Household poverty and food insecurity in Kenya is a major challenge especially in rural areas. The situation has been exacerbated by poor economic performance, nationwide drought and the global financial crisis. Nevertheless, numerous efforts are being made both by the government and non-governmental organizations to mitigate the situation in rural areas notably in Nyeri district which is a relatively poor district in central province with poverty rate standing at 30% (CBS, 2008). Nyeri district has been a target of a non-governmental organization, The International Smallgroup and Tree Planting Program (TIST). Its objective is to help locals increase their income and conserve environment by planting trees. Farmers are increasingly joining the program substituting land previously allocated for food crops with tree planting enterprise. Despite presence of TIST, poverty still remains high in the region, with the statistics showing an increase in the population living below the poverty line from 29% in 2000 to 33% in 2006 (CBS, 2008). The main objective of the study was to determine the effects of TIST program on household income, environmental services and to determine factors that influence participation through a survey questionnaire. Multistage sampling was applied in selection of 120 farmers and analysis was done using SPSS and STATA. Heckman two stage sample selection model was used in analyzing factors that determine participation and log-log model was used to quantify the contribution of the program to household income upon participation. Descriptive statistics were used to determine the effects of the program on environmental services. The results showed no significant difference in income between participants and nonparticipants. Other socioeconomic and farm specific factors showed significant difference in household incomes. Farm sizes, access to micro-finance and program awareness positively influenced participation and were significant at 5% level of significance. Farm size p>(z) value was 0.048, access to micro finance(0.018) and program awareness(0.029). The program had a positive effect on a variety of environmental services. The study recommends that TIST program should adopt a better tree planting management approach to avoid total substitution of crops with tree planting enterprise and collaborate with the government and other stakeholders in linking farmers with market for their produce so as to enable them market their surplus and further research be done to explore issues that this study was not able to capture like technical issues regarding carbon sinking, the cost being presently incurred and potential benefits in the future and opportunity cost of the foregone agricultural output.