Ontogenetic shifts in interactions among annual plants

Interactions among plants strongly influence the structure and dynamics of plant populations and communities. However, most empirical studies of plant-plant interactions failed to make repeated measures of responses to neighbouring individuals and thereby neglected possible changes in interactions throughout the life history of the plants. We tested the hypothesis that competition between annual species intensifies from early to late life-history stages, by sequentially measuring interactions in neighbour-removal experiments at three study sites located along a rainfall gradient in Israel. Two annual species, Biscutella didyma and Hymenocarpos circinnatus, grew with and without neighbours in their natural habitats. Five response variables representing consecutive life-history stages (seedling survival, juvenile biomass, adult survival, number of seeds and final biomass) were recorded throughout the whole growing season. The direction and intensity of interactions varied considerably between environments and life stages. On average, growth-related response variables indicated higher competition intensity at the productive end of the climatic gradient, while survival indicated either facilitation at the dry end or no trend along the gradient. Temporal changes occurred, with moderate facilitation soon after germination shifting to strong competition at the end of the growing season. Our results demonstrate that the outcome of experimental studies on plant-plant interactions may depend not only on the environmental productivity but more so on the life stage at which the target plant is studied.