Restoration at the roots: Nurse plant and soil addition as catalysts for seedling establishment and plant growth in Ozark glades
Many grassland plant species depend on symbiotic relationships with arbuscular mycorrhizal fungi (AMF), and AMF/plant interactions are often plant species specific. Use of soil from intact grasslands as a primer for restoration could be helpful. However, benefits likely depend on proximity to mycorrhizal nurse plants to maintain interactions. We experimentally transferred soil and AMF host plants to plots in three early restoration glades in a factorial design. We selected nurse plants with high AMF affinity: Rudbeckia missouriensis (an early-successional species) and Schizachyrium scoparium (a mid-successional species). For soil amendment, we combined soils from two intact glade communities. We controlled for possible disturbance effects of experimental manipulations by excavating and refilling “mock” planting holes and by sterilizing soil in half of the treatments before addition. Within each destination plot, plant growth was monitored for 15 mos. for extant individuals of Schizachyrium scoparium, Rudbeckia missouriensis and Ruellia humilis (mid-successional). Each growing season, we transplanted seedlings (phytometers) of R. missouriensis and S. scoparium into plots approximately 0.5 meters from treatment locations and measured seedling growth. Because most S. scoparium seedlings died in the second growing season, we only analyzed growth of R. missouriensis seedlings that year.
Treatment effects on shoot architecture of seedlings varied between species and for R. missouriensis, differed between years. Results suggest that host plants and soil microbes have transient, species specific effects on seedling shoot growth and developement. Extant plants of S. scoparium grew more rapidly when provided with glade soil and proximity to conspecific nurse plants, suggesting positive, endosymbiont-mediated density dependence for this mid-successional species. Growth of R. missouriensis plants did not vary among treatments. Surprisingly, extant R. humilis benefitted from the combination of inoculum with a heterospecific nurse plant, producing few flowers in treatments where sterile soil was added and nurse plants were absent.. Results support the idea that benefits of soil microbes increase in later successional species but vary with life cycle stage, indicating their potential as a restoration tool depends on age and composition of the plant community.