- and intra-specific competition between plants for external resources is a critical process for plant growth in natural and managed ecosystems. We present a new approach to simulate competition for the resources light, water, and nitrogen between individual plants within a canopy. This approach was incorporated in a process-oriented plant growth simulation model. The concept of modelling competition is based on competition coefficients calculated from the overlap of occupied crown and soil volumes of each plant individual with the occupied volumes of its four nearest neighbours. The model was parameterised with data from a two-year phytotron experiment with juvenile beech and spruce trees growing in mono- and mixed cultures. For testing the model, an independent data set from this experiment and data from a second phytotron experiment with mixed cultures were used. The model was applied to analyse the consequences of start conditions and plant density on plant-plant competition. In both experiments, spruce dominated beech in mixed cultures. Based on model simulations, we postulate a large influence of start conditions and stand density on the outcome of the competition between the species. When both species have similar heights at the time of canopy closure, the model suggests a greater morphological plasticity of beech compared with spruce to be the crucial mechanism for competitiveness in mixed canopies. Similar to the experiment, in the model greater plasticity was a disadvantage for beech leading to it being outcompeted by the more persistent spruce.