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The role of agroforestry systems in biodiversity conservation was investigated in the_semiarid Tehuacan Valley, Central Mexico. Richness and diversity of native plant species were compared between agroforestry systems (6 sampling sites) and the following forests (6 sampling sites) dominated by :columnar cacti: (i) "chichipera" dominated by Polaskia chichipe; (ii) dominated by Escontria chiotilla; and (iii) "garambullal" dominated by Myrtillocactus schenckir. Our information on genetic variation of dominant arboreal species in the study sites was reviewed and included in the analysis. Factors influencing household’s decisions to maintain vegetation cover were compiled through a survey and interviews and analyzed. All the samples of the agroforestry systems studied maintained on average nearly 59% plant species and 94% genetic variation of dominant cacti occurring in the forests, although their ability to preserve endemic rare species is limited. Social factors favoring maintenance of perennial species in agricultural plots include collective rules, households traditions, use of the plants maintained in the systems, and the environmental information gathered from NGOs, the local Biosphere Reserve, and researchers. However, agroforestry systems are losing their capability to maintain vegetation cover, mainly because of (i) decreasing amount of land managed by households, determined by a progressive fragmentation of the land area given to new families, (ii) adoption of technologies to intensify agriculture, and (iii) governmental programs penalizing the presence of vegetation patches within agricultural lands since they are considered "useless" areas. Necessary policies to stop degradation of the agroforestry systems and to improve their conservation capacity are discussed.