I have read this book some time, ago. It may give you some ideas for the student project: http://www.publicbookshelf.com/public_html/A_Year_in_the_Wonderland_of_Trees_/whatcause_dd.html

It is probably essential to have a long period of rainfall records covering the decades before, as well as after the trees were planted (or removed !) That was available when demonstrating that the filling up of Lake Kariba (between Zimbabwe and Zambia) seemed to have increased precipitation in surrounding areas. [There must, surely, be comparable studies for parts of the Amazon basin, where deforestation has been particularly severe]. Even so, conclusions have to be based.on *statistical probability*; i.e., you can never really claim proof with 100% certainty... If (as you say) only a small area of trees is involved, then the task will become more difficult. Strong wind-flow in the region concerned will carry much of the transpired moisture away before it can contribute to increased rainfall - so the overall effect may not always be easy to detect.