"In situ" conservation means "the conservation of ecosystems and natural habitats and the maintenance and recovery of viable populations of species in their natural surroundings and, in the case of domesticated or cultivated species, in the surroundings where they have developed their distinctive properties". "In situ" conservation comprises both conservation of trees and crop wild relatives "in situ" in the wild, and conservation of landraces and neglected and underutilized species on farm in farmers' fields. Conserving agrobiodiversity "in situ" has the benefit that species can continue to evolve in response to natural and human pressures. In the case of crops, a large amount of diversity is retained in developing countries by smallholder farmers, particularly for many crops in their centers of domestication and diversity. There, farmers continue to grow landraces and maintain traditional knowledge and seed management practices in a process known as de facto conservation. Home gardens too are repositories of high levels of species diversity, and traditional landraces contain wide genetic diversity. For forest trees, "in situ" conservation is considered the most appropriate method since most tree seeds cannot be conserved "ex situ", and because there are 60,000 tree species, each with multiple populations, so too many to identify and collect.