Facilitation at Early Growth Stages Results in Spatial Associations and Stable Coexistence in Late Growth Stages of Two Long‐lived, Dominant Shrubs

While models of species coexistence largely focus on how competition defines biological communities, over recent decades, a number of studies show positive plant–plant species interactions (facilitation) can also promote stable coexistence. The long‐lived, co‐dominant shrubs California buckwheatEriogonum fasciculatumand California sagebrushArtemisia californicashare a well‐documented positive association at the habitat level in their native California coastal sage scrub ecosystem, but mechanisms underlying their interactions remain unclear at finer spatial scales. Here, a hypothesis thatE. fasciculatumacidifies CSS's alkaline soils and facilitatesA. californicathrough amelioration of alkalinity stress is tested in a greenhouse experiment and association tests in the field. Greenhouse results demonstrate facilitation at early growth stages. In late growth stages, water competition is known to determine the shrubs' interactions with each other, but here, field observations of the shrubs in late growth stages show positive associations betweenA. californicaandE. fasciculatumthat have a positive linear relationship to increasing soil pH. These results highlight the importance of understanding lifecycle‐long interactions among species in evaluating facilitation's impacts on community structure.