Biomass explains the intensity of facilitative – not competitive – interactions: three intraspecific tests with annuals

摘要

Despite efforts to discern the role of plant size in resource competition,the circumstances under which size-dependent plant-plant interactions occurare still unclear. The traditional assumption is that competitionintensifies with increasing neighbour size. However, recent studies suggestthat the size (biomass) dependence of competitive interactions is strongestat very low biomass levels and becomes negligible after a certain thresholdneighbour biomass has been reached. We searched for the generality of suchpatterns for three common annual plant species in Israel. We monitoredtarget and neighbour biomass along their entire lifecycle using aneven-aged, intraspecific and intrapopulation competition screenhouseexperiment under water-limited conditions. For all focal species, neighbourpresence had a net negative effect on vegetative biomass at harvest.However, this was not explained by increasing neighbour biomass over time,as a consistent pattern of size-dependent facilitative, rather thancompetitive, interactions was observed at all life stages. We explain theseobservations in terms of co-occurring aboveground facilitation and dominantbelowground competition for water. Since our findings are the first of theirkind and contradict theoretical predictions of biomass dependence of netnegative interactions, we advocate further experiments addressingsize dependence in interactions among plants. In particular, theoreticalmodels addressing size dependence of positive interactions must bedeveloped.