Asymmetric competition in plant populations

Light-Gap disturbances, recruitment limitation, and tree diversity in a neotropical forest

Light gap disturbances have been postulated to play a major role in maintaining tree diversity in species-rich tropical forests. This hypothesis was tested in more than 1200 gaps in a tropical forest in Panama over a 13-year period. Gaps increased seedling establishment and sapling densities, but this effect was nonspecific and broad-spectrum, and species richness per stem was identical in gaps and in nongap control sites. Spatial and temporal variation in the gap disturbance regime did not explain variation in species richness. The species composition of gaps was unpredictable even for pioneer tree species. Strong recruitment limitation appears to decouple the gap disturbance regime from control of tree diversity in this tropical forest.

Asymmetric competition between plant species

Despite extensive interest in the role of plant size in competition, few formal attempts have been made to quantify the magnitude of asymmetric competition, particularly for interactions between members of different species. This paper introduces the concept of asymmetric interspecific competition at the population livel (i.e. mean plant performance) in mixtures of species. It proposes an index of interspecific competitive asymmetry which allows for a progressively greater asymmetric effect as the average size differences between competing species increase, and allows for such an effect whether individuals of focal species are larger or smaller, on average, than competitors. This index of competitive asymmetry is evaluated in the study of interactions between two widely coexisting annuals of disturbed habitats, Stellaria media and Poa annua. An experiment was conducted in which the density, relative frequency and relative seedling sizes (emergence times) of Poa and Stellaria individuals were varied. The relative growth rate (RGR) for both species was measured over a 22-day period. An inverse linear model was fitted for each species, relating the RGR of the focal species to the initial biomass of each species. Each response model included an asymmetry coefficient (β) to assess whether the impact of a unit of initial biomass of the associate species changed with the relative sizes of seedlings of the two species. A zero value of β implies symmetric competition between the two populations; i.e. the competitive effect of a unit of associate species biomass does not change with its initial seedling size. If β is positive the smaller the initial relative size of seedlings of the associate species, the smaller their per unit biomass effect on the response of the focal species. The model fitted our data for Stellaria and Poa well and was validated by an alternative modelling approach. Asymmetry coefficients were estimated as 0.508 (P<0.05) for the effect of Poa in the Stellaria model, and 0.0001 (NS) for the effect of Stellaria in the Poa model; i.e. the effect of Poa on Stellaria was asymmetric while the effect of Stellaria on Poa was symmetric. Differences in interspecific species asymmetric competitive effects are discussed within the context of shoot architecture, and the relative importance of competition for light versus soil resources. Finally, we discuss the relationship of this model to earlier models of competitive asymmetry, and consider the implications of interspecific competitive asymmetry for a number of current theories of plant competition and community organisation.

Genetic engineering of harvest index in tobacco through overexpression of a phytochrome gene

The phytochrome photoreceptor family regulates plant architecture in response to environmental light signals. Phytochromes mediate the shade avoidance syndrome, in which plants react to far-red radiation reflected from neighbors by elongation growth, occurring at the expense of leaf and storage organ production. We show that transgenic overproduction of phytochrome A in tobacco suppresses shade avoidance, causing proximity-conditional dwarfing. At high densities in the field, assimilates show an enhanced allocation to leaves, with a concomitant increase in harvest index. Transfer of this approach to other crop plants could provide significant improvements in productivity.

On the cause of offspring superiority conferred by mycorrhizal infection of Abutilon theophrasti

In previous studies we demonstrated that for Abutilon theophrasti Medic, offspring of mycorrhizal plants grew more rapidly than did offspring of non-mycorrhizal plants. This observation was verified in the current study. We hypothesized that the faster growth of offspring of mycorrhizal plants was caused by a superior ability to absorb nutrients, since offspring of mycorrhizal plants produced faster-growing root systems with higher phosphatase activities. We tested the hypothesis by growing offspring plants in distilled water. Despite the absence of any nutrients short-term growth rates of offspring of mycorrhizal plants were still significantly greater than those of offspring of non-mycorrhizal plants. We conclude that the faster growth of offspring of mycorrhizal plants is nor caused by a superior ability to acquire external resources.