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Tree species in agroforestry ecosystems contribute to the livelihoods of rural communities and play an important role in the conservation of biodiversity. Unless agroforestry landscapes are productive, however, farmers will not maintain or enhance the range and quality of tree species in them, and both income opportunities and biodiversity will be lost. Productivity depends on both tree species diversity and genetic (intra-speciWc) variation, but research on the latter has until recently not received the recognition it deserves. Worse, when knowledge on tree genetic variation in agroforestry systems has become available, it has not generally been linked in any systematic way with management, indicating a disjunction between research and Weld-level practice. In this essay, we attempt to bridge this gap by considering three questions: why is genetic diversity important in tree species? What is our current state of knowledge about intra-speciWc variation in trees in agroforestry systems? And, Wnally, what practical interventions are possible to support the conservation of this diversity in agricultural landscapes, while enhancing farmers' livelihoods? A wide genetic base in agroforestry trees is essential to prevent inbreeding depression and allow adaptation to changing environmental conditions and to altering markets for tree products. Recent evidence shows, however, that many species are subject to poor germplasm collection practice, occur at low densities in farmland, and are found in highly aggregated distributions, all of which observations raise concerns about productivity and sustainability. A range of germplasmaccess based interventions is necessary to improve current management, including the enhancement of community seed-and seedling-exchange networks, and the development of locally based tree domestication activities. Equally necessary, but more diYcult to address, is Biodivers Conserv (2009) 18:969-986 1 C the development of markets that support genetic diversity in tropical tree species; we discuss approaches by which this may be undertaken.

Anthropogenic climate change has significant consequences for the sustainability and productivity of agroforestry ecosystems upon which millions of smallholders in the tropics depend and that provide valuable global services. We here consider the current state of knowledge of the impacts of climate change on tree genetic resources and implications for action in a smallholder setting. Required measures to respond to change include: (1) the facilitated translocation of environmentally-matched germplasm across appropriate geographic scales, (2) the elevation of effective population sizes of tree stands through the promotion of pollinators and other farm management interventions; and (3) the use of a wider range of 'plastic' species and populations for planting. Key bottlenecks to response that are discussed here include limitations in the international exchange of tree seed and seedlings, and the absence of wellfunctioning delivery systems to provide smallholders with better-adapted planting material. Greater research on population-level environmental responses in indigenous tree species is important, and more studies of animal pollinators in farm landscapes are required. The development of well-functioning markets for new products that farmers can grow in order to mitigate and adapt to anthropogenic climate change must also consider genetic resource issues, as we describe.

Farmers use and conserve a large variety of tree species. In Meru, a tree census on 35 farms covering 60 ha was conducted. This study included farmer interviews and biological measurements, with about 63,000 trees and 297 species being recorded. This paper discusses tree densities per species and germplasm sources for trees and species. The low densities and limited influx of germplasm from outside the farming community for some species, may result in an increased vulnerability to inbreeding and genetic erosion. This paper aims to provide some baseline data for understanding genetic resource management in agroforestry systems. It also provides suggestions for interventions to lower the vulnerability for species in Meru. Farmers need to have increased access to germplasm to diversify their farms in terms of species evenness, by substituting trees of more common species with trees of rarer species, or by increasing trees of rarer species.