Trees and crops can help each other grow. Some trees have a light, thin canopy which allows adequate light to filter through to the crops below. Crops growing with the trees save their own moisture because protection by the tree cuts down their rate of evapotranspiration. Many trees can fix nitrogen, enriching the soil when their residues decompose. This benefits subsequent non-leguminous crops, which do not have this capability.
Trees also improve the soil in other ways.
Leaf litter decomposes to add nutrients, and even the root system, with its rhizobia and related communities of organs, releases nutrients and improves soil structure when it decomposes. Some trees capture nutrients lying deep in the soil, too deep for crops to reach, and bring these nutrients back up to the surface and back to the soil as litter, where the crops can utilise them when the litter decomposes. In a simultaneous system, trees and crops or animals grow together, at the same time and on the same piece of land. In a sequential system, crops and trees take turns in occupying much of the same space. Simultaneous systems include, boundary plantings, contour hedges, live hedges and fences, parklands systems.