Effects of litter and standing biomass on growth and reproduction of two annual species in a Mediterranean old‐field

Abstract

Summary We tested the hypothesis that the competitive ability of a plant changes during its lifetime, by evaluating variations in the relative influence of standing biomass and litter on plant performance. Seedlings of two annual herbaceous species (‘targets’), of contrasted life‐forms (Crepis foetida and Bromus madritensis) were transplanted into an old‐field. The competitive ability of seedlings, vegetative and reproductive plants towards neighbours was recorded over one growing season. Fourteen traits related to plant morphology, growth and reproduction were measured to assess the competitive ability of targets. Relationships among traits were characterized to identify a set of traits as a surrogate of target competitive ability. The two target species responded similarly. Early growth was facilitated by litter and, to a greater degree, by vegetative biomass. Thereafter, the effect of neighbouring vegetation on target performance was negative, with a maximum depressing effect on reproduction (especially seed number and date of flowering) despite a weak facilitative effect of litter. Plant basal diameter, measured at peak growth period, and total number of inflorescences per plant, were found to predict growth and reproductive components of competitive ability, respectively. Assessing the effects of competition for population success from the response of vegetative plants will underestimate its importance because competition appears to exert its maximal impact on seed production.