, 2007). The seed production of many agroforestry trees is often informal and very few countries have included these species in their tree improvement programmes. Germplasm of exotic tree species, typically from introductions of unknown provenance and uncharacterised performance, is often collected by smallholders directly for their find more own planting.

Lillesø et al. (2011), for example, identified five sources for farmers’ tree planting material (farmland, natural forest, plantations, seed orchards and vegetative propagules) and indicated heavy reliance on the first source, with natural forest sources being underutilised. Farmers and local seed dealers often prefer to collect seed from previously learn more introduced exotic trees in farmland rather than source externally because the transaction costs are lower, even when better-performing seed sources of the same trees may be available elsewhere (Lengkeek et al., 2005 and Muriuki, 2005). In recent decades, there has been a greater focus on the cultivation of indigenous tree species in agroforestry systems, with the involvement of local people in carrying out genetic selection for tree characteristics of importance

to them. One such approach, known as participatory domestication, has been developed in Africa on indigenous fruit trees (see Dawson et al., 2014, this special issue). The advantage of this approach is that genetic quality as a concept is explicitly considered, and local wild stands provide significant genetic variation that is a pool for selection

(Tchoundjeu et al., 2006). The risk of spreading pests and diseases while transferring reproductive material is often considerable. Pests and diseases travel in different substrates and it is challenging anti-PD-1 antibody inhibitor to monitor the way they spread; for example, to reconstruct the exact pathways of their past movements. In Europe, Santini et al. (2013) reconstructed the most probable pathways of alien invasive forest pathogen spread since 1800. They found that living plants (57% of all pathogen introductions) and wood (10%) were likely major vectors for introductions, while the share of any other pathway, such as bark, seed, soil and cuttings, was less than 10% over the last two centuries. According to the same authors, over the last few decades, the invasion rate of alien forest pathogens has increased exponentially in Europe, with soil recently becoming a major transfer substrate second to living plants. In the USA, a similar study attributed 69% of the introductions of non-native forest insects and pathogens since 1860 to the trade in living plants (Liebhold et al., 2012). These studies confirm the need for phytosanitary regulations and their careful implementation while transferring tree germplasm. However, they also show that the pathogen risk associated with transferring seed is considerably lower than the risk connected with transferring other materials such as living plants or wood.