Light availability, not soil texture, correlates with shrub community structure in mesic forest gaps

Woody shrubs are important components of temperate forest understories, contributing to forest structure, diversity, and succession. However, the extent to which variation in canopy openness and soil texture influence shrub occurrence, diversity, and composition remains unclear. Because woody tissues require comparatively high light availability and sandy soils may favor shrubs over trees, we hypothesized that shrub richness and cover would increase in canopy gaps and sandier microsites. To address this, we quantified soil texture, canopy gap structure, and recorded shrub abundance in twenty randomly stratified plots within sixteen different mesic forests in Wisconsin, USA. Using a mixed-effects modeling approach, we determined that shrub richness, Shannon diversity, and coverage increased significantly in more open canopy gaps, while soil sand content had no significant impact on these metrics. Canopy gaps also had a significant effect on the composition of shrub communities at the plot level. At the stand level, neither sandier nor more open-canopied forests had more diverse, dense shrub layers, though more heterogenous canopies had a weak negative correlation with shrub richness. Broadly, shrub abundance and cover increased in more open gaps, though individual species demonstrated significantly different preferences in terms of light availability and soil sand content. Our results demonstrate the importance of local light availability for shrub growth. However, species-specific responses to soil texture and light availability and the modeled importance of unmeasured site-level factors suggest that additional environmental and biotic factors are likely to impact this diverse temperate guild.