Jacob Weiner

Asymmetric competition in plant populations.

Recently there has been much interest in the hypothesis that competition between individual plants is asymmetric or onesided: larger individuals obtain a disproportionate share of the resources (for their relative size) and suppress the growth of smaller individuals. This has important implications for population structure, for the analysis of competition between plants at the individual, population and community levels, and for our understanding of competition as a selective force in the evolution of plant populations.

Mechanisms determining the degree of size asymmetry in competition among plants

Abstract When plants are competing, larger individuals often obtain a disproportionate share of the contested resources and suppress the growth of their smaller neighbors, a phenomenon called size-asymmetric competition. We review what is known about the mechanisms that give rise to and modify the degree of size asymmetry in competition among plants, and attempt to clarify some of the confusion in the literature on size asymmetry. We broadly distinguish between mechanisms determined primarily by characteristics of contested resource from those that are influenced by the growth and behavior of the plants themselves. To generate size asymmetric resource competition, a resource must be “pre-emptable.” Because of its directionality, light is the primary, but perhaps not the only, example of a pre-emptable resource. The available data suggest that competition for mineral nutrients is often size symmetric (i.e., contested resources are divided in proportion to competitor sizes), but the potential role of patchily and/or episodically supplied nutrients in causing size asymmetry is largely unexplored. Virtually nothing is known about the size symmetry of competition for water. Plasticity in morphology and physiology acts to reduce the degree of size asymmetry in competition. We argue that an allometric perspective on growth, allocation, resource uptake, and resource utilization can help us understand and quantify the mechanisms through which plants compete.

Allocation, plasticity and allometry in plants

Abstract Allocation is one of the central concepts in modern ecology, providing the basis for different strategies. Allocation in plants has been conceptualized as a proportional or ratio-driven process (‘partitioning’). In this view, a plant has a given amount of resources at any point in time and it allocates these resources to different structures. But many plant ecological processes are better understood in terms of growth and size than in terms of time. In an allometric perspective, allocation is seen as a size-dependent process: allometry is the quantitative relationship between growth and allocation. Therefore most questions of allocation should be posed allometrically, not as ratios or proportions. Plants evolve allometric patterns in response to numerous selection pressures and constraints, and these patterns explain many behaviours of plant populations. In the allometric view, plasticity in allocation can be understood as a change in a plants allometric trajectory in response to the environment. Some allocation patterns show relatively fixed allometric trajectories, varying in different environments primarily in the speed at which the trajectory is travelled, whereas other allocation patterns show great flexibility in their behaviour at a given size. Because plant growth is often indeterminate and its rate highly influenced by environmental conditions, ‘plasticity in size’ is not a meaningful concept. We need a new way to classify, describe and analyze plant allocation and plasticity because the concepts ‘trait’ and ‘plasticity’ are too broad. Three degrees of plasticity can be distinguished: (1) allometric growth (‘apparent plasticity’), (2) modular proliferation and local physiological adaptation, and (3) integrated plastic responses. Plasticity, which has evolved because it increases individual fitness, can be a disadvantage in plant production systems, where we want to optimize population, not individual, performance.

The meaning and measurement of size hierarchies in plant populations

SummaryThe term “size hierarchy” has been used frequently by plant population biologists but it has not been defined. Positive skewness of the size distribution, which has been used to evaluate size hierarchies, is inappropriate. We suggest that size hierarchy is equivalent to size inequality. Methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations. A measure of inequality such as the Gini Coefficient will usually be more appropriate than a measure of skewness for addressing questions concerning plant population structure.

Size Hierarchies in Experimental Populations of Annual Plants

The effects of inter- and intraspecific interference on size hierarchies (size inequali- ties) were investigated in populations of the annual plants Trifolium incarnatum and Lolium multiflorum. Variables experimentally manipulated included plant density, species proportions, soil fertility, and spatial pattern of plantings. Densities were below those for extensive density-dependent mortality. Size inequality always increased with increasing density. Plants grown individually showed very low inequality, while plants grown at the highest density had the most developed hierarchies. Size inequality usually increased with an increase in productivity when interference was occurring. When dominant in mixtures, Lolium showed less size inequality than in monoculture, while the suppressed species, Trifolium, usually displayed an increase in inequality. Spatial pattern appeared to be less important than other factors in causing size inequalities; plants sown in a uniform spatial pattern showed significantly lower size inequality than plants sown in a random pattern in only one out of four cases. Inequality in reproductive output of Trifolium, as estimated by dry mass of flower heads, was always greater than inequality in plant dry mass. The results support a model of plant interference in which large plants are able to usurp resources and suppress the growth of smaller individuals more than they themselves are suppressed. While interference decreases mean plant mass, it increases both the relative variation in plant mass and the concentration of mass within a small fraction ofthe population.

