Nucleation sites and forest recovery under high shrub competition

Forests currently face numerous stressors, raising questions about processes of forest recovery as well as the role of humans to stimulate recovery through planting trees that might not otherwise regenerate. Theoretically, planted trees can also provide a seed source for further recruitment once the planted trees become reproductive, acting as “nucleation” sites; however, it is unclear whether changing site conditions over time (e.g., through the growth of competitors like woody shrubs) influences establishment in the long term, even if seed availability increases. We tested the nucleation concept in a system where shrub competition is known to influence tree establishment and growth, performing an observational study of sites within and close to newly reproductive planted stands in yellow-pine and mixed-conifer ecosystems in the Sierra Nevada, CA. We surveyed and then modeled both seedling occurrence and density as a function of distance to seed sources, competing woody vegetation, and...