Size-asymmetric competition and size-asymmetric growth in a spatially explicit zone-of-influence model of plant competition

Size-asymmetric competition among plants is usually defined as resource pre-emption by larger individuals, but it is usually observed and measured as a disproportionate size advantage in the growth of larger individuals in crowded populations (“size-asymmetric growth”). We investigated the relationship between size-asymmetric competition and size-asymmetric growth in a spatially explicit, individual-based plant competition model based on overlapping zones of influence (ZOI). The ZOI of each plant is modeled as a circle, growing in two dimensions. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. We grew simulated populations with different degrees of size-asymmetric competition and at different densities and analyzed the size dependency of individual growth by fitting coupled growth functions to individuals. The relationship between size and growth within the populations was summarized with a parameter that measures the size asymmetry of growth. Complete competitive symmetry (equal division of contested resources) at the local level results in a very slight size asymmetry in growth. This slight size asymmetry of growth did not increase with increasing density. Increased density resulted in increased growth asymmetry when resource competition at the local level was size asymmetric to any degree. Size-asymmetric growth can be strong evidence that competitive mechanisms are at least partially size asymmetric, but the degree of size-asymmetric growth is influenced by the intensity as well as the mode of competition. Intuitive concepts of size-asymmetric competition among individuals in spatial and nonspatial contexts are very different.     

Size-asymmetric root competition in deep,nutrient-poor soil

Aims There is much evidence that plant competition below ground is size symmetric, i.e. that competing plants share contested resources in proportion to their sizes. Several researchers have hypothesized that a patchy distribution of soil nutrients could result in size-asymmetric root competition. We tested this hypothesis.     

The effects of density, spatial pattern, and competitive symmetry on size variation in simulated plant populations

Patterns of size inequality in crowded plant populations are often taken to be indicative of the degree of size asymmetry of competition, but recent research suggests that some of the patterns attributed to size-asymmetric competition could be due to spatial structure. To investigate the theoretical relationships between plant density, spatial pattern, and competitive size asymmetry in determining size variation in crowded plant populations, we developed a spatially explicit, individual-based plant competition model based on overlapping zones of influence. The zone of influence of each plant is modeled as a circle, growing in two dimensions, and is allometrically related to plant biomass. The area of the circle represents resources potentially available to the plant, and plants compete for resources in areas in which they overlap. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. Theoretical plant populations were grown in random and in perfectly uniform spatial patterns at four densities under size-asymmetric and size-symmetric competition. Both spatial pattern and size asymmetry contributed to size variation, but their relative importance varied greatly over density and over time. Early in stand development, spatial pattern was more important than the symmetry of competition in determining the degree of size variation within the population, but after plants grew and competition intensified, the size asymmetry of competition became a much more important source of size variation. Size variability was slightly higher at higher densities when competition was symmetric and plants were distributed nonuniformly in space. In a uniform spatial pattern, size variation increased with density only when competition was size asymmetric. Our results suggest that when competition is size asymmetric and intense, it will be more important in generating size variation than is local variation in density. Our results and the available data are consistent with the hypothesis that high levels of size inequality commonly observed within crowded plant populations are largely due to size-asymmetric competition, not to variation in local density.     

Dynamics of biomass partitioning in two competing meadow plant species

The optimal allocation theory predicts that growth is allocated between the shoot and the roots so that the uptake of the most limiting resource is increased. Allocation is dynamic due to resource depletion, interaction with competitors, and the allometry of growth. We assessed the effects of intra- and inter-specific competition on growth and resource allocation of the meadow species Ranunculus acris and Agrostis capillaris, grown in environments with high (+) or low (−) availability of light (L) and nutrients (N). We took samples twice a week over the 7 weeks experiment, to follow the changes in root-to-shoot ratios in plants of different sizes, and carried out a larger scale harvest at the end of the experiment. Of all the tested factors, availability of nutrients had the largest effect on the growth rate and shoot-to-root allocation in both species, although both competition and light had significant effects as well. The highest root-to-shoot ratios were measured from the L+N− treatment, and the lowest from the L−N+ treatment, as predicted by the optimal allocation theory. Competition changed resource allocation, but not always toward acquiring the resource that is most limiting to growth. We thus conclude that the greatest variation in shoot-to-root allocation was due to the resource availability and the effects of competition were small, probably due to low density of plants in the experiment.     

A trade-off between scale and precision in resource foraging

Summary There is widespread uncertainty about the nature and role of morphological plasticity in resource competition in plant communities. We have assayed the foraging characteristics of leaf canopies and root systems of eight herbaceous plants of contrasted ecology using new techniques to create controlled patchiness in light and mineral nutrient supply. The results are compared with those of a conventional competition experiment. Measurements of dry matter partitioning and growth in patchy conditions indicate a consistent positive association between the foraging characteristics of roots and shoots, supporting the hypothesis of strong interdependence of competitive abilities for light and mineral nutrients. Differences are identified in the abilities of dominant and subordinate plants to forage on coarse and fine scalcs. It is suggested that a trade-off exists in the scale (“high” in dominants) and precision (high in subordinates) with which resources are intercepted and that this trade-off contributes to diversity in communities of competing plants.     