The effect of nutrient availability on biomass allocation patterns in 27 species of herbaceous plants

Abstract We investigated allocation to roots, stems and leaves of 27 species of herbaceous clonal plants grown at two nutrient levels. Allocation was analyzed as biomass ratios and also allometrically. As in other studies, the fraction of biomass in stems and, to a lesser extent, in leaves, was usually higher in the high-nutrient treatment than in the low-nutrient treatment, and the fraction of biomass in roots was usually higher under low-nutrient conditions. The relationship between the biomass of plant structures fits the general allometric equation, with an exponent 1 in most of the species. The different biomass ratios under the two nutrient conditions represented points on simple allometric trajectories, indicating that natural selection has resulted in allometric strategies rather than plastic responses to nutrient level. In other words, in most of the species that changed allocation in response to the nutrient treatment, these changes were largely a consequence of plant size. Our data suggest that some allocation patterns that have been interpreted as plastic responses to different resource availabilities may be more parsimoniously explained as allometric strategies.

Competition and Allometry in Three Species of Annual Plants

Comparisons between competing and noncompeting populations of three annual plant species demonstrate that plant allometry is altered by competition. When plants are grown in isolation, relationships between stem diameter, height, and plant mass generally show simple allometry (i.e., the relationships are linear on log-log scale). When plants are competing, however, these relationships are curvilinear or discontinuous. We attempt to clarify the relationship between interindividual allometry (the usual source of allometric data) and allometric growth. When plants are not competing, the allometric relationships among individuals of different sizes at one point in time and the allometry of individual plants as they grow appear to be similar, but these two classes of allometric relationships are very different for competing plants. We present a simple model that explains both static and dynamic patterns of plant allometry in terms of (a) the allometric responses of individual plants to competition, and (b) the size dependence of growth after the onset of competition. Our results illustrate the importance of reciprocal interactions between competition and allometry, and emphasize the difficulty in making inferences from one size measure to another. We conclude that the commonly held assumption that plant size is a single entity, which can be reflected by any of several measures, may not be justified.

Neighbourhood interference amongst Pinus rigida individuals

(1) Recent yearly bole growth of individual trees, as estimated from height and annual growth ring measurements, is considered as a function of the number, distance and size of neighbours in a young Pinus rigida stand in New Jersey. (2) To measure the annual increase in tree bole volume, an allometric model of tree bole growth was developed. In the model, the cross-sectional area of annual growth rings is constant along the length of the bole, which is constructed as concentric ellipsoids. A complete ring profile of one individual tree is consistent with this model. (3) Significant correlations between individual plant growth rate and several measures of local interference demonstrate that interference is occurring. (4) The size of neighbours, estimated from height and girth measurements, was the most important single variable in the regressions on individual plant growth; the number and distance of neighbours was significant but of less importance. The angular dispersion of neighbours within 2 m did not make a significant contribution to the variation in individual tree growth. (5) The results are consistent with a model in which the growth of an individual is inversely related to the total effect of interference, and the contribution of each neighbour to this effect is additive in proportion to its size and inversely proportional to the square of its distance. (6) While the results show, as expected, that the effect of a neighbour decreases with its distance, they do not allow one to distinguish between alternative formulations with confidence. However, a modified version of the model in which the effect of a neighbour decreases with its distance always resulted in a slightly improved fit over the original formulation in which a neighbours effect decreases with the square of its distance.

A Neighborhood Model of Annual‐Plant Interference

A model is constructed in which seed production by individual annual plants within a population is a function of the number and species of individuals within each of several concentric neighborhoods. The effect of increasing competition is to reduce seed production in a hyperbolic fashion, and the contribution of each individual to this effect is inverse proportion to the square of its distance from the test individual. A simple monospecific version of this model was tested on populations of two annual knotweeds. A least—squares fit of the model accounted for over 80% of the variation in seed production. This model provides an alternative to density in describing plant populations. A monospecific aggregated version can be seen as an extension of the inverse—yield law, which has been widely applied to monocultures. See full-text article at JSTOR

DESCRIBING INEQUALITY IN PLANT SIZE OR FECUNDITY

Lorenz curves have been used to describe inequality in plant size and fecundity, where the total inequality is summarized by the Gini coefficient. Here we propose a second and complementary statistic, the Lorenz asymmetry coefficient, which characterizes an important aspect of the shape of a Lorenz curve. The statistic tells us which size classes contribute most to the populations total inequality. This may be useful when interpreting the ecological significance of plant size or reproductive inequality.