Nitrogen fixation is actually accomplished by bacteria living in the roots of host legumes. The usual practice is to directly apply bacterial inoculum to seeds just prior to planting rather than gamble that sufficient amounts of appropriate bacteria already exist in the soil. Nitrogen fixing trees such as alders, black locust,or mesquites also form bacterial associations that make them useful as sources of nitrogen for other plants and animals in silvopastures. Competition between trees and pasture is reduced by selecting pasture plants which either grow at a different time of year, or are more shallowly rooted than trees. In the Pacific Northwest, shallowly rooted subterranean clover-ryegrass pastures actually reduce summer moisture stress in trees compared to ungrazed naturalized grasses which are more deeply rooted and grow longer into the dry season.
Trees reduce forage production by competing for soil moisture, nutrients and light.
The major factor reducing forage production in open canopy forests, such as young timber stands, is probably soil resources rather than light. Forage plants which grow primarily during the rainy season, and those with high drought tolerance are most likely to compete successfully with trees in established young agroforests. As tree canopies begin to coalesce into a closed canopy, tolerance of low light conditions becomes important in selecting forage species. Livestock Component Trees in pasture provide shelter for livestock during periods of inclement weather. This can significantly improve animal performance during particularly hot or cold times of the year.