Neighbour interactions strengthen with increased soil resources in a northern hardwood forest

1. The relationship between soil resource availability and competitive interactions remains unclear in forest ecosystems. If competition shifts from below-ground to above-ground across soil resource gradients, then competitive interactions should constrain growth across soil fertility levels. Alternatively, competition may be less important under stressful conditions associated with low soil resources. 2. We developed individual-based growth models as functions of local tree neighbourhood and measurements of soil resources for 10 common tree species from sites established across a soil resource gradient in north-west Lower Michigan, USA. We hypothesized that tree growth should increase with soil resource availability (rarely measured at local scales), but decrease with the density, size and proximity of neighbouring trees. 3. Correlations of growth to neighbourhood effects were strongest for species occupying primarily high-resource sites. Correlations of growth to soil resources were positive for species associated with high and intermediate fertility. In contrast, correlations of growth to soil water were negative for species associated with low fertility, suggesting that competitiveness of these species decreased with higher soil resources and concomitant decreases in irradiance. 4. Relationships between mean site-level tree growth and soil resources were much stronger than individual growth-local resource relationships. Weaker species-specific, individual-level trends likely arose from limited species distributions across each soil resource domain. 5. Synthesis. Neighbourhood interactions were more prevalent in species associated with high soil fertility sites, where canopy transmission of irradiance was low. For species dominant at low fertility, where canopy transmission of irradiance was relatively high, neighbourhood interactions were absent or negligible. The growth of intermediate-fertility species was negatively correlated with soil water, but decreasing site-level canopy openness with soil water suggests that these species were out-competed for irradiance as soil resources increased. Thus, irradiance likely mediated the stronger competitive interactions at higher-fertility sites.