Any plant that helps to cover the soil surface acts as a ground cover plant that benefits the entire forest garden by protecting the soil. In a skoghage , however, it makes sense to use plants with many functions - whether they are edible, have medicinal value, collect minerals, fix nitrogen or attract beneficial insects. 

Often there is only one plant layer under the shrubs in the forest garden and that is the perennial plants. But perennial plants can be quite tall and generally perform poorly as ground covers, so it would make sense to introduce two or more layers of ground covers. 

Very often the lowest plant layers need to be shade tolerant since they can have quite a few layers of plants above them. Another important factor is that many do not tolerate being walked on too often, so it can be a good idea not to walk on the same places all the time. 

If you co-plant in this layer, it's a good idea to combine plants with deep roots with plants with shallow roots. This avoids competition between the plants. Remember that several types of grass are very competitive with almost everything in the forest garden.