COS 144-5 - Soil microbes and competition limit conservative forb establishment in a restored temperate woodland

Abstract: Plant community reassembly in disturbed habitats is limited by dispersal barriers and environmental selection. In ecological restoration, these barriers can be overcome by translocating plants to suitable habitats, but establishment may still be limited by biotic barriers, such as a lack of soil microbial symbionts or competition with established vegetation. The goal of this study was to compare the effects of soil microbial composition and plant competition on the establishment of seven conservative forbs (i.e., forbs sensitive to habitat disturbance) in a chronosequence of restored woodlands in eastern Missouri, USA. We sowed seeds of each forb species into sterile medium inoculated with two teaspoons of live soil from either a remnant woodland or a restored woodland. We grew the seedlings in a greenhouse for 20 weeks before transplanting them (n = 832) into three restored woodlands differing primarily in time since the initiation of restoration actions (7-29 years), including a prescribed fire regime, invasive species control, and tree thinning. Transplanted forbs were protected from herbivory by deer fencing and were randomly assigned a factorial combination of two treatments – competition present versus competition removed (using glyphosate) and inoculation of soil microbes originating from a remnant woodland or from the transplantation site. After three years, we found that survival did not vary strongly across experimental treatments, but relative growth rates were additively affected by competition removal (+17-32%, p < 0.001) and to a lesser extent by remnant soil inoculation (+4-11%, p = 0.067), such that relative growth was significantly > 0 only with competition removal and remnant soil inoculation, and relative growth was significantly < 0 only with competition present and soil inoculation from the transplantation site. We also found that experimental removal of competition improved the odds of flowering for two of seven species (odds increased by 3.6× for Heuchera americana, p < 0.001; 2.6× for Solidago caesia, p = 0.016), whereas remnant soil inoculation increased fruiting probability for all species (2.6×, p = 0.014). Our results indicate that both competition and soil microbial origin impacted the establishment and reproduction of conservative forbs in a restored temperature woodland and that these effects varied across life history stages (e.g., flowering vs. fruiting) and plant species. For restoration practitioners, this study highlights that remnant soil inoculation during nursery propagation and repeated competition removal were more important than the age of restored woodlands for determining transplantation success.