Abstract: Density-dependent competition is one of the most fundamental concepts in plant population ecology yet is not consistently detected in wild populations. This apparent incongruity could signify lesser ecological importance, but density dependence may be missed if its effects and spatial grain vary across life history stages or vital rates. We tested for density-dependent juvenile size and survival to flower at fine spatial scales over four years in a wild population of the biennial Sabatia angularis. We further derived ecological neighborhoods of juveniles empirically using density-based spatial clustering and quantified size inequalities to infer competition symmetry. Juvenile size depended on conspecific density within 2.5cm. Juvenile size, in turn, positively influenced survival to flower, suggesting a potential indirect, negative impact of density on survival. Density also had a direct negative effect on survival, detectable at larger spatial scales (5-10 cm). Neighborhood sizes and densities were highly variable, revealing a patchy, heterogeneous spatial distribution of juveniles that may partly reflect fine-scale resource heterogeneity. The positive effect of nearest neighbor size on focal plant size supports this interpretation. Although some neighborhoods exhibited evidence of moderate size inequality, inequality did not increase with neighborhood density. This pattern could signify largely symmetric competition but may also arise from other demographic or spatial processes. Our work highlights the shifting spatial scale of density dependence, and thus ecological neighborhoods, within populations. It further demonstrates the ecological importance of fine-scale negative density dependence for juvenile growth and survival in S. angularis amidst spatial heterogeneity, low survival, and temporal variation.

Key words: Density dependence, intraspecific competition, ecological neighborhoods, asymmetric competition, size inequalities, spatial heterogeneity, juvenile survival