Abstract. We tested the prediction from spatial competition models that intraspecific aggregation may promote coexistence and thus maintain biodiversity with experimental communities of four annual species. Monocultures, three-species mixtures, and the fourspecies mixture were sown at two densities and with either random or intraspecifically aggregated distributions. There was a hierarchy of competitive abilities among the four species. The weaker competitors showed higher aboveground biomass in the aggregated distribution compared to the random distribution, especially at high density. In one species, intraspecific aggregation resulted in an 86% increase in the number of flowering individuals and a 171% increase in the reproductive biomass at high density. The competitively superior species had a lower biomass in the aggregated distribution than in the random distribution at high density. The data support the hypothesis that the spatial distribution of plants profoundly affects competition in such a way that weaker competitors increase their fitness while stronger competitors are suppressed when grown in the neighborhood of conspecifics. This implies that the spatial arrangement of plants in a community can be an important determinant of species coexistence and biodiversity.

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 * E-mail: jw@kvl.dk. † E-mail: stoll@sgi.unibe.ch. ‡ E-mail: helene@eno.princeton.edu. § E-mail: ajasen@concentric.net. 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.Keywords: asymmetric competition, individual-based models, population structure, size inequality, spatial effects, zone of influence.Competition among individuals usually increases size variation within plant populations, but there is controversy over the mechanisms through which this occurs. This controversy reflects a fundamental disagreement about the nature of competition among individual plants. Some studies have concluded that a major factor generating size variation in crowded plant populations is the "size asymmetry" of competition: larger plants have a disproportionate advantage (for their relative size) in competition with smaller plants, suppressing their growth (Begon 1984;Weiner 1990;Schwinning and Weiner 1...

Abstract. Although accumulating evidence indicates that local intraspecific densitydependent effects are not as rare in species-rich communities as previously suspected, there are still very few detailed and systematic neighborhood analyses of species-rich communities. Here, we provide such an analysis with the overall goal of quantifying the relative importance of inter-and intraspecific interaction strength in a primary, lowland dipterocarp forest located at Danum, Sabah, Malaysia. Using data on 10 abundant overstory dipterocarp species from two 4-ha permanent plots, we evaluated the effects of neighbors on the absolute growth rate of focal trees (from 1986 to 1996) over increasing neighborhood radii (from 1 to 20 m) with multiple regressions. Only trees 10 cm to Ͻ100 cm girth at breast height in 1986 were considered as focal trees. Among neighborhood models with one neighbor term, models including only conspecific larger trees performed best in five out of 10 species. Negative effects of conspecific larger neighbors were most apparent in large overstory species such as those of the genus Shorea. However, neighborhood models with separate terms and radii for heterospecific and conspecific neighbors accounted for more variability in absolute growth rates than did neighborhood models with one neighbor term. The conspecific term was significant for nine out of 10 species. Moreover, in five out of 10 species, trees without conspecific neighbors had significantly higher absolute growth rates than trees with conspecific neighbors. Averaged over the 10 species, trees without conspecific neighbors grew 32.4 cm 2 in basal area from 1986 to 1996, whereas trees with conspecific neighbors only grew 14.7 cm 2 in basal area, although there was no difference in initial basal area between trees in the two groups. Averaged across the six species of the genus Shorea, negative effects of conspecific larger trees were significantly stronger than for heterospecific larger neighbors. Thus, high local densities within neighborhoods of 20 m may lead to strong intraspecific negative and, hence, density-dependent, effects even in species-rich communities with low overall densities at larger spatial scales. We conjecture that the strength of conspecific effects may be correlated with the degree of host specificity of ectomycorrhizae.

Patients showed statistically significantly lower parenchymal enhancement in menstrual cycle days 7-20 than in days 21-6. Patients aged 35-50 years yielded higher parenchymal enhancement than did younger patients and older patients.

No abstract

No abstract

Summary 1We used simulated and experimental plant populations to analyse mortality-driven pattern formation under size-dependent competition. Larger plants had an advantage under size-asymmetric but not under symmetric competition. Initial patterns were random or clumped. 2 The simulations were individual-based and spatially explicit. Size-dependent competition was modelled with different rules to partition overlapping zones of influence. 3 The experiment used genotypes of Arabidopsis thaliana with different morphological plasticity and hence size-dependent competition. Compared with wild types, transgenic individuals over-expressed phytochrome A and had decreased plasticity because of disabled phytochrome-mediated shade avoidance. Therefore, competition among transgenics was more asymmetric compared with wild-types. 4 Density-dependent mortality under symmetric competition did not substantially change the initial spatial pattern. Conversely, simulations under asymmetric competition and experimental patterns of transgenic over-expressors showed patterns of survivors that deviated substantially from random mortality independent of initial patterns. 5 Small-scale initial patterns of wild types were regular rather than random or clumped. We hypothesize that this small-scale regularity may be explained by early shade avoidance of seedlings in their cotyledon stage. 6 Our experimental results support predictions from an individual-based simulation model and support the conclusion that regular spatial patterns of surviving individuals should be interpreted as evidence for strong, asymmetric competitive interactions and subsequent density-dependent mortality.

We studied the variation in growth of individual trees in a population of Pinus sylvestris near Zurich in relation to tree age, size, and local competition. This population established naturally when the drainage pattern in a peat bog was altered after the Second World War. An unusual opportunity to study stand development was afforded when the trees were harvested in 1987 as part of a restoration program. All trees stumps were mapped, and growth since establishment was measured as cross—sectional area of wood in 5—yr intervals, using disks from the base of each tree. The distribution of cross—sectional area was lognormal over most of the period of growth. Size at the beginning of a time interval was the primary determinant of individual growth during that interval, but age and local competition also had significant effects. Younger, smaller trees were better fitted by an exponential growth model, whereas older, larger trees were better fitted by a Gompertz (sigmoidal) model. Some of the result were very different from previous studies: (1) The spatial distribution of trees was not significantly different from random. (2) Size variability decreased during the course of stand development, even though there was no self—thinning. This was because (3) the density of natural establishment was low, and competition, although clearly detectable, was still relatively weak and symmetric after 45 yr of stand development. Thus, there was no initial advantage in competition, and the younger trees, which were still growing exponentially, were able to catch up with the older trees, whose relative growth rates were declining.