Encroachment of woody species is a significant threat in native grasslands. This study exams the interaction between shrub-sapling overstory and ground layer species and functional group (FG) cover, richness, and diversity along a woody encroachment gradient. The study area is a 65 ha mosaic of tallgrass prairie and shrubland in northeastern Illinois. The study questions were: (1) how do plant species and FG cover, richness, and diversity respond to increasing levels of woody encroachment, (2) are there levels of encroachment relevant to restoration opportunities, and (3) are there patterns of FGs associated with increasing levels of encroachment that can serve as ecological indicators? Data on ground layer and overstory structure including stem density, canopy cover, and leaf area index (LAI) were recorded; LAI explained the most variance in the ground layer data.
There was a significant decline in several ground layer species and FG parameters with increasing woody encroachment (LAI) and percent bare ground increased. For most parameters, change followed a linear response; however, native species diversity and FG density declined only after LAI reached intermediate levels and the changes are best fit by a quadratic function. Cover and species richness of perennial dicot (PD) forbs, C3 and C4 grasses, legumes and hemi-parasites declined with increasing LAI. Native FGs most strongly associated with the lowest LAI levels were hemi-parasites, C4 grasses, legumes, and PD forbs. Efficacious opportunities for restoration remain at this site following low-to-intermediate levels of woody encroachment and results highlight ordered patterns of decline in cover and richness of FGs that may guide evaluating restoration potential of sites undergoing woody encroachment.