Dear Magda -
These messages are important. To add, I just wanted to draw attention to a recent paper in the Forest Policy and Economics Journal (http://www.sciencedirect.com/science/article/pii/S1389934116304592) which documents the extent of on farm tree growing across a number of African countries, using the LSMS-ISA data .
Malawi is one of the countries covered. The data are open access and the exercise may be useful by way of baseline in examining future endeavours (for an early WP version see http://documents.worldbank.org/curated/en/819461471962574880/pdf/WPS7802.pdf).