Trees hold nutrients in an efficient, closed cycling system. Their deeper roots tap nutrients from lower soil levels that are inaccessible to forage species, and nitrogen-fixing trees can raise the nutrient levels of pasture soils. Roots combat the compaction of soil and cut soil and organic matter losses to erosion, thereby improving soil structure and soil biological activity. Shade and reduced wind velocity raise the moisture level of soils by reducing evaporation. Trees planted in a living snow fence can raise soil moisture by harvesting and distributing snow. Some species can remedy soil toxicities that limit livestock forage production, such as salinity.