Asymmetric competition as a natural outcome of neighbour interactions among plants: results from the field-of-neighbourhood modelling approach

Numerous attempts have been made to infer the mode of competition from size or biomass distributions of plant cohorts. However, since the relationship between mode of competition and size distributions may be obscured by a variety of factors such as spatial configuration, density or resource level, empirical investigations often produce ambiguous results. Likewise, the findings of theoretical analyses of asymmetric competition are equivocal. In this paper, we analyse the mode of competition in an individual-based model which is based on the new field-of-neighbourhood approach. In this approach, plants have a zone of influence that determines the distance up to which neighbours are influenced. Additionally, a superimposed field within the zone of influence defines phenomenologically the strength of influence of an individual on neighbouring plants. We investigated competition at both individual and population level and characterised the influence of density and of the shape of the field-of-neighbourhood on occurrence and degree of competitive asymmetry. After finding asymmetric competition emerging in all scenarios, we argue that asymmetric competition is a natural consequence of local competition among neighbouring plants.     

Intraspecific interactions in the annual legume Medicago minima are shaped by both genetic variation for competitive ability and reduced competition among kin

Documenting if plants exhibit kin competition avoidance in intraspecific plant interactions is relevant both to improve crop growth, and to understand diversity and composition in natural plant communities. However, a number of confounding mechanisms complicates detecting kin competition avoidance from experiments comparing plants growing with kin and non-kin neighbors. We conducted complementary greenhouse experiments using genotypes from four populations of the annual Medicago minima, which in a previous study showed higher survival when interacting with kin relative to non-kin. We show that genotypes vary in kin competition avoidance, and in competitive ability, but find no indication of complementary resource use. Importantly, from our first experiment of root growth behavior, we know that some genotypes exhibit kin competition avoidance. Yet, the variation in competitive ability we find in our second experiment, where plants grow in mini communities together with either kin or unrelated genotypes, can alone explain the variation we observe in growth and biomass among communities. In our case, the genotypes with highest competitive ability were also those that showed kin competition avoidance. This confounding effect obscured the disentangling of mechanisms underlying difference in growth between kin and non-kin interactions. When silencing root exudates by adding activated carbon to a subset of our genotype combinations, we found increased size asymmetry of plants grown together, and mostly in kin communities. This suggests that plants recognize the identity of neighbors via root exudates, and compete less with neighbors recognized as kin. To detect kin competition avoidance we suggest designing experiments that pair unrelated genotypes with similar competitive abilities. Such design, combined with silencing root exudates would be powerful to detect whether plants show kin competition avoidance or not.     

Decomposition Analysis of Competitive Symmetry and Size Structure Dynamics

An analysis is introduced, based on the decomposition of relativegrowth rates, to examine the mode of competition (i.e. whethercompetition is symmetric or asymmetric), the size-dependenceof growth, and their interdependence. In particular, the basisfor two commonly held views is examined: (1) that the type ofresource limitation determines the mode of competition, and(2) that asymmetric competition always leads to size-divergencebetween unequal competitors. It is shown that in field-grownmillet plants, competition for light was symmetric at low densityand asymmetric at high density. However, size variation at lowdensity decreased during growth, because small plants had greaterrelative growth rates than larger plants. Size variation stayedconstant at high density, since plants of all sizes had equalaverage relative growth rates. Based on these results and ageneral discussion, it is proposed that the type of resourcelimitation does not determine the mode of competition. Competitionfor light can be symmetric, and foraging for heterogeneouslydistributed soil resources can produce asymmetric competitionbelow-ground. Furthermore, the mode of competition alone doesnot determine size structure dynamics. Size-dependence of resourceconversion efficiency and allocation can modify the effectsof resource uptake on growth. Pennisetum americanum‘Custer ’; mode of competition; size structure dynamics; plant growth analysis     

Competitive asymmetry, foraging area size and coexistence of annuals

Individuals allocate resources to the expansion of their foraging area and those resources are no longer available for the traits that determine how well those individuals are able to protect their foraging area against competitors. The resulting trade‐off between foraging area size and the traits associated with the ability to compete for the resources within the foraging area applies to ecological scenarios as different as territorial defence by individuals and colonies, and light competition in plants. Whether the trade‐off affects species performance in competition for resources at the area of overlap between foraging areas depends on the symmetry of resource division. In symmetric competition resources are divided equally between the competitors, while in asymmetric competition the individual with the smallest foraging area, and consequently the greatest competitive ability, gains all the resources. Competition may also be a combination of the symmetric and asymmetric processes. I studied the effects of competitive asymmetry on population dynamics and coexistence of two annual species with different sized foraging areas using an individual‐based spatially explicit simulation model. Symmetric competition favoured the species with the larger foraging area and did not allow coexistence. Competitive asymmetry favoured the species with smaller foraging area and allowed coexistence, which was due to the consequences of losing an asymmetric competition being more severe than losing a symmetric competition. The mechanism of coexistence is the larger foraging area's superiority in low population densities (little competition) and the smaller foraging area's ability to win a large foraging area when competition was intense. Competitive asymmetry and small size of both foraging areas led to population dynamics dominated by long‐term fluctuations of small intensity. Symmetric competition and large size of the foraging areas led to large short‐term fluctuations, which often resulted in the extinction of one or both of the species due to demographic stochasticity.